Impact of early hydrocarbon charge on the diagenetic history and reservoir quality of the Central Canyon sandstones in the Qiongdongnan Basin, South China Sea

Impact of early hydrocarbon charge on the diagenetic history and reservoir quality of the Central Canyon sandstones in the Qiongdongnan Basin, South China Sea

Journal of Asian Earth Sciences 185 (2019) 104022 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.el...

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Journal of Asian Earth Sciences 185 (2019) 104022

Contents lists available at ScienceDirect

Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes

Impact of early hydrocarbon charge on the diagenetic history and reservoir quality of the Central Canyon sandstones in the Qiongdongnan Basin, South China Sea

T



Guangxu Bia,b, Chengfu Lyua, Chao Lia, , Guojun Chena, Gongcheng Zhangc, Qianshan Zhoua,b, Chengze Lia,b, Yilin Zhaoa,b a b c

Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China Research Institute of China National Offshore Oil Corporation, Beijing 100028, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Diagenesis Reservoir Central Canyon sandstones Qiongdongnan Basin Early hydrocarbon charge

The Central Canyon sandstones, located in the Ledong–Lingshui Depression, Qiongdongnan Basin, South China Sea, are the main reservoirs of the deep-water area. The present study aimed to examine the diagenetic processes and reservoir quality of these sandstones. Using an integrated approach of examination of casting thin sections, X-ray diffraction, porosity and permeability measurements, scanning electron microscopy, carbon and oxygen stable isotope analyses, fluid inclusion analyses and electron microprobe analysis, we analysed the diagenetic features of the sandstones and the reservoir quality of the LD30-1 and LS17-2 sandstones, respectively. The results showed that the sandstones consist of lithic arkoses and sublitharenites, the main pore systems consist of primary intergranular pores and secondary pores, and the main cements consist of authigenic quartz, clay minerals, and carbonates. By combining the analyses of burial history and hydrocarbon charging history, we discovered that the LD30-1 sandstone reservoirs and the LS17-2 sandstone reservoirs are quite different. A significant negative shift in δ13C and δ18O (ranging from −4.88‰ to −2.27‰ and from −11.83‰ to −5.96‰, respectively) is evident in the carbonate cements of the LD30-1 sandstones, and oxidation of organic matter had an important role in the occurrence of the carbonate cements. However, relatively high and concentrated δ13C and δ18O values in the LS17-2 sandstones, ranging from −1.76‰ to −0.48‰ and −5.04‰ to −2.95‰, respectively, are closer to the original isotopic compositions. Based on casting thin sections, the abundance of quartz overgrowths in the LD30-1 sandstones is higher than that in the LS17-2 sandstones. The development of each clay mineral in the LD30-1 and LS17-2 sandstones is quite different. The LS17-2 sandstones have higher reservoir quality than the LD30-1 sandstones. Early hydrocarbon charge is an important factor causing the differences of diagenetic minerals and reservoir quality between the LS17-2 sandstones and the LD30-1 sandstones.

1. Introduction In recent years, there has been great interest in studies of submarine canyons because of their potential for hydrocarbon exploration and the discovery of many great deep-water submarine canyon oil and gas fields, such as the North Midway–Sunset, California, and West Africa (Schamel et al., 1998; Khain and Polakova, 2004; Mayall et al., 2006; Bourget et al., 2008). Submarine canyons not only function as primary conduits for sediment transport by gravity flows from the shelf and upper slope into the deep basin (Kneller and Buckee, 2000; Shanmugam, 2006; Babonneau et al., 2013; Gales et al., 2014; Martín-



Merino et al., 2014; Bayliss and Pickering, 2015; Qin et al., 2016) but also provide storage space for the gravity flow deposits. Gravity flow deposits, often accumulating large amounts of oil and natural gas resources, are usually mostly composed of coarse-grained sedimentary rocks, including gravelly sandstones and massive or graded sandstones, along with some relatively fine-grained rocks, including siltstones and silty mudstones, in the deep-water region of the canyon (Macaulay et al., 1993; Jobe et al., 2018; McHargue et al., 2011; Talling, 2014; Bayliss and Pickering, 2015; Zhou et al., 2015; Corella et al., 2016). Diagenesis and reservoir-quality evolution pathways of the sandstones are complex, being governed by numerous interrelated parameters such

Corresponding author at: No. 382 West Donggang Road, Chengguan District, Lanzhou 730000, China. E-mail address: [email protected] (C. Li).

https://doi.org/10.1016/j.jseaes.2019.104022 Received 25 October 2018; Received in revised form 8 September 2019; Accepted 8 September 2019 Available online 09 September 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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Basin, with a length of 520 km, extends parallel to the continental slope and has the rare characteristic of being developed along the basin axis (Fig. 1). The Central Canyon reservoir is composed of channel sandstones superimposed by multiple gravity flows, which have been cut by a later watercourse and reconstructed by mass flows. Except for the sand layer on the top of the channel, the lateral distribution of other sand layers is continuous and stable. The gross thickness of the reservoirs in the channel is 200–300 m, and the thickness of a single layer is 20–60 m (Xie et al., 2016).

as detrital composition, depositional facies, climatic conditions, tectonic setting, burial history, as well as the chemical composition and flow patterns of basin fluids (Stonecipher et al., 1984; Wilson and Stanton, 1994; Morad et al., 2000). Deep-water hydrocarbon exploration for submarine canyons in the Central Canyon of the Qiongdongnan Basin has achieved great success in recent years. The sandstone reservoirs in the Central Canyon have been proven prolific by wells drilled on the LS22-1 and LS17-2 structural traps, with an estimated gas resource of more than 113.2 × 109 m3 (4 TCF) (Xie, 2014; Huang et al., 2016), indicating great gas exploration potential in the deepwater area of this basin. Thermal fluid-related diapirs provided effective pathways for the migration of gas generated by coal measure strata of the Yacheng Formation to the Central Canyon sandstones (Huang et al., 2016; Zhang et al, 2016; Gan et al., 2018). However, the evolution of the chemical compositions of inorganic minerals in hot oil–water–mineral systems involves interactions among pore water, gases, and minerals in systems with various compositions. Their alteration, which includes the physicochemical processes of dissolution, transfer of dissolved solutes, and concomitant precipitation of secondary minerals, occurs ubiquitously from shallowly to deeply buried rocks (Lasaga and Luttge, 2001; Kampman et al., 2009; Bjørlykke and Jahren, 2012). Feldspar dissolution pores, carbonate cements, and authigenic quartz and clays are common in the Central Canyon sandstones (Zhao et al., 2015; Zhong et al., 2018). However, the import and export of materials related to chemical diagenesis and the impact on reservoir quality remain subjects of intense debate. Thus, developing quantitative understandings of the diagenetic processes, sources of cements, and sinks of dissolved minerals in the sandstones is critical for quality prediction of the sandstone reservoirs (Taylor et al., 2010). Therefore, the objectives of this study were to investigate the diagenesis and reconstruct the diagenetic history of the Central Canyon sandstones and identify the sources of the cements in these sandstones. Based on petrographic and geochemical analyses, the impacts of early hydrocarbon charge on diagenetic minerals were explained and the controls of different diagenetic processes on reservoir quality were evaluated. The results provide a reliable basis for improving the geological theory of the gravity flow reservoir in a deep-water area.

3. Samples and methodology In this study, porosity and permeability were obtained from 260 core plug samples in the Central Canyon sandstone. The data were acquired from the China National Offshore Oil Corporation (CNOOC). A total of 23 core samples from the Central Canyon sandstone were collected from key wells (LD30-1, LS17-2) in the Central Canyon. All the well data and core samples were obtained from the CNOOC. We selected 12 samples to make casting thin sections to highlight porosity and facilitate the observation of pore types, features, distribution, and relationships with framework grains. The thin sections were stained with a mixed solution of alizarin red and potassium ferricyanide to distinguish carbonate minerals. In addition, the samples were observed under a scanning electron microscope (SEM). Carbon and oxygen isotope compositions of carbonate cements of 23 samples were analysed using a Thermo Finnigan MAT253 mass spectrometer at the Beijing Createch Testing Technology Co., Ltd. A minor amount of samples of 200 mesh powders were reacted with 100% orthophosphate at a temperature of 90 °C to obtain CO2. Replicate measurements of the internal laboratory standard gave a total analytical precision of ± 0.02‰ for both carbon and oxygen measurements. The results of the carbon and oxygen isotope analyses were reported relative to the Pee Dee Belemnite (PDB) standard and the Standard Mean Ocean Water (SMOW) standard, respectively. The δ18OV-SMOW values were calculated using the equation δ18OV-SMOW = 1.03086 × δ18OVPDB + 30.68 of Friedman and O'Neil (1977). Six representative samples were selected for in situ element analysis using an electron microprobe analyser (EMPA; JEOL JXA 8100) at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. The EMPA was operated under an accelerating voltage of 15 kV and a beam current of 12 nA using a beam spot of 10 μm and counting time of 5–10 s. The precisions for the major elements (Ca, Fe, Mg, Mn) of the carbonate cement analyses were better than 2%. Semi-quantitative and qualitative mineralogical compositions of bulk rock samples were determined by X-ray diffraction analysis (XRD). Twenty-three samples were analysed for whole-rock (bulk) and clay fraction (< 2 μm) mineralogy using XRD at the Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences. Qualitative analysis was done based on the JCPDS table (1974) and semi-quantitative analysis was performed by the software of ADM-V7, RMS Kempten.

2. Geological setting The Qiongdongnan Basin is located in the north-western part of the South China Sea. The basin is bounded by the Yinggehai Basin to the west and the Pearl River Mouth Basin to the east and neighbours Hainan Island to the north and the Xisha Uplift to the south (Fig. 1). There are four tectonics belts, which include the Northern Depression Belt, Central Uplift Belt, Central Depression Belt, and Southern Uplift Belt (Zhao et al., 2015). The Qiongdongnan Basin is one of the Cenozoic gas-bearing rift basins developed on Mesozoic basement. It formed in response to the rifting associated with the opening of the South China Sea. As illustrated in Fig. 2, the Qiongdongnan Basin contains sediments of the Eocene–Oligocene rift stage and the Neogene–Quaternary postrift thermal subsidence stage (Gong et al., 1997; Zhu et al., 2009; Hoang et al., 2010; Morley, 2016). The Qiongdongnan Basin covers an area of 82,900 km2 and contains a 6000–12,000 m thick sequence of Tertiary to Quaternary sediments (Zhu et al., 2009; Hu et al., 2013). Eight Cenozoic stratigraphic units occur from bottom to top: the Eocene Formation, Yacheng Formation in the early Oligocene, Lingshui Formation in the late Oligocene, Sanya Formation in the early Miocene, Meishan Formation in the middle Miocene, Huangliu Formation in the late Miocene, Yinggehai Formation in the Pliocene, and Ledong Formation in the Quaternary (Fig. 2) (Gong et al., 1997; Su et al., 2012; Li et al., 2007; Cao et al., 2015). The Central Canyon in the Qiongdongnan Basin was developed during the late Miocene and Pliocene. In contrast to most of the submarine canyons elsewhere on Earth, the Central Canyon in the Qiongdongnan

4. Results 4.1. Sandstone petrography The Central Canyon sandstones in Ledong Depression and Lingshui Depression are mainly sandstone debris flows deposits and turbidity current deposits, and the rock compositions vary. The Central Canyon sandstones in Ledong Depression are classified as lithic arkose and feldspathic litharenite (Fig. 3, red points). Monocrystalline quartz is the most abundant mineral, ranging from 36% to 49% with an average of 44.9%. Feldspar content varies from 20.5% to 38.9%, with an average of 30.1%, and comprises plagioclase and potassium feldspars. Rock fragments range in abundance from 14.7% to 29.8%, with an average of 25%. The sandstones consist of both coarse-grained and fine-grained 2

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Fig. 1. The location map of the basin showing the distribution of the sags and the location of the Central Canyon (outlined by the red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. The sketch showing the sequence classification, lithology character, biological fossil, depositional environment, relative change of sea level and tectonic stage of the Qiongdongnan Basin. The ages of the sequence boundary, the lithology and the relative change of sea level in the Qiongdongnan Basin were provided by Research Institute of China National Offshore Oil Corporation, the biological data was adopted from Gong et al. (1997), and the global eustatic curve was taken from Haq et al. (1987). 3

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4.3. Diagenetic mineralogy Aside from grain size and composition, the most important factor determining sandstone reservoir quality is diagenesis. On the basis of thin section observations, the significant diagenesis in the Central Canyon reservoirs includes mechanical compaction, dissolution, and cementation by calcite, quartz, clay minerals, and minor amounts of framboidal pyrite. 4.3.1. Carbonate cements Carbonate cements have very active chemical properties and can be dissolved and precipitated easily. Calcite and ferrocalcite are the primary carbonate cements in the Central Canyon sandstones (Fig. 7), ranging in abundance from trace amounts to 44% (from XRD data). Dolomite and ankerite are relatively less common, ranging from trace amounts to 2% (from XRD data). Multiple stages of carbonate cementation were recognized through petrographic and elemental analyses. The early calcite occurs as intergranular pore-filling cement (Fig. 7a, b, e), which formed in an alkaline environment of calcium carbonate oversaturated deposition. It formed before compaction. Later stage cements are common in intragranular pores of detrital grains (Fig. 7d, f) and in replaced feldspars and siliceous fragments (Fig. 7c, d). Hence, these formed in the late stage of diagenesis. The EMPA data, provided in Table 1, show that the carbonate cements in the samples studied are mainly calcite, and the average amounts of CaO, FeO, MgO, and MnO are 48.7%, 4.1%, 1.02%, and 1.14%, respectively. Stable C and O isotopes of the carbonate cements revealed a range of δ13CV-PDB values of −7.52‰ to −0.48‰ and a range of δ18OV-PDB values of −11.83‰ to −2.95‰, respectively (Table 2). These are helpful for inferring reactions and material sources during cement precipitation. Generally, the carbon isotopes can constrain the carbon source in diagenetic fluids (Macaulay et al., 1993; Fayek et al., 2001), and the oxygen isotopic compositions of carbonate cements can be used to estimate the temperature during cement precipitation (Friedman and O'Neil, 1977) and distinguish the formation environment (freshwater or marine water) (Zhang, 1985).

Fig. 3. Rock composition of the Central Canyon sandstones (sandstone classification after Folk 1974). 1 = Quartz arenite; 2 = Subarkose; 3 = Sublitharenite; 4 = Arkose; 5 = Lithic Arkose; 6 = Feldspathic Litharenite; 7 = Litharenite.

sandstones and are poorly sorted (Fig. 4a). The Central Canyon sandstones in Lingshui Depression are classified as sublitharenite (Fig. 3, blue points). Monocrystalline quartz is the most abundant mineral, ranging from 76% to 80% with an average of 79.6%. Feldspar content varies from 6% to 11.4%, with an average of 8.3%. Rock fragments range in abundance from 8% to 14%, with an average of 12%, and consist mainly of metamorphic rock fragments and volcanic rock fragments. The sandstones consist of fine- to very fine-grained sandstones and are well sorted (Fig. 4b). Generally, feldspar is replaced by carbonate (Fig. 4c) or dissolved (Fig. 4d). Low amounts of biotite (Fig. 4e), glauconite (Fig. 4f), chlorite, and fossil remains (foraminifera, mollusc fragments) (Fig. 4d, f) occur as detrital components in both sandstones. The fossils are reworked, partly disintegrated, and recrystallised.

4.2. Pore systems

4.3.2. Clay minerals As determined by SEM and XRD analyses of the Central Canyon sandstones, chlorite was present in almost all the analysed samples. However, the occurrence of illite and illite/smectite was limited. Based on XRD analysis, the proportion of chlorite in the total clay mineral content averages 91% (ranges from 85% to 94%) in the LD30-1 sandstones and averages 26.7% in the LS17-2 sandstones (Table 3). Chlorite is the predominant authigenic clay mineral, and two forms of chlorite, grain-coating and replacement of framework grains, were identified through petrological and mineralogical observations. The grain-coating chlorite is attached to surfaces of quartz, feldspar, and rock fragments. Generally, the chlorite is Fe-rich and euhedral, featuring radial orientations, and is located in primary pores and intercrystallite pores (Fig. 8a). These chlorite rims are sometimes engulfed by quartz overgrowths or dissolved by pore fluids (Fig. 8b). Moreover, the chlorite can also be observed on the intragranular edges of dissolved detrital grains (Fig. 8c). Lastly, chlorite replacing quartz grains was detected by SEM observations (Fig. 8d). I/S and kaolinite were not observed, and the XRD data showed that their content in the LD30-1 sandstones is 0% (Table 3). The proportion of illite in the total clay mineral content of the LD30-1 sandstones ranges from 4% to 15% (average of 8.8%) (Table 3). However, the proportion of illite in the total clay mineral content is higher, with an average of 35.9%, in the LS17-2 sandstones (Table 3). The XRD data showed contents of I/S and kaolinite in the LS17-2 sandstones of 11–26% and 15–21% (Table 3), respectively.

Pores in the Central Canyon sandstones were classified into four types: (1) primary intergranular pores between detrital grains, (2) secondary intergranular pores, (3) secondary intragranular pores, and (4) mouldic pores (Fig. 5). Based on observations made by microscope, there are no fractures in the sandstones. The dissolved grains are mainly feldspars and a small amount of monocrystalline quartz. Most of these grains are partially dissolved, forming intragranular pores, but others are completely dissolved, producing mouldic pores. Some secondary intragranular pores are surrounded by carbonate cement that shows no evidence of dissolution (Fig. 5c). These demonstrate that the dissolution of feldspars predates the formation of carbonate cements and that the pH of the fluid in the reservoir changed. However, detrital grain edges or cements, such as the early-stage carbonate cements, dissolved to form secondary intergranular pores, and secondary intergranular pores were often observed in the micrographs of casting thin sections of the sandstones. The Central Canyon sandstone reservoirs have ultrahigh porosity and high–ultrahigh permeability. According to the conventional plug sample analysis, the porosity ranges from 5% to 36%, with most samples falling within the range of 19–35%. The permeability ranges from 0.05 to 1300 mD, with most samples within the range of 50–1100 mD (Fig. 6). The correlation between permeability and porosity displays a log-linear relationship (coefficient of determination: R2 = 0.8113). Most of the sandstones are characterized by a loose and crumbly consistency and have low contents of matrix (averaging 6%) and cement (averaging 1%). Some samples having low permeability and porosity had undergone extensive carbonate cementation.

4.3.3. Quartz cements Quartz cements in the sandstones occur mainly as quartz 4

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Fig. 4. The lithology characteristics of the Central Canyon sandstones. (a) A section of core from Well LD30-1, 3250.25 m. (b) A section of core from Well LS17-2, 3342.20 m. (c) Photomicrograph acquired under crossed polarisers of a sample from Well LD30-1, 3428.36 m. The feldspar grain is replaced by carbonate. (d) Photomicrograph of a casting thin section (Well LD30-1, 3428.36 m). The feldspar grain has been dissolved. (e) Photomicrograph acquired under crossed polarisers of a sample from Well LD30-1, 3438.96 m. Fine sand grains have undergone intensive mechanical compaction and biotite is deformed. (f) Photomicrograph of a casting thin section (Well LS17-2, 3343.20 m). A small amount of glauconite and fossil fragments can be observed. Quartz is dissolved. Q = Quartz; FD = feldspar dissolution; F = feldspar; Fo = fossil fragments; Cal-R = calcite replaced; R = rock fragment; Gla = glauconite.

XRD analyses, the presence of halite is limited, with an average content of 0.45 vol%, and halite cement was recognised only in Well LS17-2 (Fig. 9f).

overgrowths around quartz grains. Generally, the precipitation of quartz overgrowths occurs after effective burial and compaction. The quartz overgrowths in the sandstones partially or completely fill pores and are up to 16 μm thick (Fig. 9a). They are easy to discriminate from detrital quartz grains under a microscope due to the existence of dust rims and clay minerals. SEM images suggest that the size of the single crystal quartz is less than 10 μm (Fig. 9b), and it is sometimes interjacent to crystallised chlorite rims (Fig. 9c). The authigenic quartz often occupies primary pores, significantly reducing the porosity and blocking narrow pore throats. Occasionally, euhedral micro-quartz, with sizes generally less than 1 μm, can be observed filling pores (Fig. 9d).

5. Discussion 5.1. Impact of early hydrocarbon charge on diagenetic minerals Hydrocarbon gas was enriched mainly in the turbidite channel sandstones of the Central Canyon, and the LS17-2 gas field was discovered in the Yinggehai–Huangliu Formation (Xie, 2014). Research on formation pressure has shown that high pressure is prevalent in the Mesozoic–Neocene Oligocene formations beneath the LS17-2 section of the Central Canyon, and the high pressure provides a strong impetus for natural gas migration (Liu et al., 2017). Thermal fluid-related diapirs provided effective pathways for the migration of gas generated by coal

4.3.4. Pyrite and halite cements Pyrite is minor cement that was developed in most of the samples studied. It occurs as framboids (Fig. 9e). As determined by SEM and 5

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Fig. 5. Porosity in the Central Canyon sandstones (pores are pointed out by red arrows). (a) Micrograph of a casting thin section (Well LS17-2, 3343.20 m). The sandstones underwent weak compaction and primary intergranular pores were preserved. (b) Micrograph of casting thin section (Well LD30-1, 3438.97 m). Quartz grains edge and cements dissolved to form secondary intergranular pores. (c) Micrograph of casting thin section (Well LD30-1, 3428.36 m). Intragranular pores in feldspars. (d) Micrograph of casting thin section (Well LD30-1, 3428.36 m). The grain is completely dissolved to produce moldic pore. Cal-C = calcite cement. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

pressure driving force (Gan et al., 2018). 5.1.1. Origin and distribution of carbonate cements Carbonate cements are diagenetic minerals that are widespread in siliciclastic rocks within different types of sedimentary basins (Sun et al., 2002; Loyd, Corsetti, Eiler, & Tripati, 2012), and their formation can be used as a mineral indicator for the variation of fluid composition. Particularly, the material origin of carbonate cements and the geochemical environment can be determined using carbon and oxygen isotopic ratios (Huang et al., 2002; Melezhik et al., 2003; Wang et al., 2005). The relatively high δ13C values VPDB (between −7.52‰ and −0.48‰) and δ18O values VPDB (between −11.83‰ and −2.95‰) are characteristic for marine diagenesis (Table 2). The following empirical formula was proposed by Keith and Weber (1964):

Z= 2.048 × (δ 13 C+ 50) + 0.498 × (δ 18 O+ 50)(PDB standard), where Z can be used to indicate ancient salinity. Z < 120 indicates fresh water, and Z > 120 indicates marine water. The Z values of the sandstones in LD30-1 range from 107.1 to 119.6, all less than 120, indicating that the diagenetic fluids of the carbonate cements were mainly freshwater fluids with some admixture of salt water; those in LS17-2 range from 121.7 to 125.0, all more than 120, indicating that the diagenetic fluids of these carbonate cements were mainly marine fluids (Table 2). This indicates that the salinity of the diagenetic fluids in the LD30-1 sandstones is higher than that of the LS17-2 sandstones. The precipitation of early carbonate cements is more likely to occur in LS17-2. The carbon isotope and oxygen isotope compositions of carbonate cements are plotted in Fig. 11. The LS17-2 sandstones are relatively concentrated in carbon and oxygen isotopic compositions, implying that the carbonate minerals had similar formation temperature and

Fig. 6. Cross-plot of permeability and porosity of the Central Canyon sandstones. Data for well Y35-1/2, LS22-1, LS17-1, 2, 3, 4, 7, 8, are from the CNOOC Research Centre.

measure strata of the Yacheng Formation to sand bodies of the Yinggehai–Huangliu Formation, and the bottom temperature of the source rock in the Yacheng Formation can exceed 250 °C (Huang et al., 2016; Zhang et al, 2016; Gan et al., 2018;). As shown in Fig. 10 and Table 4, the temperature of the Huangliu Formation is mostly lower than 100 °C, and the homogenisation temperatures of the aqueous fluid inclusions are higher than those of reservoirs since the Pliocene–Upper Miocene. This may be due to the rapid filling of deep thermal fluids under high 6

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Fig. 7. Characteristics of the carbonate cements of the Central Canyon sandstones. (a) Photomicrograph of a casting thin section of a sample from Well LS17-2, 3344.20 m. Calcite cementation (red) fills intergranular pores. (b) SEM image showing authigenic micro-calcite preceding poorly crystallised pore-lining chlorite, LD30-1, 3263.15 m. (c) Photomicrograph acquired under crossed polarisers of a sample from Well LD30-1, 3438.97 m. The feldspar grain is replaced by carbonate. (d) Photomicrograph acquired under crossed polarisers of a sample from Well LD30-1, 3263.16 m, showing partial replacement. (e) EPMA BSE image (LD30-1, 3251.30 m) showing calcite cement filling intergranular pores. (f) EPMA BSE image (LS17-2, 3343.20 m) showing ankerite cements filling intragranular pores. Chl = chlorite; Cal = calcite; E-Cal = euhedral micro-calcite; Cal-R = calcite replaced; Ank = ankerite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

diagenetic fluids. The carbon and oxygen isotopic compositions of the LD30-1 sandstones have positive relationships and are both lighter than those of the LS17-2 sandstones. As shown in Table 2 and Fig. 11, the δ13C and δ18O values of the carbonate cements in the LD30-1 sandstones are within the range of −7.52‰ to −2.27‰ and −11.83‰ to −5.96‰, respectively. This indicates that oxidation of organic matter has as an important role in the occurrence of carbonate cements in the sandstone reservoirs. With increasing temperature in the LD30-1 sandstones due to increased burial depth, CO2 with lighter carbon isotope composition is generated by the thermal decarboxylation of organic material. Additionally, the much larger gap of δ18O suggests that carbonate cements are formed in the deep hot subsurface at the late stage of diagenesis, which is consistent with the results of casting thin section observations (Figs. 4c, 7c). The analysis of δ18OV-PDB followed

Friedman and O'Neil (1977), and the precipitation temperatures of carbonate minerals probably range from 32 °C to 43 °C and from 40 °C to 70.9 °C in LS17-2 and LD30-1, respectively (Fig. 12). The temperatures of carbonate minerals suggest that the precipitation of carbonate cements in the LS17-2 sandstones is relatively early. Previous research (Lubomir and Urrea, 1990) has shown that the δ13C values of carbonate cements precipitated from marine water are within the range of −3‰ to 0‰, whereas the δ13C values of carbonate cements precipitated from meteoric water are generally within the range of −4‰ to −1‰. According to Fig. 11, the relatively high and concentrated carbon isotope and oxygen isotope values in the LS17-2 sandstones are closer to the original isotope compositions, ranging from −1.76‰ to −0.48‰ and −5.04‰ to −2.95‰, respectively. In comparison with the LD30-1 sandstones, the temperatures of 7

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Table 1 Chemical composition (Ca, Fe, Mg and Mn) of carbonate cements in the Central Canyon sandstones. Samples

Table 3 The proportion of main constituents in total clay mineral content in the Central Canyon sandstones from XRD.

Content (in wt%)

Well

Well

Depth, m

No.

CaO

FeO

MgO

MnO

Total

LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2

3248.10

1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 1 2 3 4 5 1 2 3 4 5 6 1 2 3 4 5 6 7

55.77 51.87 52.00 52.17 53.30 56.03 31.66 51.05 51.68 51.50 51.43 50.85 52.42 53.31 51.83 51.45 51.39 51.94 53.03 51.60 52.17 51.86 52.47 55.47 53.25 52.40 53.13 53.19 55.21 52.49 4.14 5.07 4.64 54.38 52.95 53.90 53.47 54.04 54.40 52.64

0.01 0.77 0.91 0.81 0.08 0.05 5.81 1.22 0.79 0.88 0.99 0.64 0.19 0.67 0.82 1.40 1.50 1.33 1.06 1.35 1.27 1.49 1.12 0.13 0.94 1.39 1.06 0.01 0.10 0.73 44.58 40.85 45.06 0.32 0.99 0.23 0.57 0.49 0.49 1.05

0.44 0.30 0.25 0.32 1.04 0.06 6.83 0.42 0.27 0.31 0.32 0.19 1.02 0.44 0.24 0.46 0.54 0.45 0.37 0.45 0.46 0.49 0.32 0.19 0.35 0.56 0.25 0.56 0.11 0.75 6.79 7.60 6.23 0.22 0.31 0.25 0.13 0.11 0.11 0.39

0.00 1.28 1.18 0.87 0.08 0.01 11.06 1.81 1.33 1.56 1.76 2.08 0.28 0.50 1.70 1.44 1.44 1.38 0.94 1.40 1.37 1.41 0.51 0.00 0.73 0.18 0.36 0.01 0.00 0.36 1.27 3.37 2.03 0.25 0.37 0.14 0.28 0.23 0.24 0.39

56.22 54.22 54.33 54.17 54.49 56.14 55.36 54.51 54.07 54.25 54.49 53.76 53.91 54.92 54.59 54.75 54.87 55.11 55.40 54.80 55.27 55.25 54.42 55.79 55.27 54.52 54.80 53.78 55.42 54.32 56.77 56.88 57.96 55.17 54.62 54.52 54.45 54.87 55.23 54.46

3255.95

3428.36

3342.70

3343.20

3344.20

LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2

Depth (m)

3248.10 3251.30 3253.90 3255.95 3260.90 3263.15 3423.10 3428.36 3430.55 3434.45 3436.71 3438.97 3340.70 3341.20 3341.70 3342.20 3342.70 3343.20 3343.70 3344.20 3344.70

Clay mineral content (%) I/S

Illite

11 26 19 26 23 18 16 20 17

4 8 13 6 6 4 11 8 8 11 15 12 41 32 39 28 31 42 34 35 41

%S

Kaolinite

Chlorite

I/S

21 16 16 19 19 16 21 18 15

96 92 87 94 94 96 89 92 92 89 85 88 27 26 26 27 27 24 29 27 27

10 10 5 7 7 8 8 9 5

C/S

I/S: Illite/Smectite; C/S: Chlorite/Smectite.

precipitation of carbonate cements are lower and early carbonate cements are more abundant in the LS17-2 sandstones. These results indicate that the evolution of carbonate cements is inhibited during burial. This could also be explained by the fact that Fe and Mn contents of carbonate cements are relatively higher in the LS17-2 sandstones than in the LD30-1 sandstones (Fig. 13). The molar ratios of Fe/Ca and Mn/Ca are approximately 3.55 and 5.38 mmol/mol in the early carbonate portion in the Central Canyon sandstones. With increasing burial depth, the formation of organic acids facilitates the dissolution of feldspar and lithic fragments in the normal cementing process, which releases Fe and Mn ions. In the LS17-2 carbonate cements, the molar ratio of Fe/Ca ranges from 0.23 to 19.88 mmol/mol and the molar ratio of Mn/Ca ranges from 0 to 13.73 mmol/mol, close to those of early carbonate cements. However, the molar ratios of Fe/Ca (ranges from 12.51 to 29.21 mmol/mol) and Mn/Ca (ranges from 9.42 to

Table 2 Stable carbon and oxygen isotopic compositions of carbonate cements in the Central Canyon sandstones. Well

No.

Depth (m)

δ13CPDB (‰)

δ18OPDB (‰)

δ18OSMOW (‰)

Z (‰)

LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LD30-1 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10

3248.10 3251.30 3253.90 3255.95 3260.90 3263.15 3423.10 3428.36 3430.55 3434.45 3436.71 3438.97 3340.70 3341.20 3341.70 3342.20 3342.70 3343.20 3343.70 3344.20 3344.70 3345.20

−2.27 −2.47 −2.50 −2.98 −3.00 −3.01 −3.15 −7.52 −3.79 −4.16 −3.62 −4.88 −1.36 −1.56 −1.65 −1.29 −0.23 −1.76 −1.26 −0.48 −0.73 −0.93

−5.96 −7.96 −8.28 −8.16 −8.15 −8.12 −9.40 −9.58 −9.96 −10.16 −9.12 −11.83 −3.52 −3.86 −4.40 −3.78 −3.69 −4.05 −4.23 −2.95 −5.04 −4.33

24.54 22.48 22.15 22.27 22.28 22.31 20.99 20.80 20.41 20.20 21.27 18.48 27.05 26.70 26.15 26.79 26.87 26.51 26.32 27.64 24.86 26.22

119.69 118.28 118.05 117.13 117.09 117.10 116.17 107.13 114.57 113.72 115.34 111.41 122.76 122.18 121.72 122.79 124.98 121.69 122.61 124.86 122.99 123.24

8

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Fig. 8. The chlorite cements characteristics of the Central Canyon sandstones. (a) Micrograph of SEM showing the authigenic calcite grains is replaced by chlorite, LD30-1, 3248.10 m. (b) Micrograph of SEM showing chlorite rims are engulfed by quartz overgrowth, LD30-1, 3248.10 m. (c) Micrograph of SEM showing the chlorite on the intragranular edge of dissolved K-feldspar grains, LD30-1, 3248.10 m. (d) Micrograph of SEM showing chlorites replace quartz grains, LD30-1, 3248.10 m.

of the LD30-1 sandstone reservoirs. Chlorite cementation could occur from eodiagenesis to mesodiagenesis in the Central Canyon sandstones. However, the chlorite observed in the LS17-2 sandstones typically formed in the early stages of diagenesis. The proportions of chlorite and illite in the total clay mineral content in the LD30-1 sandstones range from 85% to 94% (average of 91%) and 4% to 15% (average of 9%), respectively, and illite/smectite is not present. This is a sign of late diagenesis. However, the proportions of illite/smectite (average of 19.6%) and smectite (average of 7.6%) indicate that the process of the LS17-2 sandstones only reached the medium stage of mesodiagenesis. In the LS17-2 sandstones, the proportion of chlorite in the total clay mineral content ranges from 24% to 29% (average of 26.5%), but the proportion of illite is relatively higher (average of 36%). In comparison with the LD30-1 sandstones, the early hydrocarbon charge events changed the pore fluid properties; hence, the development of chlorite was inhibited in the LS17-2 sandstones. Illitisation was promoted due to the early hydrocarbon charge, causing high temperatures in the LS17-2 sandstones.

40.94 mmol/mol) are relatively higher in the LD30-1 carbonate cements, indicating a higher content of late carbonate cements. Based on the history of hydrocarbon charge and the aforementioned analysis, early hydrocarbon charge into pores inhibited the cementation of carbonate minerals in the LS17-2 sandstones. 5.1.2. Origin and distribution of clay minerals The origin and precipitation of chlorite cement are controlled by pore fluid, material source, early clay minerals, and temperature (Aagaard et al., 2000; Beaufort et al., 2015; Billault et al., 2003; Huang et al., 2004). It has been proven by laboratory experiments that with increasing temperature and pressure chlorite will recrystallise to form thick, continuous coatings of Fe-rich chlorite (Aagaard et al., 2000). As chlorite/smectite mixed layer mineral has not been found in the Central Canyon reservoirs and as honeycomb residues of clay mineral transformation in the process have also not been found, it is considered that the chlorite coating is mainly new precipitation from pore water. On the basis of SEM analysis, chlorite in the Central Canyon sandstones also occurs between grains, suggesting that the chlorite rims formed during eodiagenesis at a low temperature and before mechanical compaction. The reservoirs are close to the mudstone, so the main mechanism of chlorite formation is reprecipitation of Fe ions released during mudstone diagenesis. With increasing burial depth and pH, the chlorite is transformed from kaolinite under Fe-rich and Mg-rich diagenetic conditions. The chlorite on the intragranular edges of dissolved K-feldspar grains (Fig. 8c, Eqs. (1), (2)) indicates that chlorite rims can grow continuously during the late stage of mesodiagenesis in the LD30-1 sandstones. This is an important factor causing permeability decreased

2KAlSi3O8 (K-feldspar) + 2H+ + H2O = Al2Si2O5(OH)4 + 2 K+,

(quartz)

(kaolinite)

+ 4SiO2 (1)

3.5Fe2+ + 3.5 Mg2+ + 9H2O + 3Al2Si2O5(OH)4 + (kaolinite) = Fe3.5Mg3.5Al6Si6O20(OH)16 (chlorite) + H ,

(2)

Smectite + K → Illite + SiO2 + H2O,

(3)

+

KAlSi3O8 (K-feldspar) + Al2Si2O5(OH)4 lite) + 2 SiO2 + H2O. 9

(kaolinite)

= KAlSi3O10(OH)2

(il-

(4)

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Fig. 9. (a) Micrograph of casting thin section (Well LD30-1, 3434.45 m). Quartz overgrowths fill intergranular pores. (b) Micrograph of SEM showing quartz overgrowth, LD30-1, 3248.10 m. (c) Micrograph of SEM showing pore-filling authigenic quartz formed after euhedral-crystallised pore-lining chlorite, LD30-1, 3248.10 m. (d) Micrograph of SEM showing euhedral micro-quartz, LS17-2, 3342.70 m. (e) Micrograph of SEM showing pyrite, LS17-2, 3343.20 m. (f) Micrograph of SEM showing pore-filling halite cements, LS17-2, 3341.20 m. Qo = quartz overgrowth; Qz = micro-quartz; Py = pyrite; Ha = halite cements.

The transition from smectite to illite occurs at temperatures around 70 °C (Eq. (3)), and the transition from kaolinite to illite begins at 125 °C (Eq. (4)) (Worden and Morad, 2009; Ma et al., 2017). Based on the XRD analysis, the proportion of illite in the total clay mineral content is relatively high in the Central Canyon sandstones. This suggests that illitisation is a source of silica for quartz overgrowths. Dissolution of both feldspars and clay minerals, which provides silica for quartz cement, is likely promoted by organic acids produced by the decarboxylation of organic matter. Recent research on the transport of silica shows that the mass transport of SiO2 in solution is very small and that most of the dissolved ions never leave the column, instead

5.1.3. Origin and distribution of quartz cements The sources of SiO2 in sandstones may include biogenic silica in marine sediments, dissolution of feldspars, alteration of clay minerals, and pressure solution of detrital quartz grains (Worden and Morad, 2009; Kim and Yong, 2005; Taylor et al., 2010; Maast et al., 2011; Xi et al., 2015). However, biogenic silica-related quartz cement was not observed in the samples studied, and pressure solution of detrital quartz grains was weak due to the high sedimentation rate and short burial time of the Central Canyon sandstones (Hoang et al., 2010; Zhao et al., 2015; Zhong et al., 2018). Dissolution of K-feldspar grains (Eq. (1)) and illitisation (Eqs. (3) and (4)) are important sources of silica. 10

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early carbonate cement. Generally, this phase of diagenesis occurs at temperatures lower than 70 °C, and pore-water features are controlled by depositional or meteoric waters (Morad et al., 2000). Mechanical compaction played a vital role in reducing primary porosity during the eodiagenesis phase. Progressive burial and mechanical compaction during mesodiagenesis continued to reduce the primary porosity of the sandstones and resulted in the deformation of ductile grains. Meanwhile, the first stage of hydrocarbon charge was a small-scale charge, and the weak acidity of the fluid resulted in weak dissolution of feldspars. The second, large-scale, phase of hydrocarbon charge occurred during mesodiagenesis, and there was greater production of acidic fluid. At this phase, the dissolution of feldspars and carbonates not only generated secondary porosity and increased porosity but also provided material for the authigenic mineral precipitation that occurred during the development of quartz overgrowths, ferrocalcite cement, porefilling kaolinite, and so on. Meanwhile, the dissolution of quartz and quartz overgrowths occurred during this interval of alternating acidic and alkaline conditions. The diagenetic parasequence of the Central Canyon sandstones can be summarised as follows: early carbonate cements → early clay coatings → mechanical compaction → invasion of organic acids → dissolution of feldspar and lithic fragments → secondary porosity → smectite-to-illite reactions → kaolinite and quartz overgrowths → dissolution of quartz and other silicate minerals → displacement of feldspar grains by late carbonate cements. 5.3. Impact of diagenesis on reservoir quality Based on the petrographic observations, different diagenetic processes control reservoir quality. These include mechanical compaction, dissolution, cementation, precipitation of clay minerals, and others. Generally, mechanical compaction and cementation play major roles in porosity reduction. Dissolution not only generates secondary porosity and increases porosity but also provides material for cements. Because of the good sorting and low mud content in the sandstones, the primary porosity was relatively high and the sandstones were more difficult to compact during the eodiagenesis phase. In particular, there is a high volume of rigid grains (quartz > 79.6%) in the LS17-2 sandstones, making compaction more difficult. Based on the statistics of the casting thin sections, the apparent compaction rate varies from 14.6% to 24.18%, with an average of 20.15%. This indicates that the LS17-2 sandstones are weakly compacted, and the weak compaction is also related to the high sedimentation rate and short burial time (Hoang et al., 2010; Zhao et al., 2015; Zhong et al., 2018). Cementation, especially the carbonate cementation, decreased reservoir porosity, but compaction was inhibited during the eodiagenesis phase. At the same time, acidic fluids migrated predominantly through the sand bodies, causing dissolution of the cements, transporting the products of dissolution, and inhibiting reprecipitation because of the relatively higher primary porosity. The content of quartz overgrowths in the LS17-2 sandstones is lower than that in the LD30-1 sandstones based on the casting thin sections. Based on the statistics of these casting thin sections, the apparent cementation rate varies from 18.7% to 25.6%, with an average of 22.18%; hence, the LS17-2 sandstones are weakly cemented. This is an important factor explaining why the porosity of the LS17-2 sandstones is higher than that of the LD30-1 sandstones. It is accepted that chlorite coatings preferentially formed in this area and subsequently hindered quartz cementation. However, evidence for quartz overgrowth after chlorite coating occurs in some samples (Fig. 8b). Based on this result, the effect of chlorite for efficient porosity preservation is limited. With increasing burial depth and pH, chlorite is transformed from kaolinite under Fe-rich and Mg-rich diagenetic conditions. This is an important factor explaining why the permeability of the LD30-1 sandstones is lower than that of the LS17-2 sandstones. It is important that the hydrocarbon accumulation event hindered cementation and thus preserved porosity in the sandstones. This may be

Fig. 10. (a) Histogram of the homogenization temperatures of aqueous inclusions in the LS17-2, 3 sandstones of the Central Canyon and (b) burial and thermal histories buried history of well LS17-2 in the Qiongdongnan basin (modified from Huang et al., 2016).

forming secondary minerals (Bjørlykke and Jahren, 2012; Bjørlykke, 2014). However, the rapid filling of the early hydrocarbon caused fluid to be discharged from pores and changed the fluid properties in the LS17-2 sandstones; hence, quartz overgrowths were hindered. This is probably a factor explaining why the content of quartz overgrowths is higher in the LD30-1 sandstones than in the LS17-2 sandstones based on casting thin sections. In addition, quartz overgrowths and the dissolution of quartz can be observed due to the change in diagenetic environment, respectively. 5.2. Coupling relation between diagenetic sequence and early hydrocarbon charge According to the petrographic analysis, stable isotope compositions, and textural relationships of authigenic minerals, the relative timing of the primary diagenetic features in the Central Canyon sandstones is reconstructed in Fig. 14. Comprehensive study of the regional petroleum accumulation has shown that there were two stages of petroleum accumulation in the Central Canyon sandstones in the Lingshui Depression (Huang et al., 2017; Gan et al., 2018; Zhang et al., 2019): the first hydrocarbon charge at about 2.0 Ma, the and second hydrocarbon charge at about 0.8–0 Ma (Fig. 14). The significant early diagenetic features in the Central Canyon sandstones were the formation of authigenic clay coatings around detrital grains and the precipitation of 11

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Table 4 Micro-thermometric data of aqueous fluid inclusions in the Central Canyon sandstones (data from the CNOOC Research Centre). Th: homogenization temperature. Well

Depth, m

Position

LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-2 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3 LS17-3

3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3220.5 3225 3225 3225 3225 3225 3225 3225 3225 3225 3225 3225 3225 3229 3229 3229 3229 3229 3229 3229 3229 3229 3229 3229 3370 3370 3370 3370 3370 3370 3370 3370 3370 3370 3370 3390 3390 3390 3390 3390 3390 3404 3404 3404 3404 3477 3477 3477 3477 3485.5 3485.5 3485.5 3483.6 3483.6 3483.6 3483 3483 3483 3482 3482 3482

healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed healed

microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures microfractures

of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of of

quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz quartz

Type

Size, μm

Th, °C

Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water Salt-water

8.2 3.9 4.1 4.6 3.4 9.6 4.3 6.6 6.2 6.4 9.5 4.9 10.8 5.1 5.7 6.3 12.2 3.5 3.3 8.1 2.9 9.8 3.4 6.9 7.2 3.7 3.7 3.9 5.7 2.9 8.2 5.1 3.4 4.2 4.1 4.6 4.9 3.2 2.2 3.1 4.9 4.6 3.9 7.2 4.1 2.9 5.3 5.6 4.5 4.7 4.9 5.6 3.4 2.9 6.1 4.9 3.9 10.3 4.2 3.1 3.3 3.7 6.4 6.9 3 4.1 2.6 2.9 2.9 3.1 2.5 4 3.2 7.6

126.1 149.6 135.9 122.7 118.5 182.3 178.6 162.9 173.6 135.2 147.3 218.7 173.6 169.2 177.2 183.6 135.9 171.9 165.8 175.3 171.9 167.7 225.7 172.5 217.6 207.6 153.9 135.2 156.3 192.7 164.6 178.2 213.7 207.8 113.6 196.9 211.5 157.1 146.9 162.5 169.8 170.3 167.4 177.2 215.7 169.5 141.6 206.2 138.6 169.7 172.3 223.7 161.9 131.5 183.6 215.7 199.8 123.1 138.2 141.4 136.2 155.5 119.8 120.8 118.2 142.5 144.5 143.1 162.8 178.1 174.3 182.3 172.4 184.3

(continued on next page) 12

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Table 4 (continued) Well

Depth, m

Position

LS17-3 LS17-3 LS17-3 LS17-3 LS17-3

3482 3482 3481 3481 3481

healed healed healed healed healed

microfractures microfractures microfractures microfractures microfractures

of of of of of

quartz quartz quartz quartz quartz

Type

Size, μm

Th, °C

Salt-water Salt-water Salt-water Salt-water Salt-water

3.2 3.1 3.3 5.1 5.3

169.1 168.5 133.4 185.1 178.3

Fig. 12. Temperature of carbonate precipitation based on oxygen isotope composition, calculated using equation 103lnα = 18.03 × 103 T−1- 32.42 (Kim and O’Neil, 1997), where α is the isotope fractionation factor and T is temperature in Kelvin. SMOW = standard mean ocean water.

Fig. 11. The distribution of carbon and oxygen isotopic composition of the carbonate cements in the Central Canyon sandstones (modified from Yang et al., 2010).

explained by the high maturity of the framework grains, which include a high content of quartz. Because the contents of feldspars and detritus were low, dissolution was weak after hydrocarbon entered the reservoir. Meanwhile, the hydrocarbon reduced the concentrations of inorganic ions in pore fluids and hindered the exchange of ions. Therefore, dissolution and cementation were reduced. Based on the above, the early hydrocarbon charge controlled the development of diagenetic minerals (carbonate cements, clay minerals, and quartz overgrowths) and hence was an important factor affecting the LS17-2 sandstone reservoir quality.

Fig. 13. The molar ratios of Fe/Ca versus Mn/Ca for carbonate cements.

permeability in the eodiagenesis stage. Later, dissolution associated with hydrocarbon charge improved the reservoir quality of the sandstones. The hydrocarbon charge hindered reprecipitation of carbonate cements and quartz overgrowths. (3) Formation of chlorite in the Central Canyon sandstones started in the eodiagenesis stage and continued during mesodiagenesis at higher pressure–temperature conditions. Chlorite coatings contributed to the preservation of primary porosity in the sandstones. In comparison with the LD30-1 sandstones, hydrocarbon charge hindered the alteration of clay minerals in the LS17-2 sandstones. (4) Weak compaction and cementation and early hydrocarbon charge are the main factors controlling the formation of high-quality reservoirs in the LS17-2 sandstones.

6. Conclusions (1) The Central Canyon sandstones in the Ledong Depression and Lingshui Depression comprise mainly lithic arkoses and sublitharenites. The average porosity and permeability values are 19 vol% and 10 mD and 30 vol% and 488 mD, respectively. The LS17-2 sandstones have better reservoir quality than the LD30-1 sandstones. (2) Cementation and dissolution were the most important diagenetic process. Carbonate cements decreased reservoir porosity and 13

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Fig. 14. The major diagenetic sequence of the Central Canyon sandstones sandstones based on diagenetic alteration relations, isotopic composition, and burial history curve.

Declaration of Competing Interest

Bjørlykke, K., 2014. Relationships between depositional environments, burial history and rock properties. Some principal aspects of diagenetic process in sedimentary basins. Sed. Geol. 301, 1–14. Bourget, J., Zaragosi, S., Garlan, T., Gabelotaud, I., Guyomard, P., Dennielou, B., EllouzZimmermann, N., Schneider, J.L., 2008. Discovery of a giant deep-sea valley in the Indian Ocean, off eastern Africa: The Tanzania channel. Mar. Geol. 255, 179–185. Cao, L.C., Jiang, T., Wang, Z.F., Zhang, Y.Z., Sun, H., 2015. Provenance of Upper Miocene sediments in the Yinggehai and Qiongdongnan basins, northwestern South China Sea: Evidence from REE, heavy minerals and zircon U-Pb ages. Mar. Geol. 361, 136–146. Corella, J.P., Loizeau, J.L., Kremer, K., Hilbe, M., Gerard, J., Dantec, N., Stark, N., Gonzalez-Quijano, M., Girardclos, S., 2016. The role of mass-transport deposits and turbidites in shaping modern lacustrine deepwater channels. Mar. Pet. Geol. 77, 515–525. Fayek, M., Harrison, T.M., Grove, M., McKeegan, K.D., Coath, C.D., Boles, J.R., 2001. In situ stable isotopic evidence for protracted and complex carbonate cementation in a petroleum reservoir, North Coles Levee, San Joaquin Basin, California, U.S.A. J. Sediment. Res. 71, 444–458. Folk, R.L., 1974. Petrology of Sedimentary Rocks. Hemphills Publ. Co, Austin. Friedman, I., O'Neil, J.R., 1977. Compilation of stable isotope fractionation factors of geochemical interest. Data Geochem. 440kk, KK1–KK2. Gales, J.A., Leat, P.T., Larter, R.D., Kuhn, G., Hillenbrand, C.D., Graham, A.G.C., Mitchell, N.C., Tate, A.J., Buys, G.B., Jokat, W., 2014. Large-scale submarine landslides, channel and gully systems on the southern Weddell Sea margin, Antarctica. Mar. Geol. 348, 73–87. Gan, J., Zhang, Y.Z., Liang, G., Yang, X.B., Li, X., Yang, J.H., Guo, X.X., 2018. On accumulation process deep water area and dynamic mechanism of natural gas in the of Central Canyon, Qiongdongnan Basin (in Chinese with English abstract). Acta Geol. Sin. 92, 2359–2367. Gong, Z.S., Li, S.T., Xie, T.J., Zhang, Q.M., Xu, S.C., Xia, Q.Y., Yang, J.M., Sun, Y.C., Liu, L.H., 1997. Continental Margin Basin Analysis and Hydrocarbon Accumulation of the Northern South China Sea. Science Press, pp. 193–256 (in Chinese). Haq, B.U., Hardenbol, J., Vail, P.R., 1987. Chronology of fluctuating sea levels since the triassic. Science 235, 1156–1167. Hoang, L.V., Clift, P.D., Schwab, A.M., Huuse, M., Nguyen, D.A., Zhen, S., 2010. Largescale erosional response of SE Asia to monsoon evolution reconstructed from sedimentary records of the Song Hong-Yinggehai and Qiongdongnan basins, South China Sea. Geological Society, London, Special Publications. 342, 219–244. Huang, B.J., Tiam, H., Li, X.S., Wang, Z.F., Xiao, X.M., 2016. Geochemistry, origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin, South China Sea. Mar. Pet. Geol. 72, 254–267. Huang, S.J., Shi, H., Zhang, M., Shen, L.C., Wu, W.H., 2002. Application of strontium isotope stratigraphy to diagenesis research. Acta Sedimentol. Sinica 20, 359–366 (in Chinese). Huang, S.J., Xie, L.W., Zhang, M., Wu, W.H., Shen, L.C., Liu, J., 2004. Formation mechanism of authigenic chlorite and relation to preservation of porosity in nonmarine Triassic reservoir sandstones, Ordos Basin and Sichuan Basin, China. J. Chengdu Univ. Technol. 31, 273–281 (in Chinese). Huang, H.T., Huang, B.J., Huang, Y.W., Li, X., Tian, H., 2017. Condensate origin and

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This paper was supported financially by the National Science and Technology Major Project of the Ministry of Science and Technology of China (Grant No. 2016ZX05026-007-005). We are grateful to the CNOOC Research Centre for providing all of the samples and the basic data and permitting the publication of the results of this study. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jseaes.2019.104022. References Aagaard, P., Jahren, J.S., Harstad, A.O., Nilsen, O., Ramm, M., 2000. Formation of grain coating chlorite in sandstones. Laboratory synthesized vs. natural occurrences. Clay Miner. 35, 261-261. Babonneau, N., Delacourt, C., Cancouët, R., Sisavath, E., Bachelery, P., Mazuel, A., Jorry, S.J., Deschamps, A., Ammann, J., Villeneuve, N., 2013. Direct sediment transfer from land to deep-sea: Insights into shallow multibeam bathymetry at La Réunion Island. Mar. Geol. 346, 47–57. Bayliss, N., Pickering, K.T., 2015. Transition from deep-marine lower-slope erosional channels to proximal basin-floor stacked channel-levée-overbank deposits, and synsedimentary growth structures, Middle Eocene Banastón System, Ainsa Basin, Spanish Pyrenees. Earth Sci. Rev. 144, 23–46. Beaufort, D., Rigault, C., Billon, S., Billault, V., Inoue, A., Inoue, S., Patrier, P., 2015. Chlorite and chloritization processes through mixed-layer mineral series in lowtemperature geological systems a-review. Clay Miner. 50, 497–523. Billault, V., Beaufort, D., Baronnet, A., Lacharpagne, J.C., 2003. A nanopetrographic and textural study of grain-coating chlorites in sandstone reservoirs. Clay Miner. 38, 315–328. Bjørlykke, K., Jahren, J., 2012. Open or closed geochemical systems during diagenesis in sedimentary basins: constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs. AAPG Bull. 96, 2193–2214.

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G. Bi, et al.

1998. Optimization of heavy- oil production by steam flood from a shallow sandstone reservoir, Midway-Sunset Field, southern San Joaquin Basin, California. AAPG Ann. Convention Extended Abstracts, 576. Shanmugam, G., 2006. Deep-water Processes and Facies Models: Implications for Sandstone Petroleum Reservoirs. Handbook Petrol. Explorat. Product. Stonecipher, S.A., Winn Jr., R.D., Bishop, M.G., 1984. Diagenesis of the Frontier Formation, Moxa Arch: a function of sandstone geometry, texture and composition, and fluid flux. In: McDonald, D.A., Surdam, R.C. (Eds.), Clastic Diagenesis. AAPG Memoir. 37, 289–316. Su, L., Zheng, J.J., Chen, G.J., Zhang, G.C., Guo, J.M., Xu, Y.C., 2012. The upper limit of maturity of natural gas generation and its implication for the Yacheng formation in the Qiongdongnan Basin, China. J. Asian Earth Sci. 54–55, 203–213. Sun, Y.S., Shen, Y.M., Xu, X., Yang, F., 2002. Evaluating and Predicting Heterogeneous Reservoirs and Its Oil-bearing Properties by the analysis technique of the diagenetic lithofacies-taking Hadexunarea in Tarim basin as an example. Acta Sedimentol. Sinica 20, 55–60 (in Chinese). Taylor, T.R., Giles, M.R., Hathon, L.A., Diggs, T.N., Braunsdorf, N.R., Birbiglia, G.V., Kittridge, M.G., Macaulay, C.I., Espejo, I.S., 2010. Sandstone diagenesis and reservoir quality prediction: Models, Myths, and reality. AAPG Bull. 94, 1093–1132. Talling, P.J., 2014. On the triggers, resulting flow types and frequencies of subaqueous sediment density flows in different settings. Mar. Geol. 352, 155–182. Wang, Y.Q., Zhang, X.Y., Arimoto, R., Cao, J.J., Shen, Z.X., 2005. Characteristics of carbonate content and carbon and oxygen isotopic composition of northern China soil and dust aerosol and its application to tracing dust sources. Atmos. Environ. 39, 2631–2642. Wilson, M.D., Stanton, P.T., 1994. Diagenetic mechanisms of porosity and permeability reduction and enhancement. In: Reservoir Quality Assessment and Prediction in Clastic Rocks. SEPM Short Course Notes. 30, 59–117. Worden, R.H., Morad, S., 2009. Quartz Cementation in Oil Field Sandstones: A Review of the Key Controversies. In: Quartz Cementation in Sandstones. Blackwell Science Publishing, pp. 1–20. Xie, Y.H., 2014. Significant breakthrough in proprietary deepwater natural gas exploration in the northern South China Sea and its inspiration. Nat. Gas Ind. B 1, 221–229 (in Chinese). Xie, Y.H., Li, X.S., Fan, C.W., Tan, J.C., Liu, K., Lu, Y., Hu, W.Y., Li, H., Wu, J., 2016. The axial channel provenance system and natural gas accumulation of the Upper Miocene Huangliu Formation in Qiongdongnan Basin, South China Sea. Pet. Explor. Dev. 43, 570–578. Xi, K.L., Cao, Y.C., Jahren, J., Zhu, R.K., Bjorlykke, K., Zhang, X.X., Cai, L.X., Hellevang, H., 2015. Quartz cement and its origin in tight sandstone reservoirs of the Cretaceous Quantou formation in the southern Songliao basin, China. Mar. Pet. Geol. 66, 748–763. Yang, Z., Zou, C.N., He, S., Li, Q.Y., He, Z.L., Wu, H.Z., Cao, F., Meng, X.L., Wang, F.R., Xiao, Q.L., 2010. Formation mechanism of carbonate cemented zones adjacent to the top overpressured surface in the central Junggar Basin, NW China. Sci. China-Earth Sci. 53, 529–540. Zhang, G.C., Zeng, Q.B., Su, L., Yang, H.Z., Chen, Y., Yang, D.S., Ji, M., Lv, C.F., Sun, Y.H., 2016. Accumulation mechanism of LS 17–2 deep water giant gas field in Qiongdongnan Basin. Acta Petrolei Sinica 37 (supplement 1), 34–46 (in Chinese). Zhang, X.L., 1985. Relationship between carbon and oxygen stable isotope in carbonate rocks and paleosalinity and paleotemperature of seawater. Acta Sedimentol. Sinica 3, 17–30 (in Chinese). Zhang, Y.Z., Li, X.S., Xu, X.D., Gan, J., Yang, X.B., Liang, G., He, X.H., Li, X., 2019. Genesis, origin, and accumulation process of the natural gas of L25 Gas Field in the western deepwater area, Qiongdongnan Basin. Mar. Origin Petrol. Geol. 1–10 (in Chinese). Zhao, Z.X., Sun, Z., Wang, Z.F., Sun, Z.P., Liu, J.B., Zhang, C.M., 2015. The high resolution sedimentary filling in Qiongdongnan Basin, Northern South China Sea. Mar. Geol. 361, 11–24. Zhong, J., You, L., Zhang, Y.Z., Qu, X.Y., Dai, L., Wu, S.J., 2018. Diagenesis and porosity evolution of the Huangliu Formation canyon-channel reservoir in Ledong-Lingshui Sag, Qiongdongnan Basin. Nat. Gas Geosci. 29, 708–718. Zhou, W., Gao, X.Z., Wang, Y.M., Zhuo, H.T., Zhu, W.L., Xu, Q., Wang, Y.F., 2015. Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern South China Sea. Mar. Pet. Geol. 68, 449–469. Zhu, W.L., Huang, B.J., Mi, L.J., Wilkins, R.W.T., Fu, N., Xiao, X.M., 2009. Geochemistry, origin and deep-water exploration potential of natural gases in the Pearl River Mouth and Qiongdongnan Basins, South China Sea. AAPG Bull. 93, 741–761.

hydrocarbon accumulation mechanism of the deepwater giant gas field in western South China Sea: A case study of Lingshui 17–2 gas field in Qiongdongnan Basin, South China Sea. Pet. Explor. Dev. 44, 380–388. Hu, B., Wang, L.S., Yan, W.B., Liu, S.W., Cai, D.S., Zhang, G.C., Zhong, K., Pei, J.X., Sun, B., 2013. The tectonic evolution of the Qiongdongnan Basin in the northern margin of the South China Sea. J. Asian Earth Sci. 77, 163–182. Jobe, Z.R., Bernhardt, A., Lowe, D.R., 2018. Facies and Architectural Asymmetry in a Conglomerate-Rich Submarine Channel Fill, Cerro Toro Formation, Sierra Del Toro, Magallanes Basin, Chile. J. Sediment. Res. 11, 327–496. Kampman, N., Bickle, M., Becker, J., Assayag, N., Chapman, H., 2009. Feldspar dissolution kinetics and Gibbs free energy dependence in a CO2-enriched groundwater system, Green River, Utah. Earth Planet. Sci. Lett. 284, 473–488. Keith, M.L., Weber, J.N., 1964. Isotopic composition and environmentalclassification of selected limestones and fossils. Geochim. Cosmochim. Acta 28, 1787–1816. Khain, V.E., Polakova, I.D., 2004. Oil and gas potential of deepand ultra-deepwater zones of continental margins. Lithol. Min. Resour. 39, 610–621. Kim, S.T., O’Neil, J.R., 1997. Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates. Geochimica et Cosmochimica Acta 61 (16), 3461–3475. Kim, Y., Yong, I.L., 2005. Origin of quartz cement in the Lower Ordovician Dongjeom formation, Korea. J. Asian Earth Sci. 24, 327–335. Kneller, B., Buckee, C., 2000. The structure and fluid mechanics of turbidity currents:a review of some recent studies and their geological implications. Sedimentology 47, 62–94. Lasaga, A.C., Luttge, A., 2001. Variation of crystal dissolution rate based on a dissolution stepwave model. Science 291, 2400–2404. Liu, Z., Xia, L., Wang, Z.S., Zhang, W., 2017. Study on vertical migration height of hydrocarbon in overpressure basin. Acta Geol. Sin. 91, 1634–1640. Li, X.X., Liu, B.M., Zhao, J.Q., 2007. Paleogene sequence configuration, depositional filling patternand hydrocarbon-generation potential in Qiongdongnan basin. China Offshore Oil Gas 19, 217–223 (in Chinese). Loyd, S.J., Corsetti, F.A., Eiler, J.M., Tripati, A.K., 2012. Determining the diagenetic conditions of concretion formation: assessing temperatures and pore waters using clumped isotopes. J. Sediment. Res. 82, 1006–1016. Lubomir, F., Urrea, V.H.N., 1990. Geology and diagenetic history of overpressured sandstone reservoirs, venture gas field, Offshore Nova Scotia, Canada. AAPG Bull. 74, 1640–1658. Maast, T.E., Jahren, J., Bjørlykke, K., 2011. Diagenetic controls on reservoir quality in Middle to Upper Jurassic sandstones in the South Viking Graben, North Sea. AAPG Bull. 95, 1937–1958. Macaulay, C.I., Haszeldine, R.S., Fallick, A.E., 1993. Distribution, chemistry, isotopic composition and origin of diagenetic carbonates: magnus sandstone, North Sea. J. Sediment. Petrol. 63, 33–43. Ma, P.J., Lin, C.Y., Zhang, S.Q., Dong, C.M., Xu, Y.F., 2017. Formation of chlorite rims and the impact of pore-lining chlorite on reservoir quality: a case study from Shiqianfeng sandstones in upper Permian of Dongpu Depression, Bohai Bay Basin, eastern China. Aust. J. Earth Sci. 64, 825–839. Martín-Merino, G., Fernández, L.P., Colmenero, J.R., Bahamonde, J.R., 2014. Masstransport deposits in a Variscan wedge-top foreland basin (Pisuerga area, Cantabrian Zone, NW Spain). Mar. Geol. 356, 71–87. Mayall, M., Jones, E., Casey, M., 2006. Turbidite channel reservoirs—Key elements in facies prediction and effective development. Mar. Pet. Geol. 23, 821–841. McHargue, T., Pyrcz, M.J., Sullivan, M.D., Clark, J.D., Fildani, A., Romans, B.W., Covault, J.A., Levy, M., Posamentier, H.W., Drinkwater, N.J., 2011. Architecture of turbidite channel systems on the continental slope: Patterns and predictions. Mar. Pet. Geol. 28, 728–743. Melezhik, V.A., Fallick, A.E., Smirnov, Y.P., Yakovlev, Y.N., 2003. Fractionation of carbon and oxygen isotopes in 13C-rich Palaeoproterozoic dolostones in the transition from medium-grade to high-grade greenschist facies: A case study from the Kola Superdeep Drillhole. J. Geol. Soc. 160, 78–82. Morad, S., Ketzer, J.M., Ros, L.F.D., 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology 47 (Supplement s1), 95–120. Morley, C.K., 2016. Major unconformities/termination of extension events and associated surfaces in the South China Seas: Review and implications for tectonic development. J. Asian Earth Sci. 120, 62–86. Qin, Y.P., Alves, T.M., Constantine, J., Gamboa, D., 2016. Quantitative seismic geomorphology of a submarine channel system in SE Brazil (Espírito Santo Basin): Scale comparison with other submarine channel systems. Mar. Pet. Geol. 78, 455–473. Schamel, S., Forster, C., Deo, M., Sprinkel Douglas, A., Olson K., Simmons M., Jenkins C.,

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