Geothermal analysis of boreholes in the Shenhu gas hydrate drilling area, northern South China Sea: Influence of mud diapirs on hydrate occurrence

Geothermal analysis of boreholes in the Shenhu gas hydrate drilling area, northern South China Sea: Influence of mud diapirs on hydrate occurrence

Journal of Petroleum Science and Engineering 158 (2017) 424–432 Contents lists available at ScienceDirect Journal of Petroleum Science and Engineeri...

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Journal of Petroleum Science and Engineering 158 (2017) 424–432

Contents lists available at ScienceDirect

Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol

Geothermal analysis of boreholes in the Shenhu gas hydrate drilling area, northern South China Sea: Influence of mud diapirs on hydrate occurrence Zhifeng Wan a, c, *, Xing Xu b, Xianqing Wang a, Bin Xia a, Yuefeng Sun c a School of Marine Sciences, Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering/Key Laboratory of Offshore Oil Exploration and Development of Guangdong Higher Education Institutes, Sun Yat-sen University, Guangzhou 510006, China b Guangzhou Marine Geological Survey, Ministry of Land and Resources, Guangzhou 510075, China c Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Geothermal analysis Mud diapir Gas hydrates Shenhu area Northern South China Sea

Mud diapirs developed on the northern South China Sea continental slope are important indicators for gas hydrate exploration. The Guangzhou Marine Geological Survey (GMGS) of the Chinese Ministry of Land and Resources targeted mud diapirs and deployed gas hydrate drilling in the Shenhu area. Although an obvious bottomsimulating reflection (BSR) was detected below borehole number SH5, no gas hydrates were found at this borehole. Scientists encountered several contradictions when they analyzed the formation mechanism of hydrates related to mud diapirs. It was important to resolve this problem by sophisticated detection and thermodynamic analysis. In this study, we analyzed the thermodynamic mechanism of mud diapirs and their relationship to the occurrence of gas hydrates based on seismic profiles, mineral identification, sediment thermal conductivity measurements, and other available datasets. Our study concludes that gas hydrates were not detected at SH5 because a mud diapir pierced into the gas hydrate stability zone, leading to the decomposition of hydrates. The evolution of a mud diapir could be divided into three stages with a continuous geological process that controlled the accumulation of gas hydrates. The seismic profile and lithologic analysis showed that the mud diapir below SH5 was in the late stage of mud diapir evolution. The gas leakage system of the mud diapir was evident in the seismic profile, and the typical V-AMP (velocity-amplitude anomaly) structure displayed free gas accumulation under the BSR. The anaerobic oxidation of leaking methane promoted the formation of authigenic carbonate minerals. Thus, the deep layer of SH5 contained a relatively high percentage of carbonates and had a low thermal conductivity. In this stage, higher-temperature liquid migrated into the gas hydrate stability zone. The in situ temperature measured in this borehole was higher than the critical temperature of hydrate formation and decomposition. The liquid changed the temperature field of the region and altered the geothermal environment of hydrate accumulation, which led to the dissociation of hydrates.

1. Introduction Gas hydrates are crystalline solids that form when water molecules trap gas, commonly methane, within a crystal lattice cage (Sloan, 1990) under certain temperature and pressure conditions. Gas hydrates are widely distributed, especially in continental slope areas (Kvenvolden, 1995; Makogon et al., 2007). The gas hydrate stability zone is mainly affected by the bottom water temperature, geothermal gradient, pressure, gas composition and fluid salinity (Ginsburg et al., 1984; Wang et al., 2013). The bottom water temperature and geothermal gradient are the most important parameters for determining the thickness of the

hydrate stability zone. Thus, marine heat flow measurements and their related research attracted the most attention during the investigation of gas hydrates (Xu et al., 2005; Li et al., 2010; Liao et al., 2014). Some special geological structures, aspects of rock physics, and associated geochemical characteristics often accompany gas hydrate accumulations. The mud diapir/volcano is an important indicator for gas hydrate exploration, which develops widely in continental slopes (Ginsburg et al., 1984; Feseker et al., 2014). A mud diapir/volcano is the result of overpressured fluid released from deep strata (Kopf, 2008; Normile, 2008; Egorov and Rozhkov, 2010; Wan et al., 2015). Many gas hydrates are found in areas that developed mud diapirs/volcanoes, such

* Corresponding author. School of Marine Sciences, Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering/Key Laboratory of Offshore Oil Exploration and Development of Guangdong Higher Education Institutes, Sun Yat-sen University, Guangzhou 510006, China. E-mail address: [email protected] (Z. Wan). http://dx.doi.org/10.1016/j.petrol.2017.08.053 Received 17 January 2017; Received in revised form 13 August 2017; Accepted 25 August 2017 Available online 26 August 2017 0920-4105/© 2017 Elsevier B.V. All rights reserved.

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Fig. 1. Location map of the Shenhu gas hydrates drilling area in the Zhu II depression of the Pearl River Mouth Basin, northern South China Sea (after Zhu and Mi, 2010).

drilling in the Shenhu area (Zhang et al., 2007). Eight boreholes were drilled, and gas hydrates were revealed in the core samples from 3 sites (SH2, SH3 and SH7) (Zhang et al., 2007; Wu et al., 2008), while there were no gas hydrates in SH5, even though a clear bottom-simulating reflection (BSR) was detected in the seismic profiles at the location of SH5 (Wu et al., 2007). The scientific explanation of disparity of gas hydrates discovery at SH5 from the other three sites in the area, namely, SH2, SH3 and SH7, has remained a challenging problem even since the GMGS expedition in 2007. However, detailed studies of seismic velocity profiles across SH5 revealed typical mud diapir features at and around the vicinity of SH5 (Wang et al., 2010). Nevertheless, the thermal structure of the mud diapir at SH5 has not been investigated, not to mention its relation to the absence of gas hydrates in SH5. Inspired by the findings of Feseker et al. (2009) for the Dvurechenskii mud volcano in the Sorokin Trough on the northern margin of the Black Sea, SE of the Crimean Peninsula, we in this paper aim to reveal the thermal structure of the mud diapir at SH5 and analyze the thermodynamic mechanism in the evolutionary process of mud diapir and its relationship to the gas hydrate occurrences, integrating all data sets available including seismic profiles, mineral identification, sediment thermal conductivity measurements and other information. Furthermore, comparative studies of thermal structure at the nearby site SH2 where gas hydrates were found and mud diapir is absent could shed light on the geothermal control of gas hydrates occurrence in the region.

as the Caspian Sea, the Black Sea, and the Gulf of Mexico (Sauter et al., 2006; Franek et al., 2014). Feseker et al. (2009) analyzed the temperature distribution in the sediments and bottom water at the Dvurechenskii mud volcano in the Sorokin Trough on the northern margin of the Black Sea, and found that gas hydrate dissociation and formation at mud volcano were closely related to heat flow changes due to the changes in seepage rates. Milkov (2000) estimated that the amount of methane in all gas hydrates associated with submarine mud volcanoes is 1010~1012 m3. A mud diapir/volcano is easy to identify, and its related hydrate resources are rich. Thus, it is necessary to study the influence of mud diapirs/volcanoes on the occurrence of hydrates. Gas hydrate resources are very rich in the continental slope of the northern South China Sea (Zhang et al., 2007; Luo et al., 2013; Sha et al., 2015). Recently, many studies have been conducted regarding the formation and occurrence of gas hydrates there, including such topics as physical and chemical properties, output conditions, distribution, exploration technology, economic evaluation and environmental impact (Yu et al., 2014; Yang et al., 2015; Liu et al., 2015). However, because of the complex topography, tectonic and sedimentary environments, and high heat flow background in this area, hydrate exploration and research are very difficult. These factors also affect many key scientific issues on hydrate accumulation, such as the formation mechanism and controlling factors of gas hydrate. There are many mud diapirs/volcanoes in the northern South China Sea continental slope (Wan et al., 2012; Chen et al., 2015). Geologists who analyzed the formation mechanism of hydrates related to mud diapirs/volcanoes using temperature, pressure and fluids data encountered several contradictions. It is critical to solve this problem by sophisticated detection methods and thermodynamic analysis. In 2007, the Guangzhou Marine Geological Survey (GMGS) of the Chinese Ministry of Land and Resources deployed the first gas hydrate

2. Geological setting The South China Sea, located at the junction of the Eurasian plate, the Indo-Australian plate and the Pacific plate, is a marginal sea that has 425

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Fig. 2. Bathymetric map of the study area. Boreholes are marked on the map. The location of this area is shown in Fig. 1.

and faults. Since the Cenozoic, the Zhu II depression has experienced three evolutionary stages, the early rift, the fault depression and the depression stages (Sun et al., 2009; Zhu and Mi, 2010). The sedimentary environment gradually evolved from continental facies to shallow sea and slope to a deep-water environment. During the rift stage, there were many halfgrabens with river and lake sedimentary environments. Large-scale lake facies developed during the fault depression stage. During the depression stage, the sedimentary environment slowly changed from shelf shallowwater to slope deep-water. Thick deep-water sedimentary strata accumulated in the study area with a high deposition rate. The maximum thickness of the deep-water area is approximately 12 km (Shi et al., 2005; Pang et al., 2007; Xie et al., 2014). The Shenhu gas hydrates drilling area is located between Shenhu Shoal and the Dongsha Islands. The seafloor topography of this studied area is complex. The overall trend is inclined from north to south, with the water deepening gradually from 1 000 m to more than 1700 m. Submarine canyons are well developed in this area (Li et al., 2014; He et al., 2014; Qiao et al., 2015). There are many different geomorphologies, including sea mounds, valleys, erosion ditches, antitrench slopes, and bottom grooves (Fig. 2). The difference in depth between canyon ridges and grooves is approximately 300 m.

developed oceanic crust (Hamilton, 1979; Tapponnier et al., 1982; Briais et al., 1993; Li, 2011). The geological characteristics of this region are very complex. The continental margin varies, comprising a rift to the north, a shear zone to the west, a subduction accretionary wedge to the east and a platform to the south (Nissen et al., 1995; Clift et al., 2002; Xia et al., 2006; Sun et al., 2009). The continental margins have good thermodynamic conditions and geological tectonic environments for forming gas hydrates (Yao, 2001; Luo et al., 2013). Since the Late Cretaceous, the northern continental margin of the South China Sea has experienced multiple episodes of rifting. It evolved into a passive continental margin with the final rupture of the lithosphere and the onset of seafloor spreading during the Oligocene. It developed a broad flat shelf and a slope where the water depth increases rapidly to a deep-sea basin (Yan et al., 2001; Hao et al., 2008; McIntosh et al., 2014; Gao et al., 2015). The Shenhu area is located in the Zhu II depression of the Pearl River Mouth Basin in the northern South China Sea (Fig. 1). Faults developed recently in the Zhu II depression, which is the active area of neotectonic movement in the northern South China Sea. Numerous structural traps formed and, at the same time, mud diapirs/ volcanoes developed (Shi et al., 2005; Sun et al., 2012). The South China Sea has a high heat flow, with an average value of 78 ± 23 mW/m2 (He et al., 2001; Shi et al., 2003). Heat flow distribution is uneven, however, with an overall trend as follows: the heat flow increases gradually from the north to the central basin, decreases from the central basin to the east, and increases from south to west. The heat flow is highest in the central basin. The heat flow values of the east sub-basin, northwest sub-basin, and southwest sub-basin are 93 mW/m2, 89 mW/ m2, and 95 mW/m2, respectively, while the average heat flow of the northern margin of the South China Sea is 75 mW/m2 (Yuan et al., 2009; Tang et al., 2014). The heat flow of the Shenhu area is high, within the range of 80–90 mW/m2 (Xu et al., 2012). This may be due to the fact that the location is close to the central basin and is also influenced by uplifts

3. Data and methods Eight boreholes were drilled by the Guangzhou Marine Geological Survey (GMGS) in 2007. The drilling depths of SH2 and SH5 are 250 m and 208 m, respectively. An FPWS system (Fugro Pore Water Sampler) was used to measure the in situ temperature during the process of hydrate drilling, which was designed for sample collection and temperature measurement in deepwater areas, with an applicable range of water depth of 3 000 m. The

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Fig. 3. The in situ temperatures-depth plots of the gas hydrate target layers (a. SH2, b. SH5).

measured in the laboratory of the GMGS. We used a TK04 Thermal Conductivity Meter to test the thermal conductivity of the samples. The standard sample was used for calibration before each measurement, and the test error was controlled to less than 5%. We also adjusted the measurement results according to the laboratory temperature. There were 38 and 84 thermal conductivity measurements from SH2 and SH5, respectively. The concentrations of major mineral phases of the samples were determined by X-ray diffraction (XRD) analysis in Peking University. The XRD analysis was performed on an X'Pert PRO DY2198 at the GMGS. Approximately 150 g of the samples was ground to approximately 100 mesh in an agate mortar, pressed into the specimen plane, and then measured for main minerals by XRD. The seismic profiles analyzed in this study were collected by the “FENDOU SI HAO” of GMGS in 2006. The vessel was equipped with a 160-cubic-inch air-gun array towed at a depth of 3 m below sea level. Shots were fired at intervals of 25 m. The common depth point (CDP) of measurement was 12.5 m. To ensure complete data acquisition, seismic records with a duration of 6 s were acquired with a sampling rate of 1 ms. 4. Results During the drilling process, the in situ temperatures of gas hydrate target layers were tested (Fig. 3). According to the seismic data analysis, the hydrate target layer is 200–220 mbsf in borehole SH2. Where the minimum in situ temperature is 1.30  C. The maximum temperature is 8.20  C. And the average temperature is 3.18  C (Fig. 3-a). The depth of bottom-simulating reflection (BSR) in borehole SH5 is 170 mbsf. In the depth range of 165–175 mbsf, the minimum and maximum in situ temperatures are 15.00  C and 20.80  C respectively. And the average temperature of which is 17.69  C (Fig. 3-b). The seafloor heat flow values of SH2 and SH5 are 65.7 and 71.4 mW/m2, respectively. The geothermal gradient of SH5 is 67.6  C/km, and that of SH2 is 46.95  C/km. Changes in thermal conductivity with depth are shown in Fig. 4. There is an obvious difference between the shallow and deep values at SH2. The values are low at shallow depths at approximately 1.0 W/(m⋅K) and high in deep areas with a range of 1.3–1.5 W/(m⋅K). Meanwhile, the thermal conductivity value of SH5 is approximately 1.0 W/(m⋅K) from top to bottom. X-ray diffraction measurements indicate that the sediments from SH2 and SH5 are mainly composed of detrital, clay and carbonate minerals. The content variations of the three components are shown in Fig. 5-a. The mineral components of the two boreholes mainly trend toward detrital minerals, which shows that the sediments are mostly composed of terrigenous clastic minerals. The content of clay minerals is the lowest. Detrital grains are mainly light minerals, such as quartz, plagioclase, orthoclase and muscovite (Fig. 5-b). The proportion of heavy minerals is

Fig. 4. Thermal conductivity variations with depth in SH2 and SH5.

equipment for temperature measurement was a Fenwall 112–102 EAJB01 with a range of 2 ~ þ40  C, a resolution of 0.005  C, an accuracy of ±0.01  C, and a reaction time of less than 1 s. During logging, the FPWS test system descended to the bottom of the hanging rope into its drilling position. Then, the temperature probe was inserted into the surrounding soil layer to measure the temperature. Real-time data were recorded and transmitted to the shipboard workstation for monitoring. After the test, the temperature measurement device was brought back up to the deck by a suspension cable. There were 64 and 88 in situ temperatures measured at SH2 and SH5, respectively. Sediment cores were collected in the process of drilling. The thermal conductivity and mineral composition of the sediment samples were

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Fig. 5. Lithology triangle maps of SH2 and SH5. a. Detrital minerals - clay minerals - carbonate minerals triangle map; b. Light minerals - heavy minerals - carbonate minerals triangle map.

between the two boreholes. At borehole SH2, the thermal conductivity is low in the shallow part and high in the deep part. At SH5, the content of epicontinental clastic minerals decreases with depth, while the content of carbonate minerals is low in the shallow part and high in the deep part. Thus, the thermal conductivity has little overall change at SH5. The deep part of SH5 has high contents of carbonate minerals, while in contrast, gas hydrates occur in the deep part of SH2 with relatively low carbonate minerals. Many leakage structures have developed in the study area, such as faults, fractures and mud diapirs. Cold seep systems are active. The main fluids of the cold seep system are water and gas (mostly methane). Part of the leaking methane is transformed into gas hydrates combined with water in the hydrate stability zone. The rest is changed into bicarbonate ions (HCO3) by anaerobic oxidation of methane, which promotes the formation of authigenic carbonate minerals (Peckmann et al., 2001; Peckmann and Thiel, 2004; Mansour and Sassen, 2011). No gas hydrates are found at SH5, but the carbonate content is higher. This observation indicates that the methane leaking from the cold seep promotes the formation of carbonate minerals.

extremely low, and the species are fewer, mainly pyrite and hornblende. At SH2, the light mineral contents such as quartz and feldspar change little in the Pleistocene interval (Fig. 6-a). The content of carbonate grains increases with depth from 20 m to 35 m. At the 35-m horizon, it reaches its highest value, but the carbonate content is relatively stable with increasing depth. The contents of white mica and clay minerals decline slowly with increasing depth from 20 m to 35 m, and their fluctuation is very small below 35 m. Overall, the terrigenous clastic minerals of this borehole are stable. The carbonate minerals are relatively high at 30–40 m, up to 40%. It is worth noting that the hydrate layer (191–225 m) is high in the terrigenous minerals quartz and feldspar (approximately 40%) and low in carbonate minerals (approximately 12%) compared to the adjacent horizons. SH5 shows different characteristics in its mineral contents (Fig. 6-b). From top to bottom, the contents of detrital and clay minerals decrease gradually, while carbonate minerals increase gradually. In the layer between 0 and 23 m, the content of detrital minerals is as high as 60%, mainly quartz, feldspar, hornblende and white mica. The content of carbonate minerals is the lowest in the borehole, in the range of 10% to 20%. In all of the layers from the Pleistocene and the Holocene (0–99 m), the borehole contains mainly epicontinental clastic sediments. The content of clay minerals is stable, and the content of carbonate minerals is low. From 130 m to the core bottom, the content of detrital minerals falls sharply with depth, while the content of carbonate minerals increases gradually up to 50%.

5.2. Characteristics of the geothermal field in the two boreholes Temperature and pressure are the two important factors for natural gas hydrate formation and occurrence. So the study of pressure, temperature and the temperature-related geothermal gradient, thermal conductivity and heat flow, can predict the range of natural gas hydrate stability zone. In general, the thickness of gas hydrate stability zone is calculated by the hydrate phase equilibrium. Dickens and Quinby-Hunt (2013) measured the stability of methane hydrate in seawater, and obtained the temperature-pressure linear equation for the stable presence of methane hydrate as follows:

5. Discussion 5.1. Characteristics of the thermal physical properties of the two drill cores Thermal conductivity of sedimentary rocks is affected by many properties, including rock fabric, porosity, water saturation, permeability, temperature, and pressure. It increases with the increase of buried depth, because porosity reduces and the rocks become more compacted (Allen and Allen, 2005). And the thermal conductivity of sediments is directly related to the contents of quartz and feldspar. Generally, the thermal conductivity value is high as content of quartz and feldspar increases (Beardsmore and Cull, 2001). This observation could explain the difference in thermal conductivity

T 1 ¼ 3:79  103  2:83  104 log p

(1)

where P is pressure (MPa), T is temperature (K). The water depth of borehole SH2 is 1 238 m. And the depth of BSR of SH2 is 203 mbsf. According to the equation (1), the corresponding temperature of gas hydrate occurrence should be under 15.8  C. As to SH5, the water depth is 1 425 m, and the depth of BSR is 170 mbsf. The calculated maximum temperature of gas hydrate occurrence was 16.8  C. 428

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5.3. Characteristics of seismic stratigraphy around the two boreholes The existence and depth of a hydrate deposit is often indicated by the presence of a bottom-simulating reflection (BSR). A BSR is a seismic reflection indicating the lower limit of hydrate stability in sediments due to the different densities of hydrate saturated sediments, normal sediments and those containing free gas. Some submarine formations with BSRs around the world do not have gas hydrates, while others with gas hydrates do not have BSRs. Because the seismic data could have multiple solutions. And the multiple reflections are often easy to be seen as BSR (Shipley et al., 1979; Hyndman and Spence, 1992; Holbrook et al., 1996). The seismic profiles across SH2 and SH5 are analyzed. In the seismic profile of SH2, a BSR is very obvious at a depth of 1 441 m (203 mbsf) (Fig. 7-a), where gas hydrates are also found. Gas source of hydrates in Shenhu area mainly comes from in situ microbial origin methane. So some scholars believe that its occurrence type belongs to the diffusion of hydrate reservoiring pattern. But according to the natural gas hydrate reservoiring conditions and geophysical characteristics, leakage type hydrate also exists (Zhang et al., 2007; Wu et al., 2008). In the lower part of SH5, the depth of BSR is 1 595 m (170 mbsf), which is the lower limit of hydrate stability (Fig. 7-b). But a mud diapir is also exists. The seismic profile shows that the reflection is discontinuous within the mud diapir. The interface at its bottom is clear, while the top surface undulates, and the layers are warped. The boundary line between surrounding layers and the mud diapir is evident. At the same time, many faults can be identified above the mud diapir. They reach the bottom of the BSR, forming the gas leakage system. The bottom of the BSR is marked by the enhanced amplitude of the reflector, while the area of leakage displays a weak amplitude reflection. The reflection phase axis is dropped down, which is a typical V-AMP (velocity-amplitude anomaly) structure, displaying free gas accumulation under the BSR (Scholl et al., 2009). Above the BSR, the layers appear blank because of the difference in acoustic impedance. In addition, two shallow faults developed upon the mud diapir. A weak reflection phase can be seen inside the faults, which are filled by free gas migrating from the mud diapir. Affected by the fault channels and fluids from the mud diapir, SH5 has an unusually high geothermal gradient and a high heat flow value. 5.4. Influence of mud diapir activity on occurrence of gas hydrates According to the CSMHYD program (Sloan, 1990), we calculated the bottom boundary and thickness of the gas hydrate stability zone of SH2 and SH5, based on the data of water depth, submarine temperature, geothermal gradient, heat flow and gas source. The phase diagram of SH2 (Fig. 8-a) shows that the calculated thickness of gas hydrate is 294 m. In borehole SH5, the calculated thickness of gas hydrate is 410 m from the phase diagram (Fig. 8-b). And The corresponding temperature is below 16.8  C. While SH5 didn't get gas hydrate sample because of the mud diapir development. Mud diapirs developed in the northern South China Sea and have been discovered in Yinggehai, Qiongdongnan, Taixinan, and the Shenhu area of the Pearl River Mouth basin (Wan et al., 2012; Chen et al., 2015). The Shenhu area is located west of the Philippine plate, which formed by a collision between the Luzon arc and South China plate. Since the Late Miocene, two phases of collision occurred, at approximately 5 Ma and 2 Ma. Thus, the study area is an active region of neotectonic movement in the South China Sea (Yan et al., 2001; Hao et al., 2008; McIntosh et al., 2014; Gao et al., 2015). Affected by neotectonic movement, faulting increased during the thermal subsidence stage of the Zhu II depression, and the peak of faulting appeared at 1.5–2 Ma (Shi et al., 2005; Sun et al., 2012). At the same time, neotectonic movement induced plastic flow of argillaceous rocks in the deep overpressure layers. Then, the mud diapirs formed, pierced the upper layers and generated many high-angle faults and a vertical fracture system, which constitute the main channels for fluid migration. Mud diapirs are closely related to natural gas hydrates because they

Fig. 6. Lithology variations with depth in the two boreholes. a. SH2; b. SH5.

The temperature at the seafloor is approximately 2.2–2.5  C in the study area. However, the in situ temperature measured during the drilling reveals that the geothermal fields are obviously different in the deep parts of this area. The in situ temperature of the BSR layer does not exceed 15.8  C at SH2. Combined with the geothermal gradient and the thermal conductivity, the calculated heat flow is 59 mW/m2. Meanwhile, the geothermal gradient of SH5 is as high as 67  C/km. The average temperature of the predicted hydrate stability zone (165–175 m) is 17.69  C in SH5. The heat flow data confirm that the geothermal properties are different in the two boreholes. They also reflect that the temperature and pressure are obviously different in the two locations. This difference is the main influence on the hydrate reservoir conditions. The critical temperature for hydrate formation and decomposition is approximately 16.8  C in SH5. Therefore, if the temperature is higher, as it is at SH5, the methane in the strata can exist only in the form of free gas and it is unlikely to form hydrates.

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Fig. 7. Seismic profiles across the two boreholes. a. SH2; b. SH5.

Fig. 8. Phase equilibrium curve of gas hydrate stabilily zone (GHSZ) in the study area (a.SH2, b. SH5).

mud diapir migrating from the deep layer into the shallow one, its development can be divided into early, middle and late stages. The three stages form a continuous geological process that controls the reservoir evolution of gas hydrates. During the early stage, the fault channels of the mud diapir have

can provide sufficient methane for gas hydrate formation. At the same time, they can change the geothermal field and affect the stability of gas hydrates (Milkov, 2000; Sauter et al., 2006; Franek et al., 2014). This mutually restricting relationship may be due to differences in the evolutionary periods of a mud diapir. According to the characteristics of a 430

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Fig. 9. The evolution characteristics of a mud diapir and their influence on gas hydrate accumulation (a. Early stage; b. Middle stage; c. Late stage). And the temperature curves show the changes of geothermal field in different evolutionary stages.

6. Conclusion

already developed (Fig. 9-a), but they do not reach the gas hydrate stability conditions. The power of mud diapir activity comes from internal high-pressure gas in the pore spaces. The thermal conductivity of the gas is lower than that of the surrounding normal mudstone layers, and the gas has high thermal resistance. Thus, the upper layer of the mud diapir lies in a low-temperature field during the early stage. This is conductive to the formation of gas hydrates. During the middle stage, the fault channels have extended to the natural gas hydrate stability field (Fig. 9b). Usually the substances leaking from the mud diapir include gas and liquid. Because gas migrates much faster than liquid, it arrives first at the bottom of the gas hydrate stability zone and becomes the gas source for the hydrates. The unit heat capacity of gas is low because it has lost most of its heat to the surroundings in the process of upward migration. Thus, the temperature of the gas is close to that of the stability zone and does not disturb the geothermal field in the middle stage. During the late stage, liquid from the mud diapir begins to invade the gas hydrate stability zone (Fig. 9-c). The formation of gas hydrates is decided by the heat balance exchange between the liquid and layers in the stability zone. Because of the high unit heat capacity of liquid, the whole temperature field of the surrounding layers increases significantly when the mud diapir pierces upwards. This high heat flow leads to decomposition of the gas hydrates. Therefore, in the early stage of the mud diapir development, gas migrated along the fault channels, and part of leaking methane promoted the formation of authigenic carbonate minerals. So borehole SH5 has high carbonate contents and low percentages of quartz and feldspar. This is the reason that SH5 has relatively low thermal conductivity at the bottom of the borehole. In the late stage, the higher-temperature liquid erupted rapidly into the shallow strata, which would change the geothermal field of the region. The reason SH5 did not find gas hydrates may be that the mud diapir had pierced through during the late stage, leading to gas hydrate decomposition, even though there is an obvious BSR. The seismic profile shows that the fault system is well developed in and above the mud diapir. These faults are effective channels for fluid migrating from the deep part to the shallow part. The hot deep fluid changes the temperature of the hydrate occurrence layer, and this affects the formation of hydrates.

The deep part of SH5 has a relatively high percentage of carbonates and low thermal conductivity. The in situ temperature measured in this borehole is higher than the critical temperature for hydrate formation and decomposition. The seismic profile shows that a mud diapir developed in the lower part of SH5. The gas leakage system of the mud diapir is obvious, and the typical V-AMP structure displays free gas accumulation under the BSR. The evolution of the mud diapir can be divided into three stages within a continuous geological process controlling the gas hydrate reservoir. The mud diapir below SH5 is in the late stage when hightemperature liquid migrated into the gas hydrate stability zone. It changed the temperature field of the region and destroyed the geothermal environment for hydrate accumulation, which led to decomposition of hydrates. Further, the anaerobic oxidation of the leaking methane resulted in the formation of authigenic carbonate minerals. This is the reason SH5 did not find gas hydrates, even though an obvious BSR was detected below the borehole. Acknowledgements This work was supported by National Nature Science Foundation of China (No. 41776056, 91428205), Science and Technology Program of Guangzhou (No. 201607010214), the Fundamental Research Funds for the Central Universities (No. 17lgjc10), Geology Investigation Project of China Geological Survey (No. DD20160158 and KD 20160208) and the China Scholarship Council. We are also grateful to Dr. Y. Chen at Los Alamos National Laboratory of US for the valuable suggestions. The authors also thank the anonymous reviewers for their helpful comments. References Allen, P.A., Allen, J.R., 2005. Basin Analysis: Principles and Applications, second ed. Blackwell Scientific Publications, Oxford, pp. 349–402. Beardsmore, G.R., Cull, J.P., 2001. Crustal Heat Flow: a Guide to Measurement and Modelling. Cambridge University Press, pp. 293–302. Briais, A., Patriat, P., Tapponnier, P., 1993. Updated interpretation of magnetic anomalies and seafloor spreading in the South China Sea: implications for the tertiary tectonics of southeast Asia. J. Geophys. Res. 98, 6299–6328. 431

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