Compound hydrocarbon accumulation and enrichment in southwestern Huizhou Sag, Pearl River Mouth Basin, South China Sea

Compound hydrocarbon accumulation and enrichment in southwestern Huizhou Sag, Pearl River Mouth Basin, South China Sea

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 42, Issue 2, April 2015 Online English edition of the Chinese language journal Cite this article as: PETR...

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PETROLEUM EXPLORATION AND DEVELOPMENT Volume 42, Issue 2, April 2015 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2015, 42(2): 215–222.

RESEARCH PAPER

Compound hydrocarbon accumulation and enrichment in southwestern Huizhou Sag, Pearl River Mouth Basin, South China Sea CHEN Weitao1, DU Jiayuan1,2,*, SHI Hesheng1, HE Min1 1. Shenzhen Branch of CNOOC Ltd., Guangzhou 510240, China; 2. Ministry of Education Key Laboratory of Oil and Gas Resources and Exploration, Yangtze University, Jingzhou 434023, China

Abstract: Aimed at four major issues in the oil and gas exploration of the southwestern Huizhou area in Pearl River Mouth Basin, in-depth study on geologic features and oil and gas reservoir forming pattern has been carried out to find out the exploration potential and direction of compound oil and gas reservoirs in this area. There develops the compound formation structure of basement, rifting and depression layers in this area, and a composite hydrocarbon transport system comprised by faults, sandstone layers and the sequence boundaries, which are connected to the several hydrocarbon generation subsags in the adjacent Huixi half graben. Controlled by the subsag-controlling boundary faults, long inherited paleo-uplifts and ancient structural ridges, oil and gas accumulate in multiple series in vertical direction, multiple connected traps horizontally, forming compound reservoirs of various traps. According to the three types of structural zones controlling oil and gas enrichment, the southwestern Huizhou area can be divided into seven compound oil and gas accumulation zones in three categories, which combine into a large compound hydrocarbon accumulation province. Key words: compound oil and gas accumulation play/zone; southwestern Huizhou area; Pearl River Mouth Basin; reservoir forming features; accumulation pattern; exploration potential

1.

Study background

As a natural expansion of the Dongsha Uplift to the Zhu I Depression, the southwestern Huizhou Sag is located in the southwest of the Huixi Half-graben in the Zhu I Depression of the Pearl River Mouth Basin. The main tectonic elements of the southwestern Huizhou Sag mainly include the Huixi Low-uplift, Southwest Huizhou Sag and the Northwest Dongsha Uplift[1] (Fig. 1). The southwestern Huizhou Sag is a significant area for oil and gas exploration and development in the Pearl River Mouth Basin. However, after more than thirty-year exploration, some major issues have already emerged: (1) Oil and gas exploration has mostly focused on structural traps for a long term, while other types of hydrocarbon reservoirs were somehow ignored. (2) The Zhujiang Formation in the Miocene has been always the major target interval, while other layers have not been systematically investigated yet. (3) Due to the high cost of offshore exploration and development, lots of relatively isolated oil-bearing structures would hardly turn into commercial oil and gas fields. (4) The complexity of newly discovered hydrocarbon targets increase the difficulty of exploration. Consequently, it is very

critical to investigate the petroleum geology and hydrocarbon accumulation patterns deeply, which is significant to get out of the current dilemma and provide guidance for future hydrocarbon exploration in this area.

2. Geology of the compound hydrocarbon accumulations 2.1.

Multi-layered compound stratigraphic structure

As one part of the Pearl River Mouth Basin, the southwestern Huizhou Sag has multi-layered compound formation structures, consisting of the basement, rifting and depression layers from lower to upper strata, providing favorable conditions for the compound hydrocarbon accumulation in the deep, middle and shallow intervals (Fig. 2). The basement structural layer refers to the sedimentary basement of this basin, mainly dominated by four types of lithology: the Paleozoic metamorphic rocks, Mesozoic magmatic rocks, Mesozoic sedimentary rocks as well as the lower Cenozoic volcanic and pyroclastic rocks[1]. Two sets of fault systems, which orient NE and NW respectively, break and cut the basement of this basin, forming the basic framework of

Received date: 02 Aug. 2014; Revised date: 13 Feb. 2015. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2011ZX05023-002). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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Fig. 1.

Structural elements and major subsags distribution of southwestern Huizhou Sag in the Pearl River Mouth Basin.

Fig. 2.

Stratigraphic column of the southwestern Huizhou Sag.

the rifting lake basin during the Paleogene period. In the southwestern Huizhou Sag, basement buried hills might be formed in the basement structural layer between depressions and in their peripheries. When the hydrocarbon source supply was sufficient, the buried hill reservoirs might be formed in “upper-generation and lower-storage” model. The rifting structural layer, also referred to the “lower structural layer”, is dominated by the alluvial deposits formed during the rifting phase, including the Wenchang Formation,

the Enping Formation and the Zhuhai Formation. The Wenchang Formation and the Enping Formation are mainly lacustrine deposits of the rifting stage, forming most of the source rocks in Pearl River Mouth Basin. Meanwhile, a series of nearshore underwater fans and fan deltas are developed at the steep slope belts around the lake basin, and the braided-river deltas are developed in the fault transform zones and the gentle slope belts, while the beach bars are developed in some special settings (such as the underwater highs etc.). High-

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quality reservoirs are developed in the above depositional systems. Additionally, as they are close to hydrocarbon sources, these reservoirs are the dominant intervals to form “self-generation and self-storage” reservoirs. Since the deposit stage of the Zhuhai Formation, transgression occurred to form sand and mud deposits in transitional facies. They are important intervals for lateral hydrocarbon migration due to a high sand ratio. The depression structural layer, also referred to the “upper structural layer”, is mainly dominated by the marine sediments deposited during the depression stage since late Oligocene period, mainly containing the Zhujiang Formation, the Hanjiang Formation, the Yuehai Formation and the Wanshan Formation. During the deposit stage of the Zhujiang Formation, the water depth rapidly increased with the dramatic marine transgression, leading to the formation of delta deposits in transitional facies and neritic shelf deposits in the basin on quasi-passive margins in this structural layer[1]. The paleo-Pearl River delta depositional system predominated most of the depression structural layer. Controlled by cyclic relative sea level fluctuations, multiple sets of reservoir-seal assemblages were formed[2]. The hydrocarbon generated in the Paleogene source rocks migrated upwards along faults, resulting in the important hydrocarbon-bearing intervals in “lower-generation and upper-storage” mode from external sources. Hereinto, the Zhujiang Formation and the lower Hanjiang Formation have the best hydrocarbon-bearing situations. 2.2.

Hydrocarbon source conditions

Several petroliferous subsags are developed in the Huixi Half-graben (Fig. 1), providing sufficient hydrocarbon supply for the southwestern Huizhou Sag. Covering an area over 3 000 km2, the Huixi Half-graben has a total generated hydrocarbons of 320×108 t, hydrocarbon-generating intensity over 1 000×104 t/km2, the total resources of approximately 20×108 t and resource abundance of 65×104 t/km2. It is a typical petroliferous half-graben. Among the six hydrocarbon generation subsags in this half-graben, the Huizhou 26, Xijiang 30 and Xijiang 23 subsags are all characterized by “thicker Wenchang Formation and thinner Enping Formation” (i.e., mudstones in semi-deep to deep lake facies are developed in the Wenchang Formation)[1], which have significant potential in generating oil and gas. For example, the average hydrocarbon-generating intensity of Huizhou 26 subsag is up to 1 800×104 t/km2. Two sets of source rocks are developed in the Huixi Half-graben, including the Wenchang Formation and the Enping Formation. Geochemical analysis indicated that the lacustrine deposits of the Wenchang Formation are dominated by type Ⅱ1 kerogen, with minor portion of Ⅰ and Ⅱ2 types. Generally, high-quality source rocks are distributed near the boundary faults of the subsags. In contrast, low-quality and thin source rocks are distributed between subsags. The Enping Formation is characterized by shallow water deposits, leading to the formation of coal-measure source rocks, and its kerogen

type is mainly Ⅱ2, with minor Ⅱ1 and Ⅲ types. Favorable hydrocarbon generating areas are located in the subsag centers, with sandy sediments occurring in the region of boundary faults[3]. According to the analyses of buried history and thermal evolutions, it is speculated that the source rocks of the Wenchang Formation entered hydrocarbon threshold since 32 Ma, with extensive hydrocarbon generation since 25 Ma. While, the source rocks of the Enping Formation entered hydrocarbon threshold since 18 Ma, with extensive hydrocarbon generation since about 15 Ma and until nowadays[4]. 2.3. “Ternary coupling” compound hydrocarbon migration system Fault systems, sand layers and unconformities are the most common hydrocarbon migration pathways[5−7]. Faults serve as the vertical hydrocarbon migration pathway and are critical for forming vertically stacked multi-layer compound accumulations. Sand layers and unconformities typically serve as the lateral and long-distance migration pathways along the structural ridges, which is significant to the continuous hydrocarbon accumulations on plan[8−11]. In the southwestern Huizhou Sag, a compound hydrocarbon migration system consists of “ternary coupling” (including fault systems, sand layers and unconformities which mainly conduct as the sequence boundaries), which is the key factor of the compound hydrocarbon accumulations. The reservoir of rifting structural layer in the southwestern Huizhou Sag is adjacent to the source rocks and prior to catch and store oil and gas, especially in the Wenchang Formation and the Enping Formation. At first, hydrocarbons were prone to enter the adjacent sand layers and subsequently migrated towards high locations along the sand layers or sequence boundaries. Later tectonic events and hydrocarbons generated during later stages might modulate and complement the accumulations formed previously. In the basement structural layer, the buried hills located in the center or around the subsags are generally the favorable directional zone for hydrocarbon migration. Especially, the inherited paleo-highs close to the subsags, due to long-term positive landform and multiphase uplifting events, which made numerous fissures have developed, have better hydrocarbon accumulation conditions. Hydrocarbon reservoirs in the depression structural layer are all the “lower-generation and upper-storage” mode. Faults served as the principal vertical migration pathways. Laterally, hydrocarbons migrated for a long distance along sand layers with high sand ratios in the Zhuhai Formation and the lower Zhujiang Formation and the sequence boundaries on the structural ridges. For the upper Zhujiang Formation, the sand ratio significantly decreases, and sandstones are distributed along the upper and lower sections of the sequence boundaries. Hydrocarbons would preferentially migrate along the sequence boundaries and the high-quality sand layers nearby after they entered the upper Zhujiang Formation. Hence, sequence boundaries have been important lateral hydrocarbon migration pathways. Regional

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studies show that numerous porous fan deltas are prone to forming along the downthrown sides of early subsag-controlling boundary faults. These fan deltas served as the transfer station for the hydrocarbon from source rocks to faults. The subsag-controlling boundary faults directly contact with sandstone translocation layersin the Zhuhai and lower Zhujiang Formations which have high sand ratios and the sequence boundaries in the upper Zhujiang Formation. After hydrocarbons migrated from the source rocks to the region adjacent to the boundary faults, they subsequently migrated upwards along faults to the sandstone translocation layers and vicinity of sequence boundaries, and along which they further migrated for a long distance. Meanwhile the later formed faults (including the early faults with later reactivation), together with the sand layers they cut through, could further redistribute hydrocarbons in vertical direction and transport hydrocarbons in lateral direction (Fig. 3). Therefore, a compound hydrocarbon migration system was composed of the subsag-controlling boundary faults, fan deltas at their downthrown sides, the sandstone translocation layers in the Zhuhai Formation and the lower Zhujiang Formation, the sequence boundaries with more sand contents in the upper Zhujiang Formation, later faults and the sand bodies connected by these faults, then forming the compound hydrocarbon accumulation features in southwestern Huizhou Sag.

3. Compound hydrocarbon accumulation and enrichment patterns 3.1.

Compound hydrocarbon accumulation

Hydrocarbon accumulations in the southwestern Huizhou Sag are characterized by compound features, which are represented by “superposition of multiple layers in vertical direction, overlap of multiple traps in lateral direction, and composite of multiple types of traps”. “Superposition of multiple layers in vertical direction” means that the reservoirs of the compound hydrocarbon reservoir are not only limited to a

single “principle” layer, but composed of layers with different epochs and genetic types. Owing to multi-layered compound layer structure, the southwestern Huizhou Sag has three vertically superposed reservoir sets, namely buried hills at the bottom, terrestrial deposits around rift lake basin in the lower layer and marine delta deposits in the middle-upper layers. Each of the hydrocarbon reservoirs has different traps and accumulation features. “Overlap of multiple traps in lateral direction” means that, within a compound hydrocarbon accumulation zone, hydrocarbon reservoirs in different layers and types generally make up a hydrocarbon reservoir group, and various hydrocarbon reservoirs have overlapping feature on plane[12]. In the southwestern Huizhou Sag, this feature is represented by the following three aspects: (1) On the downthrown sides of the subsag-controlling boundary faults near the source rocks, the Paleogene structural traps, stratigraphic overlapping traps, and Neogene structural traps and lithologic traps together formed a compound hydrocarbon accumulation zone. (2) On the long-term inherited paleo-uplifts distributed at the rim of the subsag, the basement buried hills, drape anticlines and lithologic traps on the structure flanks formed another type of compound hydrocarbon accumulation zone. (3) On the northwest pitching end of the Dongsha Uplift, Neogene structural traps and lithologic traps make the last type of compound hydrocarbon accumulation zone. “Composite of multiple types of traps” means that, there are various traps in a compound hydrocarbon accumulation zone; on a certain structural position, the types of the traps generally vary in different layers, while in most cases, one or more certain types dominate. For example, the main types of traps in the compound hydrocarbon accumulation zones formed at the downthrown side of the Paleogene subsag-controlling boundary faults are reverse dragging anticline traps and Paleogene stratigraphic overlap traps, while the Neogene lithologic traps only occupy a second place. Drape anticline traps and lithologic traps on the rim of uplifts are the major exploration

Fig. 3. Schematic map of the oil and gas migration pattern in the southwestern Huizhou Sag of the Pearl River Mouth Basin (see location in Fig. 1).

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Table 1.

The types and characteristics of compound hydrocarbon accumulation zones in the southwestern Huizhou Sag, Pearl River

Mouth Basin. Types of accumulation zones

Major depositional systems

Compound hydrocarbon accumulation zone at downthrown side of subsag-controlling boundary fault

Paleogene fan deltas, Paleogene braided river deltas, Neogene paleoPearl River Delta Basement buried hills, Paleogene braided river deltas and beach bars, Neogene paleo-Pearl River Delta

Compound hydrocarbon accumulation zone at longterm inherited paleo-uplift Compound hydrocarbon accumulation zone at paleo-structural ridge

Beach, Neogene paleo-Pearl River Delta

Types of traps

targets in the long-term inherited paleo-uplift compound hydrocarbon accumulation zone, while Paleogene beach bar lithologic traps occupy the second place. Moreover, Neogene anticline traps and lithologic traps are the main exploration targets in the compound hydrocarbon accumulation zone formed at the pitching end of the Dongsha Uplift. 3.2.

Dominant traps

Occurrence regions

Reverse dragging anticline, fault Reverse dragging The south rim of Xijiang anticline, stratitrap, stratigraphic overlap trap, 30 Subsag, the south rim graphic overlap lithologic trap, fault-lithologic of Huizhou 26 Subsag trap compound trap Xijiang 23-Xijiang 24 Paleo-buried hill, stratigraphic Drape anticline, buried hill region, Huizhou overlap trap, drape anticline lithologic trap trap (Paleogene +Neogene), 21 buried hill region, lithologic trap Huizhou 25 region Huixi Low-uplift, the Drape anticline trap ( Neogene), Drape anticline, northwest rim of lithologic trap lithologic trap Dongsha Uplift

Hydrocarbon enrichment patterns

There are abundant oil and gas resources in the southwestern Huizhou Sag, with various types of hydrocarbon reservoirs. Though the factors controlling hydrocarbon accumulation are very complicated, hydrocarbon accumulation was mainly controlled by three types of structural belts: subsagcontrolling boundary faults around the hydrocarbon-enriched subsags, inherited paleo-uplifts and paleo-structural ridges. In these structural belts, sedimentary sandbodies and structural traps are overlapped in vertical direction, alternated and connected in lateral direction. Oil and gas migrated, transported, accumulated and enriched through faults, sand layers and unconformities, eventually forming several compound hydrocarbon accumulation zones[12] (Table 1). The subsag-controlling boundary faults at the rim of the subsag not only controlled the development of reservoirs in nearshore subaqueous fans or fan deltas on Paleogene steep-slopes, but also led to the formation of the reverse dragging anticline traps, fault traps and fault-lithologic compound traps on the downthrown sides of these faults. Paleogene fans on steep slope contact with the source rocks directly, and have favorable hydrocarbon source conditions, but the poor reservoir properties owing to deep burial depth and close to sedimentary source limit the enrichment of oil and gas. Higher-yield hydrocarbon reservoirs may occur when the underlying reservoir beds are sealed by thick mudstones with overpressure, or secondary pore belts are developed because of the dissolution by organic acid. When reverse dragging anticline traps or fault traps are formed on the downthrown sides of subsag-controlling boundary faults, the hydrocarbon-bearing intervals often span the layers of the Paleogene and Neogene, then resulting in multi-layered and widely dis-

tributed compound hydrocarbon accumulations. Moreover, fault-sealed and lithologic pinchout combination traps can also be formed on the downthrown sides of subsag-controlling boundary faults, but their exploration risk is relatively higher because of the uncertainties of the effective sealing of the faults and lithologic pinchout which are indispensable for such compound traps. Long term inherited paleo-uplifts are favorable positions for oil and gas migration and accumulation. Buried hill hydrocarbon reservoirs can be formed on basement paleo-uplifts. Beach bars were often formed on the top or flanks of paleo-uplifts during the depositing period of the Wenchang Formation. As beach bars were better developed at the beginning of lake transgression and covered by thick mudstones deposited during lake-flooding period, not only they connected with source rocks directly, but also overpressure could be formed on the top of mudstone to protect reservoirs in beach bars. Therefore, the accumulation condition in beach bars is predominant for both lithologic and structural traps. Stratigraphic overlap traps might be formed on the flanks of the buried hills during the depositing period of the Wenchang Formation. As the overlapping formations are generally distal braided river delta or meandering river delta deposits, their reservoir properties are better than those on steep slope. Owing to the high proportion of sand content and development of fissures and small faults caused by later uplifting at the top of the paleo-uplift, the drape layers above the paleo-uplifts in the Enping Formation and the Zhuhai Formation are not likely to act as hydrocarbon reservoirs but as upward hydrocarbon migration pathways. The Neogene drape layer on the paleo-uplift is mainly composed of paleo-Pearl River Delta deposits and the reservoir-seal assemblage is advantageous. The anticline traps in the Neogene were generally formed at the highs of paleo-uplift, while the lithologic traps were usually formed on the flanks of paleo-uplift. Furthermore, owing to the deposit direction of the paleo-Pearl River Delta was basically from west to east, the traps on the west flank of the paleo-uplifts was more favorable to form lithologic pinchout ones than those on the east flank.

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Structural ridges are positive structure hinges formed by the deformation of strata attitude. In the southwestern Huizhou Sag, it is represented by the Huixi Low-uplift which is a part of the Dongsha Uplift between the Huizhou Sag and the Xijiang Sag. Because of the differential compaction and subsidence between the sag and uplift areas, the drape layers above the uplift dip downward to north and south, forming a structural ridge at the center of the uplift gradually upwards the Dongsha Uplift. The oil and gas in this structural ridge mainly came from Huizhou 26 Subsag and Xijiang 30 Subsag of the Huixi Half-graben. The oil and gas provided by the well-developed source rocks in these two subsags are quite abundant so that they could migrate for a long-distance to carbonate platform on the Dongsha Uplift (Fig. 3). At present, all structural highs on this structural ridge have been drilled, discovering many anticline and up-dip pinchout reservoirs, which have made this area the most hydrocarbon discoveries and the highest prospecting degree in the southwestern Huizhou Sag or even the entire Pearl River Mouth Basin. However, there are still great exploration potential in lithologic up-dip pinchout traps along the structural ridges and lithologic traps on the predominant oil and gas migration pathways of the north side.

4. Exploration potential of compound hydrocarbon reservoirs in southwestern Huizhou Sag It is considered that the southwestern Huizhou Sag has huge exploration potential in compound hydrocarbon reservoirs based on the above analyses of petroleum geological conditions and hydrocarbon accumulation patterns. Seven compound oil and gas accumulation zones identified in this

area are summarized to three categories according to the three different structural belt types which controlled the hydrocarbon accumulation (Table 1, Fig. 4). The first category is the compound hydrocarbon accumulation zones at the downthrown sides of the Paleogene subsag-controlling boundary faults, which include the compound hydrocarbon accumulation zone at the downthrown side around the south rim of Xijiang 30 Subsag and the one at the downthrown side around the south rim of Huizhou 26 Subsag. Some reverse dragging anticlinal reservoirs were discovered in these two compound hydrocarbon accumulation zones. The hydrocarbon reservoir found at the downthrown side of Xijiang 30 Subsag-controlling boundary fault has a large scale and has already been put into production. Currently, the exploration of stratigraphic overlap traps and lithologic traps in the Paleogene fan deltas in the steep slope zones of these two compound hydrocarbon accumulation zones are still insufficient. The current exploration only focuses on those areas where structural traps may develope. The main exploration risks are the poor physical properties of the reservoirs and low test productivity which may be not up to the standard for offshore oil and gas exploration. The exploration targeting the fault traps and Neogene fault-lithologic combination traps in these two hydrocarbon accumulation zones has higher risk as well, which includes the pinchout reliability of the lithologic traps and the conflict between the hydrocarbon vertical transportation by the later activated faults and their lateral sealing. Generally speaking, the south rim of Huizhou 26 Subsag is farther to the sedimentary source than that of Xijiang 30 Subsag during Neogene period, and locate at the distal end of the paleo-pearl river delta front, indicating that the traps, both

Fig. 4. Subdivision of compound hydrocarbon accumulation zones in southwestern Huizhou Sag, the Pearl River Mouth Basin. I—Downthrown side of Xijiang 30 Subsag-controlling boundary fault compound hydrocarbon accumulation zone; II—Downthrown side of Huizhou 26 Subsag-controlling boundary fault compound hydrocarbon accumulation zone; III—Xijiang 23-Xijiang 24 buried-hill compound hydrocarbon accumulation zone; IV—Huizhou 25 compound hydrocarbon accumulation zone; V—Huizhou 21 buried-hill compound hydrocarbon accumulation zone; VI—Huixi Low-uplift compound hydrocarbon accumulation zone; VII—Northwest Dongsha Uplift compound hydrocarbon accumulation zone.

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fault traps and lithologic traps, formed in the former area have better effects. The second category is the compound hydrocarbon accumulation zones at long-term inherited paleo-uplifts, which include Xijiang 23-Xijiang 24 buried hill, Huizhou 25 and Huizhou 21 buried hill compound hydrocarbon accumulation zones. The Xijiang 23-Xijiang 24 compound hydrocarbon accumulation zones, in the south of Xijiang 23 Subsag and Xijiang 24 Subsag and the north of Xijiang 30 Subsag, with a quite large scale and adequate hydrocarbon sources, have already found several drape anticline hydrocarbon reservoirs. This buried hill compound hydrocarbon accumulation zone was close to the sedimentary source and mainly located from the delta plain to the proximal part of the delta front during Neogene period. Therefore, it has a high percentage of sandstone content, which made oil and gas migrate to shallower layers. The present discoveries of oil and gas are mainly in the upper part of the Zhujiang Formation and the lower part of the Hanjiang Formation. Besides higher sand ratio, the Xijiang 23-Xijiang 24 buried hill presents east-north dipping feature. The down-dip direction is consistent with the pinchout direction of the paleo-Pearl River delta, which is unfavorable for forming lithologic up-dip pinchout traps. At present, though, there is no sufficient explorations aiming at the buried hill traps, the adjacent lithologic traps and stratigraphic overlap traps in the Wenchang Formation yet, it is supposed that these traps will be important prospecting targets in the future for their closing to the hydrocarbon sources and great exploration potential. Huizhou 25 compound hydrocarbon accumulation zone is surrounded by Xijiang 30 Subsag, Xijiang 24 Subsag and Huizhou 26 Subsag. Some fault block reservoirs of beach bar and stratigraphic overlap reservoirs of braided river delta in the Wenchang Formation have been found in this compound hydrocarbon accumulation zone. It is the best area for hydrocarbon discoveries in the Paleogene at present. Besides some drape anticline traps, there are some up-dip pinchout traps developed in the west part of this area in Neogene as this structural belt plunges northeastward. Huizhou 21 buried hill compound hydrocarbon accumulation zone is a long-term inherited paleo-uplift located on the eastern margin of Huizhou 26 Subsag. Some Neogene drape anticlinal reservoirs have been found at the highs of this buried hill. Meanwhile, due to it being located at the distal end of the paleo-pearl river delta front during the Neogene period, there are some up-dip pinchout traps in the delta front and lithologic traps in striped sandstones reformed by tide in the delta front. Moreover, the top layer of the buried hills, some beach-bar lithologic traps and stratigraphic on-lap traps in the Wenchang Formation, drape anticline traps in the Enping Formation and Zhuhai Formation have been proven to have hydrocarbon indication or identified as oil layers, which indicate that this compound hydrocarbon accumulation zone has great exploration potential in the shallow, middle and deep structural layers. The third category is the compound hydrocarbon accumu-

lation zones at paleo-structural ridges, consisting of Huixi Low-uplift and Northwestern Dongsha Uplift compound hydrocarbon accumulation zones. Because located on the Dongsha Uplift, the strata of these compound hydrocarbon accumulation zones only deposited since late Zhuhai Formation. Its main hydrocarbon-bearing interval is in the Zhujiang Formation. The Huixi Low-uplift compound hydrocarbon accumulation zone is the most prosperous exploration area in the Pearl River Mouth Basin. Almost every structural high in this structure belt is a Neogene drape anticlinal oil reservoirs. Moreover, some larger lithologic up-dip pinchout oil reservoirs have been discovered in the lower and upper sections of the Zhujiang Formation. At present, this structural belt already has very high exploration degree, and all the large structural traps at a certain scale have been drilled. The exploration potential is mainly in the lithologic up-dip pinchout traps along the paleo-structural ridge and lithologic traps on the hydrocarbon migration pathways on the north flank of paleo-structural ridge. In the lower part of the Zhujiang Formation, the paleo-Pearl river delta front and nearby Dongsha Uplift onshore developed interactively in the northwestern rim of Dongsha Uplift compound hydrocarbon accumulation zone, forming structural traps as well as lithologic traps. In the upper part of the Zhujiang Formation, paleo-Pearl river delta distal front and Dongsha Uplift carbonate platform occurred alternatively in this zone. Because the coarse clastic sediments transported by paleo-Pearl River Delta was little, the formation formed during this period has lower sand content, thus resulting in poorer physical properties. Though several hydrocarbon-bearing structures have been found in this zone their scales are usually small. However, two large scale carbonate oil reservoirs (LH4-a and LH11-a) which have been proven that the oil and gas came from the Huizhou Sag, have been discovered in the area far away from the Huizhou Sag (Fig. 4). So it can be concluded that hydrocarbon ever migrated along this compound hydrocarbon accumulation zone for a long distance during the Neogene period. Furthermore, this can indicate that those traps on the long distance migration paths of hydrocarbon have great exploration potential. In addition, the potential of southern part around Huizhou 22 and Huizhou 24 subsags remains to be confirmed. According to statistics, there have been more than 3.7×108 3 m oil reserves and 67×108 m3 natural gas reserves discovered in structural traps of the southwestern Huizhou Sag. The experiences of land oil fields which have mature exploration indicate that the reserves of structural traps only account for one-third of the total reserves in a petroliferous area. In other words, that means there are still two times as much hydrocarbon resources as that already have been discovered. This portion of resources need to be discovered during the exploration stage of compound hydrocarbon reservoirs and lithologic hydrocarbon reservoirs. According to this principle, with the increase of exploration degree, (3−5)×108 m3 of oil equivalent reserves will be found during the exploration of compound

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hydrocarbon reservoirs including the Neogene lithologic traps, Paleogene, paleo-buried hills, shallow layers, and potential exploiting of old oil fields etc. in the southwestern Huizhou Sag. This resources will be found Hereinto, the exploration of compound hydrocarbon reservoirs focused on the Neogene lithologic traps has obtained favorable results in the Huizhou 21 buried hill and Huixi Low-uplift compound hydrocarbon accumulation zones. For another example, the exploration of compound hydrocarbon reservoirs focused on the Paleogene has made breakthroughs in the Huizhou 25 compound hydrocarbon accumulation zone. What’s more, the downthrown block of Huizhou 26 Subsag-controlling boundary fault compound hydrocarbon accumulation zone also shows very great exploration potential. All these exploration practices and understandings not only point out the direction for exploration, but also reveal favorable exploration prospects of compound hydrocarbon accumulation zones in the Southwest Huizhou Sag.

5.

the future exploration according to the features of compound hydrocarbon accumulation zones.

References

Conclusions

The southwestern Huizhou Sag has some favorable conditions for the compound hydrocarbon accumulation: the basement structure layer, rifting structure layer and depression structure layer form a compound formation structure; multiple petroliferous hydrocarbon generating subsags developed in this sag; and there is a compound hydrocarbon migration system comprised by “ternary coupling” which are fault systems, sandstone layers and sequence boundaries. The enrichment of hydrocarbon in southwestern Huizhou Sag is mainly controlled by three types of structural zones, namely subsag-controlling boundary faults around the petroliferous subsags, long-term inherited uplifts and paleo-structural ridges. The hydrocarbon accumulations are characterized by compound features which are “superposition of multiple layers in vertical direction, overlap of multiple traps in lateral direction, and composite of multiple types of traps”. According to the three types of structural zones controlling hydrocarbon enrichment, it can be divided into seven series of compound hydrocarbon accumulation zones among three categories in the southwestern Huizhou Sag. All these hydrocarbon accumulation zones comprise a compound hydrocarbon accumulation play with huge exploration potential. The theory of compound hydrocarbon accumulation zone/play is the theoretical basis for the rolling exploration in mature province and exploration breakthrough in difficult structural belts. Thus, the systematical studies on structure, sedimentation, reservoir, trap, hydrocarbon migration and their mutual relationships should be comprehensively considered and enhanced during

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