Introduction
By the end of 2022, nearly 15×108 t of oil equivalent had been collectively identified as proven reserves in the eastern part of the Pearl River Mouth Basin (PRMB), with approximately 83% of these reserves concentrated in the Neogene. The Huizhou Sag, as the most significant oil and gas production area, contributes nearly 50% of the total oil and gas reserves within the Pearl River Mouth Basin. With the continued exploration, however, the size of remaining traps is diminishing, and exploration efficiency is decreasing in shallow Neogene layers. It is thus inevitable to expand exploration targets towards deeper layers. In the past five years, the Eocene layers and buried hills ("Double Ancient" domains) have become the main exploration targets in the Huizhou Sag. Through exploration in the Eocene layers and buried hills, some large- and medium-sized oil and gas fields, such as Huizhou 26-6, have been discovered, but oil and gas are mainly endowed in the Eocene Wenchang Formation and the buried hills [1⇓⇓⇓⇓⇓⇓-8]. Following the discovery of Huizhou 26-6 field, however, exploration on the Wenchang Formation and buried hills in the Huizhou Sag has been less favorable, resulting in a situation where "oil is found in wells, but without commercial production". Consequently, exploration on the new layer (i.e., the Eocene Enping Formation) has received increasing attention. Previous studies have suggested that the Enping Formation in the Huizhou Sag is characterized by a shallow-water, broad-basin environment which is dominated by the development of large, braided river delta. Notably, the Enping Formation has a sand content of 70% to 90% and therefore lacks effective regional cap rocks. Moreover, the main trap type in the Enping Formation is fault block type, with significant risks in lateral sealing capacity [9⇓⇓⇓⇓-14]. To address these challenges, previous studies have extensively investigated key reservoir-forming conditions such as source rock characteristics, the genesis of high-quality reservoirs, and the effectiveness of traps in the Huizhou Sag [1⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-14]. However, these studies have mainly focused on regional distribution patterns, and cannot meet the sophisticated requirements in actual exploration. To fulfill the research needs of exploration on the Enping Formation, a comprehensive study was conducted in the steep slope zones of the southern Huizhou 26 subsag. Based on the recently processed, ocean bottom nodes (OBN) high-precision, three-dimensional seismic data, an area of approximately 1 500 km2 with seven wells was targeted, source rocks within the Wenchang Formation were assessed, the source-to-sink system in the Enping Formation was analyzed, and the differential hydrocarbon enrichment pattern was systematically investigated. The research findings lay a solid foundation for the exploration and discovery of large- and medium-sized glutenite-rich oil and gas fields in the Enping Formation in the steep slope zones of the southern Huizhou 26 subsag, marking a new oil and gas exploration chapter in the Enping Formation in the Zhu-1 depression of the PRMB.
Based on abundant drilling, core samples, geochemical analyses, and the recent fully covered three-dimensional seismic data of the Enping Formation in the Huizhou 26 subsag, the fundamental conditions for forming glutenite- rich oil and gas reservoirs are analyzed, and the reservoir- forming characteristics and hydrocarbon enrichment mechanisms of oil and gas fields in the steep slope zones of the southern Huizhou 26 subsag are identified, in order to advance practical exploration efforts for glutenite-rich oil and gas reservoirs in the Huizhou Sag of Enping Formation in the near-shore basins of the eastern South China Sea.
1. Regional geological setting
The PRMB in the eastern South China Sea, positioned south of the Guangdong mainland, is situated on the extensive continental shelf and slope between the islands of Hainan and Taiwan. The NE-SW-oriented PRMB, covering an area of approximately 26×104 km2, is one of the largest oil and gas producing regions in the South China Sea. The Zhu-1 depression, located in the northern part of the basin in the near-shore shallow water area, has undergone the Paleogene rifting and the Neogene depression stages. During the rifting stage, the sapropelic source rocks developed in the semi-deep to deep lake subfacies of the Paleogene Wenchang Formation are the main hydrocarbon source rocks in the basin [9⇓⇓⇓⇓-14]. During the depression stage, sand bodies from the paleo-Pearl River delta cover most of the basin from north to south and constitute high-quality reservoir intervals [9⇓⇓⇓⇓-14] (Fig. 1 ).
Fig. 1 Structural units in Zhu-1 depression and Huizhou sag (a) and comprehensive stratigraphic column (b). |
The Huizhou Sag, as one of the proven richest hydrocarbon-bearing sags, is located in the central part of the Zhu-1 depression, with the Lufeng Sag and the Xijiang Sag to the east and west, respectively. Based on the distribution of basement faults and the thickness of Paleogene sediments, the Huizhou Sag can be further divided into 11 subsags, including the Huizhou 26 subsag, Xijiang 30 subsag, and Xijiang 24 subsag (Fig. 1 ). There are mainly two groups of faults (nearly NEE and NWW) [4⇓⇓⇓⇓⇓⇓⇓-12,14 -15] in the Paleogene, which control the structural and sedimentary filling characteristics of the sag. In particular, the sag margin is characterized by the development of multiple structural transfer zones including Huizhou 27, Huizhou 26, Huizhou 25, and Xijiang 30. Notably, these transfer zones dominate the deposition and distribution of sand bodies. Therefore, under conditions of abundant hydrocarbon supply from source rocks, these transfer zones or steep fault slope zones might develop “lateral variation” or "down-generation and up-storage" reservoirs with proximal glutenites laterally juxtaposed with semi-deep lake mudstones (source rocks). This provides a solid geological foundation for oil and gas exploration in the Huizhou Sag.
The study area is located in the southwestern part of the Huizhou Sag, specifically in the steep slope zones of the southern Huizhou 26 subsag, encompassing the southern and northern blocks of Huizhou 26-6. The northern block of Huizhou 26-6 is subdivided into North Block 1, North Block 2, and North Block 3, from east to west (Fig. 2 ). The Huizhou 26-6 field is situated in the southern block, while the Huizhou 26-6 North field is located in the northern block. Both of the two fields have oil and gas shows. Fig. 2 shows the oil- and gas-bearing area on the top of the Enping Formation (T70).
Fig. 2 Faults overlay with oil- and gas-bearing area on the top of the Enping Formation (T70) in the southern margin of Huizhou 26 subsag. |
In 2010, the Eocene and buried hills gradually became key exploration targets in the Huizhou Sag. However, considering the difficulty of large-scale accumulation in fault traps in the setting of glutenite-rich Enping Formation, exploration practices prior to 2020 had been targeting on the deeply buried Paleogene Wenchang Formation and buried hills. In this process, 7 wells were drilled, leading to the discovery of Huizhou 26-6 field. Early exploration practices indicate that large and steep fan deltas dominate the Wenchang Formation where fault activity was strong. These fan delta sand bodies, often buried more than 4 000 m below the subsurface, generally exhibit poor reservoir properties, significantly constraining further exploration of the Wenchang Formation.
In 2020, a detailed analysis focusing on the petroleum geological conditions was conducted for Well B5-2 in the Huizhou Sag. This analysis challenges conventional knowledge and recognizes that the Enping Formation has three favorable conditions for developing large- and medium- sized oil and gas fields: proximity to the oil source, the development of large-scale high-quality reservoirs, and local pressurization conducive to fault-related sealing. Based on these understandings, drilling was deployed for glutenite- rich sand bodies in the northern block of Huizhou 26-6 in the steep slope zones of the southern Huizhou 26 subsag, leading to a significant breakthrough in the oil and gas exploration of the Enping Formation in the Huizhou Sag.
2. Oil and gas characteristics of the Enping Formation in the steep slope zones of the southern Huizhou 26 subsag
The Huizhou 26-6 and Huizhou 26-6 North fields are located on the downthrown side of the sag-controlling fault in the Huizhou 26 subsag. The traps in the two fields are mainly fault-block traps which inheritably develop from the Wenchang Formation to the Enping Formation. In the early stage, seven wells (A6-1 to A6-7) were drilled in the Huizhou 26-6 field, wells A6-9, A6-10 and A6-11 were drilled in the North Block 2, and well A6-8 was drilled in the North Block 3 (Fig. 2 ). The lithology of the Enping Formation, the main oil- and gas-bearing interval, is gravelly, medium-coarse sandstone, with the oil and/or condensate gas layer of 55-70 m thick.
The oil reservoirs and condensate gas reservoirs discovered in the Enping Formation in the steep slope zones of the southern Huizhou 26 subsag are structural and structural-lithological reservoirs. They are alternately distributed, with the condensate gas reservoirs predominantly in the structural highs. The reservoirs are under normal temperature and pressure. The oil layer has an overall moderate burial depth which ranges from 2 850 m to 4 000 m. The net-to-gross ratio of the oil layer varies greatly, from about 0.1 at the bottom to about 0.8 at the top of the Enping Formation. The crude oil samples, being conventional black oil, overall exhibit low density, low viscosity, high wax, and low sulfur, specifically with a dissolution gas-oil ratio of 71-354 m3/m3, a volume factor of 1.505-2.437, a surface oil density of 0.817-0.862 g/cm3 at 20 °C, a reservoir oil viscosity of 0.127-0.581 mPa·s, a wax content of 15.3%-16.6%, and a sulfur content of about 0.04%-0.05%. The condensate gas samples
are characterized by high condensate oil content, low density, low viscosity, medium wax, and low sulfur, with a gas-oil ratio of 1 425.4 m3/m3, a surface condensate oil density of 0.785 g/cm3 at 20°C, a surface condensate oil content of 517.8 g/m3, a reservoir crude oil viscosity of 0.081 mPa·s, a wax content of 5.7%, and a sulfur content of about 0.01%. In natural gas, the alkane gas content is approximately 99.6%, the CO2 content is 0.27%, and no H2S is detected, belonging to a gas with low CO2 content and no H2S.
Comprehensive oil and gas resource estimation shows that the proven oil and gas reserves of Enping Formation in the Huizhou 26-6 field exceed 5 000×104 m3, achieving a major exploration breakthrough in the new layer.
3. Reservoir-forming conditions of the Enping Formation in the steep slope zones of the southern Huizhou 26 subsag
3.1. Source rocks as the foundation of large-scale, deep-seated oil and gas accumulation
High-quality source rocks are the material foundation for the deep oil and gas exploration. The Huizhou 26 subsag among others is the largest in terms of geological resources, proven reserves, and exploration maturity within the Huizhou Sag, which is the most prolific hydrocarbon-generating sag in the PRMB [4⇓⇓-7,16 -17]. The Huizhou 26 subsag covers an area of approximately 588 km2, with the Wenchang Formation reaching a maximum thickness of 2 676 m, an average thickness of around 1 034 m, and a maximum burial depth of approximately 7 400 m. In this subsag, the widely distributed black-gray mudstones of semi-deep to deep lake subfacies of the Wenchang Formation serve as the main source rocks. Geochemical analysis on adjacent sampled wells reveals such source rocks having the total organic carbon (TOC) values ranging from 2.3% to 8.1%, with an average of 4.1% (Fig. 3a ), a hydrogen index (HI) ranging from 312 mg/g to 602 mg/g, with an average of 432 mg/g, and an organic matter type classified as II1-I (Fig. 3b ). Considering the characteristics of biomarkers, these source rocks are identified as typical oil-prone, high-quality source rocks. Moreover, basin modeling results indicate that the main source rocks of the Wenchang Formation entered the mature stage at around 16 Ma before the present (during early deposition of the Hanjiang Formation), with the vitrinite reflectance (Ro) values of 0.6% to 1.0%. By 10 Ma during late deposition of the Hanjiang Formation, they reached the mid-to- late mature stage, with a Ro value of 0.8% to 1.3%. Currently, the source rocks have entered a high-maturity stage, predominantly producing gas, with a Ro value ranging from 1.2% to 1.8%. Recent resource calculations indicate that the Huizhou 26 subsag has a hydrocarbon expulsion intensity of approximately 21.3×106 t/km2. Notably, oil expulsion primarily occurred at approximately 10-23 Ma, with an estimated volume of about 42.44×108 t, and the primary gas expulsion period was during 0 to 10 Ma, with an estimated volume of around 23.37×108 t. In general, the source rocks exhibit "large oil generation in the early stage and significant gas expulsion in the late stage", typical characteristics of oil-prone source rocks, and provided an abundant material foundation for hydrocarbon accumulation in the deep Paleogene formations in the peripheral areas of the Huizhou 26 subsag.
Fig. 3 Hydrocarbon generation potential of source rocks of the Wenchang Formation in Huizhou 26 subsag (N=34). |
3.2. Near-source large-scale glutenite-rich reservoirs as a favorable condition for oil and gas accumulation
In the relay ramp where source area is relatively large and slope gradient is low, continental deltas characterized by coalesced fans and lobes have developed under stable sediment supply at the basin margins [18]. For glutenite-rich reservoirs within the area, the mud content is low (2.5% on average), and the pores are relatively well-developed (average porosity of 12.0%). In contrast, glutenite-rich reservoirs in the steep slope zones have a higher mud content (10.4% on average), and less-developed pores. A comparison of the transfer zones and the steep slope zones in the Huizhou 26 subsag enables a more precise prediction of the tectonic-geomorphic control on sedimentation in the rifting stage and the distribution and reservoir property of glutenites [19]. Coupled with structural analysis on fault traps, this approach aids in correlating high-quality source rocks with large-scale glutenite reservoirs, creating favorable conditions for the near-source oil and gas accumulation [20-21].
3.2.1. Paleogeography during Enping Formation deposition and mechanisms of transfer zone controlling sand
During deposition of the Enping Formation in the Huizhou 26 subsag, there were two major provenances, the Dongsha uplift and the Huixi lower uplift (Fig. 4a, 4b ), where the Mesozoic granite and intermediate to acidic volcanic rocks were dominant [9]. Under the combined effect of the steep slope zones and the transfer zones, a glutenite-rich composite sedimentary system characterized by "fan delta in the early stage, and braided delta in the late stage" was developed (Fig. 4c-4e ).
Fig. 4 Paleogeomorphology during the deposition of the Enping Formation and the evolution sequence of gravel- and sand-rich sediments in the Huizhou 26 subsag. |
(1) During deposition of the early Enping Formation (the third member of the Enping Formation), fault activity was strong and sediment deposition was dominated by the fault-controlled steep slope zones. At the time, the source area was relatively small and exported coarse- grained sediments, mostly accumulating glutenites and gravelly coarse sandstones. In this fault-controlled steep slope setting (the slope is greater than 20°), a set of fan deltas characterized by "small delta-plains, and large delta-fronts" was developed (Fig. 4c ).
(2) During deposition of the mid-late Enping Formation (the early stage of the first and second members of the Enping Formation), fault activity gradually weakened, and sediment deposition was controlled by both the steep slope zones and the transfer zones. At the time, the source area experienced headward erosion and therefore had increased transport distances and exported finer sediments, primarily accumulating gravelly coarse sandstones and medium-coarse sandstones. The fan deltas in the fault-controlled steep slope zones diminished; by contrast, fan deltas in the transfer zones enlarged, with an overall increase in the area of the fan-delta plain and a trend to extend and advance towards the deep sag (Fig. 4d ).
(3) During deposition of the late Enping Formation (the late stage of the first and second members of the Enping Formation), fault activity was stagnated, and the topography became gentler (a slope of nearly 2°). The extensive erosion gradually erased the "shielding" effect of the proximal uplift and the Dongsha Uplift in the hinterland began supplying sediments to the sag. As the source area enlarged and transport distances increased again, braided river deltas characterized by "large delta-plains, and small delta-fronts" were developed in a ramp setting (Fig. 4e ).
During deposition of the Enping Formation, the steep slope zones and the transfer zones combined to control the continued deposition of fan deltas, giving rise to thick layers of stacked channels, which serve as the main oil and gas reservoirs in the area. Based on planar fault combination patterns in the sag margin, two sand controlling mechanisms are proposed for the glutenite-rich sedimentary bodies in the Huizhou 26 subsag: concave faults (A) and a single fault with planar and straight geometry (B) (Fig. 5 ).
Fig. 5 The steep slope zones and transfer zones combined sand control model (a) and 3D geomorphology and sedimentation coupling model (b) for the Enping Formation in the Huizhou 26 subsag. |
(1) Concave fauts
In areas where two faults connect, rivers transporting sand and water converge to develop large-scale fan delta systems, such as the Huizhou 26-A structure (Fig. 5b ). In the sag margin, the grain size is coarse, and glutenites are dominant, with chaotic seismic reflection patterns. This is often illustrated by low-amplitude, serrated GR logs, indicating poor sand-mud differentiation. Towards the interior of the basin, the grain size gradually becomes finer, and gravelly coarse sandstones and medium-fine sandstones are dominant. Seismic events show good stratification, characterized by mid- to low-frequency, medium-amplitude, moderate-continuity progradational patterns. GR logs often exhibit serrated-box and box shapes (Fig. 5a ).
(2) A single fault with planar and straight geometry
Sediments migrate along the steep slope plane and accumulate in the immediate lake to develop area-restricted fan deltas, such as the Huizhou 27-A structure (Fig. 5b ). In the sag margin, the grain size is coarse, and glutenites are dominant, with chaotic seismic reflections. GR logs often exhibit low-amplitude, serrated shapes. Towards the interior of the basin, the grain size gradually becomes finer, but the extension distance is relatively limited (Fig. 5a ).
3.2.2. Characteristics and main controlling factors of glutenite-rich reservoirs
Through a comprehensive analysis of sidewall cores, drilling cores, and thin sections of samples from five key wells (A6-1, A6-2, A6-3, A6-5, and A6-7) that drilled into the Enping Formation in the Huizhou 26 subsag, it is found that the rock samples are lithologically composed of variegated glutenite, light gray gravelly coarse sandstone, and medium-coarse sandstone. In particular, the sandstones are predominantly lithic feldspar sandstone and feldspar lithic sandstone (62.5%), followed by feldspar quartz sandstone, lithic quartz sandstone, and lithic sandstone. The detrital components of the reservoirs have 65% to 85% quartz (average 71%), 15% to 20% feldspar (average 18%), and 10% to 30% lithic fragments (average 16%) (Fig. 6a ). Moreover, the lithic fragments mainly comprise granite and intermediate-acidic volcanic rock fragments (Fig. 6b ). The interstitial materials are mainly muddy matrix and clay minerals, followed by tuffaceous components, carbonate minerals, and small amounts of authigenic quartz, zeolite minerals, and gypsum, among others. In general, the studied reservoirs have moderate textural maturity, with mostly sub-angular to sub-rounded particles.
Fig. 6 Ternary plots of sandstone detrital components and rock fragments, and the porosity-permeability characteristics of various facies belts in the Enping Formation in Huizhou 26 subsag. |
The porosity and permeability results reveal that the reservoirs in the Enping Formation in the Huizhou 26 subsag are characterized by medium-low porosity and low permeability (Fig. 6c ). The porosity of the reservoir ranges from 1.5% to 18.7%, with an average of 9.2%. About 67.5% of rock samples have porosity greater than 10%. Furthermore, the permeability ranges from 0.006× 103μm² to 732.700×10-3 μm², with an average of 12.9×10-3 μm². Approximately 56.1% of rock samples have a permeability greater than 1×10-3 μm².
The drilled wells reveal that the reservoirs of the Enping Formation in the Huizhou 26 subsag exhibit strong heterogeneity of physical properties. Therefore, it is imperative to clarify the genesis and distribution patterns of the glutenite-rich reservoirs of the Enping Formation (Figs. 7 -8 ). The results suggest the development of glutenite-rich reservoirs of the Enping Formation is controlled hierarchically by three factors, i.e. favorable facies zone, grain component, and microfracture.
Fig. 7 Thin sections and scanning electron microscope (SEM) images of the Enping Formation rock samples from typical wells in the Huizhou 26 subsag. (a) Well A6-5, 3 120.47 m, subaqueous distributary channels, showing coarse grains in dominance, followed by medium grains, with a porosity and permeability of 11% and 94.6×10-3 μm2; (b) The SEM of (a), showing K-feldspar intensely dissolved along cleavages, and filamentous illite as well as worm-like kaolinite distributed on the surface of the grains; (c) Well A6-5, 3 125.15 m, subaqueous levees, showing mud filling in an interstitial or streak-like manner between the grains with a porosity and permeability of 8.8% and 1.18×10-3 μm2; (d) The SEM of (c), showing the pore development and poor connectivity overall, with numerous pores and throats filled with mud, while a small number of pores and throats well developed. (e) Well A6-1, 3 235.00 m, showing dissolution within the feldspar grains, with secondary pores; (f) Well B5-2, 3 770.00 m, showing tuffaceous matters filling between the particles, and dissolution observed; (g) Well A6-1, 3 181.50 m, showing pressure-induced fractures in grains; (h) Well A6-1, 3 216.00 m, showing pressure-induced fractures in grains. |
Fig. 8 A scatter plot of cement content and negative cement porosity (the porosity if all cement is removed) in the area adjacent to the fault zones of the Enping Formation in Huizhou 26 subsag. |
(1) Favorable tectono-sedimentary environments contribute to the development of sand bodies [22⇓-24]. Fan-delta plains deposited near the fault-controlled steep slope zones are often characterized by mixed sand and mud deposits, resulting in generally poor reservoir properties. However, the fan-delta-front sand bodies exhibit higher compositional maturity and textual maturity, reduced mud content, and significantly improved reservoir properties, for long-distance transportation. In the study area, distances greater than 10 km would facilitate grain size sorting. Therefore, the distance of transportation is one of the factors influencing reservoir properties. Combining thin section analysis and reservoir property analysis, the subaqueous distributary channels are more favorable than subaqueous natural levees of the glutenite-rich fan (fan delta and braided river delta) in the fault-controlled steep slope zones. This is because the former have uniform grain size, low mud content (less than 5%), and are conducive to the preservation of primary reservoir porosity and later large-scale dissolution (Fig. 7a-7d ).
(2) High content of rigid particles and coarse grain sizes have significant advantages in retaining pores during compaction [23]. To a certain extent, the original texture components of rock (rigid particles and grain size) determine the resistance of reservoirs to compaction. Generally, high content of rigid particles and coarse grain sizes result in strong resistance to compaction, which is favorable for preserving primary porosity [25-26]. Thin section observation indicates that the reservoirs have the porosity reduction controlled by compaction, with compaction rates ranging from 49.2% to 82.1%, averaging 63.6%, and by cementation locally, with cementation rates ranging from 5.8% to 27.2%, averaging 14.1% (Fig. 8 ). Therefore, the resistance to compaction is a prerequisite for the development of high-quality reservoirs. Moreover, acidic dissolution pores of different particle components act as the major contributor to the storage space in glutenite reservoirs. In particular, pores due to feldspar dissolution are the most developed, reaching 8% to 10%. The second are dissolution pores in coarse tuffaceous components (grain size greater than 2 mm), accounting for 2% to 5%. Fine tuffaceous components can lead to reservoir densification. Therefore, in areas with rigid, coarse-grained, and high-feldspar sand bodies and well-developed coarse tuffaceous intervals, large-scale acidic dissolution is more likely to occur, thereby improving reservoir properties to become effective reservoirs on a large scale (Fig. 7e, 7f ).
(3) Microfractures improve the reservoir permeability. Thin sections of core samples reveal that the existence of microfractures enables the three-dimensional pore connectivity, thereby increasing the reservoir permeability and promoting large-scale acidic dissolution, ultimately improving the physical and oil-bearing properties of deep sandstone reservoirs (Fig. 7g, 7h ). Notably, these microfractures often develop in subaqueous distributary channel sand bodies. These sand bodies, characterized by coarse grain size, low mud content, and high rigid particle content (Fig. 8 ), have strong resistance to compaction, which is conducive to the development of microfractures in the reservoir.
In summary, the development of favorable glutenite- rich reservoirs in the Enping Formation in the Huizhou 26 subsag is subjected to a hierarchical control by three factors (favorable facies zone, particle component, and microfracture), and glutenites of subaqueous distributary channel with uniform grain size, low mud content (less than 5%), high feldspar component content, and near the fault system are conducive to the development of “sweet spot” reservoirs.
3.3. Oil and gas preservation
3.3.1. Lake flooding mudstone as a basin-wide, favorable cap rock
The lake flooding mudstone of the Enping Formation is a favorable cap rock for its basin-wide distribution and stable development. In the steep slope zones of the southern Huizhou 26 subsag, oil and gas in the Paleogene Enping Formation are sealed primarily by the mudstone cap rock. Previous studies suggest that medium-thin high-quality cap rock (~2 m thick, low porosity and permeability) can effectively seal oil and gas in the underlying reservoirs. In particular, this sealing capacity becomes evident in depths ranging from 1 500 m to 4 000 m within the study area [6⇓⇓-9,27 -28]. In the steep slope zones of the southern Huizhou 26 subsag, numerous stacked thin layers of mudstone are observed both above and within reservoirs. The mudstone typically has a thickness of 2-5 m, or even up to 20 m locally. The cap rock in the study area is consistently buried at 2 900 m to 4 000 m, an optimal depth range for sealing. Additionally, the cap rock exhibits a high differential drainage pressure which is sufficient to seal against a relatively high column of oil and gas. Consequently, the distribution range of the cap rock is the key factor that limits the hydrocarbon accumulation scale in the study area.
During deposition of the Enping Formation, the basin floor was relatively gentle, and the water was shallow. Thus, even minor lake level fluctuation could impact a large area, developing thin but relatively stable mudstone that acts as an effective cap rock [6-7,27]. Therefore, the unique depositional environment in the southern Huizhou 26 subsag has resulted in a good reservoir-cap rock assemblage in this area.
3.3.2. Transtensional faults as relatively favorable lateral seals
Previous studies have indicated that stress type controls the lateral sealing capacity of faults, thereby influencing the scale of hydrocarbon accumulation. In general, the lateral sealing capacity of compressional, transtensional and extensional faults gradually decreases [29]. The tectonic activity in the PRMB is jointly controlled by extensional and horizontal shear stress, leading to a comprehensive structural response under a composite stress background. Under the combined effect of extension and strike-slip stresses, the Neogene fault activity is primarily characterized by nearly EW- and NWW-trending normal faults, as well as the sustained activation of early NE- trending faults. Overall, the faults are predominantly extensional, with a secondary strike-slip component [14-15,30 -31].
In the current tectonic stress field of the Huizhou Sag, the maximum horizontal principal stress is oriented NWW (Fig. 2 ). Notably, the current maximum horizontal principal stress direction is perpendicular to the NE-trending faults, and parallel to the NWW-trending faults. The exploration results of over 20 fault-block traps in the Huizhou region indicate that drillings near the nearly NW-trending, trap-controlling faults show abundant oil and gas, but with a smaller quantity of oil layers, and the proven reserves are relatively small (typically less than 50×104 m3 for a single structure). On the other hand, NE- and nearly EW-trending trap-controlling faults reveal more oil and gas shows, multiple oil layers, and high hydrocarbon columns (as high as 400 m in the Huizhou B-5 structure, for example), with the maximum proven reserves for a single structure being approximately 2 000× 104 m3. The exploration results of fault block traps in the Huizhou Sag agree well with the recent discoveries of large- and medium-sized oil and gas fields in the Neogene of the Yangjiang Sag in the PRMB [32]. By combining exploration practices and theoretical analysis, it is believed that the nearly NE- and EW-trending, trap-controlling faults are characterized by transtensional stress and have relatively good lateral sealing capacity, while the nearly NW-trending, trap-controlling faults are under extensional stress, resulting in a less effective lateral sealing effect. For example, the trap-controlling faults in the Huizhou 26-6 North field mainly trend NE. The fault orientation indicates a good sealing capacity of faults, which plays a crucial role in controlling hydrocarbon accumulation in glutenite-rich reservoirs.
3.4. Oil and Gas charging process and distribution of glutenite-rich reservoirs
3.4.1. Geneses and differential accumulation process of oil and gas in the Enping Formation
More than 10 fluid samples, including crude oil and condensate gas, were collected from the Enping Formation in the Huizhou 26-6 field for phase state analysis (PVT).
For the crude oil samples, in terms of biomarker characteristics, the pristane/phytane ratio (Pr/Ph ratio) is relatively low (2.2-3.1); the ratio of rich C30 4-methyl steranes to C29 regular steranes is 1.1-1.9; the content of bicadinanes is generally low, and the bicadinanes/C30 hopane ratio is 0.1-0.5 (Fig. 9 ). The carbon isotopic compositions of the whole oil range from -27.2‰ to -25.9‰. In addition, the methylphenanthrene index, an aromatic parameter of crude oil, indicates that the maturity is 1.0%-1.1%. Notably, the geochemical characteristics of the crude oil are similar to that of the revealed mudstones, indicating that the crude oils were sourced from the semi-deep to deep lake source rocks in the oil window.
Fig. 9 Characteristics of biomarker in crude oil from oil reservoirs in the Enping Formation and source rocks in the Wenchang Formation. EP22—the second sand group of the second member of the Enping Formation; EP23—the third sand group of the second member of the Enping Formation; WC41—the first sand group of the fourth member of the Wenchang Formation. |
In addition, crude oil and natural gas samples obtained through gas-liquid separation were analyzed to determine the source of the condensate gas. It is found that the condensate oil shares similar geochemical characteristics with conventional black oil: rich C30 4-methyl steranes, the C30 4-methyl steranes/C29 regular steranes ratio of 1.1, generally low content of bicadinanes, the bicadinanes/C30 hopane ratio of 0.1, and the carbon isotopic compositions ranging from -26.7‰ to -26.4‰. In such gas, the alkane gas content is approximately 99.6%- 99.8%, the CO2 content is 0.1%-0.3%, and there is no H2S, belonging to a natural gas with low CO2 content and no H2S. The dryness coefficient (C1/C1-5) of natural gas is 0.84 and the carbon isotopic compositions of methane, ethane and propane range from -40.75‰ to -40.70‰, from -28.09‰ to -27.88‰, and from -25.63‰ to -25.12‰, respectively. The maturity of natural gas, calculated based on previous research findings, is approximately 1.25%, which is consistent with the maturity determined from light hydrocarbon parameters. Considering the biomarker characteristics of the crude oil obtained from retrograde condensation, the condensate gas is inferred to source from the high-maturity stage of the semi-deep to deep lake source rocks (Fig. 10 ) [33-34].
Finally, the molecular geochemical and reservoir fluid inclusion analysis results indicate that the oil and gas of the Enping Formation in the steep slope zones of the southern Huizhou 26 subsag were sourced from the semi-deep to deep lake source rocks of the Wenchang Formation. In general, the hydrocarbon charging process appeared in three phases: (1) oil charging (13.8-19.1 Ma); (2) oil charging (10.0-13.8 Ma); and (3) condensate charging (0-10 Ma) [4⇓⇓-7]. The first phase crude oil originated from the low-maturity stage of the deep lake source rocks (maturity of 0.5%-0.7%). The crude oil appears widely in the Paleogene, albeit with a limited amount; rather, it primarily accumulates in the shallow Zhujiang Formation reservoirs. The second phase crude oil originated from the peak oil generation period of the semi-deep lake source rocks (maturity of 0.7%-1.3%). This phase represents the main reservoir-forming period in the Paleogene, with the crude oil predominantly accumulating in the deep Paleogene reservoirs near the source rocks. The third phase involves highly mature condensate gas (maturity greater than 1.3%), which exerted selective secondary alteration to the second-phase oil reservoir.
Under reservoir conditions, the density of condensate gas, black oil and formation water is around 0.3 g/cm3, 0.6 g/cm3 and 1.0 g/cm3, respectively. In the water-wet dominated reservoirs, the density contrast between condensate gas and black oil is about 40% of the density contrast between condensate gas and formation water. Therefore, the resistance of natural gas displacement against black oil reservoirs is significantly greater than that against formation water. By analyzing the porosity- frequency distribution curves for effective and non-effective natural gas reservoirs [35], the lower petrophysical cutoff is determined as the intersection point of the two curves. Coupling with actual pressure measurements and sampling results in the study area, the minimum porosity is estimated to be around 8% for primary gas reservoirs where natural gas displaced formation water and 10% for gas reservoirs alternated by gas invasion. The proximity between the steep slope zones of the southern Huizhou 26 subsag and the hydrocarbon generation center enables large-scale oil charging in the early stage, leading to widely distributed oil-filled layers and natural gas reservoirs that distribute mainly in the zone where high-quality reservoirs are well developed.
3.4.2. Oil and gas distribution patterns in glutenites of the Enping Formation
Due to differences in reservoir properties and hydrocarbon charging processes, glutenite-rich reservoirs of the Enping Formation can be vertically divided into three hydrocarbon enrichment units: low-fill zone, transition zone, and high-fill zone. In the low-fill zone, the oil reservoirs are situated at depths ranging from 2 800 m to 3 200 m, featuring a relatively favorable porosity within the range of 14% to 20%. In addition, the reservoirs are mainly composed of oil-bearing water layers, with only some oil layers observed at certain structural highs. The trap fill level is mostly less than 0.5, indicating less favorable sealing conditions which results in significant adjustments of oil and gas to higher positions. In the transition zone, the oil reservoirs are situated at depths ranging from 3 200 m to 3 400 m, featuring a moderate porosity within the range of 10%-14%. The reservoirs are rich in oil and gas layers and characterized mainly by structural traps which have a trap fill level of 0.5-0.7. In the high-fill zone, the oil reservoirs are situated at depths ranging from 3 400 m to 4 000 m, featuring a relatively unfavorable porosity within the range of 8% to 12%. The reservoirs have alternating oil layers and dry layers and characterized mainly by structural-lithological traps which have a trap fill level of 0.7-0.9 (Figs. 11 and 12 ). In short, exploration practices and hydrocarbon charging process indicate that the oil is vertically zoned in terms of distribution and differentially enriched in terms of accumulation in the southern Huizhou 26 subsag.
Fig. 11 Three-dimensional reservoir-forming model for the Enping Formation in the steep slope zones of the southern Huizhou 26 subsag. |
Fig. 12 Reservoir property characteristics and oil-water distribution of sand groups in the Huizhou 26-6 field (the section location is shown in |
Through the comprehensive analysis, it is believed that the proximal channels in the glutenite lobe (Figs. 4 -5 ) have the most favorable coupling relationship between reservoir-caprock assemblage and the trap-controlling faults (Fig. 11 ). Under the oil and gas distribution pattern, such channels are the favorable areas for large-scale hydrocarbon accumulation.
4. Exploration significance and implications
The discovery of near-source glutenite-type reservoirs in the Enping Formation in the steep slope zones of the southern Huizhou 26 subsag marks a significant breakthrough in exploration within a new stratigraphic layer, providing a new battlefield to support the stable production of 2 000×104 t in the eastern South China Sea. The discovery confirms that the Paleogene exploration is feasible in petroleum prolific subsags. The transfer zone is prone to developing large “fan-braided river” delta sedimentary systems in the Enping Formation [18,36], which serves as the “golden zone” for the formation of large- and medium-sized oil and gas fields. The study findings provide important insights for the exploration of deep Paleogene hydrocarbons in the Huizhou Sag.
4.1. Prolific subsags provide abundant hydrocarbons for developing large- and medium-sized glutenite-type fields
During deposition of the Enping Formation in the Huizhou Sag, the rifting gradually weakened, with faults developed in the early stage. The structural characteristics provide favorable conditions for “hydrocarbon generation in lower intervals, storage in upper intervals, and near- source accumulation”. From the perspective of hydrocarbon accumulation, exploration in adjacent areas of prolific subsags for the Enping Formation is expected to have a significantly higher success rate. The resource assessment results have indicated that Huizhou 26 subsag, Xijiang 30 subsag, and Xijiang 24 subsag are three confirmed hydrocarbon-prolific subsags, around which multiple large- to medium-sized oil fields have been discovered (Fig. 13 ). The Xijiang 24 subsag, covering an area of approximately 300 km2, features a semi-deep to deep lake source rock area of about 224 km2; the Wenchang Formation has an average thickness exceeding 1 000 m, and the hydrocarbon expulsion amount in the subsag is approximately 86×108 t. The Xijiang 30 subsag, with an area of about 280 km2, has a semi-deep to deep lake source rock area of about 150 km2; the Wenchang Formation has an average thickness of approximately 700 m, and the hydrocarbon expulsion amount in the subsag is approximately 16×108 t. Consequently, the peripheral areas of the three subsags have excellent petroleum accumulation conditions, presenting enormous exploration potential within the Paleogene. Hydrocarbon-prolific subsags represent a meaningful destination for searching large- and medium-sized oil and gas fields in the Paleogene.
Fig. 13 Hydrocarbon expulsion intensity overlay with basement structures and predicted exploration potential in the Huizhou Sag. |
4.2. Large-scale transfer zones are favorable for oil and gas accumulation in large- and medium-sized, glutenite-type fields
The relatively stable large-scale transfer zones facilitate long-distance transportation of terrestrial clastic materials, enabling the development of significant fan deltas or braided-river deltas. As these depositional systems extensively overlap with the underlying high-quality source rocks of the Wenchang Formation, a large structural- lithological trap would develop. The basin-controlling faults can cut deep into the source rocks of the Wenchang Formation and large-scale glutenites would accumulate in the transfer zones where faults are less active. During hydrocarbon generation and expulsion phases, oil and gas would migrate along the fault zones of the subsag-controlling faults to the overlying glutenite-rich reservoirs. Moreover, favorable accumulation zones can develop under the sealing effect of multiple sets of normal faults with strike-slip components. Thus, the unique petroleum accumulation pattern of the Enping Formation is proposed as “strongly active faults control migration, weakly active faults control deposition, and multiple sets of faults combine to control traps” in the study area (Fig. 11 ). In addition to the Huizhou 26 subsag, Xijiang 30 and Huizhou 25 are two large transfer zones that develop around the petroleum-prolific Huizhou Sag. In these zones, large-scale braided river delta and fan delta are developed with progradation distances exceeding 40 km. Additionally, the reservoirs, 3 000-4 000 m below the subsurface, are prone to develop as sweet spots with large-scale accumulation. Along with the transition from rifting to depression stages, the main stress direction in the area rotates clockwise. The extensional stress with strike-slip components facilitates the development of structural-lithological traps, such as the Huizhou 19 fault zone, and Xijiang 24 fault zone. These structural zones have enormous potential and are targets of future exploration.
5. Conclusions
The exploration discovery of glutenite-rich reservoirs in the Enping Formation in the steep slope zones of the southern Huizhou 26 subsag confirms the feasibility of the exploration concept of “focusing on prolific subsags and targeting new layers (the Enping Formation)”. This also validates the enormous exploration potential of the transfer zones around prolific subsags where large-scale glutenite develops. These zones represent high-quality destinations for developing large- and medium-sized oil and gas fields in the near-shore basins of the eastern South China Sea.
The fault-controlled steep slope zones and transfer zones within the sag control the variations in physical properties of proximal glutenite-rich reservoirs. The type and scale of depositional systems are influenced by factors such as the scale and lithology of feeder drainage basin, the type of transfer zones, and geomorphic features. A hierarchical evaluation system incorporating three factors (favorable facies zone, particle component, and microfracture) is proposed. In addition, subaqueous distributary channel type glutenites with uniform grain size, low mud content (less than 5%), high feldspar component, and proximity to the fault, are demonstrated to be conducive to the development of “sweet spot” reservoirs.
During the rifting-depression transition period, the basin rested as a shallow wide basin, with high sand and gravel contents. The fault-block traps are dominant, and those traps controlled by NE- or EW-trending normal faults with a strike-slip component have the most effective sealing capacity. In additional to sealing capacity, the activity of faults and reservoir properties together control the glutenite-rich reservoirs of the Enping Formation which can be vertically divided into three oil-water systems: low-fill zone, transition zone, and high-fill zone. In this system, the channel-dominated areas where the coupling between reservoir properties and trap-controlling faults is optimal and saturation is high, are favorable for large-scale hydrocarbon accumulation.
Nomenclature
GR—natural gamma ray, API;
HI—hydrogen index, mg/g;
S1—content of free hydrocarbon, mg/g;
S2—content of pyrolyzed hydrocarbon, mg/g;
Tmax—maximum pyrolysis peak temperature, °C;
TOC—total organic carbon content, %.