Wenchang Formation Research Articles

Biodegradation and water washing within a series of petroleum reservoirs of the Panyu Oil Field

A suite of crude oils across 10 discrete reservoirs at different depths in the Panyu Oil Field (Pearl River Mouth Basin, South China Sea) were geochemically appraised as having a common lacustrine shale source and of similarly low thermal maturity. The δ13C values and biomarker composition indicate that the Wenchang Formation is the major source of these oils. However, a progressive variation in the distribution of some hydrocarbon compounds (e.g., n-alkanes, isoprenoids, polycyclic aromatic hydrocarbons) across several of the oils is indicative of secondary alteration by biodegradation and possibly also water washing. Biodegradation contributed to the progressive removal of increasingly susceptible hydrocarbons, largely in accordance with previously defined conventions, and was most pronounced in the shallowest oils. A decline in extent of biodegradation with increasing production unit depth may reflect their increase in temperature (72 to 98°C), with the higher temperatures of the deeper oils more hostile to microbial activity. Minor changes were also observed in the hydrocarbon composition of the deeper oils, where biodegradation impacts were negligible. This could be evidence of secondary alteration with water washing a potential influence, but might also reflect a variance in the secondary source contribution of the coaly Enping Formation. There was a notable decline in the relative abundance of several compounds with a potentially high susceptibility to water washing, including dibenzofuran (cf. non-heteroatom aromatics) and β-methylated polycyclic aromatic hydrocarbons, in contrast to α-methylated forms.

Secondary migration of hydrocarbons in the Zhujiang Formation in the Huixi half-graben, Pearl River Mouth Basin, South China Sea

The process and mechanisms of secondary hydrocarbon migration in the Huixi half-graben, Pearl River Mouth Basin, were investigated on the basis of geological analysis of the strata and study of the porosity and permeability of the reservoir rocks, fluid potential, oil properties, and geochemistry of oil–source correlation. The results suggest that the hydrocarbons of the Zhujiang Formation in the Huixi half-graben were derived from source rocks of the Eocene Wenchang Formation and the Eocene–Oligocene Enping Formation in the Huizhou Sag. The hydrocarbons migrated laterally from northeast to southwest. The sandstone in the upper member of the Zhujiang Formation exhibited superior physical properties (porosity and permeability) and connectivity than the lower member. Thin sandstone beds with good physical properties and stable distribution in the upper member of the Zhujiang Formation were the main carrier beds for lateral hydrocarbon migration.

Applying seismic geomorphology to delineate switched sequence stratigraphic architecture in lacustrine rift basins: An example from the Pearl River Mouth Basin, northern South China Sea

Switched sequence stratigraphic architectural units were developed in the Eocene Wenchang Formation stratigraphic section of the Pearl River Mouth Basin (PRMB), northern South China Sea. Utilizing a high-quality 3D seismic data set, well logs and restored paleogeomorphology, the architecture and genesis of switched sequence stratigraphic units have been systematically investigated. The Wenchang Formation, a second-order sequence, can be subdivided into seven third-order sequences (from base to top: SQ1, SQ2, SQ3, SQ4, SQ5, SQ6, and SQ7). The sequence stratigraphic architecture of the Wenchang Formation is characterized by continuous lateral stacking patterns from sequences SQ1 to SQ7. Sequences SQ1–SQ4 mainly developed in the HZ26 sag, whereas sequences SQ5–SQ7 mainly developed in the XJ24 sag. The depositional centres of the Wenchang Formation appear to have migrated from the HZ26 sag to the XJ24 sag-along the northwest direction from sequences SQ1 to SQ7. Multiple tectonic activation episodes or alternating tectonic subsidence of the HZ26 and XJ24 sags resulted in the distinctive geomorphological features that effected the development of the switched sequence stratigraphic architecture in the study area. The switched sequence stratigraphic architecture presented in this study may provide new insights into a better understanding of sequence stratigraphic stacking patterns in continental lacustrine rift basins.

Hydrocarbon Generation in the Lacustrine Mudstones of the Wenchang Formation in the Baiyun Sag of the Pearl River Mouth Basin, Northern South China Sea

The lacustrine mudstones of the Eocene Wenchang Formation are the primary source rocks of natural gas in the Baiyun Sag of the Pearl River Mouth Basin, China. The hydrocarbon generation kinetic parameters and the characteristics of lacustrine mudstones sampled from the Eocene Wenchang Formation at well LF13-2-1 were investigated on the basis of a thermocompression simulation experiment in a closed system. In addition, combined with the burial history and paleothermal history, the gas generation process of the Eocene Wenchang Formation source rocks in the Baiyun Sag was investigated through the hydrocarbon generation equations established from parameters of hydrocarbon generation kinetics. The gas generation process of the Eocene Wenchang Formation lacustrine mudstones has three thermal evolutionary stages: the middle mature stage (39–30 Ma), the late mature stage (30–23.5 Ma), and the main gas generation stage (23.5–0.0 Ma). The main hydrocarbon generation stage in the central area of the Baiyun Sag occurred from 24 Ma to 22 Ma, while the main hydrocarbon generation stage in the eastern and northern areas occurred at ∼15 Ma.

Provenance identification and sedimentary analysis of the beach and bar systems in the Palaeogene of the Enping Sag, Pearl River Mouth Basin, South China Sea

During deposition in the Palaeogene, the Pearl River Mouth Basin (Enping Sag) was filled by sediments derived from northern and southern source regions. The northern sources are the fold belt Caledonian and Hercynian metamorphic rocks that are widely exposed along the South China block and stacked with Mesozoic igneous rocks, and the southern sources are Mesozoic igneous rocks from the southern Panyu lower uplift. We analysed the characteristics of the provenance and sediment-dispersal patterns of the beach and bar systems in the Palaeogene third-order sequence framework on the basis of detritus, thin section, zircon and the geophysical response test data. The source and deposition areas were distinguished based on their palaeogeomorphology. The sedimentary material composition of the deposition area was determined based on the characteristics of the detrital assemblage and cathode luminescence, and the characteristics of the age distribution were determined in detail using detrital zircon U–Pb dating. This study suggests that the provenance system in the Palaeogene mainly consisted of Palaeozoic metamorphic rocks and Mesozoic igneous rocks, with a supplement of Proterozoic metamorphic rocks. The transport capacity of three large fault troughs that delivered the northern provenances gradually increased from the Wenchang Formation (E2w) to the Enping Formation (E3e), whereas the southern provenances were transported by four or five incised valleys in E2w and then submerged in E3e. In the 3D seismic area, the features of the beach and bar sedimentary systems were described and interpreted based an analysis of the well-based sedimentary facies, seismic reflection characteristics and multiple attribute clustering. The results show that the beach and bar systems by the short-axis braided deltas mainly developed in the upper member of E2w and that the beach and bar systems on the subaqueous high in E3e developed during the Palaeogene. In addition, the bedrock-beach bar sedimentary system in the lower member (E3eL) was transferred to the braided delta–beach bar sedimentary system in the upper member (E3eU).

The origin and distribution of crude oil in Zhu III sub-basin, Pearl River Mouth Basin, China

The origin of thirteen major oil fields in Zhu III sub-basin, Pearl River Mouth Basin, was studied by using the results of Rock-Eval pyrolysis on 340 samples and biomarker analysis on 18 source rock samples and 33 oil samples. The two possible source rock intervals have different biomarker assemblages and were deposited in different environments. The Eocene Wenchang Formation (E2w, 49.5–35 Ma) is characterized by high hopane/sterane ratio (>37.84), high 4-methyl sterane index (0.28–1.06), low gammacerane index (<0.09), low extended tricyclic terpane ratio [ETR=(C28 + C29)/(C28+C29 + Ts), 0.13–0.20] and bicadinane/C30 hopane (<0.02), which was deposited in fresh-brackish water and dominated by algae. On the contrary, Oligocene Eenping Formation (E3e, 35–30 Ma) has high gammacerane index (>0.10), high bicadinane/C30 hopane (up to 73.62) but low hopane/sterane ratio (<12.50), and it was deposited in relative saline water environment with significant high terrestrial plant debris input. Because of complex geologic settings, oils have different distribution. By using oil-source correlation analysis and hierarchical cluster analysis, two source-related oil groups were identified. Oils found in the Wenchang A depression and Yangjiang uplift, which are classified to oil group 1, were derived from E3e. Oils found in the Wenchang B depression and Shenhu uplift, which are classified to oil group 2, were derived from both E2w and E3e source rocks. But the Qionghai uplift received supply from both the Wenchang A and B depression. This distribution pattern is also proven by using a simple three-dimensional model called PathWay™. An explanation was put forward in this paper about the oils in Zhu III sub-basin and four potential petroleum accumulations were roughly predicted where no well drilled.

Source rock characteristics and hydrocarbon expulsion potential of the Middle Eocene Wenchang formation in the Huizhou depression, Pearl River Mouth basin, south China sea

The Middle Eocene Wenchang (WC) formation in the Huizhou depression of the Pearl River Mouth basin is an important geological element for petroleum exploration in the South China Sea. In this study, the geological and geochemical characteristics of the WC source rocks, including the distribution, sedimentary environment, organic matter type, hydrocarbon generation potential and thermal maturity were investigated. Hydrocarbon generation history and hydrocarbon expulsion intensity and quantities were evaluated through one dimensional basin modelling and an improved hydrocarbon generation potential methodology. The WC source rocks are widely distributed and have an elevated thickness (mostly thicker than 200 m, with a maximum thickness greater than 1000 m); they were deposited in a lacustrine weak oxidation–weak reduction sedimentary environment with low salinity. The source rocks have low to high organic matter abundance (TOC mainly ranges from 0.27 to 3.59 wt% with an average of 1.39 wt%) dominated by II1–type and II2–type kerogen, resulting in significant hydrocarbon generating potential under moderate–high thermal evolution (Ro mainly ranges from 0.65% to 1.2% on the margins of sags and greater than 1.2% in the centre of sags). The source rocks became mature (Ro = 0.5%) in the early Miocene and began to generate hydrocarbons. They reached the hydrocarbon expulsion threshold at 0.8% Ro and the hydrocarbon expulsion rate became greatest at 1.1% Ro. The expulsion intensities in the H26 sag, H21 sag, X24 sag and X23 sag are relatively large, with their greatest expulsion intensities being 104 × 106, 71 × 106, 62 × 106 and 53 × 106 t/km2, respectively. The comprehensive hydrocarbon expulsion efficiency was approximately 75%. About 690.1 × 108 t of hydrocarbons were expulsed from the WC source rocks in the Huizhou depression.

Origin and occurrence of crude oils in the Zhu1 sub-basin, Pearl River Mouth Basin, China

The origin of the seventeen major oil fields in the Zhu1 sub-basin, Pearl River Mouth Basin (PRMB) was studied based on the results of Rock–Eval pyrolysis on more than 370 samples and biomarker analysis on 31 source rock samples and 63 oil samples. The two possible source rock intervals have different biomarker assemblages and were deposited in different environments. The Wenchang Formation (E2w, 56.5–32Ma) is characterized mainly by low C19/C23 tricyclic terpane (<2.0), low C24 tetracyclic terpane/αβC30 hopane (<0.06), low bicadinane-T/αβC30 hopane (<2.0) and high 4-methyl steranes/∑C29 steranes (most >0.4) ratios, and were deposited in anoxic to suboxic environments with important contribution from Pediastrum and Dinoflagellates. The Enping Formation (E3e, 32–30Ma) has high C19/C23 tricyclic terpane, high C24 tetracyclic terpane/αβC30 hopane, widely variable yet overally high bicadinane-T/αβC30 hopane and low 4-methyl steranes/∑C29 steranes ratios, and were deposited in freshwater lacustrine to swamp conditions with significant terrigenous organic matter input. According to oil-source correlation, three oil classes can be identified in the Zhu1 sub-basin. Class 1 oil is E2w-derived and occurs widely. Class 2 oil is E3e-derived and refers to oils from F field in the north of the Huizhou depression. Class 3 oil is a mixture of oils generated from E2w and E3e, only distributed in the Huizhou depression and on its southern margin. The petroleum distribution pattern is mainly controlled by the distribution of source rocks and the migration pathways of oils. This research has important implications for future exploration.

Novel tetracyclic terpanes in crude oils and source rock extracts in pearl river mouth basin and their geological significance

Novel tetracyclic terpanes X and Y namely C24-des-A-oleanane and C27 tetracyclic terpane were detected in crude oils and source rock extracts in Zhu 1 depression in Pearl River Mouth Basin by GC-MS analysis technology. These compounds are similar to oleanane in the structure, and their relative abundance in m/z 191 partial mass chromatogram has a good correlation with oleanane. Here compounds X and Y are considered to be derived from the des-A degradation of oleanoid precursor. The ratio of X/(X+C24) and Y/(Y+C24) increase with the increasing inputs of terrigenous organic matter in crude oils. Wenchang Formation middle-deep lacustrine source rocks with planktonic algae organic matter inputs and oil generating from it contain low abundance of compounds X and Y, while Enping Formation coal measures source rocks with terrigenous higher plants organic matter inputs and oil generating from it usually have higher compounds X and Y. In general when two terrigenous compounds C19 tricyclic terpane and bicadinane-T are high in crude oils, there are also a corresponding high abundant compounds X and Y. Relative abundance of compounds X and Y is closely related to the source of organic matter and it can indicate the input of terrigenous organic matter.

Resource Potential Assessment of Series of Strata with Few Drilled Wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea

Huizhou sag is located in the depression zone in the north of the Pearl River Mouth basin. In recent years, great progress has been made in the exploration focused on the deep Paleogene, opening up new areas of exploration and necessitating the evaluation of both the hydrocarbon generation potential and the exploration potential of the Paleogene. The Enping Formation and the Wenchang Formation have become new exploration series of strata. The research utilizes paleotectonic recovery, sequence stratigraphy analysis, basic parameter prediction of hydrocarbon source rock and basin modeling, as well as other techniques. It is carried out to examine the tectonic inversion effect of the Huidong low uplift, the sedimentary facies of the Wenchang Formation, hydrocarbon source rock thickness and organic carbon content in the series of strata that have not been extensively drilled, with the calculation of both the hydrocarbon generation intensity and amount from the source rock. It is considered that: (1) after sedimentation, the Wenchang Formation suffers from tectonic inversion and severe uplift and denudation, especially the Huidong low uplift, changing the sedimentary pattern; (2) during the sedimentation of the Wenchang Formation, LF13 low-lying to Huizhou 14 low-lying is at the center of the lake basin, dominated by deep lake – semi-deep lake facies, with hydrocarbon source rock developing; (3) in total, 659.46×108 t of hydrocarbon is generated in Huizhou sag, with total oil and gas resources with the Wenchang Formation as source rock at about 34.3232×108 t; (4) comparing with the parameters and results of the previous resources assessment and considering the exploration practices in the area, it is believed that the deep part of Huizhou sag still has great exploration potential as a sag with large amount of remaining oil and gas resources in Zhu I depression.

Cenozoic volcanic rocks in the Pearl River Mouth and Southeast Hainan Basins of South China Sea and their implications for petroleum geology

Drilling, gravity, magnetic observation and seismic facies are combined to identify the Cenozoic volcanic rocks in the Pearl River Mouth Basin and the Southeast Hainan Basin in the northern South China Sea, and to analyze how their lithology, distribution characteristics and volcanic activity influence on hydrocarbon accumulation. As a result, more than 70 volcanic bodies are ascertained. They are divided into two types: (1) laminated lava and pyroclastic rocks in the Cenozoic sedimentary sequence; and (2) conoid, mounded or wedged basalt with rigid basement under the Cenozoic sedimentary sequence. In the early Cenozoic (E1-E2), the acidic, neutral and basic volcanic rocks developed, while in the mid-late Cenozoic (E3-Q), single basic rocks existed intensively. The volcanic lithofacies mainly include the lower and middle effusive subfacies and the pyroclastic flow deposit explosive subfacies. Spatially, from the Southeast Hainan Basin to the Pearl River Mouth Basin, the Cenozoic volcanic activity gradually enhances; from shallow water area to the Central Basin of the South China Sea, the scale of Cenozoic volcanic rocks increases gradually, and the age tends to be younger. These activities improve the formation temperature, reduce the hydrocarbon generation threshold, and promote the hydrocarbon accumulation. Volcanic rocks in Wenchang Formation (E2w) to Zhujiang Formation (N1z) provide favorable conditions for hydrocarbon accumulation, so they are important targets in the deep exploration.

Characteristics of the turbidite fan in the Wenchang Formation of the Enping Sag, Pearl River Mouth Basin, China and its hydrocarbon significance

A turbidite fan in the Eocene upper Wenchang Formation in the Enping Sag, Pearl River Mouth Basin (PRMB) has been studied using seismic, logging and borehole data. The fan is characterized by parallel progradation on the dip seismic profile and is mound-shaped or lenticular-shaped on the strike seismic profile. The study of the core and logging data from well EP17-3-1, which is located in the front side of the turbidite fan, shows that this fan is a set of normal grading sand beds, interbedded within thick dark grey mudstones of semi-deep to deep lake deposits in the Wenchang Formation. The fan is interpreted as a sand/mud-rich turbidite fan that has an area of over 140km2 and a maximum thickness of over 340 m. Combined with a study of the regional geological background and previous provenance analysis of the Eocene Wenchang Formation, the main potential provenances for the turbidite fan are considered to be the Panyu low-uplift and northern fault terrace zone. The Enping Sag is considered to be a half graben-like basin whose north side is faulted and whose south side is overlapped. Basement subsidence in the Eocene was mainly controlled by boundary faults which dip relatively steeply on the north side, causing the subsidence center of the Enping Sag in this stage to be close to the north boundary faults. Sustained faults developed in the Enping Sag during the Eocene caused an increase of the relative height difference between the north and the south uplift zone in the Enping Sag. Affected by the second episode of the Zhuqiong movement (39–36 Ma) in late Eocene, sediments which had accumulated on the Panyu low-uplift zone were triggered and moved toward the subsidence center of the Enping Sag and formed the turbidite fan. The second episode of the Zhuqiong movement is the most important triggering factor for the formation of the turbidite fan in the Wenchang Formation. Seismic attribute characterization shows that the low frequency energy is enhanced and high frequency energy is weakened when seismic waves propagate through the oil-bearing zone in this fan. Amplitude versus offset (AVO) anomalies are observed in the seismic data and abnormally high pressure is encountered. The turbidite fan in the Wenchang Formation has provided important information for sedimentary evolution in deep layers of the Enping Sag and pointed to a new direction for the hydrocarbon exploration in the study area.

Tracing early-charged oils and exploration directions for the Wenchang A sag, western Pearl River Mouth Basin, offshore South China Sea

The Wenchang A sag is the largest petroleum producing sub-depression in the western Pearl River Mouth Basin, offshore South China Sea. The Eocene Wenchang Formation and the early Oligocene Enping Formation are the two major source rocks in the sag. The oil discoveries in this region mainly originated from the Enping Fm. How to objectively estimate the petroleum potential of the Wenchang Fm. and trace the Wenchang oils has long been the focus of research and exploration in this region. In this paper, inclusion oils obtained from reservoir sandstones, as well as typical reservoir oils and potential source rocks, were investigated by systematic geochemical methods. Oil–oil and oil–source rock correlations indicate that inclusion oils in the central and marginal areas of the Wenchang A sag had similar geochemical characteristics and mainly originated from the Wenchang source rocks. The reservoir oils in the central area are derived mainly from the Enping source rocks, while in the marginal areas they have mixed characteristics of both the Enping and Wenchang formations. Combining the tectonic-sedimentary evolution and petroleum generation histories, it is concluded that the two source rocks in the Wenchang A sag have generated oils, but in the central areas the main stage of oil generation of the Wenchang Fm. was much earlier than the formation time of the traps, resulting in the dissipation of most of the early generated Wenchang sourced oils. In the marginal areas, the main stage of oil generation of the Wenchang Fm. was later and the Wenchang oils not only could be captured as fluid inclusions in the reservoir quartz grains, but also migrated and accumulated into oil pools in which they mixed with the Enping sourced oils. The early lithological and structural traps in the central and marginal areas of the Wenchang A sag, especially in the northern part of the Shenhu uplift, are the most prospective areas for further exploration of the sag.

Source controls on geochemical characteristics of crude oils from the Qionghai Uplift in the western Pearl River Mouth Basin, offshore South China Sea

The Qionghai Uplift is a sub-uplift tectonic unit in the western Pearl River Mouth Basin. Though lacking source rocks itself, it adjoins the Wenchang A sag, the Wenchang B sag and the Qionghai sag which contain significant source rocks. For the special geological background, the Qionghai Uplift has become an important accumulation area for oil and gas, and the oils in the uplift show significant variation in important geochemical parameters. In this study, the geochemistry of oils in the Qionghai Uplift oil pools has been analyzed systematically, revealing geographic differences. Three groups of oils were identified in the northern part of the Qionghai Uplift: Group I oils, represented by the WC8-3E oil pool located in the northeast; group II oils in the middle represented by the WC13-1 oil pool, and group III oils in the southwest represented by the QH18-1 oil pool. According to oil-source correlations, the oils of the three groups have mixed geochemical characteristics between typical oils sourced from the Enping Formation in the Wenchang A sag, and those sourced from the Wenchang Formation in the Wenchang B sag. We show that based on knowledge of hydrocarbon generation and evolution of source rocks around the Qionghai Uplift, as well as the formative features of traps within the uplift, the geochemical characteristics of the oils are mainly controlled by the distribution of mature hydrocarbon kitchens surrounding the Uplift. Oils in the northeast of the northern Qionghai Uplift were mainly sourced from the Enping Formation in the Wenchang A sag with a secondary contribution from the Wenchang Formation in the Qionghai sag. Oils in the middle section are of mixed origin from the Enping Formation in the Wenchang A sag and the Wenchang Formation in the Wenchang B sag. Oils in the southwest part of the northern Qionghai Uplift were mainly generated from the Wenchang Formation in the Wenchang B sag. This research provides guidance for future exploration of the Qionghai Uplift.

Petroleum System and Hydrocarbon Accumulation Characteristics in the Wenchang and Enping Formations in the Huizhou Sag, Pearl River Mouth Basin, China

Conventional oil and gas reservoirs are in the Oligocene and later formations in the Huizhou Sag, Pearl River Mouth Basin, South China, and oil &amp; gas were generated by the source rocks in the Eocene Wenchang and Eocene-Oligocene Enping Formations. However, exploration paradigm shift has been made in the Sag with the depletion of conventional reservoirs and recent oil and gas discoveries in the Wenchang and Enping Formations, and Wenchang and Enping become the new targets of exploration. With more detailed researches to evaluate the exploration potential in the two conventional source rock formations, basic factors of the petroleum system are analyzed based on seismic data, well drilling, and lab measurements. We used sequence stratigraphy and statistics to research thickness trend of source rock. Different from discontinuous distribution of lake basin during the Wenchang formation deposition suggested by predecessors, as the major source rock in the Wenchang Formation, SQ2 whose dominant facies are deep lakes and shallow lakes is present widely with thickness varies from 200 meters to 1000 meters, and the thickest source rocks locate in the Huixi, Luxi and Huidong half-grabens. Both the Wenchang and Enping formations contain multiple source rock-reservoirseal combinations. We conduct a detailed study on the petroleum system in the deeper reservoir-cap combinations in the Huizhou Sag. The study results in the hydrocarbon generation and migration timing. Reservoirs are classified based on the multi-stage structural movements and multi-stage hydrocarbon generations. We address the controlling effect of the structure evolution on hydrocarbon accumulation. Finally, we build a relationship between reservoir quality and structural units in the study area, and helps reduce the risks of the exploration and production in the Huizhou Sag.

CONDENSATES IN THE PY30‐I STRUCTURE, PANYU UPLIFT, PEARL RIVER MOUTH BASIN, SOUTH CHINA SEA: EVIDENCE FOR HYDROTHERMAL ACTIVITY ASSOCIATED WITH PETROLEUM MIGRATION AND ACCUMULATION

Condensates are present in the PY30–1 structure in the Panyu Uplift, Pearl River Mouth Basin. Biomarkers and compound‐specific stable carbon isotope ratios of three condensate and two source rock samples indicate that the condensates were generated by lacustrine mudstones and coals in the Oligocene Enping Formation with a minor contribution from mudstones in the Eocene Wenchang Formation. Elevated vitrinite reflectance values, high smectite‐illite transformation ratios, and elevated fluid inclusion homogenization temperatures (about 100®C higher than normal borehole temperatures) point to the influence of hydrothermal fluids at the PY30–1 structure. Hydrocarbon migration was found to have occurred at the same time as hydrothermal activity.Modelling of formation pressure evolution in the Wenchang and Enping Formation source rocks in the Baiyun Depression, adjacent to the south of the Panyu Uplift, suggest that there were three episodes of overpressure release at approximately 40–37 Ma, 33–31 Ma and 16–10 Ma. Overpressure release was probably induced by uplift and erosion during the Zhuqiong, Nanhai and Dongsha phases of tectonic deformation, respectively. The third episode of overpressure release coincided with the main phase of hydrocarbon migration.The accumulation of condensates at the PY30–1 structure probably followed hydrocarbon expulsion from source rocks as a result of overpressure release in the adjacent Baiyun Depression. Vertical migration into overlying reservoir rocks occurred through faults associated with a fluid diapir which is present at the core of the PY30‐I structure. The faults are pathways along which petroleum can migrate up to shallow reservoirs.

CHARACTERISTICS AND EVOLUTIONARY HISTORY OF OVERPRESSURE IN PANYU 4 SAG, PEARL RIVER MOUTH BASIN

Overpressure,which is closely related to the generation,migration and preservation of hydrocarbon,plays an important role in petroleum accumulation dynamics.This paper confirms the existence of overpressure in the Panyu 4sag,Pearl River Mouth Basin based on logging,drilling,geological and geochemical data,discusses its forming mechanism,divides the area of overpressure,and reconstructs pressure evolution process in a two-dimensional framework.It suggests that overpressure,which is mainly controlled by lithology,exists widely in Panyu 4sag.Its forming mechanism includes both hydrocarbon generation and the compaction disequilibrium.The history of pressure evolution is complex.Overpressure in Wenchang Formation includes two stages:the eaily stage is due to compaction disequilibrium,and the later stage is related to hydrocarbon generation since 15Ma.

Generation and accumulation of oil and condensates in the Wenchang A Sag, western Pearl River Mouth Basin, South China Sea

AbstractThe Pearl River Mouth (PRM) Basin is one of four Cenozoic basins in the South China Sea, and the Wenchang A Sag is a secondary depression in the western part of the basin. Both the Wenchang and Enping formations contain good source rocks in the western PRM Basin; however, only the latter has been considered a likely source of the discovered oil and gas. New data from fluid inclusions and the analysis of oil–source rock correlations for the WC10‐3 oil and gas pools indicate two stages of petroleum charging, the earlier originating from the Wenchang Formation and the later from the Enping Formation. Kinetics of petroleum generation and structural evolution modeling were employed to further investigate the mechanism of formation of the WC10‐3 oil and gas pools. It was shown that the crucial condition for the formation of pools is the time of development of the structural trap. The Wenchang Formation source rocks generated oil from 25 to 14 Ma in the possible source area of the WC10‐3 oil and gas pools in the Wenchang A Sag, so that only traps formed earlier than this period could capture oil sourced by the Wenchang Formation. The Enping Formation source rock experienced its oil window from 18 Ma to the present with the main stage of oil generation from 15 to 5 Ma. During this period structural traps in the sag continued to form until movements became weak, so that most pools in the Wenchang A Sag originated from the Enping Formation source rock. The likely dissipation of oil and gas from the earlier stage of charging should be taken into account in assessing the oil potential of the Wenchang A Sag.

Aromatic hydrocarbons as indicators of origin and maturation for light oils from Panyu lower uplift in Pearl River Mouth basin

Aromatic fractions of six light oils and two source rock samples from Panyu (番禺) lower uplift of Pearl River Mouth basin were analyzed using GC-MS (gas chromatography-mass spectrometric) technique. Thirteen aromatic series of hydrocarbons detected such as biphenyls, naphthalenes, phenanthrenes, dibenzothiophenes (more than two hundred aromatic hydrocarbon compounds) were used to analyze the origin and maturity of the light oils. This study indicates that the distribution of aromatic hydrocarbon compounds in the light oils of wells L1 and P1 differs from those of wells P2, P3 and P4. The light oil samples from wells L1 and P1 contain more dibenzofuran and fluorence hydrocarbons and less naphthalene hydrocarbons. The contents of fluorene, dibenzothiophene and dibenzofuran in the two groups of light oils also show their difference in sedimentary environment. Moreover, the diversity of the relative distributions of biphenyl and naphthalene is apparent between the two groups of light oils. As mentioned above, the origin of the light oils from wells L1 and P1 varies from that of wells P2, P3 and P4. According to the result of oil-source correlation, the light oils from wells P2, P3 and P4 mainly originated from the source rocks in the Enping (恩平) Formation. Accordingly, the light oils from wells L1 and P1 may have been derived from the lacustrine shales in the Wenchang (文昌) Formation or from the mixed source of the Wenchang Formation and the Enping Formation. Applying maturity parameters of methylnaphthalenes, methylphenanthrenes and methyldibenzothiophenes from the aromatic hydrocarbons to the study of the maturity of light oils from Panyu lower uplift indicates that the maturity has reached the high mature stage.

Tracing of deeply-buried source rock: A case study of the WC9-2 petroleum pool in the Pearl River Mouth Basin, South China Sea

The identification of a deeply-buried petroleum-source rock, owing to the difficulty in sample collection, has become a difficult task for establishing its relationship with discovered petroleum pools and evaluating its exploration potential in a petroleum-bearing basin. This paper proposes an approach to trace a deeply-buried source rock. The essential points include: determination of the petroleum-charging time of a reservoir, reconstruction of the petroleum generation history of its possible source rocks, establishment of the spatial connection between the source rocks and the reservoir over its geological history, identification of its effective source rock and the petroleum system from source to trap, and evaluation of petroleum potential from the deeply-buried source rock. A case study of the W9-2 petroleum pool in the Wenchang A sag of the Pearl River Mouth Basin, South China Sea was conducted using this approach. The W9-2 reservoir produces condensate oil and gas, sourced from deeply-buried source rocks. The reservoir consists of a few sets of sandstone in the Zhuhai Formation, and the possible source rocks include an early Oligocene Enping Formation mudstone and a late Eocene Wenchang Formation mudstone, with a current burial depth from 5000 to 9000 m. The fluid inclusion data from the reservoir rock indicate the oil and the gas charged the reservoir about 18–3.5 Ma and after 4.5 Ma, respectively. The kinetic modeling results show that the main stages of oil generation of the Wenchang mudstone and the Enping mudstone occurred during 28–20 Ma and 20–12 Ma, respectively, and that the δ 13C 1 value of the gas generated from the Enping mudstone has a better match with that of the reservoir gas than the gas from the Wenchang mudstone. Results from a 2D basin modeling further indicate that the petroleum from the Enping mudstone migrated upward along the well-developed syn-sedimentary faults in the central area of the sag into the reservoir, but that the petroleum from the Wenchang mudstone migrated laterally first toward the marginal faults of the sag and then migrated upward along the faults into shallow strata. The present results suggest that the trap structure in the central area of the sag is a favorable place for the accumulation of the Enping mudstone-derived petroleum, and that the Wenchang mudstone-derived petroleum would have a contribution to the structures along the deep faults as well as in the uplifted area around the sag.