The upper Eocene to lower Oligocene Enping Formation provides the primary source rocks for hydrocarbon accumulation in the Baiyun Sag, Pearl River Mouth Basin, South China Sea. Nevertheless, the depositional environment of this formation has been a subject of debate. This study confirms that the lower to middle members of the Enping Formation predominantly consist of lacustrine deposits, and a marine transgression occurred during the deposition of the Enping Formation's upper member in the Early Oligocene period. This transgression is supported by an increase in paleosalinity proxies (B/Ga, Sr/Ba), greater negative S isotopic values of pyrite, and an increase in triaromatic dinosteroid concentrations in the upper member samples. A series of indicators (chemical alteration index, V/Cr and Ni/Co ratios) suggest that mudstones from the lower-middle member and the upper member were all deposited under warm-humid climate with the dysoxic-oxic water conditions. The mudstones from the lower-middle member and upper member were formed with low-tomoderate primary productivity under relatively strong dilution condition. The paleoredox and paleoproductivity were minimally influenced by marine transgression. Additionally, there is a weak correlation between the total organic carbon content and indicators of these depositional conditions. The input of terrigenous organic matter is a pivotal factor in regulating the accumulation of organic material within the Enping Formation. The multi-stage deltas likely played a significant role in supplying terrigenous organic matter, thereby influencing the distribution of organic-rich mudstone in the Baiyun Sag.

The Potwar Basin (PTB) is a prominent geological feature located in northern part of Pakistan, and is considered as one of the active and productive regions for petroleum and gas exploration in Pakistan. In this study, eight crude oil specimens originating from three distinct fields within the PTB were comprehensively examined to decipher the environmental conditions, source of organic matter (OM) and oil-oil correlations using gas chromatography coupled with flame ionization detection (GC-FID) and gas chromatography-mass spectrometry (GC -MS). The molecular marker and hierarchical cluster analysis of PTB oils reveal two different crude oil families. Family-I showed relatively low values of Pr/Ph, C 19 TT/C 23 TT and C 25 TT/C 24 TeT, C 27 - C 29 regular steranes, dibenzothiophene (DBT), fluorene (FL) and dibenzofuran (DBF), as well as C 26 /C 28 TAS (20S) and C 27 / C 28 TAS (20R). These results suggest that Family-I crude oils derived from marine environment of suboxic water bodies with higher contribution from green algae or planktonic microbes. However, Family II contains comparatively high values of the aforementioned molecular parameters, indicating that crude oils mainly originated from a lacustrine environment with a higher contribution of plant organisms under oxidizing conditions. Saturated hydrocarbon maturity parameters such as CPI and OEP, and C 31 22S/(C 31 22S + C 31 22R) and C 32 22S/(C 32 22S + C 32 22R) indicate PTB crude oils are thermally mature, while aromatic indices reveal that Family-II crude oils are high mature. The present research defines that PTB crude oils contributed from mixed organic matter sources of marine and lacustrine majorly from marine sedimentary environment with the bloom of algae and/or higher plants.

Three abundant rearranged oleananes detected in Tertiary oils from the Baiyun Sag, South China Sea, were designated as 5(4-+ 3) abeo-3 alpha(H), 58(H), 18 alpha(H)-oleanane (I), 3 alpha, 58-dimethyl-23 alpha, 25-dinor-108(H), 18 alpha(H)-oleanane (II) and 1(10-+ 5)abeo-38-methyl-248-nor-18 alpha(H)-oleanane (III), using gas chromatography-mass spectrometry (GC-MS) and GC-MS-MS. The relative abundances of different rearranged oleananes correlated with each other, and also showed strong positive correlations with those of oleanane or des-A-oleanane. This is consistent with rearranged oleananes, oleanane (OL) and des-A-oleanane originating from functionalized ole-anane triterpenoids, but by different formation processes. The distribution of rearranged oleananes in oils from the study area is controlled mainly by angiosperm inputs and the diagenetic conditions of their source rocks, rather than thermal maturity. Oligocene marine transgression in the study area may have played an important role in the expression of saturated oleanoids. Moreover, oxic and acidic peat swamp conditions in the Panyu Low Uplift favoured the formation of rearranged oleananes compared to oleananes. In addition, oils from the Baiyun Sag can be classified into two oil families, i.e., A and B, using the relative abundance of rearranged oleananes and other source-related indicators. The Family A oils in the eastern and northeastern portions of the sag are char-acterized by relatively low pristane/phytane (Pr/Ph) ratios, low rearranged oleananes/oleanane ((I + II + III)/ OL) and bicadinane-T/C30 hopane (T/C30H) ratios. The Family B oils in the Panyu Low Uplift and northeastern portion of the sag have higher Pr/Ph, (I + II + III)/OL and T/C30H ratios. The related parameters of rearranged oleananes were therefore shown to be effective indicators for oil-oil or oil-source rock correlation in the study area.

The Baiyun Sag in the Pearl River Mouth Basin is a hydrocarbon-rich depression in which oil and natural gas fields coexist. The complex characteristics of oil and condensate in the sag have been long-term issues, and one of the questions is the formation stage of black oil, volatile oil and gas condensate in the sag. In this study, to accurately estimate the thermal maturity of oil in the Baiyun Sag, maturity parameters derived from normal alkanes, terpanes, steranes and aromatic hydrocarbons were systematically investigated. In addition, adamantane parameters were used to verify both the oil and condensate maturity. The results show that the saturated hydrocarbon biomarker parameters are not suitable for providing a complete understanding of the oil maturity in the Baiyun Sag. A qualitative evaluation method for various maturity stages of oil was proposed based on the methylphenanthrene distribution fractions, phenyldibenzothiophene maturity parameters, and diamantane concentration. Subsequently, the maturity was quantitatively evaluated using empirical equations based on methyl diamantine. The equations were selected by comparing the calculated equivalent reflectance (Rc) with the qualitatively determined maturity stage of the crude oil. Finally, the calculated vitrinite reflectance values indicated an overestimation of the maturity of light oils and condensate oils, which were thought to be produced in the late oil window (Ro: 1.0%∼1.3%) to wet gas stage (Ro: >1.3%). The new results indicate that the black oils with Rc values ranging between 0.72% and 1.05% was generated during the peak oil window. The Rc values of gas condensates in the eastern and northeastern structural belts range from 1.20% to 1.24%, suggesting that they are products of the late oil window. The maturity levels of gas condensates in the Panyu Low Uplift are the highest (Rc: 1.14%–1.46%) in the sag. They were produced in the late oil to wet gas stages. The oil maturity characteristics confirm the oil exploration potentials of the eastern and northeastern structural belts of the Baiyun Sag.

The Sohnari Member of the Early Eocene Laki Formation is massively deposited in the Southern Indus Basin of Pakistan and is considered a potential source rock to generate hydrocarbons. However, the detailed paleoclimatic, paleoweathering, and depositional conditions of the Sohnari Member have not been studied earlier. This research mainly discusses the detailed mineralogical (bulk and clay) and elemental geochemistry of the Laki Formation from two outcrop sections (Jhimpir and Lakhra) in the Southern Indus Basin, Pakistan. The bulk minerals, including quartz (low), hematite, calcite, halite, gypsum, and clay minerals such as kaolinite, chlorite, smectite and illite have been discussed here. These results demonstrate the paleo-environment of studied area was arid with enhanced saline and weak to strong oxidizing depositional conditions. The chemical index of alteration (CIA) values in Jhimpir and Lakhra sections are in the ranges of 41.30–97.93 and 22.30–96.19, respectively, indicating that the Sohnari sediments experienced weak to intense chemical weathering in the source area. The interpretation of the A–CN–K ternary diagram is consistent with the clay mineral contents in the studied sediments, which is characterized by the predominance of kaolinite, gibbsite and chlorite, demonstrating the weak to strong weathering state under warm and humid climatic conditions. The chemical indices such as Sr/Ba, δU, V/Cr, Ni/Co, and Cu/Zn, U/Th and Ba/Ga show that Sohnari rocks of Early Eocene Laki Formation underwent strong evaporation, oxic water column with warm to humid and minor contact of cold climatic conditions. Based on our present data, it can be concluded that the sediments of Sohnari Member of Laki Formation from Jhimpir and Lakhra areas of Southern Indus Basin in Pakistan are related to Indio-Eurasian collision and came from the Indian shield rocks that were deposited in a brackish water body with a minor contact of the freshwater oxidizing paleo-environment depositional conditions.

Methylated 2-methyl-2-(4,8,12-trimethyltridecyl)chromans are salinity-sensitive biomarkers that have been detected in immature – early mature petroleum and sediments. In this study, the occurrence and distribution patterns of 2-methyl-2-(4,8,12-trimethyltridecyl)chromans were investigated in a set of lacustrine sediments from Nördlinger Ries of southern Germany and marine sediments from the South China Sea. Among all of the 2-methyl-2-(4,8,12-trimethyltridecyl)chroman isomers detected, 8-Me-2-methyl-2-(4,8,12-trimethyltridecyl)chroman presented with high abundance in sediments deposited in hypersaline environments, while absent in samples from normal marine environments. In contrast, 5,7,8-triMe-2-methyl-2-(4,8,12-trimethyltridecyl)chroman was more enriched in sediments from marine environments. This study also showed that the ratio of 5,7,8-triMe-/5,8-diMe-2-methyl-2-(4,8,12-trimethyltridecyl)chroman can be applied as a potential salinity indicator on account of a positive correlation with other 2-methyl-2-(4,8,12-trimethyltridecyl)chroman salinity indicators. This ratio can be an alternative indicator of paleosalinity when 8-Me-2-methyl-2-(4,8,12-trimethyltridecyl)chroman is absent or present in quite low abundance. The content of 2-methyl-2-(4,8,12-trimethyltridecyl)chroman isomers may be affected by freshwater supply and lithology. Molecular simulations showed that 5,8-diMe-2-methyl-2-(4,8,12-trimethyltridecyl)chroman has a higher thermal dynamic stability than 7,8-diMe-2-methyl-2-(4,8,12-trimethyltridecyl)chroman. Thus, the ratio of 5,8-diMe-2-methyl-2-(4,8,12-trimethyltridecyl)chroman/7,8-diMe-2-methyl-2-(4,8,12-trimethyltridecyl)chroman may be a potential maturity parameter for sediments at a low thermal mature stage.

Crude oils from the Baiyun Sag of Pearl River Mouth Basin have high abundance of diagnostic biomarkers including novel C15 sesquiterpene, special configurated tricyclic sesquiterpenes and tetracyclic sesquiterpenes (X, Y, Z and X1, Y1, Z1), and bicadinanes, which indicate the contributions from terrestrial higher plants to the organic matters. These oils are also rich in oleanane, which is a diagnostic biomarker for angiosperm organic matter. Based on the analysis of 13 crude oils from the Baiyun Sag, the compositions and geochemical significance of typical biomarkers were revealed. Results indicate that C24-des-oleanane (Y1) and C27 tetracyclic (Z1) may share a similar biological origin. Novel C15 sesquiterpene, C24-des-oleanane and C27 tetracyclic, and bicadinanes may be derived from different higher plants according to the molecular structure and abundance correlation analysis of the related compounds. In addition, the biomarker compositions of crude oils from different regions are significantly varied, which will benefit the oil group classification in the Baiyun Sag. The distribution of novel C15 sesquiterpene and bicadinanes was influenced by sedimentary environment, and may be more enriched in oxidized environment. The distribution patterns of C24-des-oleanane, C27 tetracyclic and oleanane are principally affected by biogenic input, but are less influenced by sedimentary environment fluctuation. The relatively high abundance of C24-des-oleanane, C27 tetracyclic and oleanane in the crude oil in the northeastern and eastern Baiyun area reflects that the Dongsha Uplift and the Yunli Low Uplift may be an important source of terrigenous organic matter in this area.

在川中地区大安寨段烃源岩抽提物中检测出丰富的重排藿烷系列、重排补身烷系列、重排甾烷系列及C26+长链三环萜烷系列的分子标志物组合.结合烃源岩的有机质来源、沉积环境、热演化程度特征,探讨上述甾烷、萜烷分布特征指示的地质地球化学意义.结果表明:重排类化合物与C26+长链三环萜烷系列存在着某些相同的富集途径,重排藿烷系列富集途径多于重排补身烷系列、重排甾烷系列及C26+长链三环萜烷系列;特殊的甾烷、萜烷分布特征指示,烃源岩中有机质以湖泊内源(藻类和细菌等)为主,形成于范围局限的弱还原环境,高丰度的重排化合物指示,有机质热演化处于成熟阶段.此外,特殊的甾烷、萜烷分布特征还可能指示了特定的酸性粘土矿物、钙质沉积催化和形成环境中发育的特殊生物族群.综上,研究结果对分子标志物异常组合的地球化学应用具有重要的示范意义.

Crude oil reserves in tight Middle and Lower Jurassic reservoirs are of increasing exploration interest in the central Sichuan Basin, SW China. However, the origin of these “tight oils” is poorly understood. In this study, sixteen samples of light oils/condensates from tight Middle and Lower Jurassic reservoir rocks were analysed using gas chromatography (GC) and isotope ratio mass spectrometry to investigate the oils’ origin and to classify them into genetic families. The tight oils can be divided into two families. Family I oils occur in the Gongshanmiao oilfield where reservoir units comprise the Da'anzhai Member of the Lower Jurassic Ziliujing Formation, the Lower Jurassic Lianggaoshan Formation, and the First Member of the Middle Jurassic Shaximiao Formation. Family I oils are characterized by relatively low values of the methylcyclohexane (MCH) and cyclohexane (CH) indexes, low values of Mango's parameter K2 for light hydrocarbon composition, and relatively negative δ13C values ranging from ‐30.8‰ to ‐28.9‰. Family I oils are inferred to be self‐sourced by lacustrine shales in the Da'anzhai Member and the Lianggaoshan Formation in the study area, both of which are rich in sapropelic organic matter. These source rocks also charged reservoirs in the First Member of the Shaximiao Formation. By contrast, the newly discovered Family II oils, which occur at the Jinhua oilfield and the as‐yet undeveloped Qiulin and Bajiaochang structures, are reservoired in the Second Member of Shaximiao Formation. Family II oils have higher values of the MCH index, CH index and Mango's K2 parameter, and δ13C values varying from ‐27.5‰ to ‐25.4‰. These oils have similar light hydrocarbon compositions and δ13C values to oils derived from source rocks in the Upper Triassic Xujiahe Formation which contain dominantly humic organic matter. Family II oils are therefore inferred to be derived from the coaly mudstones in the Xujiahe Formation.The different compositions of the tight oils in the First and Second Members of the Shaximiao Formation appear to be controlled by the distribution and thickness of source rocks in the study area. Thus, the Gongshanmiao oilfield where Family I oils occur in the First Member is close to the depocentre of source rocks in the Da'anzhai Member and Lianggaoshan Formation. These source rocks are inferred to have charged the First Member reservoirs which may also be present in nearby oil‐ and gas‐bearing structures such as Nanchong and Yingshan. By contrast, Family II oils occur in tight reservoirs in the Second Member in areas with thick successions of Upper Triassic Xujiahe Formation mudstone source rocks, such as the Jinhua oilfield. In areas where both source rocks are present such as the Zhongtaishan and Lianchi oilfields, Shaximiao Formation reservoirs appear to contain both Family I and Family II oils.

Indigenous hydrocarbon biomarkers in the black shales of the 1.40 billion-year-old Xiamaling Formation (XML) from the North China Craton were systematically analyzed to reveal the redox condition and biological community. These black shales are not only rich in organic matter, but also undergone an unexpectedly low thermal evolution process, both of which contribute to well-preserved indigenous hydrocarbons and promote the convincing discovery of the Precambrian biology. The combination of the absence or low concentrations of gammacerane, dibenzothiophene series and aryl isoprenoids, abundant rearranged hopanes, the occurrence of micro-ripples, and low level of redox-sensitive trace elements suggested shallow, low salinity and suboxic to anoxic conditions during the deposition period of XML. The absence of aryl isoprenoids as diagnostic biomarkers for green/purple sulfur bacteria strongly argued against the pervasively euxinic (sulphidic) bottom water in mid-Proterozoic ocean. The scenario of large hump unresolved complex mixture and abundant monomethyl alkanes as well as absent steranes may be derived from benthic microbial reworking processes for biolipids. Here the absence of saturated and aromatic steroids in the XML sediments suggested that eukaryotic algae have little ecological contribution to primitive biomass and/or are the result of heterotrophic reworking from benthic microbial mats. Abundant presence of hopanoid, tricyclic terpane and n-alkane in these samples supported the predominant contribution of (cyano) bacteria.

川中油气区金华地区侏罗系油气勘探开发取得较大进展,侏罗系发育多套油气供烃层位.采用饱和烃气相色谱、色谱-质谱等技术,细致对比并揭示了研究区自流井组大安寨段和须家河组五段(简称须五段)烃源岩的地球化学特征,探讨了 2套主要供烃层位烃源岩的沉积环境和生源构成特征.结果表明,大安寨段烃源岩的分子地化特征与须五段烃源岩差异较大:大安寨段烃源岩中等姥植比(Pr/Ph),中等三环萜烷参数(C19+20TT/C23TT),中等降藿烷/藿烷系列化合物(C30D/C30H)、早洗脱重排藿烷/藿烷系列化合物(C30E/C30H),高降藿烷/升藿烷(C27D/C27R),相对低三芳甾烷参数(C27/C28TAS),生源构成以菌藻生源为主,兼有丰富的高等植物生源的混源输入;须五段烃源岩呈现低-中等Pr/Ph,低C19+20TT/C23TT、C30D/C30H、C30E/C30H,中等C27D/C27R,相对高C27/C28TAS,生源构成以高等植物生源为主.

天然气伴生凝析油及轻质油的轻烃地球化学参数常常被用于天然气成因和油—气对比研究,但蒸发分馏等次生作用对天然气和伴生原油的轻烃组成会产生一定的影响.基于川中地区16件侏罗系天然气与伴生凝析油或轻质油样品气相色谱分析,探讨了轻烃组成及相关地球化学参数在天然气和原油中的差异.研究表明:蒸发分馏作用对油气轻烃组成影响明显,相对于伴生原油,天然气轻烃组成具有高异构烷烃和正构烷烃相对含量,而贫环烷烃和芳香烃的特征.蒸发分馏作用对甲基环己烷指数、环己烷指数、Mango参数K1等成因参数影响不大,伴生原油的这类指标均可以较好地反映天然气的成因类型.蒸发分馏作用对成熟度参数异庚烷值影响较小,而对庚烷值、2,4-DMC5/2,3-DMC5值影响明显.轻烃化合物沸点的不同可能是导致天然气与原油中不同类型化合物相对含量存在差异的主要原因,并且沸点较低的化合物在天然气中相对含量更高.组成轻烃参数的化合物的沸点差异越大,该参数受蒸发分馏作用影响越大,天然气与伴生原油的该参数值相差也越显著.而当比值参数中组成分子与分母的化合物的沸点之差小于2℃时,该参数基本不受蒸发分馏作用的影响.

Tight gas exploration plays an important part in China’s unconventional energy strategy. The tight gas reservoirs in the Jurassic Shaximiao Formation in the Qiulin and Jinhua Gas Fields of central Sichuan Basin are characterized by shallow burial depths and large reserves. The evolution of the fluid phases is a key element in understanding the accumulation of hydrocarbons in tight gas reservoirs. This study investigates the fluid accumulation mechanisms and the indicators of reservoir properties preservation and degradation in a tight gas reservoir. Based on petrographic observations and micro-Raman spectroscopy, pure CH 4 inclusions, pure CO 2 inclusions, hybrid CH 4 –CO 2 gas inclusions, and N 2 -rich gas inclusions were studied in quartz grains. The pressure–volume–temperature–composition properties ( PVT-x ) of the CH 4 and CO 2 bearing inclusions were determined using quantitative Raman analysis and thermodynamic models, while the density of pure CO 2 inclusions was calculated based on the separation of Fermi diad. Two stages of CO 2 fluid accumulation were observed: primary CO 2 inclusions, characterized by higher densities (0.874–1.020 g/cm 3 ) and higher homogenization temperatures (> 210 °C) and secondary CO 2 inclusions, characterized by lower densities (0.514–0.715 g/cm 3 ) and lower homogenization temperatures: ~ 180–200 °C). CO 2 inclusions with abnormally high homogenization temperatures are thought to be the result of deep hydrothermal fluid activity. The pore fluid pressure (44.0–58.5 MPa) calculated from the Raman shift of C–H symmetric stretching ( v 1 ) band of methane inclusions is key to understanding the development of overpressure. PT entrapment conditions and simulation of burial history can be used to constrain the timing of paleo-fluid emplacement. Methane accumulated in the late Cretaceous (~ 75–65 Ma), close to the maximum burial depth during the early stages of the Himalayan tectonic event while maximum overpressure occurred at ~ 70 Ma, just before uplift. Later, hydrocarbon gas migrated through the faults and gradually displaced the early emplaced CO 2 in the reservoirs accompanied by a continuous decrease in overpressure during and after the Himalayan event, which has led to a decrease in the reservoir sealing capabilities. The continuous release of overpressure to present-day conditions indicates that the tectonic movement after the Himalayan period has led to a decline in reservoir conditions and sealing properties.

Gas chromatography-mass spectrometry (GC-MS) analysis has revealed extremely high abundances of rearranged hopanes in Jurassic source rocks and related crude oils in the center of the Sichuan Basin. The detected rearranged hopanes include 17α(H)-diahopanes (C27D and C29–35D), early-eluting rearranged hopanes (C27E and C29–33E), and 18α(H)-neohopanes (C29Ts and Ts). Both the 17α(H)-diahopanes and the early-eluting rearranged hopanes exhibit a distribution pattern similar to that of the 17α(H)-hopane series, with a predominance of the C30 member and the presence of 22S and 22R epimers of hopanes in the extended series (>C30). The results of this study show that the relatively high abundance of rearranged hopanes in Jurassic source rocks in the study area is associated with their depositional environments and with clay-mediated acidic catalysis rather than, as was previously thought, thermal maturity. Shallow lacustrine facies with brackish water and a suboxic to weak reducing sedimentary environment have contributed to the enrichment of rearranged hopanes, and clay-mediated acidic catalysis may also have had a positive influence on their abundance. The distribution patterns of the diahopane series indicate that the oils from Jurassic reservoirs in the Gongshanmiao Oilfield are sourced from Jurassic source rocks. Rearranged hopanes are therefore considered to be effective biomarkers for oil-source correlation in the center of the Sichuan Basin.

The GC-MS analyses of a suite of 24 Jurassic source rock samples from the center Sichuan Basin (SW China) show that they contain variable amounts of rearranged hopanes, including 17α(H)-diahopanes (D series), 18α(H)-neohopanes (Ts and C 29 Ts) and the early-eluting rearranged hopanes (E series). D series have similar distribution patterns to 17α(H)-hopane series, including a range from C 29 to C 35 for carbon numbers and presence of 22S and 22R epimers for C 31 –C 35 homologues. In particular, the 17α(H)-hopanes were not detected in several samples, which have extremely relatively higher abundance of D series. E series extend from C 29 to C 31 , and also have 22S and 22R epimers for C 31 homologues. The relative abundance of E series has a strongly linear relationship with that of D series. The slopes and intercepts for tender line in both C 30 E/C 30 H vs C 30 D/C 30 H and C 29 E/C 29 H vs C 29 D/C 29 H plots are less than 0.7 and 0.2 respectively, indicating that E series have similar forming conditions with D series, but probably following different mechanisms. In contrast, the abundance of C 29 Ts also has a significant linear correlation with that of C 29 D.

Recent years, numerous condensate and oil pools are found in the northern and eastern parts of Baiyun Sag in the Pearl River Mouth Basin (PRMB) South China Sea. However, their origin and filling history have not been determined yet. Firstly, this study investigated the oils' geochemical characteristics and determine the origin of the oils in different regions of the Baiyun Sag. Oil-oil correlation demonstrates that oils in the Panyu Low Uplift (PLU) and the Eastern Structural Belts (ESB) of the sag belong to two different oil groups. The condensates in the PLU were generated by the coal bearing source bed of the Eocene Enping (E(2)e) Formation in the Main Sub-sag (MSS), while ESB oils originated from the shallow lacustrine/deltaic source rocks in the E(2)e Formation. Secondly, based on fluid inclusion analysis and 1-D basin simulation, oil filling history of two wells from PLU and ESB were clarified, respectively. One episode of oil charging occurred between 6 Ma and 0 Ma in the Neogene Zhujiang Formation (N(1)zh) of Well P30 in the PLU. The charging of the condensate in the N(1)zh reservoirs at Well H34 mainly occurred during 6.5 Ma and 4 Ma, which is 5Ma-10 Ma later than that of the light oil in the ESB. However, according to the simulation of the hydrocarbon generation history, both light oils and condensates in the ESB were generated during the mature stage of the source rocks at about 20 Ma to 8 Ma. Thus, the difference between the charging times in the ESB implies that a special accumulation mechanism may occur in the ESB. Moreover, charging of condensates in the PLU and ESB occurred at late Miocene to Pliocene, which has a good correlation with the Dongsha Event. This characteristics provides an evidence for that Dongsha Event may play a critical role in the formation of effective migration conduit in the Baiyun Sag.

在珠江口盆地白云凹陷多个层系都发现了储层沥青.储层沥青丰度自上部的珠海组、珠江组至深部的恩平组、文昌组逐渐增加.储层沥青在物性较好的粗粒砂岩中多为零星状出现,而在细粒的粉砂岩和泥岩中主要呈脉状分布而且含量明显增加.几乎所有的储层沥青均没有显示出明显的各向异性,而且沥青反射率均小于2.0％,表明高温热裂解不是储层沥青形成的主要原因.沥青抽提物的分子地球化学组成特征显示,白云凹陷储层沥青抽提物中缺乏25-降霍烷系列,色谱基线没有明显的“UCM”鼓包、单井地层埋藏史显示储层也未遭受大幅度的抬升剥蚀,因此储层沥青也非生物降解成因.结合地质背景和油气藏特征分析认为,白云凹陷储层沥青的成因为:恩平组-文昌组烃源岩早期生成的原油聚集成藏以后,深部主要遭受了后期天然气的气洗作用.不同层位烃源岩、储层热演化的时间差异是导致白云凹陷油气藏发生调整,并转变为轻质油、挥发油等特殊油藏类型的重要原因.

A total of 25 light oils/condensates (5 condensates from 2 new exploratory wells) were collected from the Baiyun Sag in the Pearl River Mouth Basin (PRMB) in the South China Sea and investigated to establish their origins and sources. Three oil families were identified among samples following analysis of various geochemical parameters using biomarkers, stable carbon isotope compositions, and Hierarchical Cluster Analysis (HCA). Family-I oils, distributed in the northern structural belts, are characterized by significantly high pristane to phytane (Pr/Ph) ratios (4.77–7.93), moderate oleanane to C30 hopane (OL/C30H) ratios (0.18–0.97) and raised bicadinane-T/C30H (T/C30H) ratios (1.67–7.67). Family-II oils are distributed along the eastern flank of the Main Sub Sag towards the northeastern tectonic zones, and, have relatively high Pr/Ph ratios (3.14–4.68), elevated OL/C30H values (0.73–2.29) and moderate T/C30H values (1.64–4.52). The geochemical characteristics of the oils suggest that oils of both Family-I and Family-II are of terrestrial origin, but from two entirely independent sources. Oils of Family-III (a newly discovered oil group) have relatively lower Pr/Ph ratios (0.89–1.99), moderate OL/C30H values (0.13–0.82) and lower T/C30H values (0.34–1.61), indicating a reduced contribution of higher plant organic matter in related source rocks. Family-III oils are consistent in geochemical characteristics with shallow lacustrine source rocks and their related oils discovered in the Zhu I and III depressions of the PRMB, suggesting that they are likely to be derived from source rocks deposited in a shallow lacustrine environment during the Eocene Wenchang-Enping period. The oils of Family-III mainly occur in the southwest flank of the Baiyun Sag, which implies that this is the preferred direction for migration of lacustrine oils. The discovery of lacustrine oil demonstrates that the Baiyun Sag possesses enormous exploration potential in lacustrine oils and that this may become the next favored petroleum exploration target.

珠江口盆地白云凹陷原油中双杜松烷(W和T)含量丰富.基于35个原油样品系统的色谱—质谱(GC-MS)分析,对白云凹陷双杜松烷分布特征及地球化学意义进行了研究.白云凹陷原油双杜松烷相对含量分布明显呈现“北高东低”的特点.双杜松烷相对含量在成熟—高成熟原油中基本不随成熟度变化,也不受气洗作用的影响,而主要受油气来源的控制.双杜松烷相对含量与姥鲛烷/植烷(Pr/Ph)及三环萜烷(C19TT+C20TT)/C23TT均呈现正相关关系,偏氧化及高陆源输入的环境有利于双杜松烷的富集.双杜松烷相对含量是指示生源、沉积环境及油源分析的良好指标.此外,双杜松烷T/W比值是有效的成熟度参数,适用于低—中等成熟度的原油和有机质.

Phosphorite organic-rich sediments from Weng’an Doushantuo Formation preserve earliest animals and multicellular life, though it has been investigated by many researchers but in terms of biomarker analysis still need to be known. Here we have applied various specific aliphatic and aromatic biomarkers (Molecular fossils) parameters with stable carbon isotope data. The biomarker investigation of sixteen phosphoritic rocks from Weng’an Doushantuo Formation, Guizhou, South China, was carried out to identify the paleo-depositional environment, source of its organic matter and thermal maturity by using gas chromatography-mass spectrometry/ mass spectrometry (GC-MS). Particular care was taken while doing geochemical analysis in order to get rid of possible contaminations. Geochemical results indicate the source rock extracts from Weng’an Doushantuo Formation have similarity in molecular composition. All the studied samples have a lower amount of TOC, ranging from 0.07% to 0.58%. Molecular fossils revealed that the source input of organic matter was mainly from eukaryotic algae including dinoflagellates (or their ancestors) and demosponges as confirmed by steranes and terpanes. Pristane and phytane are derivative of Chlorophyll-a suggesting the availability of photosynthesis process in early ocean. Maturity based parameters suggest that the Weng’an phosphorite are highly mature, which is consistent with the thermal history of Doushantuo sediments. Isotopic compositions of δ 13 C saturated and δ 13 C aromatic hydrocarbon fractions show the marine environment. Biomarker investigation revealed that the phosphorite rocks from Weng’an Doushantuo Formation are highly mature, experienced strongly anoxic, clay rich conditions with major contribution of eukaryotic micro-organisms.