The Eocene organic-rich lacustrine shales in the South China Sea were deposited in fresh-brackish environment under tropical climate including the early Eocene Climatic Optimum (EECO, 53–49 Ma) and the Middle Eocene Climatic Optimum (MECO). In the Eocene Liushagang Formation (LF), biomarkers diagnostic for cyanobacteria and algae, together with organic petrology and inorganic geochemical data from ninety-three shale samples in different sedimentary environments were comprehensively analyzed to investigate algal and bacterial responses to the EECO. Cyanobacteria-derived C28–C34 2-methylhopanes during the EECO in shallow lake and during the subsequent 49.0–48.5 Ma in semi-deep lake exhibit higher abundance than in other Eocene time intervals. In contrast, there are remarkable decline of C28–C34 2-methylhopanes and increase of C30 4-methylsteranes from periods of 48.5–47.5 Ma to 47.5–46.47 Ma when the lower LF second member was deposited. Under the inferred terrestrial mean annual temperature > 25 °C during the EECO and the subsequent 0.5 Ma, cyanobacteria were thought to have competitive advantage over dinoflagellate-dominated algae in the shallow water and semi-deep lake. During 48.5–46.47 Ma when the lower LF second member was deposited, there were declined temperature and significantly enhanced monsoonal hydrodynamics and nutrient supply, dinoflagellate-dominated algae had higher abundance than cyanobacteria in the deep-water lake. During 46.47–39.2 Ma, there were not much cyanobacterial or algal contribution under the sustained temperature decline and decreased nutrients supply under global cooling and weakened monsoonal hydrodynamics and. These are partially supported by various telalginite and lamalginite, plant-derived vitrinite content in macerals and inorganic parameters indicative for climate and sedimentary conditions in the upper LF third member and the LF second and first members. They collectively suggest that there is higher cyanobacterial contribution relative to dinoflagellate-dominated algae during the EECO and the subsequent 0.5 Ma in shallow and semi-deep lakes. This finding provides analogues for the coeval lacustrine organic-rich shales deposited during the EECO and the subsequent 0.5 Ma in the tropical South China Sea and other ancient tropical lakes during the same time intervals.

Coal-measure source rocks may play an important role in hydrocarbon generation in petroliferous basins where coal seams are well developed. Hydrocarbon generation characteristics and potential of coal-measure source rocks have been well documented for continental petroliferous basins, while the understanding of coal-measure source rocks in offshore basins is yet to be delved into. Significant oil exploration breakthroughs have been made in the well-developed coal measures of Turpan–Hami Basin (THB), a typical continental petroliferous basin in northwestern China. In this study, a comparative analysis is conducted on the Paleogene coal seams in the Zhu I Depression (ZID), located in the northern part of the South China Sea, and the Jurassic coal seams in the THB in terms of genetic conditions, mineral composition, and hydrocarbon generation potential. The geological understandings are obtained as follows. Both the coal-forming periods during the deposition of the ZID and THB were of a warm and wet climate type. The Paleogene coal-forming environments during the deposition of the ZID mainly include peat swamp in the upper plain and interdistributary bays in the lower plain of the braided river delta, along with littoral shallow lakes. As a whole, the coal seams are characterized by multiple layers, thin single layer thickness and poor stability, while those in the upper plain peat swamp of the braided river delta have relatively larger single layer thickness but relatively fewer number of layers. The Jurassic coal-forming environments in the THB include peat swamp in the upper delta plain, lower delta plain, and inter-delta bay. The coal seams formed in the lower delta plain are the most stable, while those in the inter-delta bay are the thickest. The ZID coal has a higher vitrinite content (averaging 76.11 %) and liptinite content (averaging 10.77 %) compared to its THB counterpart, which has an average vitrinite content of 68.28 % and average liptinite content of 7.61 %. The kerogen of the ZID coal is mainly of type II1, while that of the THB coal mainly of type II2, followed by type Ⅲ. Both the ZID and THB coals have entered the oil-generation window, as indicated by their maximum vitrinite reflectance values (Ro, max, %), reflecting good oil generation capacity. However, the hydrocarbon generation potential of the ZID coal is higher than that of the THB.

Gas derived from the primary cracking of kerogen and the secondary cracking of oil has historically been the focus of petroleum geologists, given its importance as a gas source. The Wenchang A Depression within the Zhu III Sub-basin is the largest gaseous hydrocarbon-rich depression in the Pearl River Mouth Basin (PRMB), and the sources of gaseous hydrocarbons in this depression are a research focus. Mud-stones from the Eocene Wenchang Formation contain type I and type II organic matter and are oil-prone, with TOC, S1+S2, and HI values mostly ranging from 1.42% to 3.12%, 9.71 mg/g to 20.61 mg/g, and 410.71 mg/g TOC to 736.17 mg/g TOC, respectively. Data of gaseous hydrocarbon yields and carbon isotopic compositions show that the gaseous hydrocarbons generated from oil-prone mudstones are mainly derived from the secondary cracking of oil, and the plot of delta 13C2-delta 13C3 versus ln(C2/C3) effectively identified the gas source. To further assess the gas generation processes and the ratio of oil-cracking gas under geological conditions, we reconstructed the history of gaseous hydrocarbon generation in mudstones from the Wenchang Formation in the Wenchang A Depression. Results showed that gaseous hydrocarbon generation began at approximately 33 Ma, a maximum of 69% of total gaseous hydrocarbons (C1-C5) was generated by oil cracking, and total heavy hydrocarbon gases (C2-C5) were mainly generated from oil cracking (65%-81%). This study provides a deeper understanding of the characteristics of gas generated from oil-prone mudstones and is important for gas exploration in the Wenchang Depression. (c) 2025 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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 coal-measure source rock in the Chinese sea area plays a significant role as a hydrocarbon source rock, with its genetic environment, development and distribution, and hydrocarbon generation potential serving as essential factors for the exploration of coal-type oil and gas fields. This study focuses on the coal-measure source rock of the Paleogene Enping Formation in the Zhu I Depression, located in the northern South China Sea. The main geological insights obtained are as follows. The coal measures of the Enping Formation are developed in a warm and wet tropical-subtropical climate. The development environment of the coal-measure source rock in the Enping Formation includes the braided river delta upper plain peat swamp, characterized by dry forest swamp coal facies with relatively thick coal seams and a small number of layers. The braided river delta lower plain swamp-interdistributary bay of braided river delta front represents a forest edge-wetland herbaceous swamp coal facies with numerous layers of thin coal seams and poor stability. The shore swamp corresponds to an open water swamp coal facies with multiple layers of thin coal seams and poor stability. The organic matter abundance in the braided river delta upper plain is the highest, followed by the braided river delta lower plain-braided river delta front, and the shore-shallow lake. The organic matter type is predominantly type II1. Thermal evolution analysis suggests that the organic matter has progressed into a substantial oil generation stage. The hydrocarbon generation potential of the coal-measure source rock in the Enping Formation is the highest in the braided river delta upper plain, followed by the braided river delta lower plain-braided river delta front and the shore-shallow lake. Overall, this study proposes three organic facies in the coal-measure source rock of the Enping Formation: upper-plain swamp-dry forest swamp facies, lower plain-interdistributary bay-forest-herbaceous swamp facies, and lake swamp-herbaceous swamp facies.

The northern South China Sea, including the Zhujiangkou Basin and the Beibuwan Basin, developed high-quality lacustrine source rocks during the Eocene rifting period. These source rocks are vital for hydrocarbon generation in the northern South China Sea. The Zhu I depression in the Zhujiangkou Basin and the Beibuwan Basin typically exhibit high abundance of C30 4-methyl steranes. However, shales in the Eocene Wenchang Formation in the Zhu III depression of the Zhujiangkou Basin contains lower quantities of high-quality lacustrine source rocks with 4-methyl steranes, which often co-elute with some pentacyclic triterpanes in gas chromatography-mass spectrometry (GC-MS). Therefore, the single 4-methylsterane parameter based on GC-MS cannot accurately distinguish organic source in the deep to semi-deep water lacustrine source rocks of the Wenchang Formation from other source rocks, thus impeding the recognition of their contributions to petroleum reservoirs. In this study, GC-MS of aliphatic hydrocarbons, palynofacies and algal identification, as well as stable carbon isotope data of organic matter were used to identify the algal species and construct the paleoclimate during deposition of the Wenchang Formation source rocks in the Zhu III depression of the Zhujiangkou Basin. It is suggested that during the Wenchang Formation period, freshwater green algae prevailed in the lake, which is likely account for the relatively low content of 4-methyl steranes in the high-quality lacustrine source rocks. Controlled by the algal species, it is proposed that the content of C30 tetracyclic polyprenoids (TPP) can better indicate the quality of the Wenchang source rocks than C30 4-methyl steranes. Consequently, a relationship between the TPP index and the quality of the lacustrine source rocks in the Wenchang Formation of the Zhu III depression was established. A higher TPP index indicates higher organic matter abundance and hydrogen index of the lacustrine source rocks. When applied to the origin analysis of oils in the Zhu III depression, it is believed that the organic-rich deep lacustrine source rocks in the Wenchang Formation made great contribution to the transitional zone crude oils in the Wenchang A and Wenchang B depressions.

The sedimentary system of Kalimantan has undergone significant development since the Oligocene. Previous research have largely ignored the capacity of the Cretaceous-Eocene sediments to produce hydrocarbons, focusing instead primarily on the Oligocene-Miocene coal as the principal source rocks. Shales and coals from the outcrops in the northern margin of Kalimantan were analyzed with palynological and geochemical methods to characterize the palaeoenvironmental and palaeoecological differences between the Cretaceous-Eocene and the Oligocene-Miocene samples. The high proportion of Cheirolepidoaceae, Schizaeoisporites and Ephedripites in the pollen assemblage from the Cretaceous—Eocene outcrops reflects an arid tropical/subtropical climate. The relatively low abundances of gymnosperm-derived biomarkers including isopimarane, β-phyllocladane, β-kaurane, suggest the gymnosperm features in flora. High C27/C29ααα 20R sterane ratios, (C19–C29) tricyclic terpanes/C30αβ hopane and extremely low oleanane/C30αβ hopane, bicadinane T/C30αβ hopane, and diterpenoid abundance indicate that there was a dominance of algae relative to higher plants in the organic matter. The gymnosperm-derived biomarkers, including isopimarane, β-phyllocladane, β-kaurane, suggest that palaeovegetation during this period was dominated by gymnosperms. The saline and reducing conditions in the bathyal and abysmal sea, evidenced by rather low Pr/Ph and high Gammarerane index, are beneficial for the preservation of hydrogen-rich organic matter. It is presumed that the Cretaceous—Eocene shales had great hydrocarbon generation potential in the southern South China Sea. During the period of Oligocene to Miocene in the Zengmu Basin and the Baram-Sabah Basin, the climate changed to a dominant humid and warm condition, which is corroborated by abundant pollen of Florschuetzia and Magnastriatites hawardi. Low C27/C29ααα 20R sterane ratios, (C19–C29) tricyclic terpanes/C30αβ hopane, and high oleanane/C30αβ hopane, bicadinane T/C30αβ hopane suggest that the palaeovegetation was dominated by angiosperms including the mangrove plants. The extremely abundant higher plants provide ample terrigenous organic matter for the formation of coal-measures in delta facies. The low gammacerane index and high Pr/Ph indicate the fresh and sub-oxic water in delta-neriticabysmal faces, which is not beneficial for the accumulation of hydrogen-rich organic matter. Thus, the Oligocene-Miocene marine argillaceous rocks can be potential sources of natural gas.

Reservoirs of high CO2 content are rarely found in Bohai Sea, offshore of the Bohai Bay Basin, and Qinhuangdao 29 tectonic zone (QHD29) is one of the few typical reservoirs of the type. To study the correlation of hydrocarbon accumulation with the migration and accumulation of high-CO2-content gas is of great significance to oil and gas exploration and the discovery of CO2 gas reservoirs. Based on geochemical analysis of saturated and aromatic hydrocarbons, petrologic characteristics of fluid inclusions, we systematically discuss the interrelationship between CO2 charging and hydrocarbon accumulation in QHD29 tectonic zone and the genetic mechanism of hig-CO2-content reservoirs in the tectonic zone. The results are shown as follows. First, the hydrocarbons sourced from the source rocks of the first and third members of Shahejie Formation (E2s1 and E2s3) mainly accumulate in the E2s1, followed by Dongying Formation (E3d). Second, QHD29 reservoirs underwent two stages of hydrocarbon charging. The early stage features crude oil charging dominated by north-south lateral migration from the Qinnan Sag to the uplift along the sand bodies; and the late stage features instantaneous gas accumulation caused by magmatic-hydrothermal fluid as dominated by vertical migration. The supercritical CO2, fluid rich in hydrocarbons, moved upward along the deep-rooted faults, entered the major oil reservoirs, and then moved laterally from south to north. Third, QHD29 reservoirs obviously experienced gas washing under both early oil and late gas accumulation events, forming a hydrocarbon distribution pattern with heavy oil and bitumen enriched at the bottom, light oil and gas condensate in the middle, and CO2 gas dominating on top of the major reservoirs.

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 discovery of the Bozhong 19-6 gas field, the largest integrated condensate gas field in the eastern China in 2018, opened up a new field for the natural gas exploration deep strata in the Bohai Bay Basin, demonstrating there is a great potential for natural gas exploration in oil-type basins. The ethane isotope of the Bozhong 19-6 condensate gas is heavy, showing the characteristics of partial humic gas. In this paper, aimed at the source rocks of the Bozhong 19-6 gas field in the Bohai Bay Basin, the characteristics of the source rocks in the Bozhong 19-6 structural belt were clarified and the reason are explained from impact of microorganism degradation on hydrocarbon generation of source rocks why the condensate oil and gas had heavy carbon isotope and why it showed partial humic characteristics was explored based on the research of parent materials. The following conclusions were obtained: The paleontology of the Bozhong 19-6 structural belt and its surrounding sub-sags is dominated by higher plants, such as angiosperm and gymnosperm. During the formation of source rocks, under the intensive transformation of microorganism, the original sedimentary organic matter such as higher plants was degraded and transformed by defunctionalization. Especially, the transformation of anaerobic microorganisms on source rocks causes the degradation and defunctionalization of a large number of humic products such as higher plants and the increase of hydrogen content. The degradation and transformation of microorganism don't transform the terrestrial humic organic matter into newly formed "sapropel" hydrocarbons, the source rocks are mixed partial humic source rocks. As a result, hydrogen content incrased and the quality of source rocks was improved, forming the partial humic source rocks dominated by humic amorphous bodies. The partial humic source rocks are the main source rocks in the Bozhong 19-6 gas field, and it is also the internal reason why the isotope of natural gas is heavy.

渤海湾盆地发育多套优质湖相烃源岩,不同烃源岩地球化学特征相似,油源对比难度大.针对常规油源对比方法无法有效区分不同湖相烃源岩的问题,以渤中凹陷为研究靶区,引入判别分析油源对比方法,利用108件烃源岩样品建立判别模型,判识了 115个原油样品的油源,并结合成藏条件对判别结果进行合理性探讨.研究表明,利用判别分析方法筛选出的C19/C23三环萜烷、C21/C23三环萜烷、ααα-C29甾烷(20R)/C30藿烷等17个生物标志化合物参数组合,可以在多维空间有效区分渤中凹陷沙三段、沙一段和东三段3套湖相烃源岩;判别分析油源对比模型初始验证正确率达92.6％,交叉验证正确率达83.3％,提高了油源对比可靠性.该油源对比方法不仅适用于渤中凹陷,也可推广应用至其他多源供烃的含油气盆地.

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 Xihu Depression in the East China Sea Shelf Basin is a large petroliferous sedimentary depression, in which oil and gas reservoirs were mainly discovered in the Pinghu Slope and the central inversion zone. The oil-gas source correlation in the Xihu Depression was analyzed by hydrocarbon generating thermal simulation data via gold-tube pyrolysis experiments. The results indicated that the oil and gas in the Xihu Depression were mainly derived from coal measure source rocks of the Eocene Pinghu Formation. Therefore, the identification of coal seams is extremely crucial for evaluating coal measure source rocks in the Pinghu Formation in the Xihu Depression. Geochemical and petrological characterization pointed to input of terrigenous organic matter and redox conditions of the depositional environment as factors that govern the ability of the coal measure source rocks in hydrocarbon generation in the Xihu Depression. In this regard, the sedimentary organic facies in the Pinghu Formation were classified into four predominantly terrigenous and one mixed-source subfacies, which all varied in carbon and hydrogen content. The coal measure source rocks in the carbon- and hydrogen-rich tidal flat-lagoon exhibited the highest hydrocarbon generation potential, whereas the mudstone in the neritic facies was the poorest in its hydrocarbon yield. These results suggested that the coal measure source rocks in the Pinghu Formation likely developed in the Hangzhou Slope and the Tiantai Slope, both representing promising sources for oil and gas exploration.

The terrigenously-dominated marine shales which were deposited in the lower Eocene Pinghu Formation were thought to be a potential source rock in the Xihu Depression of the East China Sea Shelf Basin. However, the exceptionally high total organic carbon content (TOC, >6％ on average) of the tidal sand ridge samples was not compatible with their sedimentary environment, indicating coal-bearing sedimentary debris may have been transferred from the coast to the ocean. In this study, new sights into the origins and supply of organic materials in the coastal environment were proposed in the neritic organic matter of the Eocene Pinghu Formation. A discriminant model was developed using plynofacies analysis data to pinpoint the source of organic materials in marine source rocks. The discrimination results suggested that marine mudstones were associated with tidal flat mudstones rather than deltaic ones. The biomarker characteristics of mudstones deposited in various environments support this assertion, indicating that the supply of plant materials in tidal flats is the primary organic matter source for the marine environment. The organic matter abundance was elevated in tidal flats due to their superior preservation conditions. Additionally, the lithological assemblage of tidal flats suggests that tidal currents can scour marshes and then transport dispersed terrigenous organic materials to neritic areas. These findings indicate that coal-bearing sedimentary debris was likely transferred from the coast to the ocean, and tidal currents are thought to be the dominant mechanism driving organic matter from the tidal to the marine environment.

The increase of total organic carbon content of the late Oligocene-early Miocene terrigenously-dominated marine shales in the shallower depth intervals was reported in the Ying-Qiong Basin, South China Sea. The organic enriched lower Sanya Formation shales (early Miocene) have biomarker characteristics of tropical/subtropical plants, with abundant high molecular weight n-alkanes, angiosperm-derived oleanane, rearranged oleananes I, II, II, tricyclic/tetracyclic terpanes including des-A-oleanane, X, *, Y, Z, Z1 and bicadinanes W, T, T1, R. The biomarker characteristics are suggestive of larger influx of the dominant tropical/subtropical angiosperms in flora under a warming and more humid climate during depositions of the lower Sanya Formation (early Miocene) than the older Lingshui Formation (late Oligocene). The tropical/subtropical angiosperm input was thought as the prime control of terrigenous organic matter enrichment relative to the redox condition, and the coeval sea level changes and seafloor spreading in the South China Sea. Enrichment of the terrigenous organic matter in the early Miocene shales is likely in association with the coeval peak East Asian summer monsoon intensity in the South China Sea.

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.

对中国海区及邻域14个含煤盆地组成的巨型含煤盆地带的煤型油气进行了系统分析.该巨型含煤盆地带经历了古新世、始新世、早渐新世、晚渐新世和中新世—上新世等5个成煤期,发育了三角洲、扇三角洲和潮坪-潟湖3种成煤环境.巨型含煤盆地带以煤型烃源岩为主,煤系与海相烃源岩具有"二元结构"发育模式.通过对烃源岩、凝析油、天然气样品的实验分析,发现该巨型含煤盆地带煤成烃具有"四阶段"生烃模式,并且煤型烃源岩生气极限Ro(镜质体反射率)可达到4.38％,极大地拓展了天然气生成范围.该巨型含煤盆地带煤型烃源岩形成了凹陷边缘煤系和凹陷内部陆源海相两种油气成藏模式,在这两种成藏模式控制下,形成了多个大型煤型油气富集区.

The discovery of BZ19-6 condensate gas field in the Bozhong Sag of Bohai Sea area brought hope and confidence for giant gas fields. Intensive study of the source and genesis of natural gas in BZ19-6 gas field plays an important role for guiding natural gas exploration in the Bohai Sea. Open and sealed system pyrolysis as well as geochemical and numerical simulation show that the high-quality sapropelic lacustrine source rocks with an average TOC content of 3.93% and an IH index of 727 mg/g in the Bohai Sea area mainly generate oil, and only 10% of kerogens in the source rocks crack into gas. For humic-prone source rocks, about 30% of kerogens generate into gas when IH reaches 100 mg/g. However, the humic-prone source rocks still expel gas which is cracked from residual hydrocarbon due to the adsorption of kerogens. The natural gas in BZ19-6 gas field are mainly sourced from Ⅱ1, Ⅱ2 and Ⅲ type source rocks in the third member of Shahejie Formation during mature to high-maturity stages. They are mixtures of kerogen cracked gas and residual hydrocarbon cracking gas sourced from parts of the southwestern Bozhong Sag. The source rocks of the third member of Shahejie Formation in the hydrocarbon supply area mainly discharge oil in the early stage, and changed to gas after 15 Ma, accounting for 82% of the total gas discharge. To find large gas fields in the Bozhong Sag, the targets where source rocks have large gas/oil ratio and mainly expel gas should be focused on.

渤海湾盆地是中国东部重要的含油气盆地之一,2018年中国东部最大的整装凝析气田——渤中19-6凝析气田的发现,一举打开了该地区深层天然气勘探的新领域,展现了油型盆地天然气勘探的巨大潜力.本文利用气体同位素、金刚烷双质谱、饱和烃色谱-质谱、芳烃色谱-质谱等实验分析技术,深入分析了渤海海域渤中19-6构造带油气藏分布特征,并研究了该构造带"上油下气"纵向连续分布的形成机理.认为:渤中19-6凝析气田凝析油气成熟度均分布在Ro=1.50％左右,其中凝析油裂解程度较低,属于烃源岩在高成熟阶段生成的产物;烃源岩在成熟阶段生成的原生油气早期赋存于深层潜山圈闭中,后期由于受到新构造运动的影响,油藏被破坏,原油发生降解作用且向上逸散运移进入浅层圈闭中,形成渤中19-4油田;而烃源岩在高成熟阶段生成的凝析油气进入渤中19-6圈闭中,形成现今的渤中19-6凝析气田.

北部湾盆地涠西南凹陷和乌石凹陷流二段普遍发育油页岩.有机地球化学分析表明,北部湾盆地流二段油页岩具有高的有机质丰度,油页岩的有机碳含量下限为3％,含油率介于3.5％～10％,达到中等和优质油页岩矿品级,同时属于优质烃源岩.全岩有机显微组分、干酪根镜检和热解等分析表明,北部湾盆地流二段油页岩藻类含量丰富,藻类是有机质的主要来源,有机质类型主要为腐殖—腐泥型,部分为腐泥型,以湖相腐殖—腐泥型油页岩为主,部分为湖相腐泥型油页岩.流二段油页岩中镍和钼等微量元素含量普遍较高,镍和钼的含量与有机碳含量之间存在很好的正相关关系,说明流二段油页岩形成时期北部湾盆地具有富营养湖泊特征.北部湾盆地流二段油页岩中还原硫含量高,普遍大于1％,流二段油页岩形成于还原的沉积环境,具有很好的有机质保存条件.高有机质生产力及湖侵体系域顶部和高位体系域底部的中深湖还原环境共同控制了北部湾盆地流二段油页岩的发育.