Oil source rocks at mesozoic and cenozoic continental margins: Communication 2. Oil source rocks at continental margins in the second half of the cretaceous and in the Cenozoic

Oil source rocks represent sequences with the C_org content ranging from 3–5 to 15–20%. Sedimentary sections of large petroliferous basins usually include one or two such sequences, which generated liquid and gaseous hydrocarbons (HCs) during their long-term subsidence to the elevated temperature zone. The middle episode of the Late Cretaceous was marked by the accumulation of sediments with a high C_org content in different areas of the World Ocean. However, truly unique settings favorable for accumulation of the sapropelic organic matter (OM) appeared at continental margins that primarily faced the Tethys Ocean. The La Luna Formation is one of the best known source rock sequences responsible for the generation of liquid HCs in basins of the Caribbean region. In the Persian Gulf, the Kazhdumi Formation composed of marls and clayey limestones is considered the main oil-generating sequence. In the Paleogene after closure of the Tethys, the Pacific continental margins became the main domains that accumulated source rocks. The maximal deposition of sapropelic OM in this region corresponded to the early-middle Eocene. In the Neogene, the accumulation of source sediments was associated with deltas and submarine fans of large rivers and with upwelling zones. In basins of the Californian borderland, the main oil-generating sequences are represented by siliceous rocks of the Monterey Formation. They were deposited in a regional upwelling zone related to the cold California Current.     

Source rocks in sedimentary basins of continental margins in the early paleozoic

During the Paleozoic, epochs with the relatively cold climate alternated with epochs marked by significant warming. Moreover, cooling epochs were characterized by the substantial sea level fall, while warming was accompanied by its rapid rise. In many basins located at margins of Laurentia, Baltica, and the North China continental block, such an alternation is reflected in the structure of sedimentary sequences and the lateral/vertical distribution of reservoirs, confining beds, and source rocks. Despite the fact that sediments with high concentrations of sapropelic OM accumulated in different periods, their distribution areas on continents and their margins became highly reduced during cold epochs, when these sediments filled mostly rift troughs and foreland basins. After the colonization of land by higher plant communities in the Carboniferous and Permian sediments deposited during cold epochs, the humic material became an important constituent of OM in the source rock sequences.     

Characteristics and Natural Gas Origin of Middle−Late Triassic Marine Source Rocks of the Western Sichuan Depression, SW China

A scientific exploration well(CK1) was drilled to expand the oil/gas production in the western Sichuan depression, SW, China. Seventy-three core samples and four natural gas samples from the Middle–Late Triassic strata were analyzed to determine the paleo-depositional setting and the abundance of organic matter(OM) and to evaluate the hydrocarbon-generation process and potential. This information was then used to identify the origin of the natural gas. The OM is characterized by medium n-alkanes(n C_(15)–n C_(19)), low pristane/phytane and terrigenous aquatic ratios(TAR), a carbon preference index(CPI) of ～1, regular steranes with C_(29) C_(27) C_(28), gammacerane/C_(30) hopane ratios of 0.15–0.32, and δD_(org) of-132‰ to-58‰, suggesting a marine algal/phytoplankton source with terrestrial input deposited in a reducing–transitional saline/marine sedimentary environment. Based on the TOC, HI index, and chloroform bitumen "A" the algalrich dolomites of the Leikoupo Formation are fair–good source rocks; the grey limestones of the Maantang Formation are fair source rocks; and the shales of the Xiaotangzi Formation are moderately good source rocks. In addition, maceral and carbon isotopes indicate that the kerogen of the Leikoupo and Maantang formations is type Ⅱ and that of the Xiaotangzi Formation is type Ⅱ–Ⅲ. The maturity parameters and the hopane and sterane isomerization suggest that the OM was advanced mature and produced wet–dry gases. One-dimensional modeling of the thermal-burial history suggests that hydrocarbon-generation occurred at 220–60 Ma. The gas components and C–H–He–Ar–Ne isotopes indicate that the oilassociated gases were generated in the Leikoupo and Maantang formations, and then, they mixed with gases from the Xiaotangzi Formation, which were probably contributed by the underlying Permian marine source rocks. Therefore, the deeply-buried Middle–Late Triassic marine source rocks in the western Sichuan depression and in similar basins have a great significant hydrocarbon potential.     

柴达木盆地西部地区古近系及新近系碳酸盐岩沉积相

柴达木盆地西部地区古近系和新近系湖相碳酸盐岩主要分布于下干柴沟组上段到油砂山组,其中,下干柴沟组上段和上干柴沟组的碳酸盐岩更发育。碳酸盐岩主要岩石类型有泥晶灰岩、藻灰岩和颗粒灰岩等三大类,此外,还普遍发育由石灰质、白云质和陆源碎屑等3种组分构成的混积岩。碳酸盐岩沉积相可划分为滨湖灰泥坪、滨湖藻坪、浅湖颗粒滩、浅湖藻丘以及半深湖泥灰岩相。滨湖灰泥坪的主要岩石类型有泥晶灰岩、含陆屑泥晶灰岩、陆屑泥晶灰岩以及陆屑泥灰岩等;滨湖藻坪为藻泥晶灰岩、藻纹层灰岩、含陆屑藻泥晶灰岩;浅湖颗粒滩有亮晶或泥微晶的鲕粒灰岩、生屑灰岩和内碎屑灰岩,其次为含陆屑颗粒灰岩;浅湖藻丘为藻叠层灰岩、藻团块灰岩、藻泥晶灰岩和含陆屑藻泥晶灰岩;而半深湖泥灰岩相的主要岩石类型为泥晶灰岩、泥灰岩以及含少量陆屑泥和粉砂的泥晶灰岩或泥灰岩。碳酸盐岩沉积相表现出很强的由西南向东北的迁移性。     

Hydrocarbon source rocks in sedimentary basins of continental margins in the Middle-Late Paleozoic

The second half of the Paleozoic was marked by amalgamation of large continental blocks. The collision between the Laurentia and Baltica continents in the Devonian culminated in the formation of Laurussia. This event was followed by accretion of the Siberian and Kazakhstan continental blocks after the closure of the Uralian marine basin in the terminal Carboniferous-initial Permian. These processes were responsible for the formation of the Pangea supercontinent at the end of the Permian Period. They were accompanied by climate changes reflected in the alternation of warming and cooling epochs. One of these cooling epochs was terminated by large-scale glaciation of Gondwana at the end of the Carboniferous Period. Nevertheless, the most significant process, which drastically changed the existing paleogeographic situation, was colonization of continents by plants and animals, and, thus, accumulation of coaliferous formations in them. The lacustrine and sea basins also accumulated humic and mixed humic/sapropel organic matter (OM) in addition to pure sapropelic sediments.     

The origin of oil in the Cretaceous succession from the South Pars Oil Layer of the Persian Gulf

The origin of the oil in Barremian–Hauterivian and Albian age source rock samples from two oil wells (SPO-2 and SPO-3) in the South Pars oil field has been investigated by analyzing the quantity of total organic carbon (TOC) and thermal maturity of organic matter (OM). The source rocks were found in the interval 1,000–1,044 m for the Kazhdumi Formation (Albian) and 1,157–1,230 m for the Gadvan Formation (Barremian–Hauterivian). Elemental analysis was carried out on 36 samples from the source rock candidates (Gadvan and Kazhdumi formations) of the Cretaceous succession of the South Pars Oil Layer (SPOL). This analysis indicated that the OM of the Barremian–Hauterivian and Albian samples in the SPOL was composed of kerogen Types II and II–III, respectively. The average TOC of analyzed samples is less than 1 wt%, suggesting that the Cretaceous source rocks are poor hydrocarbon (HC) producers. Thermal maturity and Ro values revealed that more than 90 % of oil samples are immature. The source of the analyzed samples taken from Gadvan and Kazhdumi formations most likely contained a content high in mixed plant and marine algal OM deposited under oxic to suboxic bottom water conditions. The Pristane/nC₁₇ versus Phytane/nC₁₈ diagram showed Type II–III kerogen of mixture environments for source rock samples from the SPOL. Burial history modeling indicates that at the end of the Cretaceous time, pre-Permian sediments remained immature in the Qatar Arch. Therefore, lateral migration of HC from the nearby Cretaceous source rock kitchens toward the north and south of the Qatar Arch is the most probable origin for the significant oils in the SPOL.     

世界深水含油气盆地烃源岩的发育特征及对中国南海北部深水区烃源岩的启示

国外深水盆地烃源岩发育特征表明:无论是板内的断坳盆地还是板缘的先裂陷后漂移沉积盆地,在裂陷期(断陷期)及坳陷期(漂移期)均有烃源岩分布,大陆边缘盆地不同演化阶段有效烃源岩发育程度存在差别,其中裂陷期非海相(湖相)暗色页岩是最主要和最普遍的生油岩,还可能发育过渡相或近海相的煤系和泥质烃源岩;其次是板内坳陷晚期或板缘漂移期的海陆过渡相、近海相、前三角洲相等有大量的有机质供给或者长期离物源区较近的情况下,烃源岩发育;裂陷晚期和坳陷早期烃源岩一般不发育,但有大量的有机质供给的半封闭浅海—半深海环境可能发育高效烃源岩。国外主要深水盆地中新生界烃源岩发育特征对中国南海北部深水区烃源岩的认识具有重要的指导意义。中国南海北部裂陷早期阶段发育的湖相烃源岩和裂陷晚期发育的海陆过渡相烃源岩为主力烃源岩;南海北部不同盆地坳陷期中新统海相烃源岩差异较大,其中陆源有机质输入不足的琼东南盆地烃源岩稍差些,陆源有机质输入相对充足的珠江口盆地烃源岩较好一些。墨西哥湾和北苏门答腊盆地的勘探实践已经证实了半深海相烃源岩可作为有效油源岩,这对中国南海北部深水区中新统海相烃源岩认识具有很好的借鉴意义。     

鄂西古生代硅质岩的地球化学特征及沉积环境

鄂西地区的上奥陶统五峰组、下志留统龙马溪组下部、下二叠统孤峰组和上二叠统大隆组中发育有薄层硅质岩。二叠系栖霞组、茅口组和吴家坪组灰岩中发育有结核状或似层状燧石。Fe Al Mn三角图 ,Cu、Ni、Co、Cr、Zr的关系 ,U Th判别图 ,Ba、As、Sb元素和REE配分模式等地球化学指标表明 :五峰组和龙马溪组层状硅质岩属于正常海水中生物化学和化学沉积 ;孤峰组和大隆组层状硅质岩、茅口组灰岩中燧石结核不属于典型的热水沉积相 ,但受热水作用的影响 ,而孤峰组硅质岩受热水作用明显。鄂西五峰组和龙马溪组硅质岩的沉积环境是四周被古陆或台地环绕的半封闭深水相滞流盆地。鄂西孤峰组和大隆组硅质岩的沉积环境是由裂陷作用形成的碳酸盐台地内部的盆地(或台沟 )滞流还原环境。CaO/(Fe +CaO)值和 (MgO/Al2 O3 )× 10 0值表明 :五峰组、龙马溪组和大隆组硅质岩沉积环境海水略有淡化。栖霞组、茅口组和吴家坪组含结核状或似层状燧石的生物灰岩 ,形成于陆棚上快速海侵和上升洋流形成的还原环境。     

Sedimentary environments and geochemistry of Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus

Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus were examined in the most representative Chirkei section (Sulak River basin). Sharp lithogeochemical distinctions between them were revealed. The results of the study of nannoplankton demonstrated that the Eocene/Oligocene interface occurs slightly below the boundary between the Belaya Glina and Khadum formations. The studied section revealed a series of nannoplankton bioevents facilitating its stratigraphic subdivision. It has been established that organic matter (OM) in rocks of the Khadum Formation is characterized by a relatively high degree of maturity. Probably, the material of mainly marine genesis contains a terrigenous OM admixture. Positive oxygen isotope anomaly in the upper part of the Belaya Glina Formation reflects global climate changes (cooling) near the Eocene/Oligocene interface. Limitation of the anomaly by the upper boundary of the Belaya Glina Formation is likely related to changes in water salinity variations in the Early Oligocene basin and intense early diagenetic processes in rocks therein. Lithological, geochemical, and paleoecological data suggest that the Khadum paleobasin was depleted in oxygen. Such environment was unstable with periodic intensification or attenuation. Paleoecology in the Belaya Glina basin was typical of normally aerated basins.     

中祁连党河南山-木里地区早古生代火山-沉积岩系锆石U-Pb年代学、物源示踪及其地质意义SCIEI北大核心CSCD

造山带内与板块俯冲-碰撞过程相关的一系列沉积盆地对于重建造山带演化历史具有重要意义。本文以党河南山-木里地区早古生代火山-沉积岩系为研究对象,对其开展沉积序列、锆石U-Pb年代学、Hf同位素及碎屑物源综合分析,研究结果表明:(1)该火山-沉积岩系从底部吾力沟组到中部盐池湾组,整体上表现为弧后盆地的演化特征,顶部多索曲组具有向前陆盆地演化的特征;(2)获得吾力沟组玄武安山岩锆石U-Pb年龄为472±10Ma,限定盐池湾组和多索曲组沉积时代分别为467~450Ma和450~440Ma;(3)盐池湾组碎屑锆石年龄谱系和锆石ε;(t)组成说明盐池湾组碎屑物质主要来源于中祁连岩浆弧;而多索曲组的早古生代碎屑物质主要来源于伴生的火山岩或南祁连花岗岩,前寒武纪碎屑物质主要来自于中祁连。综合分析与汇聚板块边缘相关的沉积盆地特征和前人研究资料,本文认为党河南山-木里地区的早古生代火山-沉积岩系可能形成于弧后盆地及其闭合过程的前陆盆地环境。     

青藏高原北部新生代陆相盆地石油地质条件及勘探前景

青藏高原北部陆相盆地中烃源岩发育层位包括始新统风火山组、渐新统雅西措组和中新统五道梁组。古近系雅西措组烃源岩,特别是灰岩属于中好烃源岩范畴,有机质类型较好,且烃源岩主体处于成熟阶段,是藏北高原新生代陆相盆地主力烃源岩的发育层位。陆相盆地储集岩较发育,储层较丰富,发育层位包括风火山组和雅西措组,其中雅西措组是储层主力分布层位。对测区分析数据表明：通天河盆地具备一定规模的生油岩厚度,而且有机质丰度为沱沱河地区最高,表明其勘探前景较好。     

Oil families and their source rocks in the Weixinan Sub-basin, Beibuwan Basin, South China Sea

Thirty-one crude oils and 15 source rocks were selected for molecular geochemical and isotopic analyses in order to establish the genetic relationships between discovered oils and petroleum source rocks in the Weixinan Sub-basin, Beibuwan Basin, South China Sea. Three groups of oils were recognized. Group I oils are only found in the upper section of the Liusagang Formation, with a moderate abundance of C₃₀ 4-methylsteranes, low oleanane contents and lighter δ¹³C values, showing a close relation to the shale occurring in the upper section of the Liusagang Formation. Group II is represented by the majority of the discoveries and is distributed in multi-sets of reservoirs having different ages. The oils are characterized by a high abundance of C₃₀ 4-methylsteranes, low to moderate abundance of oleanane and heavy δ¹³C values, and shows a good correlation with the lacustrine shale and oil shale in the middle section of the Liusagang Formation. Group III oils occurred in the lower section of the Liusagang Formation. The oils have a lower concentration of C₃₀ 4-methylsteranes, relatively high abundance of oleananes and their δ¹³C values are intermediate. Oils of this group correlated well with the shallow lake-delta mudstone of the lower section of Liusagang Formation. These oil-source genetic relationships suggest a strong source facies control on the geographic distribution of oil groups within the Weixinan Sub-basin. The geochemical data indicate shale in the middle section of the Liusagang Formation has an excellent oil generation potential and the lower and upper sections contain dark shale and mudstone with good to fair oil potential. Future exploration or assessment of petroleum potential of the sub-basin could be improved by considering the proposed genetic relationship between the oil types and source rocks, as well as their distribution.     

Petroliferous basins at continental margins of Paleozoic paleoseas: Communication 1. Petroliferous basins at margins of Iapetus and Panthalassa

Although large marine basins governing the fabric of our planet in the Paleozoic disappeared later (whether or not they were oceans is a debatable issue), sedimentary basins formed at continental margins at that time played a crucial role as depositories of various fossil minerals, including ores, salts, phosphorites, coal, bauxites, and construction materials. Many of these basins are oil- and gas-bearing structures. Their oldest representatives are confined to margins of Proterozoic/Paleozoic paleoseas (Iapetus and Panthalassa), whereas other basins appeared after opening of the Central Asian, Uralian, and Rheic (Paleotethys) deep-marine basins. Study of specific features of the sedimentary cover of such basins, rock composition therein, rocks and associated oil- and gas-bearing systems revealed that the Paleozoic planet was divided into two parts: Gondwana, with the major portion confined to high latitudes of the Southern Hemisphere; and other smaller near-equatorial continents. This pattern significantly governed the composition and mode of post-sedimentary transformations of natural reservoirs, as well as age and spatial distribution of the major hydrocarbon (HC) source sequences. Most Paleozoic oil- and gas-bearing basins make up specific belts because of their confinement to continental margins in paleoseas of that time.     

Characteristics,differences and main controlling factors of source rocks in the central‐southern South China Sea Basins

The South China Sea is rich in oil and gas resources. With the increasing exploration of oil and gas resources in the northern South China Sea and the increasing demand for energy in the world, The central‐southern South China Sea have become important constituencies for oil and gas resources. The central‐southern basins of South China Sea can be divided into three types of basin groups, namely, the southern basin group (Zengmu Basin, Brunei‐Sabah Basin), the western basin group (Wan'an Basin, Zhong jiannan Basin) and the Central Basin Group (Liyue Basin, Beikang Basin, Nanweixi Basin and Palawan Basin). At present, the degree of exploration is relatively low, and the source rock has not yet formed a understanding of the system. The main source rock development time, source rock characteristics, hydrocarbon generation potential and control factors of each basin group are still unclear, which seriously restricts the exploration of oil and gas. Based on the sedimentary facies distribution and sedimentary filling evolution, combined with the geoche mical characteristics of the source rocks, the source age, organic matter type, hydrocarbon generation potential and main controlling factors of the basins in the central‐southern basins are discussed. By the detailed research on delta scale, provenance system, paleoclimate conditions, ancient vegetation development and hydroca rbon generation parent material, the main controlling factors of hydrocarbon generation potential of source rocks in each basin group are revealed.     

Evidence of the transgression lake of the Subei basin during Late Cretaceous and Paleocene and its geological significance

Based on analysis of the well drilling core from Subei basin, the authors conclude that during the Late Cretaceous and Paleocene, Subei basin was linked with the sea and the deposit was affected by transgression. The cause of marine transgression may be that since Late Cretaceous and Paleocene tension power had predominated ground-stress conditions of the East China Sea and developed a series of half-graben-like basins filled by a huge thick sediment of the Early Tertiary in the shelf of Huabei-Bohai gulf, Subei-South Yellow Sea and East China Sea. Consequently, seawater transgressed from the East China Sea to the Yellow Sea and linked halfgraben-like basins on the shelf to the sea within a short period. During the sedimentation of the Late Cretaceous Taizhou Formation and Paleocene Funing Formation, the Subei basin had formed the ostracoda-enriched dark shale, including predominantly the whole basin E₁ f ₂ Formation and E₁ f ₄ Formation and local K₂ t ₂ Formation, which became the main source rocks of the basin. The evidence of paleontology, minerals in rocks and geochemistry can help confirm the environment of the lake basin that developed during the Late Cretaceous and Paleocene. We generally designate this environment as “near sea lake basin” and the sea-transgressed layer and member as “transgression lake basin”.Whereas, it is generally called “inland lake and river alluvium plain” during the sedimentation of the Eocene Dainan Formation and Sanduo Formation. This research is not only significant to the paleogeographic reconstruction of the Subei basin during the Late Cretaceous and Paleocene, but also important in understanding the development and distribution of the source rocks and evaluating the potential of oil and gas generation. __________ Translated from Acta Sedimentologica Sinica, 2007, 25(3): 380–385 [译自: 沉积学报]     

青藏高原东北缘六盘山盆地烃源岩的地球化学特征

在前人认识的基础上,依据近年来的勘探成果,对六盘山烃源岩进行了评价分析,发现目前较明确的3套主要烃源岩中,白垩系下统的乃家河组、马东山组和李洼峡组源岩在盆地内分布较广,除北部埋藏较浅的区域未成熟外,大部分已趋于成熟。石炭系也是重要的源岩,由于埋藏较深,后期改造作用大,应以找气为勘查目标。三叠系烃源岩成熟度较好,但分布范围局限,勘探潜力受到影响。盆地的诸多油气显示及地面油气异常响应特征,充分证实了该盆地具有烃类的生成、运移和聚集的过程。整个盆地的油气地球化学异常响应特征明显,油气苗显示丰富,分布广泛。地球化学勘探测量发现了盆地内有3个近东西向展布的油气地球化学异常区带。根据三维荧光光谱图,对油气属性进行了判别,均表现为轻质油的地球化学特性。     

中国中西部前陆盆地烃源岩特征与油气资源潜力分析

中国中西部前陆盆地(冲断带)以发育陆相烃源岩为特征。中西部周缘和弧后前陆盆地烃源岩形成于前陆盆地沉积期,包括准噶尔西北缘、准噶尔南缘、塔里木盆地西南缘、吐哈盆地二叠系烃源岩和川西、鄂尔多斯盆地西缘三叠系烃源岩;再生前陆盆地(冲断带)烃源岩主要形成于再生前陆盆地沉积之前的三叠—侏罗系和白垩系、古近系。根据沉积环境可以将烃源岩分为被动大陆边缘海相、残留海—(泻)湖相、湖沼相、内陆坳陷淡水湖相以及内陆坳陷断陷半咸水-咸水湖相烃源岩5种类型,其中湖沼相煤系烃源岩有机质类型为III型,以产气为主,其他烃源岩有机质类型主要为II型,其次为I型,以产油为特征。中国中西部前陆盆地石油潜在资源量为89.17×108t,天然气潜在资源量为101 464×108m3,与国外典型前陆盆地相比,中国前陆盆地具有富气特征。     

Change in the degree of catagenesis and hydrocarbon generation in the sedimentary rocks of the South Sumatra Basin,Indonesia

Temperature and catagenetic history of the South Sumatra Basin in Indonesia is considered by the example of the sedimentary sequence of the Limau graben during its subsidence from the Oligocene to the present time. GALO system for basin modeling was applied for numerical reconstruction of six sedimentary successions in the area of holes Pandan-81, Petanang-1, Tepus-2, Tepus-1, Gambir-1, and Lembak-8 located along the profile cutting across the Limau graben. Modeling suggests significant cooling of the basemen for the last 15–20 Ma from the high initial heat flow of 105 mW/m², which is typical of axial zones of continental rifting, and significant heating of the basin lithosphere during the last 2–5 Ma. Examination of variations in tectonic subsidence of the basin confirms the possible extension of the lithosphere in the Oligocene-Miocene with an amplitude β increasing from 1.12 on the flanks of the Limau graben (Hole Lembak-8) to 1.32 in the central part of the graben (Tepus-1 and 2), Tectonic analysis indicates also the notable thermal activation of the basin in the Pliocene, Pleistocene, and Holocene. This activation is consistent with the high temperature gradient typical of the present-day sedimentary cover of the basin. Numerical modeling of the evolution of the organic matter maturity and hydrocarbon generation by main source formations of the basin confirms good prospects of the inferred source formations (Lemat, Talang Akar, and Gumai) of the South Sumatra basin for the generation of liquid hydrocarbons (HC) in the Limau Graben. It is also demonstrated that the source rocks of the Lemat Formation are ore-generating rocks in the main part of the Limau graben and are gas-generating rocks only in the deepest portions of the graben. The rocks at the base and roof of the Talang Akar Formation could be considered as highly oil-generating rocks, probably except for the upper horizons of the formation in the shallowest portions of the graben (Hole Lembak-8). Oil generation reached peak in the last 5–10 Ma. Modeling showed that intense oil generation by the Gumai Formation may be significant in the most part of the Limau graben and negligible only in the distant flanks of the graben (Hole Lembak-8).     

扬子地区古生代主要烃源岩有机质富集的环境动力学机制与差异

在扬子地块古生界发育了3套区域性的优质烃源岩,即:下寒武统、上奥陶统一下志留统、二叠系(中、上二叠统).下寒武统烃源岩主要由硅质岩(留茶坡组)和上部的硅质泥岩、碳质泥岩(或石煤)(牛蹄塘组)组成.早寒武世在台一盆转换带(或台地边缘)广泛发育的海底热液活动,喷流出大量的富硅流体,使大量硅岩得以在台缘外侧-盆地区沉淀;同时...     

Compound-specific carbon isotope study on the hydrocarbon biomarkers in lacustrine source rocks from Songliao Basin

Stable carbon isotopic composition of organic matter (δ¹³C_org) and compound-specific δ¹³C values of biomarkers from 15 lacustrine source rocks were analyzed to identify the original paleoenvironment and source organisms. The δ¹³C values of hopanes (δ¹³C_hop) ranged from −68.7‰ to −32‰ and exhibit strongly ¹³C-depleted values in the lower part of Member 1 of the Nenjiang Formation (K₂n¹, up to −68.7‰), suggesting an origin from predominantly methanotrophic bacteria. ¹³C-enriched δ¹³C_Ga values and significantly ¹³C-depleted δ¹³C_hop in K₂n¹, which coincide with water stratification and an intermittent anoxic photic zone, represents a shallow chemocline. The presence of an intermittent anoxic photic zone, which means that the anoxia expanded into the euphotic zone, is beneficial for OM preservation and results in high values of TOC and HI in this section. However, the absence of gammacerane and ¹³C-enrichment of δ¹³C_hop in Member 2 of Nenjiang Formation (K₂n²) reflect a deeper chemocline, corresponding to relatively oxidizing conditions and low values of TOC and HI. Moreover, the negative correlation of TOC vs δ¹³C_org and HI vs δ¹³C_org reflects the control of OM formation by sedimentary environments rather than productivity in the water column. Thus, the depth of the chemocline not only controls the abundance of OM but also affects the development of the microbial community, such as chemoautotrophic bacteria in the deep chemocline and chemoautotrophic and methanotrophic bacteria in the shallow chemocline. Moreover, δ¹³C_Ga and δ¹³C values for 4-methyl steranes are related to water salinity, with a higher salinity accompanied by ¹³C-enrichment in gammacerane and 4-methyl steranes.