南黄海盆地中部隆起CSDP-2井初步成果及其地质意义

南黄海盆地是新生代、中生代、古生代大型叠合盆地,具有良好的油气资源前景.中部隆起是南黄海盆地的二级构造单元,由于缺乏钻探资料,对中部隆起前新生代地层分布和油气资源前景的认识众说纷纭.因此,在中部隆起上实施科学钻探,探查中-古生代地层属性,对研究南黄海区域地质构造特征和油气地质条件意义重大.针对科学钻探目标,采用区域地质背景指导下的地质、地球物理综合研究的方法,拟定了科学钻探井(CSDP-2井)位置;利用叠前时间偏移成像处理剖面,采用地震相、地震速度分析和储层预测技术,进行了钻前层位预测和油气储层分析.该井由青岛海洋地质研究所负责的"大陆架科学钻探"项目实施,全取芯钻探成果表明,中部隆起上赋存了三叠系-奥陶系海相残余地层,发育中等—好级别的、厚度较大的海相中-古生界烃源岩,在三叠系青龙组、二叠系龙潭组和栖霞组、泥盆系和志留系中获得多层不同级别的油气显示.CSDP-2井的钻探发现,结束了关于中部隆起海相地层属性的争论,坚定了南黄海海相油气勘探的信心,为区域地质研究和油气地质条件分析提供了基础数据.     

Relict back-arc basins of Eurasia and their hydrocarbon potentials

Abstract Large petroliferous basins in the continental interior are characterized by very thick sedimentary sequences. It is suggested that these are not intracratonic basins, but areas underlain by oceanic crust. These include the Western Siberian, Pre-Caspian, South Caspian, North and South Kara Basins of the Commonwealth of Independent States, and Tarim, junggar and Qaidam Basins of China. The relict ocean basins are distinguished by their basement topography, by their magnetic signatures and by their elevated Moho. Their sedimentary history is distinguished by an abrupt subsidence, followed by isostastic subsidence under sedimentary load. Two circumstances have contributed to the genesis of giant hydrocarbon deposits in these basins: (1) the geochemical environment in those basins was at one time oxygen-deficient as they evolved from the open marine, through restricted marine to become inland alluvial basins; and (2) the sedimentary in-fill provide thick reservoir beds. Oil has accumulated in older structures, folds, faults, unconformities etc. on the margin of the basins, trapping early migrated hydrocarbons.     

渤海海域沙三段烃源灶演化特征研究

烃源灶是表征供烃中心的最合适的方法, 其迁移演化研究对油气勘探具有重要意义. 渤海海域为渤海湾盆地海域部分, 是中国主要富油气盆地之一. 沙河街组三段是古近系4套烃源岩中最重要的烃源岩. 本文在沉积、构造发育和热史研究成果的基础之上, 结合烃源岩地球化学参数模拟计算了沙三段烃源岩成熟生烃及生、排烃演化历史, 并以此研究渤海海域沙三段烃源灶的演化特征. 结果表明研究区具有早期的"双灶共存"和晚期"单灶为主, 多灶并存"的特征, 即早期(古近纪)为岐口和渤中凹陷烃源灶; 晚期(新近纪至今)以渤中凹陷烃源灶为主, 岐口、南堡、黄河口、辽中、辽西和秦南凹陷等烃源灶并存. 烃源灶为油气田的形成提供了物质基础, 渤海海域的大中型油气田分布在主要烃源灶周围的凸起区和斜坡带. 因而, 本文的研究可以为渤海海域油气的深入勘探决策提供基础.     

Sand veins and MISS from the Mesoproterozoic black shale (ca. 1.7 Ga) in North China: Implication for methane degassing from microbial mats

The Mesoproterozoic Chuanlinggou Formation (ca. 1.7 Ga) consists mainly of dark-gray to black shales that are widespread in the North China Platform. Abundant centimeter-scale sand veins are present within the shale layers of this unit, particularly in the middle part. Sand veins display ptygmatic shapes, perpendicular or with a high angle to bedding planes. They penetrate the black shale layers but are often terminated by thin, lenticular sandstone beds, forming small-scale ‘tepee-like’ structures. On bedding planes, sand veins are expressed as small ridges with 1–3 mm positive relief. Lack of polygonal shapes and their occurrence in thinly laminated, relatively deep-water shales preclude an origin from sand-filled desiccation cracks. Instead, their close association with microbially induced sedimentary structures (MISS) such as micro-wrinkles and gas blisters, putative bacterial fossils (possibly coccoidal cyanobacteria) and framboidal pyrites, suggests that they were formed by degassing of methane from microbial mat decay. Methane gas disrupted overlying sedimentary layers, creating fractures open to seawater. Fine-grained quartz sands, which were transported into the depositional environment by strong winds, filled the fractures. Sand-filled fractures were shortened and folded during burial compaction, forming ptygmatic shapes. The presence of dispersed dolomite and siderite in these sand veins suggests authigenic carbonate precipitation from anaerobic oxidation of methane (AOM). Sand veins are intensely distributed within the Chuanlinggou Formation and are spatially widespread in the North China Platform. If their methane origin is confirmed, they may have important implications for the Mesoproterozoic paleoclimate. With anoxic oceans and low seawater sulfate concentration during the Mesoproterozoic, methane release from microbial mat decay and/or microbial methanogenesis during shallow burial may have been proportionally higher than that of the modern marine environments, with resultant increase in the relative importance of methane in maintaining the Mesoproterozoic greenhouse climate.     

Recent developments in study of the typical superimposed basins and petroleum accumulation in China:Exemplified by the Tarim Basin

Most of petroliferous sedimentary basins in China have experienced multiple phases of tectonic evolution and deposition, and are characterized by tectonic and depositional superimposition. The term "superimposed basin" is suggested to describe those basins which consist of two or more simple prototype basins superimposing vertically and/or coalescing laterally. The characteristics of petroliferous superimposed basins are "multiple stages of basin forming and reworking, multiple layers of source rocks, multiple periods of hydrocarbon generation and expulsion, multiple periods of petroleum migration-accumulation-escape". Therefore,applying the wave process analysis method to studying the process of basin formation, hydrocarbon generation, and reservoir formation, and then establishing theory of "petroleum accumulation system" is helpful to enhancing petroleum exploration efficiency in superimposed basins.This paper will, based on case study in the Tarim basin, report the major developments in studying basin formation, hydrocarbon generation and petroleum accumulation. In study of basin formation, (1) geophysical comprehensive profiles reveal that the Tarim plate has been subducted beneath the Tianshan orogenic belt with an interfinger structure and that the deep structure in the eastern section of the Tianshan orogenic belt is different from that in the western section. (2) The vertical variation in debris and geochemical composition reveals the nature and Mesozoic-Cenozoic evolution history of the Kuqa Depression. (3) Field investigation and paleostress reconstruction show that the Kuqa Depression has undergone gravity-driven extension in sedimentary cover when the Tianshan uplifted vertically. In hydrocarbon generation study, new developments include (1) setting environmental index to judge high grade source rocks in marine carbonates, and (2) establishing the lower limit of the organic carbon content for effective carbonate source rocks. In petroleum accumulation study, (1) methods of determining paleopressure and paleotemperature of forming fluid inclusions have been established. (2) The petroleum source analysis has indicated that the crude oil in the Lunnan and Tahe oilfields are derived from the source rocks of the Middle and Upper Ordovician. (3) Three generations of oil inclusions from the Lunnan oilfield have been recognized and dated.     

Formation mechanism and model for sand lens reservoirs in the Jiyang Sub-basin, East China

The Bohai Bay basin comprises some very important and well documented subtle traps known in China, which have been the major exploration focus and have become a major petroleum play since the 1990s. However, recent exploration showed that the oil-bearing properties of some sand lens reservoirs may vary significantly and the accumulation mechanisms for these lithological subtle traps are not well understood. Based on statistical analysis of oil-bearing properties for 123 sand lens reservoirs in the Jiyang Sub-basin and combined with detailed anatomy of typical sand lens reservoirs and NMR experiments, it has been shown that the structural and sedimentary factors, hydrocarbon generation and expulsion conditions of the surrounding source rocks, as well as the petrophysical properties of sand lens reservoirs are the main controlling factors for the formation of sand lens reservoirs. The formation of a sand lens reservoir depends on the interaction between the hydrocarbon accumulation driving force and the resistance force. The driving force is made up of the differential capillary pressure between sandstones and sources rocks and the hydrocarbon diffusion force, and as well as the hydrocarbon expansion force. The resistance force is the friction resistance force for hydrocarbons and water to move through the pore throats of the sand lens. The sedimentary environment, source rock condition and sand reservoir properties can change from unfavorable to favorable depending on the combination of these factors. When these three factors all reach certain thresholds, the sand lens reservoirs may begin to be filled by hydrocarbons. When all of these conditions become favorable for the formation of sand lens reservoirs, the reservoir would have high oil saturation. This approach has been applied to evaluating the potential of petroleum accumulation in the sand lens reservoirs in the third member of the Neogene Shahejie Formation in the Jiyang Sub-basin.     

Geochemistry and possible origin of the hydrocarbons from Wells Zhongshen1 and Zhongshen1C,Tazhong Uplift

In 2013, a great breakthrough of deep petroleum exploration was achieved in the Cambrian pre-salt intervals of Wells Zhongshen1 (ZS1) and Zhongshen1C (ZS1C), Tazhong Uplift. However, the hydrocarbon discovery in the Cambrian pre-salt intervals has triggered extensive controversy regarding the source of marine oils in the Tarim Basin. The geochemistry and origin of the Cambrian pre-salt hydrocarbons in Wells ZS1 and ZS1C were investigated using GC, GC-MS and stable carbon isotope technique. These hydrocarbons can be easily distinguished into two genetic families based on their geochemical and carbon isotopic compositions. The oil and natural gases from the Awatage Formation of Well ZS1 are derived from Middle- Upper Ordovician source rocks. In contrast, the condensate and gases from the Xiaoerbulake Formation of Wells ZS1 and ZS1C probably originate from Cambrian source rocks. The recent discovery of these hydrocarbons with two different sources in Wells ZS1 and ZS1C suggests that both Middle-Upper Ordovician-sourced hydrocarbons and Cambrian-sourced petroleums are accumulated in the Tazhong Uplift, presenting a great exploration potential.     

东营凹陷波动古湖相烃源岩沉积特征

以东营凹陷牛38井为例,研究显示,该井沙河街组沙三段烃源岩的沉积特征具有明显的波动性．宏观上体现为, 构造因素控制湖盆的整体升降和沉积构造旋回,但季节性气候及其它因素的影响使湖盆呈现次级旋回的复合性沉积．微观上表现为纹层的不连续性及生物扰动构造等事件性沉积.古湖面不同幅度的波动和变化导致相对稳定的泥岩沉积的不稳定性,有机质的分布也呈现较显著的非均质性．湖泊的沉积过程影响了微量元素、有机质以及烃源物质的分布,水体较深、盐度较高的沙三段下部多数微量元素含量较高以及B／Ca、Sr／Ba呈现高值;水体较浅、盐度较低的沙三段中部各元素的分布较为稳定,B／Ca、Sr／Ba比值及Sr的含量均显著降低.波动性沉积导致烃源岩呈现明显的优劣性分布,沙三段下部中的有机质富集,为优质烃源岩;沙三段中部的有机质分布较为分散,生排烃的资源潜力有限．     

The geobiological formation process of the marine source rocks in the Middle Permian Chihsia Formation of South China

The Chihsia Formation is one of the four sets of regional marine hydrocarbon source rocks from South China.In the past two decades,detailed geochemical and sedimentological studies have been carried out to investigate its origination,which have demonstrated that the high primary productivity plays a primary role in the deposition of sediments enriched in the organic matter.However,the mechanism of this high productivity and the path of the deposition and burial of the organic matter have always been a mystery.Based on the previous studies on the Shangsi Section in Guangyuan City,Sichuan Province,we proposed that the development of the equatorial upwelling due to the sea level rise is responsible for the relatively high productivity in the Chihsia Formation.The sea waters with high nutrient were transported by the sub-surface currents along the equator.High organic carbon flux was deposited on the deeper shelf,and then decomposed by bacteria,leading to the occurrence of anaerobic respiration.The metabolism of the microorganisms consumed the dissolved oxygen in waters,which was in favor of the preservation of the organic matter.This suggested geobiological model integrating with paleoclimatology,paleoceanography and geomicrobiology will help us to understand the causes of this particular sedimentary sequence.     

Hydrocarbon generation characteristics and exploration prospects of Proterozoic source rocks in China

A large number of primary oil and gas reservoirs have been discovered in Proterozoic strata all over the globe.Proterozoic sequences are widely distributed in China, and the discovery of large Sinian-aged gas reservoirs in the Sichuan Basin and Mesoproterozoic liquid oil seepages in North China shows that attention should be paid to the exploration potential of Proterozoic strata. In this paper, the main controlling factors of Proterozoic source rocks are discussed. Principally, active atmospheric circulation and astronomical cycles may have driven intense upwelling and runoff to provide nutrients; oxygenated oceanic surface waters could have provided suitable environments for the organisms to thrive; volcanic activity and terrestrial weathering caused by continental break-up would have injected large amounts of nutrients into the ocean, leading to persistent blooms of marine organisms; and extensive anoxic deep waters may have created ideal conditions for the preservation of organic matter. Additionally, the appearance of eukaryotes resulted in diversified hydrocarbon parent material, which effectively improved the generation potential for oil and gas. Through the comparison of Formations across different cratons, seven sets of Proterozoic organic-rich source rocks have been recognized in China, which mainly developed during interglacial periods and are also comparable worldwide. The Hongshuizhuang and Xiamaling Formations in North China have already been identified previously as Mesoproterozoic source rocks. The early Proterozoic Changchengian System is highly promising as a potential source rock in the Ordos Basin. In the Upper Yangtze area, the Neoproterozoic Datangpo and Doushantuo Formations are extensively distributed, and represent the major source rocks for Sinian gas reservoirs in the Sichuan Basin. Moreover, the Nanhuan System may contain abundant shales with high organic matter contents in the Tarim Basin, although this possibility still needs to be verified. Indeed, all three cratons may contain source rocks of Proterozoic strata; thus, these strata represent major exploration targets worthy of great attention.     

The scientific connotation of oil and gas formations under deep fluids and organic-inorganic interaction

As a relatively stable craton block in the earth system, the petroliferous basin is influenced by the evolution of the earth system from the early development environment of source rocks, hydrocarbon formation, and reservoir dissolution to hydrocarbon accumulation or destruction. As a link between the internal and external factors of the basin, deep fluids run through the whole process of hydrocarbon formation and accumulation through organic-inorganic interaction. The nutrients carried by deep fluids promote the bloom of hydrocarbon-generating organisms and extra addition of carbon and hydrogen source, which are beneficial to the development of high-quality source rock and enhancement of the hydrocarbon generation potential. The energy carried by the deep fluid promotes the early maturation of the source rock and facilitates the hydrocarbon generation by activation and hydrogenation in high-mature hydrocarbon sources. The dissolution alteration of carbonate rocks and clastic reservoirs by CO_2-rich deep fluids improves the deep reservoir space, thus extending the oil and gas reservoir space into greater depth. The extraction of deeply retained crude oil by deep supercritical CO_2 and the displacement of CH_4 in shale have both improved the hydrocarbon fluidity in deep and tight reservoirs. Simultaneously, the energy and material carried by deep fluids(C, H, and catalytic substances) not only induce inorganic CH_4 formation by Fischer-Tropsch(F-T) synthesis and "hydrothermal petroleum" generation from organic matter by thermal activity but also cause the hydrothermal alteration of crude oil from organic sources. Therefore, from the perspective of the interaction of the earth's sphere, deep fluids not only input a significant amount of exogenous C and H into sedimentary basins but also improve the reservoir space for oil and gas, as well as their enrichment and accumulation efficiencies.     

下扬子中生代构造-热事件及其对海相烃源岩生烃的影响

温度是控制烃源岩有机质生烃的主要因素,构造-热事件下的高温作用对烃源岩的增熟和生烃历程具有显著影响.通过地质分析和磷灰石、锆石裂变径迹、磷灰石U-Th/He低温热年代学数据、火山岩年龄数据分析认为,下扬子中生代存在印支期(T_3-J_2),燕山期(J_3-K_1)构造-热事件,沉积盆地达到最高古热流的时间大约为130~110Ma.古温标镜质体反射率热史反演结果揭示句容地区最高地表古热流达到~94mW·m~(-2),泰兴地区为~78mW·m~(-2),热事件的强度由西至东减弱.基于EASY%Ro模型的生烃史正演结果揭示:寒武系烃源岩在常州地区主生气期为早二叠世晚期至晚三叠世末,在句容、泰兴地区主生气期为晚三叠世-早白垩世.由于T_3-J_(1-2)前陆盆地沉积和早白垩世岩浆活动热事件的双重作用,海相烃源岩有机质在早白垩世末达到最高古地温.虽然K_2-E期间在句容、泰兴和常州部分地区具有一定的沉降幅度,甚至使得部分地区海相烃源岩的埋深超过早期的埋深,但由于K_2以来大地热流降低,海相烃源层地层温度却低于早期的地温,有机质未能普遍进一步增熟生烃,即这些地区不存在大面积的二次生烃.     

“Exploring petroleum inside source kitchen”: Shale oil and gas in Sichuan Basin

The Sichuan Basin is rich in shale oil and gas resources, with favorable geological conditions that the other shale reservoirs in China cannot match. Thus, the basin is an ideal option for fully "exploring petroleum inside source kitchen" with respect to onshore shale oil and gas in China. This paper analyzes the characteristics of shale oil and gas resources in the United States and China, and points out that maturity plays an important role in controlling shale oil and gas composition. US shale oil and gas exhibit high proportions of light hydrocarbon and wet gas, whereas Chinese marine and transitional shale gas is mainly dry gas and continental shale oil is generally heavy. A comprehensive geological study of shale oil and gas in the Sichuan Basin reveals findings with respect to the following three aspects. First, there are multiple sets of organic-rich shale reservoirs of three types in the basin, such as the Cambrian Qiongzhusi Formation and Ordovician Wufeng Formation-Silurian Longmaxi Formation marine shale, Permian Longtan Formation transitional shale, Triassic Xujiahe Formation lake-swamp shale, and Jurassic lacustrine shale. Marine shale gas enrichment is mainly controlled by four elements: Deep-water shelf facies, moderate thermal evolution, calcium-rich and silicon-rich rock association, and closed roof/floor. Second, the "sweet section" is generally characterized by high total organic carbon, high gas content, large porosity, high brittle minerals content, high formation pressure,and the presence of lamellation/bedding and natural microfractures. Moreover, the "sweet area" is generally characterized by very thick organic-rich shale, moderate thermal evolution, good preservation conditions, and shallow burial depth, which are exemplified by the shale oil and gas in the Wufeng-Longmaxi Formation, Longtan Formation, and Daanzhai Member of the Ziliujing Formation. Third, the marine, transitional, and continental shale oil and gas resources in the Sichuan Basin account for 50%, 25%, and 30% of the respective types of shale oil and gas geological resources in China, with great potential to become the cradle of the shale oil and gas industrial revolution in China. Following the "Conventional Daqing-Oil"(i.e., the Daqing oilfield in the Songliao Basin) and the "Western Daqing-Oil Gas"(i.e., the Changqing oilfield in the Ordos Basin), the Southwest oil and gas field in the Sichuan Basin is expected to be built into a "Sichuan-Chongqing Daqing-Gas" in China.     

河南灵宝盆地中新生代陆相地层的新划分与对比研究

灵宝盆地位于华北板块南缘与秦岭造山带之间,是豫西北一系列北东-南西向断陷盆地之一.盆地内沉积地层主体为一套厚约2000m陆相碎屑岩夹泥灰岩、薄煤层沉积.本文根据盆地内的恐龙蛋、介形虫、腹足类、哺乳动物化石及少量孢粉等,将地层自下而上划分为下白垩统枣窳组、上白垩统南朝组、古近系古新统-下始新统项城群、中始新统川口组、上始新统庄里坡组及新近系上-中新世（组名暂未定）等6个地层单元.研究表明：下、上白垩统之间及其与项城群之间为不整合或超覆,上中新统与川口组或庄里坡组为不整合接触,整个地层是一套河流相沉积、局部洪泛洼地或小浅湖相沉积.     

Natural gas origins of large and medium-scale gas fields in China sedimentary basins

China sedimentary basins present abundant natural gas resource thanks to its unique geological settings.Marine highly-matured hydrocarbon source rocks,widespread coal-measure strata and low temperature Quaternary saline strata,etc.,indicate the wide foreground of China natural gas resources. Up to now,most of the petroliferous basins have been discovered to have wholesale natural gas accumulation from Precambrian,Paleozoic,Mesozoic to Cenozoic in the east,the central,the west and the coast of China.These large and medium-scale gas reservoirs are mainly composed of hydrocarbon gas with big dry coefficient,tiny non-hydrocarbon,wide carbon isotope distribution and varying origin types,the hydrocarbon gas includes coal-formed gas,oil-formed gas,biogenic gas and inorganic gas, etc.Coal-formed gas is the main type of China natural gas resources,in particular several explored large-scale gas fields(>100 billion cubic meter)of Kela 2,Sulige and Daniudi,etc.,they all belong to coal-formed gas fields or the gas fields consisting mostly of coal-formed gas.Oil-formed gas is also abundant in China marine basins,for example marine natural gas of Sichuan Basin generated from crude oil cracking gas.Primary and secondary biogenic gas fields were discovered respectively in the Qaidam Basin and Western Slope of Songliao Basin.In addition,inorganic gases are mainly distributed in the eastern China,in particular the Songliao Basin with abundant carbon dioxide accumulation,indicating that the eastern China present large exploration potential of inorganic gas.     

贺兰山中段震旦系盖帽白云岩的沉积特征、区域对比及意义

2011年,作者在贺兰山中段腰坝配件厂地区进行1∶5万区域地质调查时发现了震旦系盖帽白云岩,进而研究了其沉积特征、碳氧同位素组成和时代,并与该区紫花沟兔儿坑组白云岩、华南和华北地台南缘相应地层进行对比.结果表明,盖帽白云岩为含陆源细碎屑白云岩,与兔儿坑组白云岩相近;其δ13C值为-4.7‰～0.79‰,且自下而上呈下降趋势;其时代与华南震旦系下、中部相当,与华北地台南缘震旦系罗圈组的沉积特征和古地理环境相近;与全球Gaskiers冰期有相关性.这对华北地台的古构造、古地理、古气候及动物演化“源头”的研究有重要意义.     

中国近海前新生代残留盆地初探

中国近海沉积盆地按形成时代可以划分为新生代盆地和前新生代盆地。新生代陆相碎屑岩断陷盆地有良好的油气前景,而古生代还有广泛海相碳酸盐岩分布地区,只要它们经受中生代挤压,改造后还能保留下来,就具有巨大的油气潜力。初步分析中国近海的油气勘探资料及大地构造演化史表明,陆内断坳盆地下伏以古生代碳酸盐岩为主的残留盆地。而陆缘盆地并不是寻找古生代残留盆地的场所。但在台西南盆地,珠江口盆地潮汕坳陷发育海相中生代盆     

Paleoenvironments of the evolving Pliocene to early Pleistocene foreland basin in northwestern Taiwan: An example from the Dahan River section

The overriding of the Luzon volcanic arc atop the underlying Chinese rifted‐continental margin has caused the formation of the Taiwan mountain belts and a peripheral foreland basin west of the orogen since the late Miocene. In this study, lithofacies analysis and calcareous nannofossil biostratigraphic investigations of the Dahan River section in northwestern (NW) Taiwan were performed. Our results offer insights into the temporal evolution of the sedimentary environments and the competing effects of the sedimentation and basin tectonics of the NW Taiwan foreland basin from the Pliocene to early Pleistocene. Nannofossil biostratigraphic studies showed that the upper Kueichulin Formation and the overlying Chinshui Shale can be assigned to the NN15 biozone of the Pliocene age, and the Cholan Formation pertains to NN16–NN18 of the early Pleistocene. The NN15–NN16 boundary coincides roughly with the boundary of the Chinshui Shale and Cholan Formation. We recognized three major sedimentary environments in the studied foreland succession comprising the upper Kueichulin Formation, Chinshui Shale, Cholan Formation and Yangmei Formation, in ascending order. During the deposition of the upper Kueichulin Formation in the early Pliocene, the dominant environment was a wave‐ and tide‐influenced open marine setting. During the late Pliocene, the environment deepened to an outer‐offshore setting when the sediments of Chinshui Shale were accumulated. In the Pleistocene, the environment then shallowed to wave‐dominated estuaries during the deposition of the lower Cholan Formation, and the basin was rapidly filled, generating a meandering and sandy braided river environment during the deposition of the upper Cholan to the Yangmei Formation. In summary, the evolution of sedimentary environments in the studied succession shows a deepening then a shallowing and coarsening upward trend during the period from the Pliocene to the Pleistocene, spanning the age from approximately 4 to 1 Ma.     

Classification and origin of natural gases from Lishui Sag,the East China Sea Basin

Natural gases discovered up to now in Lishui Sag, the East China Sea Basin, differ greatly in gaseous compositions, of which hydrocarbon gases amount to 2%–94% while non-hydrocarbon gases are dominated by CO₂. Their hydrocarbon gases, without exception, contain less than 90% of methane and over 10% of C₂ ⁺ heavier hydrocarbons, indicating a wet gas. Carbon isotopic analyses on these hydrocarbon gases showed that δ ¹³C₁, δ ¹³C₂ and δ ¹³C₃ are basically lighter than −44‰, −29‰ and −26‰, respectively. The difference in carbon isotopic values between methane and ethane is great, suggesting a biogenic oil-type gas produced by the mixed organic matter at peak generation. δ ¹³ \( C_{CO_2 } \) values of nonhydrocarbon gases are all heavier than −10‰, indicating a typical abiogenic gas. The simulation experiment on hydrocarbon generation of organic matter in a closed gold-tube system showed that the proportion of methane in natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit is obviously higher than that in natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit, consequently the proportion of heavier hydrocarbons of the former is remarkably lower than that of the latter. Moreover, δ ¹³C₁ values of natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit are about 5‰ heavier than those of natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit while δ ¹³C₂ and δ ¹³C₃ values of the former are over 9‰ heavier than those of the latter. Currently the LS36-1 oil-gas pool is the only commercial oil-gas reservoir in Lishui Sag, where carbon isotopic compositions of various hydrocarbon components differ greatly from those of natural gases produced by the Lingfeng Formation organic matter but are very similar to those of natural gases derived from the Yueguifeng Formation organic matter, therefore, natural gases in the LS36-1 oil-gas pool are mainly derived from the Yueguifeng Formation lacustrine source rock rather than the Lingfeng Formation marine or Mingyuefeng Formation coal-measures source rocks.

     

Classification and origin of natural gases from Lishui Sag,the East China Sea Basin

Natural gases discovered up to now in Lishui Sag, the East China Sea Basin, differ greatly in gaseous compositions, of which hydrocarbon gases amount to 2%–94% while non-hydrocarbon gases are dominated by CO₂. Their hydrocarbon gases, without exception, contain less than 90% of methane and over 10% of C₂ ⁺ heavier hydrocarbons, indicating a wet gas. Carbon isotopic analyses on these hydrocarbon gases showed that δ ¹³C₁, δ ¹³C₂ and δ ¹³C₃ are basically lighter than ?44‰, ?29‰ and ?26‰, respectively. The difference in carbon isotopic values between methane and ethane is great, suggesting a biogenic oil-type gas produced by the mixed organic matter at peak generation. δ ¹³ \(C_{CO_2 } \) values of nonhydrocarbon gases are all heavier than ?10‰, indicating a typical abiogenic gas. The simulation experiment on hydrocarbon generation of organic matter in a closed gold-tube system showed that the proportion of methane in natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit is obviously higher than that in natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit, consequently the proportion of heavier hydrocarbons of the former is remarkably lower than that of the latter. Moreover, δ ¹³C₁ values of natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit are about 5‰ heavier than those of natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit while δ ¹³C₂ and δ ¹³C₃ values of the former are over 9‰ heavier than those of the latter. Currently the LS36-1 oil-gas pool is the only commercial oil-gas reservoir in Lishui Sag, where carbon isotopic compositions of various hydrocarbon components differ greatly from those of natural gases produced by the Lingfeng Formation organic matter but are very similar to those of natural gases derived from the Yueguifeng Formation organic matter, therefore, natural gases in the LS36-1 oil-gas pool are mainly derived from the Yueguifeng Formation lacustrine source rock rather than the Lingfeng Formation marine or Mingyuefeng Formation coal-measures source rocks.