River-gulf system--the major location of marine source rock formation

Petroleum was generated from sedimentary rocks. The world＇s oldest oil source rock so far was found in Proterozoic rocks. Since then, 73% to 81% of the earth＇s surface has been covered with sedimentary rocks. However, only quite a limited area is rich in oil and gas. It is found that source rocks have controlled oil and gas distribution, and they are mainly formed in two systems： （1） river-lake systems and （2） river-gulf systems. Phytoplankton is an important source of kerogen, the blooming of which is strongly dependent on nutrients. Rivers are the major nutrient provider for basins. Rivers around lakes and an undercompensation （where the sedimentation rate is less than the rate of basin subsidence） environment provide favorable conditions for phytoplankton blooming in lakes. Gulfs are usually located at the estuary of big rivers, characterized by restricted current circulation and exchange with the open sea, which benefit maintaining the nutrient density, phytoplankton levels and organic matter preservation. The river-gulf system is the most favorable place for marine source rock development. Most of the world famous marine petroleum-rich provinces are developed from river-gulf systems in geological history, such as the Persian Gulf Basin, Siberian Basin, Caspian Basin, North Sea, Sirte Basin, Nigerian Basin, Kwanza Basin, Gulf of Mexico, Maracaibo Basin and the Eastern Venezuelan Basin.

Identification of bacterial fossils in marine source rocks in South China

Based on the results of conventional geochemistry analysis including thin sections and SEM observations, different shapes of bacterial fossils, with size ranging from dozens of nanometers to several microns, were discovered in the low-mature marine source rocks and coal seams in South China, of which the Permian source rocks were dominated by the bacterial fossils derived from symbiotic sulphur bacteria with gypsum, and the Chengkou section in the Cambrian strata were occupied by abundant nanoscale bacterial fossils with rod and bar shapes. In contrast, a large quantity of possible bacterial fossils found in the high-mature Permian, Silurian, and Cambrian source rocks using SEM need to be further explored. Despite this,this study has indicated that bacterial fossils were prevalent in the source rocks, such as mudstone, siliceous rock and gypsum-bearing coal seams in South China, which has been ignored before. It also suggests that the bacterial fossils may play an important role in the formation and accumulation of shale gas in the geological history.

Formation Mechanism of High Quality Marine Source Rocks-Coupled Control Mechanism of Geological Environment and Organism Evolution

High quality marine source rock （HQMSR） is the key prerequisite for medium to large hydrocarbon accumulations. However, the forming mechanism remains unclear. On the basis of the in-vestigation for the geodynamic setting of the Middle-Upper Yangtze during the Early Cambrian in dif-ferent spatial scales and the analysis of trace elements, the main controlling factors of the development of high quality marine source rock are discussed, with specific consideration of the burial rate of the organic matter. The formation of high quality marine source rocks is suggested to be the result of a co-ordinated development and the interaction between geological environments and organism evolution during the major geological transition periods. We perceived that the burial rate of organic matter was influenced by the primary productivity and its burial conditions. The abundance of autogenetic molyb-denum gained directly by the chemical speciation analysis of rocks could be used as a proxy for the burial organic matter. The burial rate of autogenetic molybdenum and the sedimentary organics in modern marine environments could be used to estimate the sedimentary organics in ancient environ-ments effectively.

New perspective of Miocene marine hydrocarbon source rocks in deep-water area in Qiongdongnan Basin of northern South China Sea

Drilling wells reveal that the organic matter abundance of Miocene marine source rocks in shallow water area of the Qiongdongnan Basin is relatively low with poor hydrocarbon generation poten- tial. However, in some drilling wells of deep water area close to the central depression belt, Miocene marine source rocks with better organic matter abundance and hydrocarbon generation have been found, which have achieved better source rock standard based on the analysis of geochemical charac- teristics. Although there are no exploratory wells in deep water area of the research region, through the comparative analysis of geochemical data of several typical exploratory wells respectively from shallow water area in the basin, central depression belt margin in deep-water area of the basin and Site 1148 of deep sea drilling in the South China Sea Basin, it reveals that the tendency of the quality of source rocks becomes positive gradually from delta to bathyal environment, which then becomes negative as in deep oceanic environment. Owing to the lack of terrestrial organic matter input, the important controlling factors of Miocene marine source rocks in the Qiongdongnan Basin are ocean productivity and preservation conditions of organic matter. The element geochemistry data indicate that the tendency of the paleoproductivity and the preservation conditions of organic matter become positive as water depth increase from shallow area to bathyal area close to central depression belt. So it is speculated that there must exist high quality source rocks in the central depression area where the preservation conditions of organic matter are much better. Besides, in theory, in oxygen-poor zone of oceanic environment at the water depth 400–1 000 m, the preservation conditions of organic matter are well thus forming high-quality marine source rocks. The result- ing speculation, it is reasonable to consider that there are high hydrocarbon generation potential source rocks in bathyal environment of the Qiongdongnan Basin, especially at the water depth 400– 1 000 m.

Characteristics of the Upper Jurassic Marine Source Rocks and Prediction of Favorable Source Rock Kitchens in the Qiangtang Basin, Tibet

This study evaluated the hydrocarbon-bearing potential of Upper Jurassic marine source rocks in the Qiangtang （羌塘） Basin through a comprehensive organic geochemical analysis of the samples from a large number of outcrops in different structural units to predict the location of favorable hydro- carbon kitchens, based on the evaluation standards of Mesozoic marine source rocks in the Qiangtang Ba- sin. Rocks＇ depositional environment, thickness and organic geochemistry feature were analyzed in this study. The principal controlling factors of the occurrences of favorable source rocks were analyzed. Upper Jurassic Suowa （索瓦） Formation source rocks are mainly platform limestone in the Dongcuo （洞错）-Hulu （葫芦） Lake deep sag and Tupocuo （吐坡错）-Baitan （白滩） Lake deep sag. Lithologically, the Suowa Fro- mation is made up of a suite of marls in intra-platform sags, micrites and black shales, which were all de- posited in the closed, deep and static water depositional environment. Marl could form hydrocarbon-rich source rocks and its organic matter type is mainly II type in mature to highly-mature stage, the limestone forms a medium-level source rock. In addition, the favorable source kitchen of limestone is larger than that of mudstone. This study provides an important reference for the evaluation of Jurassic marine source rocks and for prediction of petroleum resources in the Qiangtang Basin.

Sedimentary Environments of Cambrian-Ordovician Source Rocks and Ultra-deep Petroleum Accumulation in the Tarim Basin

The Tarim Basin is the only petroliferous basin enriched with marine oil and gas in China.It is presently also the deepest basin for petroleum exploration and development in the world.There are two main sets of marine Source Rocks(SRs)in the Tarim Basin,namely the high over-mature Cambrian-Lower Ordovician(∈-O_(1))and the moderately mature Middle-Upper Ordovician(O_(2-3)).The characteristic biomarkers of SRs and oils indicate that the main origin of the marine petroleum is a mixed source of∈-O_(1) and O_(2-3) SRs.With increasing burial,the hydrocarbon contribution of the∈-O_(1) SRs gradually increases.Accompanied by the superposition of multi-stage hydrocarbon-generation of the SRs and various secondary alteration processes,the emergence and abnormal enrichment of terpenoids,thiophene and trimethylaryl isoprenoid in deep reservoirs indicate a complex genesis of various deep oils and gases.Through the analysis of the biofacies and sedimentary environments of the∈-O_(1) and O_(2-3) SRs,it is shown that the lower Paleozoic high-quality SRs in the Tarim Basin were mainly deposited in a passive continental margin and the gentle slope of the platform,deep-water shelf and slope facies,which has exhibited a good response to the local tectonic-sedimentary environment.The slope of the paleo-uplift is the mutual area for the development of carbonate reservoirs and the deposition of marine SRs,which would be favorable for the accumulation of petroleum.Due to the characteristics of low ground temperature,the latest rapid and deep burial does not cause massive oil-cracking in the paleo-uplift and slope area.Therefore,it is speculated that the marine reservoirs in the slope of the Tabei Uplift are likely to be a favorable area for deep petroleum exploration,while the oilcracking gas would be a potential reserve around the west margin of the Manjiaer Depression.Hydrocarbons were generated from various unit SRs,mainly migrating along the lateral unconformities or reservoirs and the vertical faults.They eventually brought up three major types of exploration fields:middle and lower Cambrian salt-related assemblages,dolomite inner reservoirs and Middle and Lower Ordovician oil-bearing karst,which would become the most favorable target of marine ultra-deep exploration in the Tarim Basin.

Presence of carboxylate salts in marine carbonate strata of the Ordos Basin and their impact on hydrocarbon generation evaluation of low TOC, high maturity source rocks

The total organic carbon （TOC） in the marine source rock of the Ordos Basin mostly ranges from 0.2% to 0.5%. The industrial standard commonly states that the TOC value has to be no less than 0.5% （0.4% for high mature or over-mature source rock） to form large petroleum reservoirs. However, gas source correlation indicates that the natural gas in the Jingbian gas field does receive contribution from marine source rocks. In order to determine the effect of Carboxylate salts （or called as organic acid salts） on TOC in highly mature source rocks with low TOC value, we sampled the Ordovician marine source rock and the Permian transitional facies source rock in one drilled well in the southern Ordos Basin and performed infrared and GC-MS analysis. It is found that both kerogen-derived organic acids and carboxylate salt-conversed organic acids exist in both marine and transitional facies source rocks. The carboxylate salt-conversed organic acids mainly come from the complete acidification of carboxylate salts, which confirms the presence of carboxylate salts in the marine source rocks. Although the C16：o peak is the main peak for the organic acids both before and after acidification, the carboxylate salt-conversed organic acids have much less relative abundance ahead of C^6：o compared with that of the kerogen-based and free organic acids. This observation suggests that the kerogen-based and free organic acids mainly decarboxylate to form lower carboxylic acids, whereas the carboxylate salt-conversed organic acids mainly break down into paraffins. By using calcium hexadecanoate as the reference to quantify the kerogen-derived and carboxylate salt-conversed organic acids, the high TOC （〉2.0%） marine source rocks have low carboxylate salt content and the low TOC （0.2%-0.5%） marine source rocks contain high content of carboxylate salt. Therefore, for the marine source rocks with 0.2%-0.5% TOC, the carboxylate salts may be a potential gas source at high maturity stage.

Geochemical characteristics and origins of natural gases in the eastern Cote d’Ivoire Basin,West Africa

The gas sources in the eastern Cote d’Ivoire Basin(Tano Basin)are seldom reported and remain controversial due to multiple sets of potential source rocks and poorly documented geochemical characteristics of natural gases.The marine source rock potential from the Upper Albian to Turonian as well as the molecular composition and the stable carbon isotope composition of natural gases in the eastern Cote d’Ivoire Basin were studied in detail to investigate the origins of natural gases.The total organic carbon(TOC),hydrogen index(HI),and generation potential(S_(1)+S_(2))of source rocks indicate that both sapropelic source rocks and humic source rocks developed during the late Albian,whereas sapropelic source rocks developed during the Cenomanian and the Turonian.The normal order ofδ^(13)CH_(4)<δ^(13)C_(2)H6<δ^(13)C_(3)H_(8)(δ^(13)C_(1)<δ^(13)C_(2)<δ^(13)C_(3)),the relationship between C_(2)/C_(3)molar ratio andδ^(13)C_(2)-δ^(13)C_(3),and the plot ofδ^(13)C_(1)versus C_(1)/(C_(2)+C_(3))collectively show that the natural gases are thermogenic due to the primary cracking of kerogen,including the typical oil-associated gases from Well D-1,the mixed oil-associated gases and coal-derived gases from Well G-1 and Well L-1.Based on the plot ofδ^(13)C_(1)versusδ^(13)C_(2)and the established relationship betweenδ^(13)C_(1)and equivalent vitrinite reflectance(Ro),we proposed that the natural gases are in a mature stage(Ro generally varies from 1.0%to 1.3%).Combined with results of basin modelling and oil-to-source correlation,the transitional to marine source rocks during the late Albian were thought to have made a great contribution to the natural gases.Our study will make a better understanding on petroleum system in the eastern Cote d’Ivoire Basin.

Genetic relationship between formation,accumulation and migration and dispersion of peat materials in Paleogene——Take the Qiongdongnan Basin as an example

Coal-type source rocks include both coal and terrigenous marine source rocks.By studying the distribution of secondary depressions,uplifts,as well as the characteristics of peat formation and accumulation in the northern marginal sea basin of the South China Sea,and combining them with coal formation characteristics observed in other basins,five genetic theories on the relationship between peat accumulation and dispersed organic matter accumulation are proposed.The northern marginal sea basin of the South China Sea is characterized by“disadvantageous coals formation and favorable terrigenous marine source rocks formation.”This paper provides a distribution map of coal seams and terrigenous marine source rocks in the Qiongdongnan Basin and determines their distribution patterns.Research shows that the migration of sedimentary facies in the basins and inner depressions led to the formation and migration of the peat accumulation centers.In addition,the vertical migration of the peat accumulation centers led to planar migration,which is actually a type of coupling relationship.Previous research results have revealed that the formation of coal-type source rock is multi-phased.The marginal sea basin is composed of several fault-depression basins,with each basin developing a second order of depression and uplift.There is no unified basin center or depositional center to be found.As a result,the concentration centers of coal-forming materials also vary greatly.Based on the distribution characteristics of coal-type source rocks in different basins within the marginal sea basins of the South China Sea,the research results have practical significance and provide guidance for exploring coal-type oil and gas reservoirs in this area.

Main Controlling Factors of Organic Matter Richness in a Permian Section of Guangyuan, Northeast Sichuan

A complete Permian section in Guangyuan （广元）, Northeast Sichuan （四川）, has been investigated, to explore the variation in organic matter richness and its main controlling factors. The research results of the detailed lithological description and organic/inorganic geochemical analysis of about 325 samples indicate that a high content of total organic carbon （TOC） occurs in calcareous mudstones and laminated marls or thin-bedded limestones. In carbonate rocks, the TOC content is negatively related to the thickness of the massive beds; the thinner the bed is, the higher the TOC content is. Marine organic matter is enriched in the outer shelf and relatively shallow basin floor environments with the maximum TOC contents of 5.07% and 14.6%, respectively. The main factors that affect the quantity and quality of marine organic matter include primary productivity, depositional processes, and redox conditions during deposition and the early diagenesis stage. Three intervals of marine good quality source rocks are identified in this section; they are lower Chihsia Formation, topmost Maokou （茅口） Formation, and the middle segment of the Dalong （大隆） Formation. They are formed in anoxic environments, in association with high primary productivity. Among these, high productivity in the lower Chihsia interval may originate from an upwelling flow area, whereas, in the rest of the intervals, it is inferred to be related to hot fluid activity due to volcanic eruption. The results of this study suggest that good quality marine source rocks in the Permian strata offer significant hydrocarbon potential.

Accumulation conditions and exploration directions of Ordovician lower assemblage natural gas, Ordos Basin, NW China

Based on drilling cores, well logging and seismic data, source rocks and reservoirs are evaluated;and the natural gas genesis is identified through the analysis of natural gas isotopes, components and fluid inclusions, to study the gas accumulation conditions of the gypsum salt rock related strata of the Ordovician lower assemblage in Ordos Basin.(1) The natural gas from Ordovician lower assemblage is high thermal evolution dry gas from marine source rock, characterized by relatively light δ^(13)C value of methane and heavy δ^(13)C value of ethane. The natural gas is identified as gas cracking from crude oil according to component analysis. Thermochemical sulfate reduction(TSR) reaction has happened between the hydrocarbon fluid and sulfate as sulfur crystals are found in the cores, hydrogen sulfide is found in the natural gas, and hydrocarbon and hydrogen sulfide fluid inclusions are widespread in secondary minerals.(2) Around the gypsum-salt lows, argillaceous rocks are extensive in the Ordovician lower assemblage, reaching a cumulative thickness of 20–80 m. The effective source rocks include argillaceous rock rich in organic laminae, algal clump and algal dolomite. Analysis shows that the source rocks have a dominant TOC of 0.1%–0.5%, 0.31% on average and 3.24% at maximum. The source rocks have an average TOC of 0.58% after recovered through organic acid salt method, indicating the source rocks have high hydrocarbon supply potential.(3) In the sedimentary period, the palaeo-uplift controlled the distribution of reservoirs. The inherited secondary palaeo-uplift in Wushenqi–Jingbian east of the central palaeo-uplift and the low uplift formed by thick salt rocks near Shenmu–Zizhou area controlled the distribution of penecontemporaneous grain shoal dolomite reservoirs. The salinization sedimentary environment of gypsum salt rock can promote the development of reservoir. There are three types of dolomite reservoirs, the one with intercrystalline pore, with dissolution pore, and with fracture;intercrystalline and dissolution pores are main reservoir spaces.(4) There are two types of cap rocks, namely tight carbonate rock and gypsum-salt rock, constituting two types of source-reservoir-cap assemblages respectively. The general accumulation model is characterized by marine source rock supplying hydrocarbon, beach facies limy dolomite reservoir, small fractures acting as migration pathways, and structural-lithologic traps as accumulation zones.(5) The third and fourth members of Majiagou Formation are major target layers in the lower assemblage. The Wushengqi–Jingbian secondary paleo-uplift area and Shenmu–Zizhou low uplift are dolomite and limestone transition zone, there develops tight limestone to the east of the uplift zone, which is conducive to the formation of gas reservoir sealed by lithology in the updip. Two risk exploration wells drilled recently have encouraging results, indicating that the two uplift zones are important prospects.

Potential petroleum sources and exploration directions around the Manjar Sag in the Tarim Basin

Since the discovery of the Tahe oilfield, it has been controversial on whether the main source rock is in the Cambrian or Middle-Upper Ordovician strata. In this paper, it is assumed that the crude oil from the Wells YM 2 and TD 2 was derived from the Middle-Upper Ordovician and Cambrian source rocks, respectively. We analyzed the biomarkers of the crude oil, asphalt-adsorbed hydrocarbon and saturated hydrocarbon in bitumen inclusions from the Lunnan and Hade areas in the North Uplift of the Tarim Basin. Results show that the ratios of tricyclic terpane C2~/C23 in the crude oil, asphalt-adsorbed hydrocarbon and saturated hydrocarbon in bitumen inclusions are less than 1.0, indicating that they might be from Upper Ordovician source rocks; the ratios of C28/（C27＋C28＋C29） steranes in the saturated hydrocarbon from reservoir bitumen and bitumen inclusions are higher than 25, suggesting that they might come from the Cambrian source rocks, however, the ratios of C28/（C27＋C28＋C29） steranes in oil from the North Uplift are less than 25, suggesting that they might be sourced from the Upper Ordovician source rocks. These findings demonstrate that the sources of crude oil in the Tarim Basin are complicated. The chemical composition and carbon isotopes of Ordovician reservoired oil in the Tarim Basin indicated that the crude oil in the North Uplift （including the Tahe oilfield） and Tazhong Depression was within mixture areas of crude oil from the Wells YM 2 and TD 2 as the end members of the Cambrian and Middle-Upper Ordovician sourced oils, respectively. This observation suggests that the crude oil in the Ordovician strata is a mixture of oils from the Cambrian and Ordovician source rocks, with increasing contribution from the Cambrian source rocks from the southern slope of the North Uplift to northern slope of the Central Uplift of the Tarim Basin. Considering the lithology and sedimentary facies data, the spatial distribution of the Cambrian, Middle-Lower Ordovician and Upper Ordovician source rocks was reconstructed on the basis of seismic reflection characteristics, and high-quality source rocks were revealed to be mainly located in the slope belt of the basin and were longitudinally developed over the maximum flooding surface during the progressive-regressive cycle. Affected by the transformation of the tectonic framework in the basin, the overlays of source rocks in different regions are different and the distribution of oil and gas was determined by the initial basin sedimentary structure and later reformation process. The northern slope of the Central Uplift-Shuntuo-Gucheng areas would be a recent important target for oil and gas exploration, since they have been near the slope area for a long time.