Thermal Effects on Composition of Rearranged Hopanes in Hydrocarbon Source Rocks

Objective The distribution characteristics and formation mechanism of rearranged hopanes in hydrocarbon source rocks are affected by various geological conditions.Among these geological conditions,thermal action has an important influence on the formation of rearranged hopanes,which has been however little documented previously.

Characteristics of Rearranged Hopanes of Hydrocarbon Source Rocks in Saline Sedimentary Environment： A Case Study of the Songliao Basin

The Songliao Basin, one of the biggest continental petroliferous basins in eastern China, is a Mesozoic- Cenozoic fault-depressed and fault-subsided basin developed on the Hercynian fold basement. Generally, rearranged hopanes are considered to be formed by clay- mediated acidic catalysis under oxic or suboxic environment, whereas high abundance of rearranged hopanes were found in hydrocarbon source rocks and crude oils that are derived from salty environment in the Songliao Basin. This phenomenon rarely happens all over the world.

Characteristics of hydrocarbon source rocks of No.3 buried-hill region in Nanpu Sag

Based on the data of 44 samples of hydrocarbon source rocks in Nanpu No.3 buffed-hill region, the kerogen type is judged through the pyrolysis and microscopic identification. At the same time, organic matter maturity and hydrocarbon generation threshold are studied by using vitrinite reflectance, pyrolysis yield and hydrocarbon abundance. Meanwhile the hydrocarbon expulsion threshold is calculated. And the characteristics of organic hydrocarbon generation and expulsion are preliminarily revealed and evaluated. The result shows that the No.3 buffed-hill region has abundant hydrocarbon source rocks with high content of organic carbon. And the primary types of kerogen are II, and lI 2. The hydrocarbon source rocks which passed biochemistry, thermolysis and thermal cracking have developed into the mature-postmature phase of different extents. And plenty of oil and gas were expelled out. It is believed the depth of oil-generating window is 3 600 m and the depth of hydro- carbon-expulsion threshold is 4 100 m. The comprehensive analysis indicates that Nanpu No.3 burried-hill region has a certain condition to generate hydrocarbon which is very promising in oil exploration and thus can become an important exploration and development target next.

Stable carbon isotopes of gaseous alkanes as genetic indicators inferred from laboratory pyrolysis experiments of various marine hydrocarbon source materials from southern China

Using high pressure and geological condition simulation vessels, we conducted hydrous pyrolysis experiments of kerogen, solid bitumen and liquid hydrocarbons in southern China in order to study the processes of gas generation and derive geo- chemical indicators of gas genesis under approximate pressure and temperature. The results indicate that gas generation productivity of different marine material decreased in the ganic matter （solid bitumen and heavy oil）, and kerogen. order of crude oil （light oil and condensate）, dispersed soluble or- Under identical temperature-pressure regimes, pyrolysates derived from kerogen and dispersed soluble organic matter display drastically different geochemical characteristics. For example, the δ13Cc02-δ13C1 values of gaseous products from dispersed soluble organic matter are greater than 20%o, whereas those from kerogen are less than 20%~. The 813C1 values of pyrolysates from different marine hydrocarbon sources generally increase with pyrolysis temperature, but are always lower than those of the source precursors. The δ13C values of ethane and propane in the pyrolysates also increase with increasing pyrolysis temperature, eventually approaching that of their sources, at peak hydro- carbon generation. At high-over mature stages, the δ13C values of ethane and propane are often greater than those of their sources but close to those of coal gases, and thus become ineffective as gas genetic indicators. Ln（CffC3） can clearly distin- guish kerogen degradation gas from oil cracking gas and Ln（CJC2）-（δ13C1-δ13C2） can be an effective indicator for distinguishing oil cracking gas from dispersed soluble organic matter cracking gas.

Combination and superimposition of source kitchens and their effects on hydrocarbon accumulation in the hinterland of the Junggar Basin,west China

In the hinterland of the Junggar Basin, there are multiple depressions with multiple sets of source rocks. Therefore, the conditions of hydrocarbon sources are complex, and the geochemical characteristics and sources of hydrocarbon vary in different structural belts. The evolution of the Che- Mo palaeohigh affected the formation of hydrocarbon source kitchens and hydrocarbon migration. We studied the combination and superimposition of hydrocarbon source kitchens, using as an example the hinterland of the Junggar Basin （including the Yongjin, Zhengshacun, Moxizhuang and Luliang uplift areas）. The study was based on geochemical analyses of crude oil and fluid inclusions, and the histories of tectonic evolution and hydrocarbon generation. The results indicated that before the Paleogene there were two hydrocarbon-generating depressions： the Western Well Penl depression and the Changji depression on the south and north sides of the Che-Mo palaeohigh, respectively. The Permian source kitchen had been generating hydrocarbon continuously since Triassic and reached high maturity stage in the Cretaceous period. After Paleogene, the adjustment of the Che-Mo palaeohigh led to the subsidence of the Changji depression and the Jurassic source rocks reached mature stage and became the main source kitchens. However, the Jurassic source rocks in the Western Well Penl depression were still in a low maturity stage and did not generate oil because of the adjustment of tectonic movements. As a result, in the central and southern parts of the Junggar Basin, Jurassic source rocks generated oil, but in the Luliang uplift, the crude oil was from the Permian source rocks in the Western Well Penl depression and the Jurassic source rocks did not contribute. The crude oil in the central Zhengshacun-Moxizhuang belt was from the Permian source rocks in two depressions, and partially from the Jurassic source rocks. The crude oil in the Luliang uplift was from the source rocks of the lower Permian Fengcheng Formation and middle Permian Wuerhe Formation, which is characterized by superimposition of two sets of source kitchens and three accumulation stages. The crude oil in the Yongjin tectonic belt was from the lower Permian, middle Permian and Jurassic source rocks, which is characterized by superimposition of three sets of source kitchens and two accumulation stages. The crude oil in the Zhengshacun tectonic belt was from a combination of source kitchens of lower Permian and middle Permian in the Western Well Penl depression in the early stage and from the superimposition of Jurassic source rocks in the Changji depression in the late stage.

Basin-filling processes and hydrocarbon source rock prediction of low-exploration degree areas in rift lacustrine basins:a case from the Wenchang Formation in low-exploration degree areas,northern Zhu I Depression,Pearl River Mouth Basin,E China

Hydrocarbon source rocks, as a main geologic factor of petroliferous systems in a sedimentary basin, play a key role in the accumulation of oil and gas and the formation of hydrocarbon accumulations. This study, which focuses on difficulties in prediction of hydrocarbon source rocks in basins or sags with low exploration degree and insufficient hydrocarbon source rock indicators, taking the Wenchang Formation of northern Zhu I Depression, Pearl River Mouth Basin as an example, proposed a hypothesis of “finding lakes and hydrocarbon source rocks”. Detailed steps include, first, determination of the lacustrine basin boundary according to analysis of seismic foreset facies, determination of the depositional area based on the compilation of strata residual thickness maps, determination of the lacustrine basin shape according to deciphering slope break belt system, determination of the fluctuation of paleo-water depth according to biogeochemical indicators of mature exploration areas, determination of the lacustrine basin scale based on analyses of tectonics intensity and accommodation space, which prove the existence of the lacustrine basin and identify the range of semi deep-deep lake;second, further analyses of tectonopalaeogeomorphology, paleo-provenance,palaeoclimate and paleo-water depth to reconstruct the geologic background of the original basin and semideep-deep lacustrine facies, to determine the distribution of semi-deep/deep lacustrine sediments in combination with studies of logging facies, core facies, seismic facies and sedimentary facies, and to rank the sags’ potential of developing hydrocarbon source rocks from controlling factors of source-to-sink system development;third, on the basis of regional sedimentary facies analysis, through identification and assessment of seismic facies types of semi-deep/deep lacustrine basins in mature areas, establishing “hydrocarbon source rock facies” in mature areas to instruct the identification and depicting of hydrocarbon source rocks in semideep/deep lacustrine basins with low exploration degree;fourth, through systematical summary of hydrocarbon-rich geological factors and lower limit index of hydrocarbon formation of the sags already revealed by drilling wells(e.g., sag area, tectonic subsidence amount, accommodation space, provenance characteristic, mudstone thickness, water body environment, sedimentary facies types of hydrocarbon source rocks), in correlation with corresponding indexes of sags with low exploration degree, then the evaluation and sorting of high-quality source rocks in areas with sparsely distributed or no drilling wells can be conducted with multi-factors and multiple dimensions. It is concluded that LF22 sag, HZ10 sag and HZ8 sag are II-order hydrocarbon rich sags;whereas HZS, HZ11 and HZ24 are the III-order hydrocarbon-generating sags.

Geological theory and exploration & development practice of hydrocarbon accumulation inside continental source kitchens

Continental shale strata in China are rich in petroleum resources and are an important area to strengthen domestic oil exploration and development.Based on the latest progress in geological research and exploration and development of petroleum inside continental source kitchens of China National Petroleum Corporation exploration areas in recent two years,we have achieved the following results:(1)The geological connotations of continental hydrocarbon accumulation inside source kitchen and"sweet spot"have been proposed.The intra-source petroleum accumulation refers to the accumulation of liquid-rich hydrocarbons retained or captured in the continental organic-rich shale strata,and"sweet spot"refers to the favorable reservoir with higher oil content,better physical properties,easier to stimulate and higher in commercial development value in the overall oil-bearing continental source rock series,they can be divided into three types,interlayer,hybrid sediment and shale.(2)High-quality shale formations in both salt water and freshwater lacustrine basins can generate hydrocarbons on a large scale,shale strata have multiple types of favorable reservoirs with large-scale storage capacity,the intra-source shale strata are overall oil-bearing and large in resource scale,and there are multiple favorable shale series for development.The exploration and development practice is propelling the formation of a series of exploration and development key technologies with"sweet spot exploration"and"volume development"as the core.Some pilot tests of these technologies have provided an important scientific basis for the economic and effective development of hydrocarbon accumulation inside source kitchen,and popularization of these technologies have achieved encouraging results preliminarily.(3)Two types of continental intra-source petroleum resources in China have great potential,including medium-high maturity with liquid-rich hydrocarbons and medium-low maturity with organic-rich matter.The Ordos,Songliao,Bohai Bay and Junggar basins are the main areas of these resources.By addressing the theoretical and technical challenges in the exploration and development,the two types of resources inside continental source kitchens will become the realistic and major strategic replacement oil resources respectively in the future.

Impact of microorganism degradation on hydrocarbon generation of source rocks:A case study of the Bozhong Sag,Bohai Bay Basin

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.

Kinetic Parameters of Methane Generated from Source Rocks in the Kuqa Depression of Tarim Basin and Their Application

In a thermal simulation experiment of gold tubes of closed-system, calculating with the KINETICS and GOR-ISOTOPE KINETICS software, kinetic parameters of gas generation and methane carbon isotopic fractionation from Triassic-Jurassic hydrocarbon source rocks in the Kuqa depression of Tarim Basin are obtained. The activation energies of methane generated from Jurassic coal, Jurassic mudstone and Triassic mudstone in the Kuqa Depression are 197-268 kJ/mol, 180-260 kJ/mol and 214-289 kJ/mol, respectively, and their frequency factors are 5.265×10^13 s^-1, 9.761×10^11 s^-1 and 2.270×10^14 s^-1. This reflects their differences of hydrocarbon generation behaviors. The kinetic parameters of methane carbon isotopic fractionation are also different in Jurassic coal, Jurassic mudstone and Triassic mudstone, whose average activation energies are 228 kJ/mol, 205 kJ/mol and 231 kJ/mol, respectively. Combined with the geological background, the origin of natural gas in the Yinan-2 gas pool is discussed, and an accumulation model of natural gas is thus established. The Yinan- 2 gas is primarily derived from Jurassic coal-bearing source rocks in the Yangxia Sag. Main gas accumulation time is 5-0 Ma and the corresponding Ro is in the range from 1.25 %-1.95 %. The loss rate of natural gas is 25 %-30 %.

Characteristics of Hydrocarbon Accumulation in the Paleozoic Donghetang Formation in the Southwestern Tahe Area

The ancient structure characteristic,correlation of the oil and the hydrocarbon source rock characteristics,hydrocarbon migration trace,types and conditions of traps,migration passages and characteristic of hydrocarbon accumulation are researched in this paper.It is shown through the analysis that two main large tectonic activities after the Early Hercynian orogeny resulted in different tectonic patterns in the study area.Two main hydrocarbon infills occurred in the Donghetang Formation,the first occurred in the Early Hercynian resulting in the ancient hydrocarbon accumulation in the northern Tahe,the second infill was a large amount that occurred in places beneficial for hydrocarbon accumulation,such as structural traps and structural-stratigraphic traps formed in the Early Himalayan orogeny after migration along the faults through source rocks and other passages.Before the earlier period of the Himalayan orogeny,the petroleum mainly migrated to the north,whereas petroleum migrated to the south and southeast because of the structural reverse in the Himalayan orogeny,so the middle and later period of the Himalayan orogeny is the key period for hydrocarbon accumulation.The model of＂oil generation formed early,hydrocarbon accumulation controlled by the faults through source rocks and structures formed late＂is proposed.It is pointed out that the south of the research area is currently the beneficial district for hydrocarbon accumulation, which provides the basis for future petroleum exploration.

SEDIMENTARY ORGANIC MATTER'S CLASSIFICATION OF OIL-GAS SOURCE ROCKS IN CHINA

According to the study on the oil-gas source rocks in China for ten years,in connection with the microscopic, submicroscopic levels, the authors used the microscope photometry together with transmission electronic microscopy, scanning electronic microscopy and proposed a new classification for sedimentary organic matters.

Production and characterization of exopolysaccharides in mycelial culture of Cordyceps sinensis fungus Cs-HK1 with different carbon sources

The effects of different carbon sources(sugars) on the production and molecular properties of exopolysaccharides(EPS) were evaluated in the mycelial liquid culture of a medicinal fungus Cordyceps sinensis Cs-HK1. Galactose or mannose was used(at 5 g·L^(-1)) as a secondary carbon source with glucose(35 g·L^(-1)) at the mass ratio of 1:7. Mannose was consumed notably since the first day of culture, but galactose was not even after glucose was exhausted.The volumetric yield of EPS in culture was increased slightly with the addition of galactose and decreased with mannose. The monosaccharide composition of EPS was also different, e.g., on day 8, the glucose contents of EPS were 76%with the addition of mannose, 59% with galactose, compared with 62% with glucose only. The molecular weight distribution of EPS was also affected by the secondary carbon source, being generally lower compared with that with glucose only. The results suggested that the addition of galactose improved the total yield of EPS in culture while mannose can improve the yield of glucan constituent of EPS.

Main controlling factors and exploration enlightenment of aluminous rock series gas reservoirs in Ordos Basin,NW China

Based on the data of outcrop,core,logging,gas testing,and experiments,the natural gas accumulation and aluminous rock mineralization integrated research was adopted to analyze the controlling factors of aluminous rock series effective reservoirs in the Ordos Basin,NW China,as well as the configuration of coal-measure source rocks and aluminous rock series reservoirs.A natural gas accumulation model was constructed to evaluate the gas exploration potential of aluminous rock series under coal seam in the basin.The effective reservoirs of aluminous rock series in the Ordos Basin are composed of honeycomb-shaped bauxites with porous residual pisolitic and detrital structures,with the diasporite content of greater than 80%and dissolved pores as the main storage space.The bauxite reservoirs are formed under a model that planation controls the material supply,karst paleogeomorphology controls diagenesis,and land surface leaching improves reservoir quality.The hot humid climate and sea level changes in the Late Carboniferous–Early Permian dominated the development of a typical coal-aluminum-iron three-stage stratigraphic structure.The natural gas generated by the extensive hydrocarbon generation of coal-measure source rocks was accumulated in aluminous rock series under the coal seam,indicating a model of hydrocarbon accumulation under the source.During the Upper Carboniferous–Lower Permian,the relatively low-lying area on the edge of an ancient land or island in the North China landmass was developed.The gas reservoirs of aluminous rock series,which are clustered at multiple points in lenticular shape,are important new natural gas exploration fields with great potential in the Upper Paleozoic of North China Craton.

Biogeochemical Records at Shangsi Section,Northeast Sichuan in China:The Permian Paleoproductivity Proxies

The marine primary producers assimilate the atmospheric CO2 to form the organic carbon in surface water. The organic carbon then settles down through the water column and is removed from the oceans by final preservation in sediments in the form of petroleum or nature gases. The reconstruction of paleoproductivity will thus improve our understanding of the biological processes in the formation of fossil energy resource and help to locate new sites for future exploration. In this study, biorelated elements P, Cd, Al, Ba, as well as redox sensitive element Mo, were analyzed in the 448 rock samples collected from Permian strata at the Shangsi （上寺） Section, Guangyuan （广元）, Northeast Sichuan （四川） in China. On the basis of the Ti content, the nondetrital contents of P, Ca, and Al, denoted as Pxs, Cdxs, and Alxs, were calculated and found to coincide with the TOC content throughout the whole section, with some enrichment intervals being found in the middle part of Chihsia Formation, topmost Maokou （茅口） Formation, and Dalong （大隆） Formation. This suggests that the biorelated elements could he used as proxies for the paleoproductivity here in this section. Baxs, a paleoproductivity indicator widely used in the paleoceanography, shows insignificant correlation with TOC, Pxs, Cdxs, and Alxs, probably arising from the loss of biological barium in anoxic conditions. Compiled with the data of TOC content and Pxs, Cdxs, and Alxs, three episodes of enhanced paleoproductivity were identified in Permian strata including the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation.

Petroleum System of the Sufyan Depression at the Eastern Margin of a Huge Strike-slip Fault Zone in Central Africa

The present paper mainly studies the petroleum system of the Sufyan Depression in the Muglad Basin of central Africa and analyzes its control of hydrocarbon accumulation. On the basis of comprehensive analysis of effective source rock, reservoir bed types and source-reservoir-seal assemblages, petroleum system theory has been used to classify the petroleum system of the Sufyan Depression. Vertically, the Sufyan Depression consists of two subsystems. One is an Abu Gabra subsystem as a self generating, accumulating and sealing assemblage. The other subsystem is composed of an Abu Gabra source rock, Bentiu channel sandstone reservoir and Darfur group shale seal, which is a prolific assemblage in this area. Laterally, the Sufyan Depression is divided into eastern and western parts with separate hydrocarbon generation centers more than 10 000 m deep. The potential of the petroleum system is tremendous. Recently, there has been a great breakthrough in exploration. The Sufyan C-1 well drilled in the central structural belt obtained high-yielding oil flow exceeding 100 tons per day and controlled geologic reserves of tens of millions of tons. The total resource potential of the Sufyan Depression is considerable. The central structural belt is most favorable as an exploration and development prospect.

Molecular Records of Primary Producers and Sedimentary Environmental Conditions of Late Permian Rocks in Northeast Sichuan,China

A series of biomarkers were identified in the aliphafic and aromatic fractions of the extracts from Late Permian Dalong （大隆） and Wujiaping （吴家坪） formations in Shangsi （上寺） Section, Northeast Sichuan （四川）, South China, on the basis of the analysis of gas chromatography-mass spectrometry （GC-MC）. The dominance of lower-molecular-weight n-alkanes throughout the profile suggests the dominant contribution of algae and bacteria to the organics preserved in the marine section. Wujiaping Formation is characterized by the elevated contribution from algae as well as other photoautotrophs such as photosynthetic bacteria as shown by the molecular ratios of hopanes to steranes or tricyclic terpanes as well as the ratio of pristane （Pr） and phytane （Ph） to C17 and C18 n-alkanes. This is in accord with the data from the microscopic measurement on the calcareous algae. In contrast, Dalong Formation is featured by enhanced contribution from bacteria and probably terrestrial organics indicated by the enhanced C24 tetracyclic terpanes relative to tricyclic terpanes. The two formations also show a distinct discrimination in sedimentary environmental conditions including redox condition and salinity. The anoxic condition was only found in the middle of the Dalong Formation as shown by the ratios of Pr/Ph and dibenzothiophene to phenanthrene, consistent with the reported data of Mo and U. An enhanced salinity indicated by the homohopane index is observed at the shallow Wujiaping Formation. On the basis of the composition of primary productivity and the redox condition, Dalong Formation is proposed, herein, to be potential hydrocarbon source rocks in the study site. It is notable that the topmost end-Permian is characterized by a large perturbauce in both the redox condition and salinity, with oxic conditions being frequently interrupted by short-term anoxia, likely showing a causal relationship with the episodic biotic crisis across the Permian-Triassic boundary.

Modes of Shale-Gas Enrichment Controlled by Tectonic Evolution

The typical characteristics of shale gas and the enrichment differences show that some shale gases are insufficiently explained by the existing continuous enrichment mode. These shale gases include the Wufeng–Longmaxi shale gas in the Jiaoshiba and Youyang Blocks, the Lewis shale gas in the San Juan Basin. Further analysis reveals three static subsystems（hydrocarbon source rock, gas reservoirs and seal formations） and four dynamic subsystems（tectonic evolution, sedimentary sequence, diagenetic evolution and hydrocarbon-generation history） in shale-gas enrichment systems. Tectonic evolution drives the dynamic operation of the whole shale-gas enrichment system. The shale-gas enrichment modes controlled by tectonic evolution are classifiable into three groups and six subgroups. Group I modes are characterized by tectonically controlled hydrocarbon source rock, and include continuous in-situ biogenic shale gas（Ⅰ_1） and continuous in-situ thermogenic shale gas（Ⅰ_2）. Group Ⅱ modes are characterized by tectonically controlled gas reservoirs, and include anticline-controlled reservoir enrichment（Ⅱ_1） and fracture-controlled reservoir enrichment（Ⅱ_2）. Group Ⅲ modes possess tectonically controlled seal formations, and include faulted leakage enrichment（Ⅲ_1） and eroded residual enrichment（Ⅲ_2）. In terms of quantity and exploitation potential, Ⅰ_1 and Ⅰ_2 are the best shale-gas enrichment modes, followed by Ⅱ_1 and Ⅱ_2. The least effective modes are Ⅲ_1 and Ⅲ_2. The categorization provides a different perspective for deep shale-gas exploration.

Formation conditions of Neogene large-scale high-abundance lithologic reservoir in the Laibei low uplift,Bohai Sea,East China

By using drilling,high-precision 3 D seismic data,data of geochemistry,logging and testing,the reservoir characteristics and accumulation conditions of the KL6-1 lithologic oilfield in the Laibei Low Uplift in the Bohai Sea are examined comprehensively.The study shows that:KL6-1 oilfield is a monolithic,high-quality,large-scale Neogene lithologic oilfield featuring shallow reservoir depth,high productivity,concentrated oil-bearing intervals,large oil-bearing area,and high reserve abundance;hydrocarbon source supply from two directions provides a sufficient material basis for the formation of large oil field;two types of"inherited structural ridge"developed under the effect of block rotation,late active faults formed by Neotectonic movement,and widely distributed contiguous sand bodies provide an efficient oil and gas transportation system for the large-scale accumulation of oil and gas;contiguous channel and lacustrine lowstand system sand bodies developed in low accommodation condition provide the basic condition for the formation of large-scale lithologic traps;deep formations structural ridge,faults(dominant migration pathways)and large-scale superimposed contiguous sand bodies constitute a"vine type"oil and gas migration and accumulation system in the Laibei Low Uplift,which is conducive to the formation of large-scale and high-abundance lithologic reservoir in this area.The successful discovery of KL6-1,100 million ton reserve order lithologic oil field,has revealed the exploration potential of Neogene large lithologic reservoirs in Bohai Sea,expanded the exploration field,and also has certain reference significance for the exploration of large lithologic reservoirs in similar areas.

Molecular Evidence for Primary Producers and Paleo-environmental Conditions in Mesoproterozoic in the Xuanlong Depression in North China

The molecular organic compounds have been identified by gas chromatography （GC） and GC-mass spectrometry （GC-MS） from Mesoproterozoic rocks in the Xuaniong （宣龙） depression in North China. The main saturated compounds are n-alkanes, monomethylalkanes, n-alkylcyclohexanes, acyclic isoprenoids, and hopanes. The dominant lower-molecular-weight n-alkanes are indicative of the main contribution of microorganisms, in particular, the chemosynthetic bacteria. The presence of abundant monomethylalkanes （mid- and end-branched） and the long chained （〉C20） acyclic isoprenoids indicates the existence of abundant bacteria and/or archaea in ancient oceans. The low abundance of pristane and phytane is suggestive of the relatively low abundance of photosynthetic autotrophs in comparison with chemosynthetic bacteria in the Mesoproterozoic oceans in North China. The sedimentary environmental condition is suboxic/anoxic, as indicated by the low value of the Pr/Ph ratio as well as the presence of abundant sulfur-bearing organic compounds, consistent with the other geochemical data in North China and elsewhere in the world. Both the composition of the primary producers and the sedimentary environmental conditions are favorable for the formation of hydrocarbon source rocks.

Geochemical Characteristics and Gas-to-Gas Correlation of Two Leakage-type Gas Hydrate Accumulations in the Western Qiongdongnan Basin, South China Sea

In recent years,a series of highly saturated leakage-type gas hydrates have been discovered in the western Qiongdongnan Basin(QDNB),South China Sea.Based on the molecular compositional and isotopic characteristics of the gas samples relevant to the gas hydrates collected from the two leakage-type gas hydrate accumulations in the GMGS5 and GMGS6 drilling zones,a detailed geochemical gas-to-gas correlation was conducted in this study,in order to further understand the geochemical characteristics and possible hydrocarbon sources of these gas hydrates.The natural gas relevant to the gas hydrates in the GMGS5 block is characterized by wet gas(67.96%<%C_(1)<98.58%,C_(1)/C_(1+)<0.9)and significant molecular and carbon isotope fractionation within the depth profile,whereas the gas samples from the GMGS6 block exhibit the characteristics of dry gas(99.25%<%C_(1)<99.81%,C_(1)/C_(1+)>0.9)and lack molecular and carbon isotope fractionation.Approximately 40%‒60%of the methane within the gas hydrate is of microbial origin,while the C^(2+)gas components are typical coal-type gas that are derived from thermogenic source rocks or deeply-buried natural gas fields.In addition,typical in situ primary microbial methane(−80.6‰<δ^(13)C-C_(1)<−67.7‰)was discovered in well W b,which was applied to estimate the contribution of the microbial gas to the gas hydrates.The gas-to-gas correlation results show that the hydrate gases within the two leakage-type gas hydrate accumulations in the GMGS5 and GMGS6 blocks are geochemically different,suggesting that they may have been derived from different source kitchens.Our results further indicate that the deeply-buried thermogenic gas significantly contributed to the shallowly-buried gas hydrates in the western QDNB and multiple effective thermogenic source kitchens provided the hydrocarbon gas in the gas hydrate accumulations.