Cretaceous source rocks and associated oil and gas resources in the world and China: A review

The Cretaceous is one of the most important stratigraphic intervals for hydrocarbon source rocks. This article summarizes the distribution, formation, and development characteristics of Cretaceous source rocks and associated oil and gas resources in the world and China, aiming at improving the understanding of this hydrocarbon enrichment and at broadening domestic exploration. Outside China, these rocks are generally formed in marine or transgressive environments during both the Upper and Lower Cretaceous. The majority of Cretaceous source rocks are located in the Persian Gulf, Mediterranean, and Gulf Coast of the USA. Kerogen types within these source rocks have distinct spatial distribution characteristics, with high-latitude Boreal Realm, Tethyan Realm and South Gondwana Realm source rocks containing type III, II, II-III kerogens, respectively. Cretaceous source rocks in China can be mainly divided into four zones： Eastern, Central, Northwest, and Qinghai-Tibet Plateau zones. The majority of Chinese source rocks formed in the Early Cretaceous, whereas the most productive source rocks are developed in the Upper Cretaceous, such as those within the Songliao Basin. Most of these basins are formed in lacustrine environments, although some may have been influenced by transgressive events： Cretaceous source rocks are formed in four distinctive ways： 1） during Oceanic Anoxic Events and associated global sea-level rises, 2） in Black Sea-type retention basins, 3） during transgression and 4） during periods of significant terrestrial input. Formation of these source rocks is controlled by four factors： paleoclimate, paleotopography, transgression, and Oceanic Anoxic Events. These four major controlling factors indicate that China＇s hydrocarbon exploration within the Cretaceous should focus on two key areas with extremely low exploration levels, the Qinghai-Tibet Plateau and the southeastern coast of China.     

Burial and thermal maturity modeling of the Middle Cretaceous–Early Miocene petroleum system, Iranian sector of the Persian Gulf

The Cretaceous Kazhdumi and Gurpi formations, Ahmadi Member of the Sarvak Formation, and Paleogene Pabdeh Formation are important source rock candidates of the Middle Cretaceous–Early Miocene petroleum system in the Persian Gulf. This study characterizes generation potential, type of organic matter, and thermal maturity of 262 cutting samples(marls and argillaceous limestones) from these rock units taken from 16 fields in the Iranian sector of the Persian Gulf. In addition, the burial and thermal histories of these source rocks were analyzed by one-dimensional basin modeling. Based on the total organic carbon and genetic potential values, fair hydrocarbon generation potential is suggested for the studied samples. Based on T max and vitrinite reflectance values, the studied samples are thermally immature to mature for hydrocarbon generation. The generated models indicate that studied source rocks are immature in central wells. The Gurpi and Pabdeh formations are immature and the Ahmadi Member and Kazhdumi Formation are early mature in the western wells. The Pabdeh Formation is within the main oil window and other source rocks are at the late oil window in the eastern wells. The hydrocarbon expulsion from the source rocks began after deposition of related caprocks which ensures entrapment and preservation of migrated hydrocarbon.     

Identification of the lower limit of high-quality source rocks and its relation to hydrocarbon accumulation— Taking the Beier Sag in the Hailaer Basin as an example

The theory of “source rock control” has evolved from source-rock-control hydrocarbon accumulation, to effective source-rock-control hydrocarbon accumulation, and to high-quality sourcerock- control hydrocarbon accumulation. However, there are problems, such as whether high-quality source rocks exist or not? What high-quality source rocks are, and how to identify them, are yet to be agreed upon. Aimed at this issue of concern to explorationists, and taking the Beier Sag in the Hailaer Basin as an example, this paper defines the high-quality source rocks and the lower limit for evaluation of high-quality source rocks, by using the inflection point on the relationship curve of hydrocarbon (oil) expulsion, which is calculated by the material balance principle, versus total organic carbon (TOC). The results show that when TOC is low, all source rocks have limited hydrocarbon expulsion and slow growth rate, thus they cannot be high-quality source rocks. However, when TOC rises to some threshold, hydrocarbon expulsion increases significantly with TOC. This inflection point should be the lower limit of high-quality source rocks: those with TOC greater than the inflection-point value are high-quality source rocks. In addition, the lower limit of high-quality source rocks is also related to the type and maturity of organic matters in the source rocks, as well as the mineral components of the source rocks affecting the residual hydrocarbons. Theoretically, the lower limit of high-quality source rocks depends on geological conditions rather than being a constant value. However, for the sake of simplicity and practicability, in this paper TOC=2.0% is regarded as the lower limit of high-quality source rocks. The examination of such standard in the work area indicates that the high-quality source rocks in members K1n2 and K1n1 of the Nantun formation contribute 76% and 82% to oil generation, and 96% and 91% to oil expulsion , respectively. The distribution of high-quality source rocks is also closely related to the distribution of hydrocarbon reservoirs in the region, demonstrating that high-quality source rocks control hydrocarbon accumulation.     

Sedimentary and geochemical characteristics of the Triassic Chang 7 Member shale in the Southeastern Ordos Basin, Central China

The Ordos Basin is the largest petroliferous basin in China, where the Chang 7 Member shale serves as the major source rock in the basin, with an area of more than 100,000 km~2 So far, sedimentary and geochemical characterizations have rarely been conducted on the shale in shallow( 1000 m) areas in the southeastern part of the basin, but such characterizations can help identify the genesis of organic-rich shale and promote the prediction and recovery of shale oil. In this paper,several outcrop sections of the Chang 7 Member in the Tongchuan area were observed and sampled, and sedimentary and geochemical characterizations were conducted for the well-outcropped YSC section. The study results show that the Chang7 Member shale is widely distributed laterally with variable thickness. The organic-rich shale is 7-25 m thick in total and exhibits obvious horizontal variation in mineral composition. In the eastern sections, the shale contains organic matter of TypeⅡ_2-Ⅲ and is low in thermal maturity, with high clay mineral content, low K-feldspar content, and no pyrite. In the western sections, the shale contains Type Ⅱ_1 organic matter and is low in thermal maturity, with high clay mineral, K-feldspar, and pyrite contents. The YSC section reveals three obvious intervals in vertical mineral composition and organic abundance.The Chang 7 Member organic-rich shale(TOC 10%) contains mainly sapropelite and liptinite, with Type Ⅱ kerogen. It is generally characterized by a hydrocarbon potential of more than 70 mg/g, low maturity, and shallow-semideep lacustrine facies. In the western sections, the shale, still in a low maturity stage, has a higher hydrocarbon potential and is optional for shale oil recovery. However, the Chang 7 Member shale in the study area is highly heterogeneous and its shale oil recovery is practical only in the organic-rich intervals.     

A geochemical investigation of the free and carbonate-bound organic matter in the clay-sized fraction of argillaceous source rocks and its significance for biogenic interpretation

The molecular composition and biomarker distribution of various occurrences of organic matter in argillaceous source rocks developed in fresh and saline lacustrine environments were revealed by successive treatments of solvent extraction followed by acid hydrolysis using gas chromatography–mass spectrometry. The free fraction obtained by solvent extraction provided abundant geochemical information concerning the sedimentary environment, thermal maturity and biogenic origin, and obvious differences existed between fresh and saline lacustrine source rock samples. Our research results indicate that the carbonate-mineral-bound(CM-bound) fraction released by successive acid hydrolysis could also serve as a significant biogenic indicator, as the bicyclic sesquiterpenoids, indicative of Botryococcus braunii origin, were specifically detected in quite high abundance in the acid-soluble fraction. In addition, the light end hydrocarbons were much better preserved in the acid-soluble fraction, and elemental sulfur was only detected in the CM-bound fraction, suggesting a relatively confined environment for the CM-bound fraction, which thus could preserve additional geochemical information compared to that of the free fraction. The CM-bound fraction also exhibited discernable differences between fresh and saline lacustrine samples. Therefore, it can be concluded that comprehensive analysis of free and CM-bound fractions in the argillaceous source rocks can provide a more authentic and objective interpretation of geologic conditions.     

Effects of U-ore on the chemical and isotopic composition of products of hydrous pyrolysis of organic matter

In order to investigate the impact of U-ore on organic matter maturation and isotopic fractionation,we designed hydrous pyrolysis experiments on Type-II kerogen samples,supposing that the water and water–mineral interaction play a role.U-ore was set as the variable for comparison.Meanwhile,anhydrous pyrolysis under the same conditions was carried out as the control experiments.The determination of liquid products indicates that the presence of water and minerals obviously enhanced the yields of C_(15+) and the amounts of hydrocarbon and nonhydrocarbon gases.Such results may be attributed to waterorganic matter reaction in the high-temperature system,which can provide additional hydrogen and oxygen for the generation of gas and liquid products from organic matter.It is found that δD values of hydrocarbon gases generated in both hydrous pyrolysis experiments are much lower than those in anhydrous pyrolysis.What is more,δD values are lower in the hydrous pyrolysis with uranium ore.Therefore,we can infer that water-derived hydrogen played a significant role during the kerogen thermal evolution and the hydrocarbon generation in our experiments.Isotopic exchange was facilitated by the reversible equilibration between reaction intermediaries with hydrogen under hydrothermal conditions with uranium ore.Carbon isotopic fractionations of hydrocarbon gases were somehow affected by the presence of water and the uranium ore.The increased level of i-C_4/n-C_4ratios for gas products in hydrous pyrolysis implied the carbocation mechanism for water-kerogen reactions.     

Distribution and geochemical significance of phenylphenanthrenes and their isomers in selected oils and rock extracts from the Tarim Basin,NW China

Twenty-two oil samples and eight source rock samples collected from the Tarim Basin,NW China were geochemically analyzed to investigate the occurrence and distribution of phenylphenanthrene(PhP),phenylanthracene(PhA),and binaphthyl(BiN) isomers and methylphenanthrene(MP) isomers in oils and rock extracts with different depositional environments.Phenylphenanthrenes are present in significant abundance in Mesozoic lacustrine mudstones and related oils.The relative concentrations of PhPs are quite low or below detection limit by routine gas chromatography-mass spectrometry(GC-MS) in Ordovician oils derived from marine carbonates.The ratio of 3-PhP/3-MP was used in this study to describe the relative abundance of phenylphenanthrenes to their alkylated counterparts-methylphenanthrenes.The Ordovician oils in the Tabei Uplifthave quite low 3-PhP/3-MP ratios(0.10),indicating their marine carbonate origin,associating with low Pr/Ph ratios(pristane/phytane),high ADBT/ADBF values(relative abundance of alkylated dibenzothiophenes to alkylated dibenzofurans),low C_(30) diahopane/C_(30)hopane ratios,and low Ts/(Ts+Tm)(18α-22,29,30-trisnorneohopane/(18α-22,29,30-trisnorneohopane+17α-22,29,30-trisnorhopane)) values.In contrast,the oils from Mesozoic and Paleogene sandstone reservoirs and related Mesozoic lacustrine mudstones have relatively higher 3-PhP/3-MP ratios(0.10),associating with high Pr/Pli,low ADBT/ADBF.high Ts/(Ts + Tm).and C_(30) diahopane/C_(30) hopane ratios.Therefore,the occuirence of significant amounts of phenylphenanthreiies in oils typically indicates that the organic matter of the source rocks was deposited in a suboxic environment with mudstone deposition.The phenylphenanthreiies may be effective molecular markers,indicating depositional environment and lithology of source rocks.     

Oil and gas resource potential of the lower members of Eh3 in the lower Tertiary of the Biyang Depression, China

The lower Tertiary E h3 is divided into two sections: the upper members of E h3 and the lower members of E h3 in the Biyang Depression. The first section is generally regarded as a key target of oil and gas exploration, but the resource potential of the lower members of E h3 has been neglected. We have obtained new knowledge about E h3 from comprehensive geological research. The lower members of E h3 are high-quality and main source rocks, which have good oil and gas resource potential. This is a new direction for oil and gas exploration. The geochemistry characteristics of source rocks of the lower members of E h3 in the lower Tertiary of the Biyang Depression were analyzed in detail. A basin modeling method was applied to hydrocarbon generation of the lower and upper members of E h3 source rocks, the oil and gas resource potential was comparatively analyzed, and then favorable tectonic zones were pointed out. In the lower members of E h3 , a set of semi-deep lake to deep lake high-quality source rocks occurs rich in algae organisms, mainly of type II 1 , with a high abundance of organic matter. Most of the source rocks are just in the peak stage of hydrocarbon generation, which is a favorable foundation for forming abundant oil and gas resources in the Biyang Depression. The comparative analysis of the hydrocarbon-generation quantities between lower and upper members of the E h3 source rocks shows that the lower members of E h3 have good oil and gas resource potential, and the hydrocarbon-generation quantity accounts for 51% of the total in E h3 . Specifically, the oil-generating quantity accounts for 50% of the total and the gas-generating quantity accounts for two thirds of the total. Therefore, source rocks in the lower members of E h3 of the Biyang Depression have good oil and gas resource potential, which is a key factor for future deep oil and gas exploration.     

Formation and distribution characteristics of Proterozoic–Lower Paleozoic marine giant oil and gas fields worldwide

There are rich oil and gas resources in marine carbonate strata worldwide.Although most of the oil and gas reserves discovered so far are mainly distributed in Mesozoic,Cenozoic,and upper Paleozoic strata,oil and gas exploration in the Proterozoic–Lower Paleozoic(PLP)strata—the oldest marine strata—has been very limited.To more clearly understand the oil and gas formation conditions and distributions in the PLP marine carbonate strata,we analyzed and characterized the petroleum geological conditions,oil and gas reservoir types,and their distributions in thirteen giant oil and gas fields worldwide.This study reveals the main factors controlling their formation and distribution.Our analyses show that the source rocks for these giant oil and gas fields are mainly shale with a great abundance of type I–II organic matter and a high thermal evolution extent.The reservoirs are mainly gas reservoirs,and the reservoir rocks are dominated by dolomite.The reservoir types are mainly karst and reef–shoal bodies with well-developed dissolved pores and cavities,intercrystalline pores,and fractures.These reservoirs arehighly heterogeneous.The burial depth of the reservoirs is highly variable and somewhat negatively correlated to the porosity.The cap rocks are mainly thick evaporites and shales,with the thickness of the cap rocks positively correlated to the oil and gas reserves.The development of high-quality evaporite cap rock is highly favorable for oil and gas preservation.We identified four hydrocarbon generation models,and that the major source rocks have undergone a long period of burial and thermal evolution and are characterized by early and long periods of hydrocarbon generation.These giant oil and gas fields have diverse types of reservoirs and are mainly distributed in paleo-uplifts,slope zones,and platform margin reef-shoal bodies.The main factors that control their formation and distribution were identified,enabling the prediction of new favorable areas for oil and gas exploration.     

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??A low productivity carbon dioxide gas reservoir was found at the well depths of 2573-2594 m of Maokou Formation-Qixia Formation?? Permian?? in well Yang-1 in Yangba Seg of Nanpanjiang Basin. It is very important for natural gas exploration in the region to find out the origins of the gas reservoir?? because this well is the first regional exploration well in the basin. On the basis of analyzing the geochemical characteristics of the natural gas?? the genesis and origins of the natural gas in the reservoir were investigated by means of the organic geochemical and organic petrological analyses of the cores of well Yang-1 and the rock samples of peripheric outcrops?? and of the thermal simulation experiments of the carbon dioxide gas source rocks. The comprehensive analysis results indicated that the hydrocarbon in the natural gas in well Yang-1 was the overmature coal??type cracking gas and came from the Permian coal measure hydrocarbon source rocks?? according to the He and Ar isotopic ratios?? the natural gas was mainly the crustal source gas?? and the carbon dioxide was mainly the petrochemical genesis?? being formed by the low temperature ????300?棩 hydrolysis of the Carboniferous-Permian carbonate rocks.     

ABSTRACTS

1 Research on Evaluation and Distribution of Source Rocks in Sikeshu Sag of the Southern Junggar Basin LIN Xiao-yun , QIN dun, CHEN Zhe , XU Ying , DENG Xiao-hui （First Author＇s Address ： Key Laboratory of Exrploration Tech nologies for Oil and Gas Resources （Yangtze University ） ,Ministry of Education ; School of Geosciences ,Yungtze University, Wuhan 430100, Hutwi , Ch ina ） Abstract： Sikeshu Sag was a secondary structure unit located in the west of southern Junggar Basin. The potential force and its distributive characters were clarified for the purpose of further analyzing its hydrocarbon accumulation mecha- nism and predicting its distributive rules. By systematically analyzing the drilling and geochemical data, source rocks in Anjihai Formation of the lower-middle Jurassic and Paleogene were comprehensively evaluated, combined with seismic and sedimentary facies analysis and basin modeling methods,the distribution of the source rocks was objectively predic ted. Study results show that the main source rocks are lacustrine dark mudstones in the sag,dark rocks in lower Jurassic Sangonghe Formation are large thickness with high organic matter abundance,good type and a high degree of thermal evolution, they are main source rocks in the area. The distribution of source rocks is obviously controlled by tectonic-sedi mentation pattern,the hydrocarbon is mainly located in the central-southeast of the sag,and its quality has been gradual- ly poor from the south to the north.     

Origin and accumulation of high-maturity oil and gas in deep parts of the Baxian Depression, Bohai Bay Basin, China

Great quantities of light oil and gas are produced from deep buried hill reservoirs at depths of 5,641 m to 6,027 m and 190 ℃ to 201 ℃ in the Niudong-1 Well, representing the deepest and hottest commercial hydrocarbons discovered in the Bohai Bay Basin in eastern China. This discovery suggests favorable exploration prospects for the deep parts of the basin. However, the discovery raises questions regarding the genesis and accumulation of hydrocarbons in deep reservoirs. Based on the geochemical features of the hydrocarbons and characteristics of the source rocks as well as thermal simulation experiments of hydrocarbon generation, we conclude that the oil and gas were generated from the highly mature Sha-4 Member （Es4） source rocks instead of thermal cracking of crude oils in earlier accumulations. The source kitchen with abnormal pressures and karsted carbonate reservoirs control the formation of high-maturity hydrocarbon accumulations in the buried hills （i.e., Niudong-1） in conjunction with several structural-lithologic traps in the ES4 reservoirs since the deposition of the upper Minghuazhen Formation. This means the oil and gas exploration potential in the deep parts of the Baxian Depression is probably high.     

Further Recognition of Petroleum Exploration Potential of Marine Carbonates in Western Tarim Basin

A series of significant discoveries in marine carbonate rocks show great petroleum exploration potential in the Tarim Basin. However, the oil and gas fields discovered in the carbonate rocks are mainly distributed around the Manjiaer Sag in the eastern Tarim Basin. Some explorations occurred and no oil or gas field was discovered around the Awati Sag in the western Tarim Basin. Information from wells and outcrops reveals that there are excellent oil and gas source rock conditions around the Awati Sag. Transformed reef-shoal reservoirs could be formed in the Ordovician carbonate rocks with paleo-geographic background and hydrothermal conditions. Therefore, it is necessary to make a systematical study and overall evaluation of the potential of the periphery of the Awati Sag in terms of source rock evolution, resource potential, high-grade reservoir formation and distribution, and main factors controlling hydrocarbon migration and accumulation.     

Hydrocarbon generation conditions and exploration potential of the Taoudeni Basin, Mauritania

The Taoudeni Basin is a typical and steady intracratonic basin in Mauritania, northwest Africa. There are six sets of potential source rocks and five regional unconformable surfaces of the Infracambrian and Paleozoic developed in the basin. We used seismic stratigraphic correlation to recover the denudation thickness of formations at a particular well location. Studies of the hydrocarbon generation history of the basin illustrate that hydrocarbon migration and accumulation occurred in the end of the Carboniferous, and after that, the whole basin suffered denudation for a long period of time. Because there is no thick Mesozoic overburden in the basin, the Silurian source rocks could not generate hydrocarbon in the Mesozoic era for the second time. Consequently, the prospects for successful hydrocarbon exploration in the basin are not good.     

The Chang 7 member of the upper Triassic in the southwest Ordos Basin： Carbon isotope studay of dispersed organic matter and its paleoclimate significance

The Taoudeni Basin is a typical steady intracratonic basin. Based on the distribution of effective source rocks in the Taoudeni Basin, combined with the structure characteristics of the basin and the distribution characteristics of reservoir beds, two petroleum systems are recognized in the basin： the infra-Cambrian petroleum system and the Silurian petroleum system. Structural uplift and timing of petroleum generation controlled the timing of petroleum charging and preservation of hydrocarbon accumulations. Maturity, evolution history, and distribution of effective source rocks controlled hydrocarbon richness. The geological key factors and geological processes controlled the type of hydrocarbon accumulations.     

Oil Source of Reservoirs in the Hinterland of the Junggar Basin

Through oil-oil and oil-source correlation and combined with the comprehensive study of hydrocarbon generation and accumulation history, the oil sources of typical reservoirs of different geologic periods in the hinterland of the Junggar basin are revealed. It is concluded that the crude oils in the study area can be classified into four types The oil in the area of well Zhuang-1 and well Sha-1 belongs to type-Ⅰ, which was generated from Cretaceous to Paleogene (K-E) and its source rocks are distributed in the Fengcheng formation of the Permian in the western depression to the well Pen-1. The oil in the area of well Yong-6 (Kltg) belongs to type-Ⅱ, which was generated from Cretaceous to Paleogene and its source rocks are distributed in the Wuerhe formation of the Permian in the Changji depression. The oil in the area of well Yong-6 (J2x) belongs to type-Ⅲ, which was generated at the end of the Paleogene and its source rocks are distributed in the coal measures of the Jurassic in the Changji depression. The oil of well Zheng-1 and well Yong-1 belongs to type-Ⅳ, which was generated in the Paleogene, and its source rocks are distributed in the Wuerhe formation of the Lower Permian and coal measures of the Jurassic. It is indicated that the hydrocarbon accumulation history in the study area was controlled by the tectonic evolution history of the Che-Mo palaeohigh and the hydrocarbon generation history of well Pen-1 in the western depression and Changji depression.     

Existence and implications of hop-17（21）-enes in the lower Cretaceous of the Saihantala Sag, Erlian Basin, China

C31- to C35-hop-17(21)-enes are identified by gas chromatography-mass spectrometry (GC-MS) analysis to exist as double isomers in most samples of the Aershan Formation and members 1 and 2 of the Tenggeer Formation from well SH3. Comprehensive organic geochemistry and organic petrology study indicates that algae and bacteria are the main biological source of lower Cretaceous sediments in the Saihantala Sag, and this is in accordance with the existence of hop-17(21)-enes. The similar distributions of hop-17(21)-enes and hopanes of these samples indicate that hop-17(21)-enes were transformed into hopanes through hydrogenation during diagenesis processes. The existence of hop-17(21)-enes means that not only the formation of organic matter is related to an anoxic environment and a biological source of algae and bacteria, but also hop-17(21)-enes are direct indicators of hydrocarbon rock at an immature to low-maturity stage. High hydrocarbon conversion ratio, algae and bacteria source and a high abundance of organic matter suggest that the Saihantala Sag has the potential to generate immature to low-maturity oil, which may be of great significance for oil exploration in the Erlian Basin.     

Optimization of shale gas reservoir evaluation and assessment of shale gas resources in the Oriente Basin in Ecuador

The petroleum geological features of hydrocarbon source rocks in the Oriente Basin in Ecuador are studied in detail to determine the potential of shale gas resources in the basin. The favorable shale gas layer in the vertical direction is optimized by combining logging identification and comprehensive geological analysis. The thickness in this layer is obtained by logging interpretation in the basin. The favorable shale gas accumulation area is selected by referring to thickness and depth data. Furthermore, the shale gas resource amount of the layer in the favorable area is calculated using the analogy method. Results show that among the five potential hydrocarbon source rocks, the lower Napo Formation is the most likely shale gas layer. The west and northwest zones, which are in the deep-sea slope and shelf sedimentary environments, respectively, are the favorable areas for shale gas accumulation. The favorable sedimentary environment formed thick black shale that is rich in organic matter. The black shale generated hydrocarbon, which migrated laterally to the eastern shallow water shelf to form numerous oil fields. The result of the shale gas resource in the two favorable areas,as calculated by the analogy method, is 55,500×10~8 m~3. This finding shows the high exploration and development potential of shale gas in the basin.     

Thermal and maturation history for Carboniferous source rocks in the Junggar Basin,Northwest China: implications for hydrocarbon exploration

The reconstruction of thermal history is an important component of basin evolution and hydrocarbon exploration. Based on vitrinite reflectance data, we integrate the paleo-temperature gradient and paleo-heat flow methods to reconstruct the thermal history of Junggar Basin. Compared with present thermal state, the Junggar Basin experienced much a higher heat flow of ca.80–120 mW/m2 during the Carboniferous. This feature can be attributed to large-scale volcanic events and related thermal effects. The hydrocarbon maturation history of Carboniferous source rocks indicates that the temperature rapidly reached the threshold of hydrocarbon generation during the Late Carboniferous and has never achieved such a high level since then.This characteristic resulted in the early maturation of hydrocarbons in Carboniferous source rocks. Meanwhile, the results reveal that hydrocarbon maturities are different among various tectonic units in Junggar Basin. The kerogen either rapidly broke through the dry gas period so that cracking of gas occurred or remained in the oil maturation window forming oil reservoirs, which depended on the tectonic background and depositional environment. In this study, we present the thermal and hydrocarbon maturation history since the Carboniferous, which has important implications for further hydrocarbon exploration in Junggar Basin.     

Genetic mechanisms of secondary pore development zones of Es_4~x in the north zone of the Minfeng Sag in the Dongying Depression,East China

The genetic mechanisms of the secondary pore development zones in the lower part of the fourth member of the Shahejie Formation(Es_4~x) were studied based on core observations,petrographic analysis,fluid inclusion analysis,and petrophysical measurements along with knowledge of the tectonic evolution history,organic matter thermal evolution,and hydrocarbon accumulation history.Two secondary pore development zones exist in Es_4~x,the depths of which range from 4200 to 4500 m and from 4700 to 4900 m,respectively.The reservoirs in these zones mainly consist of conglomerate in the middle fan braided channels of nearshore subaqueous fans,and the secondary pores in these reservoirs primarily originated from the dissolution of feldspars and carbonate cements.The reservoirs experienced ‘‘alkaline–acidic–alkaline–acidic–weak acidic',‘‘normal pressure–overpressure–normal pressure',and‘‘formation temperature increasing–decreasing–increasing' diagenetic environments.The diagenetic evolution sequences were ‘‘compaction/gypsum cementation/halite cementation/pyrite cementation/siderite cementation–feldspar dissolution/quartz overgrowth–carbonate cementation/quartz dissolution/feldspar overgrowth–carbonate dissolution/feldspar dissolution/quartz overgrowth–pyrite cementation and asphalt filling'.Many secondary pores(fewer than the number of primary pores) were formed by feldspar dissolution during early acidic geochemical systems with organic acid when the burial depth of the reservoirs was relatively shallow.Subsequently,the pore spaces wereslightly changed because of protection from early hydrocarbon charging and fluid overpressure during deep burial.Finally,the present secondary pore development zones were formed when many primary pores were filled by asphalt and pyrite from oil cracking in deeply buried paleoreservoirs.