In the past decade （2012-2021）， China's natural gas industry developed rapidly. The main reasons are： （1） The natural gas output in the past decade was 15 105.2×108 m3， which was 2.3 times of the previous ten years （2002-2011， total gas production of 6 468.38×108 m3）； （2） Natural gas reserves have soared. In the past decade， China's newly proved natural gas geological reserves increased by 84 499.58 ×108 m3， 1.5 times that of the previous decade's （55 696.57×108 m3）； （3） The exploration and development of shale gas bore fruit. In the first ten years， there were no shale gas fields. In the last ten years， seven shale gas fields were discovered and proved， and the proved geological reserves of shale gas were 2.74×1012 m3 and the productions were 920×108 m3； （4） Large gas fields with an annual output of 10 billion cubic meters， two in the first decade and three in the last decade； （5） Long gas pipelines extend in all directions. In the past ten years， 60 766.5 km of long gas pipeline was built， 2.6 times that from 1949 to 2011.
It is well accepted that organic-rich shale has been widely developed and distributed in the Lower Yangtze Region. However, no obvious breakthrough has been made in this area. In this study, two sets of organic-rich shale in the Permian Dalong Formation and Wujiaping Formation in Well WWD4 were collected, and the microscopic pores of the Permian organic-rich shale in the Wangjiang Depression of the Lower Yangtze Region were qualitatively observed and quantitatively studied. Based on this, the main controlling factors of TOC and mineral composition on the specific surface area of ?pores at different scales were discussed. The results show that the distribution of shale pores in the Dalong and Wujiaping shale is dominated by micropores and mesopores. The development of pores in the Upper Permian Dalong shale and Wujiaping shale in the Wangjiang Depression of the Lower Yangtze Region is closely related to organic matters. The positive correlation between micropore specific surface area and pore volume and TOC content proves that organic matter is the main controlling factor for the development of micropores. The weak correlation between the specific surface area and pore volume of meso-macropores and TOC content indicates that clay minerals, feldspar and other mineral could contribute to the intergranular pores and dissolution pores. Meanwhile, clay content, TOC content and specific surface area (pore volume) in Wujiaping Formation shale exist internal relations. However, no correlation between the content of quartz and carbonate minerals and the total specific surface area (pore volume) exist in the Dalong shale. However, a certain negative correlation between the quartz content and the total specific surface area (pore volume) of Dalong shale can be found, and the total specific surface area (pore volume) of shale decreases with the increase of quartz content. Overall, this study provides basic parameters and theoretical basis for the exploration and development of shale gas in the Lower Yangtze Region.
The adsorption characteristics of shale gas depend on the pore structure, mineral composition, gas pressure, reservoir temperature and so on. In this paper, the adsorption behaviors of methane in the nano-pores of several main shale components were simulated by Monte Carlo method, and the effects of gas pressure and temperature on methane adsorption were investigated. The methane adsorption in organic matter, quartz, montmorillonite, kaolinite and illite were compared, and the proportion of methane molecules in adsorbed state was discussed. The results show that the adsorption capacity of methane decreases linearly with temperature, and increases nonlinearly with pressure, while the increasing rate of methane adsorption will gradually decrease when pressure is high. The dependence between methane absorption, pressure, and temperature can be described by two combined power functions. Methane molecules in nano-pores display both adsorbed and free states, and the proportion of adsorbed state gradually decreases with pressure. The proportion of adsorbed state in the nano-pore of several main shale components is quite different, and their sequence is kaolinite > illite > montmorillonite > organic matter > quartz.
Organic-rich shale is widely developed in Carboniferous Benxi Formation in Ordos Basin, which has a good resource exploration prospect. Based on a series of studies on the organic geochemical characteristics, pore distribution characteristics and gas-bearing properties of shale in the Benxi Formation, the geological characteristics and accumulation laws of shale gas in the Benxi Formation were revealed. Jinci Member is the favorable zone in the Benxi Formation, with the following four geological characteristics: (1) the shale is characterized by high organic matter abundance (average TOC is 3.06%) and high thermal maturity (average RO is 1.84%) , its kerogen belongs to type Ⅲ; (2) micro and nanoscale pores are mainly inorganic pores, with a little organic pores, the micropores and mesoporous pores provide the main storage space, the average pore volume is 0.024 m3/g, and the average specific surface area is 11.05 m2/g. Shale gas mainly exists as adsorbed gas, and the pressure and TOC are important factors affecting the adsorbed gas volume; (3) brittle mineral content of siliceous shale is higher than 40%, indicating that the reservoir is more compressible; (4) the ideal combination accumulation characteristics provide a good preservation condition. Carbonaceous shale and siliceous shale developed between the two sets of coal, the former has good gas-bearing property, and the latter is easy to be fractured. The rules of accumulation and enrichment are summarized: Lagoon facies controls the development of source rocks; pores control the storage; favorable combination controls the accumulation and the fracturing ability of siliceous shale controls the exploration of shale gas. The results can provide a useful reference for determining favorable shale gas areas and further exploration and development of the Benxi Formation in Ordos Basin.
In order to clarify the influence of organic matter and mineral composition on the pore structure of shale reservoir in the seventh member of Yanchang Formation (Chang 7 Member), Ordos Basin, the pore structure characteristics of Chang 7 Member shale reservoir were analyzed by Argon Ion Polishing-Field Emission Scanning Electron Microscopy (FE-SEM) and Low-pressure Aitrogen adsorption (LP-N2A), and the controlling factors of pore structure were analyzed in combination with whole-rock mineral composition and organic geochemistry experiment. The results show that the Chang 7 Member shale mainly develops intercrystalline pores of clay minerals and intergranular pores, and a few organic matter pores, dissolution pores and microfractures can be seen. The LP-N2A isotherms of shale are all type Ⅱ isotherms, with H3 and H4 hysteresis loop characteristics, indicating that the mesopore are relatively developed, and the pore morphology is parallel lamellar and "ink bottle" shape. The pore structure of shale is controlled by the organic matter, clay minerals, quartz, feldspar and pyrite. The clay mineral phase transformation forms a large number of micropores and mesopores between the mineral crystal layers, which is the main source of pores in shale in the study area. Liquid hydrocarbon, solid asphalt and monoidiomorphic pyrite generated from organic matter filled the pores of inorganic minerals, which reduced the pore space of shale in Chang 7 Member to a certain extent. These results can provide a new cognition for understanding the pore structure characteristics and controlling factors of Chang 7 Member shale.
The paleogeomorphology and its impact on overlying Wufeng-Longmaxi shale in southern Sichuan Basin of China are still controversial. Based on geophysical interpretation, chronostratigraphic division and transection correlation, shale isometric map compilation, and shale mineral composition analysis, the depositional paleogeomorphology and its impact on the overlying gas-bearing shale were clarified. The results show that: (1) During Late Ordovician and Early Silurian, the southern Sichuan Basin was located on the southeastern slope of the Leshan-Longnüsi paleo-uplift. The paleo-uplift, which was a syn-depositional one and had span distance reaching up to 85 km and topographical gradient ranging from 0 to 1 m/km, extended from Weiyuan area to Luzhou area southeastward. (2) During the depositional period of the Wufeng-Longmaxi shale, there existed three significant slope breaks which divided the southeastern slope into four geomorphological units, namely, sub?aqueous high, subaqueous slope, subaqueous plain, and subaqueous sag. (3) The Wufeng-Longmaxi shale, which onlaps the southeastern slope from southeast to northwest, is wholly developed in subaqueous plain and subaqueous sag but commonly losses the graptolite zones LM1-4 in subaqueous high and subaqueous slope. (4) There exist significant differences in shale grain size, mineral composition, and TOC content of shale upon different geomorphological units. Commonly, from the subaqueous high to the subaqueous sag, the shale grain size gradually becomes finer, the TOC content and carbonate mineral content gradually decrease, the silica content gradually increases, and the clay mineral content first decreases and then increases.
The first member of Maokou Formation (Mao 1 Member) of Middle Permian in Sichuan Basin is characterized by the rapid alternate development of “eye limestone” and “eyelid limestone”, which is widely distributed in the basin. In recent years, using the idea of unconventional natural gas exploration for reference, great breakthroughs have been made in the Mao 1 Member in various regions of Sichuan Basin. Drilling and production results show that the muddy limestone (eyelid limestone) reservoir has good gas content and great exploration potential. However, since it has long been considered as a set of good source rocks, the testing and evaluation of the Mao 1 Member reservoir lacks pertinence. In this study, based on the latest drilling data of two wells in Hechuan-Tongnan area of central Sichuan Basin and the outcrop observation of the surrounding field, the characteristics of the muddy limestone reservoir in the Mao 1 Member are analyzed in detail. The results show that the reservoir space of the muddy limestone reservoir in the Mao 1 Member is mainly clay mineral pores, dissolution pores and organic matter pore,followed by micro-fractures. Pores are dominated by medium pore, mainly in the form of slit pores, followed by micropores and macropores. Pores are dispersed and developed, and the connectivity between pores is medium.The total organic carbon content is relatively high, the organic maturity is high, and the hydrocarbon generation condition is good. It is an unconventional carbonate reservoir with self-generation and self-storage characteristics.On this basis, the author establishes the evaluation criteria of marl reservoir, and makes a preliminary evaluation of muddy limestone reservoir. It is considered that the muddy limestone of the Mao 1 Member generally meets the basic conditions of reservoir accumulation, and partially meets the favorable reservoir formation conditions, which has further exploration and development value.
The Dengying Formation (Z2dn) gas reservoir in Pengtan 1 well area is another exploration achievement after the discovery of Anyue Gas Field. It is of great significance to clarify the reservoir characteristics and genetic mechanism of this gas reservoir for the distribution prediction of high-quality reservoirs, the formulation of development plan and the next exploration deployment. Based on core, physical property analysis, logging interpretation, imaging logging and seismic data, the reservoir characteristics of the second member of Dengying Formation (Z2dn2) in Pengtan 1 well area are described and the main controlling factors are analyzed. The results show that: The high quality reservoir of Z2dn2 is mainly developed in algal dolostone related to biogenesis and granular dolostone related to grain beach formation. The reservoir space includes pores, caves and fractures. According to the combination characteristics of reservoir space, the reservoir can be divided into three types: Pore type, karst cave type and fracture-cave type. The fracture-cave reservoir is the most developed, followed by karst cave type. The high quality reservoirs are mainly distributed in the middle and upper part of Z2dn2, which are generally characterized by low porosity and low permeability, and some sections are characterized by mid-porosity and mid-permeability. The development degree and spatial distribution of high-quality reservoirs are mainly affected by sedimentary microfacies, karstification during early diagenesis and Tongwan movement. The dune beach microfacies in the platform margin subfacies zone is the development facies zone of high quality reservoir. Karstification in early diagenetic stage promoted the further development of reservoir space, and Tongwan movement promoted the development of fractures and the further occurrence of atmospheric water leaching.
The distribution of source rocks is closely related to their sedimentary environment and has an important influence on the distribution of oil and gas. The evaluation of the source rocks and the study of their sedimentary environment can point out the direction for the next exploration in the Qikou Sag. In this paper, starting from the development and evolution of source rocks, the geochemical characteristics, distribution law and formation mechanism of source rocks in the Shahejie Formation in Qikou Sag are studied in depth. The geochemical analysis shows that the abundance of organic matter has reached the medium-best level and the contents of organic matter has the characteristics of increasing firstly from Es3 to Es1x and then decreasing from Es1x to Es1s. The types of organic matter are type I to type III, and the organic matter are dominated by type I and type II1 in Es1s and Es1z, while by type I to type III in Es1x, Es2, Es3, reflecting that the source of organic matter in the sedimentary period of the Shahejie Formation was lacustrine primary organic matter mixed with terrigenous organic matter. The distribution of source rocks in the Shahejie Formation on the plane has the characteristics of wide area and large thickness, and high-quality source rocks are more developed in the slope area. The paleoclimate during the sedimentary period of the Shahejie Formation was affected by moderate weathering intensity, and the overall warm and humid paleoclimate conditions during this period were conducive to the development of organisms. The high paleosalinity of the water belongs to the brackish water. There is no obvious correlation between paleosalinity and paleoclimate, and the high paleosalinity of water may be influenced by seawater intrusion. The relationship between paleosalinity and the content of organic matter shows that in the low paleosalinity stage, the abundance of organic matter increases with the increase of paleosalinity, and in the high paleosalinity stage, the abundance of organic matter decreases when the paleosalinity increases. According to the ratio of trace elements and Corg∶P molar ratio, an oxidative-suboxidative depositional environment developed during the deposition of the Shahejie Formation in Qikou Sag, and the Pr/Ph ratio show that a strong reducing environment developed intermittently in Es1x, which is conducive to the preservation of organic matter. The evaluation of the paleoproductivity at the period of the Shahejie Formation deposited shows that the lake had medium-high nutrient content and primary productivity was medium-high, while the Banqiao Sag had low-medium nutrient content and had low-medium primary productivity. The relatively high paleoproductivity and the high input of terrigenous organic matter in the Qikou Sag, with the highest value in Es1 period and the lowest value in Es2 period, provide a rich material basis for the formation of source rocks. The paleoproductivity is the main factor affecting the formation of source rocks in the Shahejie Formation in Qikou Sag. Due to the dual contribution of terrigenous organic matter and aquatic organic matter in the slope area, high-quality source rocks are more developed here.
The enrichment of organic matter is affected by many factors. It is of great significance to analyze the controlling factors of the development of organic-rich mudstone for shale oil and gas exploration. Based on the geochemical analysis of total organic carbon (TOC), major and trace elements of mudstone core samples from Middle-Lower Shahejie Formation (Es3m—l) in southwestern Bozhong Sag, the effects of paleoclimate, sedimentary water environment, sedimentation rate and paleoproductivity on organic matter enrichment during Es3m—l deposition were discussed, and the organic matter enrichment model of Es3m—l was established. The results show that Es3m—l mudstone has a high abundance of organic matter, with TOC content ranging from 1.05% to 6.37%. The CIAcorr reflects the overall warm and humid paleoclimate. The lower part of Es3l is relatively dry, and the middle and upper parts of Es3l to Es3m climate become warmer and wetter. B content and B/Ga reflect that the overall salinity is low, which is fresh-brackish water, and the salinity increases from the middle and upper parts of Es3l to Es3m The Mo-U covariation pattern and Corg/P molar ratio reflect the fluctuation of Es3m—l bottom water preservation conditions, and the overall environment is a weak redox environment. Ti/Al, K/Al, Zr/Al indicate that the terrigenous detritus input was strong during Es3m—l, with a high deposition rate, and the deposition rate decreased from the lower part Es3l to Es3m. The quantitative recovery of paleo-productivity showed that the paleo-productivity of the lake in Es3m—l period was high. The enrichment of organic matter in Es3m—l mudstone in southwestern Bozhong Sag is the result of the combined effects of paleoclimate, preservation conditions, sedimentation rate and paleoproductivity.
The piedmont structure of West Kunlun Mountains is complex, and the fluid field characteristics of different tectonic units and their relationship with oil and gas accumulation have not been revealed. By sorting out the relationship among structure, overpressure and oil and gas, this paper provides examples for the study of overpressure and oil and gas accumulation in foreland basin, and provides basis for oil and gas exploration and development deployment in front of West Kunlun Mountains. According to the measured formation pressure and drilling fluid data, the distribution characteristics of abnormal high pressure in the southern margin of West Kunlun Mountains are clarified. The main formation mechanism of overpressure is confirmed by the combination of logging curve and loading-unloading curve method combined with geological analysis. Based on the structural evolution history and hydrocarbon accumulation process, the relationship between overpressure and hydrocarbon accumulation is discussed. The results show that there is strong overpressure in the deep strata of the piedmont. There is weak overpressure in the shallow strata of the Kekeya structural belt. Tectonic compression is the main cause of overpressure formation in the study area, followed by vertical overpressure transmission along the fault. The overpressure transmission caused by oil and gas charging is the decisive factor for the development of overpressure in shallow reservoirs, which is often accompanied by the development of weak overpressure. Tectonic compression provides power and migration channels for oil and gas migration, and the resulting uplift, denudation and fault activities control the preservation conditions of regional caprocks. In the interior of the sag, the rapid increase in burial depth promotes the rapid maturation of source rocks.
China's natural gas is mainly distributed in Ordos,Sichuan,Tarim and Qaidam basins,mainly coal-type gas,with a part of oil-type gas and a small amount of bio-gas.The main oil-type basins in China are characterized by more oil and less gas.Compared with Ordos and other gas-producing basins, the proved reserves and production of natural gas are far from enough.The global natural gas is mainly oil-type gas,and the large "oil-type basin" shows the characteristics of both abundant oil and gas resources, which is inconsistent with the current exploration reality in China.Based on the investigation of domestic and foreign oil and gas exploration achievements and knowledge,the paper summarizes the knowledge of oil-type gas exploration field,compares the accumulation conditions of oil and gas basins at home and abroad,and analyzes the gas generation potential of oil-type gas in China's oil basins.It is concluded that China's oil basins have huge oil-type gas exploration potential and broad development prospects,which is one of the important directions of natural gas exploration in the future.
Many sets of source rocks are developed in the Tugeerming and surrounding areas, but the main source rocks and hydrocarbon generation potential are not clear, which restricts oil and gas exploration. Based on the comprehensive analysis of drilling, seismic, oil and gas and geochemical data of source rocks in the Tugeerming and surrounding areas, it is believed that there are four sets of source rocks in the Triassic-Jurassic strata from bottom to top in the Tugeerming and surrounding area: the mudstone of the Triassic Taliqike Formation (T3t) coal measures have thin thickness, limited distribution, medium organic matter abundance. The organic matter type is mainly type III, which is in the mature-high mature stage. The lacustrine mudstone of the first member of Yangxia Formation (J1y1 ) in Jurassic have thin thickness, stable distribution and high organic matter abundance. The organic matter is mainly type II1. The mudstone source rocks in the second to fourth members of the Yangxia Formation (J1y2-4) and the Kezilenuer Formation (J2kz) have large thickness, stable distribution, high organic matter abundance. The organic matter is mainly types II2-III. Jurassic source rocks are mainly in the low mature-mature stage. Through the hydrocarbon biomarkers and carbon isotope analysis of source rocks, it can be seen that the oil and gas in Tugeerming area mainly come from the Jurassic source rocks. The oil and gas generated by the deep source rocks in the north and south wing of the pitching of Tugeerming area, migrated through the thrust fault and the unconformity surface on the top of the Triassic and Jurassic. The oil and gas are vertically filled in the Yangxia Formation and the Kezilenuer Formation through the fault, forming two sets of rich oil and gas reservoirs. It is pointed out that the Kezilenuer Formation and the Yangxia Formation are the main target layers for oil and gas exploration in this area.
Crude oil from the Lungu area of the Tarim Basin is mainly affected by gas washing and thermal cracking, which makes the oil enriched in diamondoids. This study applied GC-MS-MS on the whole oil samples for quantitative analysis of diamondoids. It was found that, either among the diamondoid isomers or the homologs, the oils at relatively low thermal maturity show the dominant abundance of secondary-substituted diamondoid isomers, while oils with higher thermal maturity show the dominance of tertiary-substituted isomers. The abundance difference of diamondoid structural isomers from the oils with a range of thermal maturity can be potentially used as a qualitative scale for oil maturity, especially at high thermal stage. The valence bond theory is used to figure out the chemical mechanism behind the aforementioned phenomena. Compared with that one sp3 hybrid orbital is occupied by C-H single bond in the secondary-substituted isomer, the four sp3 hybrid orbitals in the tertiary-substituted diamondoid isomer are occupied by C-C single bond making the whole molecule more stable. The coincidence degree of the atomic orbits in the tertiary-substituted isomers is higher with a greater bond dissociation energy than the secondary-substituted, which could be attributed to the observation that the thermal stability of diamondoid isomer with methyl substitution at bridgehead (tertiary-substituted) is higher than that of secondary-substituted methyl isomer. At the same time, the physical trait of decreasing saturated vapor pressure of compounds with increasing diamondoid carbon number explains the phenomena of differential enrichment of diamondoid series compounds by gas washing.
Gas washing is one of the important driving forces for the late stage reformation of oil and gas reservoirs. Large-scale gas washing occurred in Xihu Sag of East China Sea Shelf Basin, but the latest research is limited to description, and there is no systematic study on the geochemical characteristics of gas washing. The geochemical analysis of the crude oil from the Pinghu Slope Belt and Tiantai Inversion Belt in the Xihu Sag shows that the sterane and terpane fingerprints and carbon isotope compositional characteristics of crude oil are similar, which can be believed to belong to the same oil family with a similar thermal maturity. However, the physical properties and distribution characteristics of light hydrocarbons, n-alkanes and diamondoids of the oils are different. Based on the calculation of the paraffinicity index, aromaticity index, n-alkane mass depletion and absolute content of diamondoid hydrocarbons, it is shown that gas washing is the main reason for the difference, not biodegradation or thermal cracking. In order to further delineate the gas washing process and genesis of the crude oil, this study discusses the controlling factors of the gas washing, and comprehensively analyzes the geological and geochemical data of oil and gas reservoirs. The results show that：(1) The oils from the northern Pinghu Slope Belt is mainly residual oil after strong evaporation fractionation. Later, it may be mixed with high-mature gas from lateral migration in the east, resulting in the increase of some light hydrocarbons and adamantane compounds in the crude oil. (2) The oils from the southern Pinghu Slope Belt are mainly affected by migration fractionation, resulting in shallow layer accumulation of condensate oil and in deep layer accumulation of light oil and waxy oil. (3) The oils from the Tiantai Inversion Belt are mainly condensates formed in the late stage of gas washing, and the residual oil has not yet been found, which implies that the deep layer may have a certain scale of light and waxy reservoirs.
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