The Leikoupo Formation of the Middle Triassic is one of the hot strata of marine natural gas exploration in western Sichuan Basin in recent years. The petroleum geologists have different opinions on the reservoir-forming conditions and the types of gas pools. The research shows that the 4th member of the Leikoupo Formation in the western Sichuan Basin has favorable hydrocarbon accumulation conditions. It not only has a large scale stable distribution of tidal flat dolomite reservoir， but also develops two sets of source rocks： the Permian source rock and the Leikoupo Formation source rock. There are three models of hydrocarbon accumulation in the 4th member of Leikoupo Formation from west to east in the western Sichuan Depression， namely， the “lower generation and upper reservoir” structural gas reservoir transported by cross-layer source faults and fractures， the “lower generation and upper reservoir” structure-formation gas reservoirs transported by the relay combination of small source faults and intra-layer fractures， and the “self-generating and self-storage” lithologic gas reservoir transported by intra-layer fractures. After the proved reserves of 100 billion cubic meters of structural gas reservoir in the 4th member of the Leikoupo Formation in the western Sichuan Basin， it is considered that the area near the pinch-out line of the 4th member of the Leikoupo Formation in the eastern depression is a favorable area for exploration of tectonic-stratigraphical gas reservoirs， and the middle and lower part of the eastern slope zone （Guanghan slope zone） in western Sichuan Depression is a favorable area to explore lithologic gas reservoirs.
The diagenesis and porosity evolution of the 3rd member of Xujiahe Formation（Xu3 member） tight sandstone reservoir in the Western Sichuan Depression were analyzed by combining data from cores， thin sections， cathode luminescence （CL）， X-ray diffraction （XRD）， scanning electron microscope （SEM）， fluid inclusion， porosity and permeability. It was found that lithic quartz sandstone and lithic sandstone are mainly developed in the Xu3 member in the Western Sichuan Depression， and the maturity of structure and composition is poor to medium. The reservoir space is mainly composed of feldspars and rock fragment dissolution pores， with porosity < 6% and permeability < 1×10-3 μm2， which belongs to typical tight sandstone reservoir. The diagenesis of the Xu3 member sandstones included compaction， cementation， dissolution and replacement. The diagenetic evolution process is compaction → early atmospheric fresh water dissolution → early quartz， feldspar， calcite and dolomite， clay minerals such as kaolinite cementation → medium-term organic acid dissolution → medium-term quartz， clay minerals such as kaolinite， ferrocalcite and ankerite cementation → late coarse-giant crystal calcite filling fractures. Through quantitative calculation of the diagenetic evolution process of sandstone reservoir in the study area， it can be found that the original porosity of sandstone is 37.30%， which decreases to 9.55% after compaction， increases to 12.15% after early dissolution， decreases to 9.49% after early cementation， and increases to 10.14% after organic acid dissolution. Finally， the porosity is reduced to 3.95% after medium-term cementation and late coarse-giant crystal calcite filling. The results indicate that although the dissolution produced secondary pores （increasing pore rate by 8.71%）， the porosity reduction effect （decreasing pore rate by 74.39% and 23.72%， respectively） caused by compaction and cementation is the key factor for the densification of the Xu3 member sandstone reservoirs.
To clarify the characteristics and main controlling factors of Xujiahe Formation gas reservoir in central Sichuan Basin， and to guide the prediction of favorable gas bearing areas effectively， this paper takes the 2nd member of Xujiahe Formation in Anyue-Moxi areas as an example. Comprehensive analysis of the gas reservoir characteristics and main controlling factors of the Xujiahe Formation reservoirs have been carried out. An effective geophysical method is used to detect the gas reservoir and predict the gas-bearing favorable zone. The results show that： （1） The 2nd member of Xujiahe Formation in Anyue-Moxi is mainly fracture developed lithologic gas reservoir， with relatively developed low porosity and low permeability pore type or fracture-pore type reservoir. It has favorable source-reservoir-cap combination conditions， and presents the distribution characteristics of large area and overall gas bearing on the plane. （2） The enrichment and high yield of gas reservoirs are mainly affected by source conditions， high-quality reservoirs， local structures and the fault distribution. Gas-bearing favorable areas are centrally controlled by the distribution of high-quality reservoirs and small and medium-sized faults. （3） The distribution of gas-bearing favorable zone can be effectively predicted by the combination of reflection characteristics comparison of near-far traces and dominant amplitude and dominant frequency technology. This method can play a great role for the favorable target exploration of this kind of gas reservoir.
In recent years， the Middle Permian in central Sichuan Basin has become a popular reservoir for natural gas exploration， with the high-yield industrial gas flow obtained from the exploration of Wells JT1， NC1， TT1， GS18 and MX42 in the Maokou and Qixia formations of the Middle Permian. The research shows that the hydrocarbon supply strata of Middle Permian natural gas in central Sichuan Basin are mainly the source rocks of Lower Paleozoic and Middle Permian， and the natural gas is mostly mixed source gas. At present， the research degree of Lower Paleozoic marine source rocks in Sichuan Basin is relatively high， but the research degree of Middle Permian marine source rocks is relatively low. Therefore， it is urgent problem to determine the distribution and hydrocarbon generation potential of middle Permian Marine source rocks. In order to describe the characteristics and hydrocarbon generation potential of the Middle Permian marine carbonate source rocks in central Sichuan Basin， this paper describes in detail the geochemical characteristics， logging response characteristics and distribution characteristics of the source rocks， and systematically evaluates the hydrocarbon generation intensity of the Middle Permian marine source rocks in central Sichuan Basin. The results show that： （1）The marine source rocks of the Middle Permian in central Sichuan are mainly poor to medium quality source rocks， and the quality of the source rocks of Maokou Formation is better than that of Qixia Formation； the organic matter types of source rocks are mainly type Ⅱ1， followed by type Ⅱ2， which are in the over-mature stage； （2）The middle Permian in the central Sichuan basin was a reductive sedimentary environment with certain salinity；（3）The logging response characteristics of organic carbon （TOC） in source rocks are clear， and the organic carbon （TOC） has the best correlation with natural gamma （GR） and acoustic time difference （AC）， so the TOC content of organic carbon can be predicted for the Middle Permian in central Sichuan Basin； （4）Vertically， the source rocks are mainly developed in the middle and lower parts of Qixia Formation （P2q1） and the middle and lower parts of Maokou Formation （P2m1-P2m2c）； Horizontally， it shows the characteristics of gradual thickening from north to east； （5）The hydrocarbon generation intensity of Maokou Formation in central Sichuan Basin is better than that of Qixia Formation， mainly ranging of （1.0-4.0）×109 m3/km2， increasing gradually from southwest to northeast， and the total hydrocarbon generation of Maokou Formation is 57.38×1012 m3.
Permian volcanic rocks in the Sichuan Basin have good prospects for exploration. However， many wells in the eastern basin have showed volcanic rocks， but no corresponding research work has been carried out. Based on the analysis of drilling， logging and seismic data， a comprehensive study of the identification mode and distribution characteristics of Permian volcanic rocks in the Longhuichang-Longmen area in the eastern Sichuan Basin are examined. The results show that when one more peak reflection is formed on the seismic profile in the Longtan Formation， it represents the development of volcanic rocks； the volcanic rocks in the area are dominated by overflow facies basalts， but tuffs are rare. The vertical direction can be divided into two eruption periods， mainly developed in the middle and lower parts of the Longtan Formation； the distribution of volcanic rocks on the plane is controlled by the paleogeomorphology and basement faults. The depression of the paleogeomorphology and near the basement faults are the main accumulation parts of volcanic rocks with a large thickness.
The shales in the 2nd member of Shanxi Formation in the Daning-Jixian blocks， east margin of the Ordos Basin were deposited in a marine-continental transitional environment during the Permian. The Shan23 sub-member is characterized by large thickness， few and thin interlayers， which is the key target for transitional shale gas exploration and development. However， there are relatively few related studies on the reservoir characteristics， especially the main controlling factors for the development of the high quality reservoirs need to be further clarified. In this paper， the reservoir characteristics of Shan23 sub-member in Daning-Jixian blocks are systematically studied and the main controlling factors of high quality reservoir development are discussed by using geochemical analysis， compositional analysis and microscopic characterization method. The results indicate that the lower section of the upper lagoon facies in the Shan23 sub-member has the characteristics of high TOC content， high brittle mineral content， high values of BET and BJH， which is the sweet spot for shale gas exploration and development. Pores developed in clay minerals and in organic matter contribute most to the porosity of the Shan23 sub-member， accounting for 76.9% and 18.7% of the total porosity respectively. SEM observation （resolution>6 nm） discloses that different components in shale have great difference in pore size distribution， and the pores developed in organic matter and calcite are mainly in meso-scale. The pore size distribution characteristics of clay minerals are similar with quartz pores where pores developed both in meso and macro scale. The pores developed in feldspar and pyrite distribute in a wide range and the distribution is relatively uniform. Single factor analysis shows that the content of the clay minerals is the dominant controlling factor for pore development in the Shan23 sub-member. The organic matter content has a certain effect on the pore development of shale， but the influence of the organic matter type on the pore development is not obvious.
The shale of marine-continental translational facies is characterized by thin bedded layers， various lithology and multiple interstratifications. Aiming at the high-resolution sequences correlations of these shales， we establish a stratigraphic delineation method combining high-resolution stratigraphic sequence and cyclic stratigraphy. Using the data from logging curve-assisted rock cores and field section， the INPEFA curve obtained from the processing of logging curve and the time-frequency features extracted by wavelet transform， the high-resolution stratigraphic interface identification and multi-well isochronous comparison of the transitional shale in the 2nd Member of the Shanxi Formation were obtained. Compared with the conventional natural GR， the medium and short-term cycles points of the GR-INPEFA curve can significantly improve the identification accuracy of sequences interfaces. Wavelet coefficient curves and time-frequency mapping analysis with different scale factors can achieve the identification and comparison of medium-term and short-term cycle interfaces. Integrated INPEFA and wavelet transform technology， the 2nd Member of the Shanxi Formation is divided into three mid-term cycles （from bottom to top： MSC1， MSC2 and MSC3） and 12 short-term cycles （from bottom to top： SSC1-SSC12）. The three medium-term cycles correspond to Sha21， Shan22 and Shan23 sub-members respectively. The short-term cycle are well coupled with the evolution of higher-order sequences and strata development. Finally， the coupling relationship of high-resolution sequence， mineral composition associated with reservoir characteristics is analyzed. It showed that SSC1 in lowest part of MSC1 is prone of excellent shale reservoirs. The medium-term and short-term cycles held significant implications for further stratigraphic correlations and exploration activities. The technic integration of multi-scale based logging cycles shows advantages for stratigraphic framework construction in a lithologically-complex shale interval.
The development of marine-continental transitional shale gas in our country is in its infancy， and the study of shale facies division and reservoir characteristics are conducive to further exploration of shale gas. A series of experiments such as analytical data of thin section， whole rock component X-ray diffraction， scanning electron microscope， high-pressure mercury intrusion， CO2 adsorption， etc. are used to conduct detailed research on the marine-continental transitional shale in the study area. Combining the mineral composition and organic matter content， the shale of the Shan23 sub-member is divided into eight different lithofacies， among which four lithofacies are mainly developed： Organic-rich argillaceous-siliceous shale （I2）， organic-rich dolomitic- argillaceous-siliceous shale （I3）， organic-rich dolomitic-siliceous-clay shale （II3）， organic-rich dolomitic-siliceous-clay shale （II4）. The results show that the organic-rich dolomitic-siliceous-clay shale （II3） is the optimal lithofacies， with good hydrocarbon generation potential， the largest pore specific surface area， and well-developed laminae； the organic-rich argillaceous-siliceous shale （I2） is the sub-optimal lithofacies， also developed laminar structure， with the largest pore volume. The organic-rich dolomitic-argillaceous-siliceous shale （I3） and the organic-rich dolomitic-siliceous-clay shale （II4） are medium lithofacies， with general laminate development， and the development degree of reservoir is lower than that of the other two lithofacies， which is not conducive to exploration and development.
Focusing on shale gas enrichment conditions of Upper Permian Longtan Formation transitional shale in Southeast Sichuan to Northwest Guizhou， this paper primarily discusses source rock quality， reservoir conditions and gas content by using measurements， e.g.， organic petrology， kerogen carbon isotope， X-ray diffractometer （XRD） and field emission scanning electron microscopy （FE-SEM）. Results show that the Longtan shale in Southeast Sichuan to Northwest Guizhou is characterized by high organic matter abundance （average TOC value is 3.50%） and high thermal maturity （average RO value is 2.23%）. The organic macerals are dominated by vitrinite， followed by inertinite， indicating type III kerogen. It is a set of high-quality gas source rocks. The shale reservoir has high physical properties with average porosity of 5.56%， the reservoir is dominated by clay mineral pores， where mesoporous and micropores among I/S mixed layers are well developed. Organic macerals are main controlling factors on the organic pore development in Longtan high to over-high organic-rich shale. Shale varies greatly in adsorbed gas content and total gas content， and the organic matter abundance is an important factor controlling adsorption capacity and gas content.
Taking the black shale in Qinshui Basin as the main research target， based on the analysis of the data， borehole sampling and experimental testing of three investigation wells， including Well Y2 in the north， Well Y5 in the middle and Well Y3 in the middle and south， the organic geochemistry and reservoir physical properties of coal measure shale gas in Qinshui Basin were systematically studied in the stratified section. The results show that the average TOC of coal measure shale samples in the study area is greater than 2.0%， and the kerogen type is mainly type Ⅲ kerogen. The brittleness coefficient of each shale is generally between 30% and 40%， and the brittleness index is Ⅳ>Ⅲ>Ⅰ>Ⅱ. The gas survey shows that the gas layer is mainly the gas layer， and the gas survey shows the grade Ⅳ>Ⅱ>Ⅲ>Ⅰ. Based on the thickness， total organic carbon content， organic matter maturity， brittleness characteristics and gas bearing characteristics of coal measure shale， the favorable sequence of shale gas exploration and development potential is Ⅱ>Ⅳ>Ⅲ>Ⅰ.
In order to investigate the fractal characteristics of micropore， mesopore and macropore in shale reservoir and the applicability of currently widely used fractal models， the shale gas reservoir of Taiyang Formation in Ordos Basin was selected as research object， and the pore structure parameters of shale were tested by using high-pressure mercury injection experiments and cryogenic nitrogen adsorption experiments. Besides， the fractal dimension of macropore in the results of high pressure mercury injection test was calculated by the MENGER sponge model， the fractal dimension of mesopore pore in the results of cryogenic nitrogen adsorption testes was calculated by the F-H-H model， and the V-S model was introduced to calculate the fractal dimension of micropore in the results of cryogenic nitrogen adsorption test. The goodness of fit （R2） was regarded as the evaluation index， and the best fractal dimension calculation model of pores were discussed with the calculated results of predecessors. The results exhibit that， D1， D2 and D3 are in the range of 2.013 6-2.294 4 （2.113 2 averages）， 2.579 3-2.762 2 （2.640 5 averages） and 2.786 3-2.998 5 （2.933 9 averages）， respectively. The fractal dimension increases with a rise in pore diameter， the pore surface converts rougher and the pore structure converts more complex. F-H-H model and V-S model are the best models to calculate the fractal dimensions of mesopore and micropore in shale， with the R2 value higher than 0.99， especially the former is the best （the R2 value is higher than 0.998）. Although the calculation process of the menger sponge model for macropore fractal dimension also exhibits a high degree of fit， the R2 value and stability are apparently worse than the former two. Results from this work are of certain reference significance to calculate the fractal dimension of transitional shale gas reservoirs.
The marine source rocks of Lower Cambrian are mainly developed in Luonan and Zhoujiahe areas in the southern margin of Ordos Basin. The lithology of source rocks is mainly shale， with local dynamic metamorphism and argillaceous slate. The organic geochemical analysis shows that the thickness of the source rock is about 0-80 m， and the total organic carbon content （TOC） is between 0.10% and 13.75%， with an average of 3.32%. 91% of the samples have TOC content greater than 1%， and 52% of the samples have TOC content greater than 2%. The type of organic matter is mainly type II1， with a small amount of type I. The source rocks have experienced a high degree of thermal evolution， with equivalent vitrinite reflectance of 2.25% - 2.71% and the methylphenanthrene ratio （F1） between 0.46 and 0.60. The overall evaluation is high quality gas source rocks. Although the Cambrian-Ordovician was uplifted and eroded by the Caledonian and Huaiyuan Movements in the Ordos Basin， the Cambrian-Ordovician strata in the Qingyang paleo-uplift and the area to the south remained generally inclined to the south， the oil and gas generated from the Lower Cambrian marine source rocks developed in the North Qinling-Luonan area can migrate and accumulate in the direction of the Qingyang paleo-uplift. The characteristics of Cambrian natural gas in the south of Ordos Basin are similar to those in Gaomo area of Sichuan Basin， and they are both carbon isotope reverse sequence oil type gas. The carbon isotope of ethane from Cambrian natural gas is the lightest in the Lower Paleozoic， and the carbon isotope distribution of methane is similar to that of kerogen in the Lower Cambrian source rocks. There are direct evidences that the Lower Cambrian source rocks have contributed to the hydrocarbon accumulation of natural gas in the southern Ordos Basin. Besides， the relationship between Cambrian reservoir bitumen and Lower Cambrian source rocks in the south of the basin is the indirect evidence of the Cambrian gas accumulation in the south of the basin.
A certain amount of H2S is found in the crude oil associated gas of Jurassic Yan'an Formation in Pengyang area of Ordos Basin. The study of its genesis is important for us to predict the distribution of H2S in Mesozoic in the basin. The crude oil of Jurassic Yan'an Formation area mainly comes from the 7th member of Yanchang Formation （Chang 7 member） source rock of Yanchang Formation. H2S is not detected in the crude oil associated gas from Chang 10 to Chang 3 members of Yanchang Formation. Therefore， the formation of H2S in the crude oil associated gas of Yan'an Formation may be related to reservoir and reservoir forming factors. The analysis shows that Jurassic formation water has high salinity and rich in divalent sulfur， which inhibits the development of sulfate reducing bacteria. The possibility of H2S generation from biologically reduced sulfate is low. The δ34S values of H2S in crude oil associated gas of Yan'an Formation are more than 25‰， which are obviously different from those of volcanic hydrothermal solution， organic matter and microbial reduction products. The δ34S values are similar to the divalent sulfur isotope value in formation water and about 10‰ lower than that of the sulfate ion in formation water. The H2S has the characteristics of sulfur isotope distribution and fractionation characteristics generated by TSR. The temperature measurement of rock inclusions in Jurassic Yan'an Formation shows that the initial charging temperature of oil and gas is higher than 100 ℃， and the maximum reservoir forming temperature is 150 ℃. The formation water of Yan'an Formation contains a large number of sulfate ions from anhydrite or buried stage. Those rich sulfate ions meets the conditions for sulfate thermochemical reduction reaction. At the same time， the rich magnesium ions in formation water catalyze the reaction. Therefore， H2S in Jurassic crude oil associated gas in Pengyang area of Ordos Basin is of TSR origin.
In recent years， major breakthroughs have been made in Feixianguan Formation of Jiulongshan gas field in northwestern Sichuan Basin， and the natural gas production of Well Long016-H2 is 131.90×104 m3/d， open flow 270.96×104 m3/d， showing its good exploration and development prospect. The study on the origin and source of natural gas can provide geological basis for expanding exploration and development achievements in the next step. According to the analysis and test data of field and downhole samples， through the analysis of natural gas composition and carbon isotope composition， combined with the comparison of biomarker compounds in source rock and reservoir asphalt， it is considered that methane is the main component of natural gas，with low contents of heavy hydrocarbons and dryness coefficient greater than 0.99，which is a typical dry gas with high evolution characteristics； The values of δ13C1， δ13C2 of natural gas are high， which belongs to the mixture of crude oil cracking gas and type II kerogen cracking gas at high-over mature stage， and kerogen cracking gas is the main gas； The biomarker compounds of reservoir asphalt are similar to the source rocks of Upper Permian Wujiaping Formation and Dalong Formation， indicating that the natural gas of Feixianguan Formation in Jiulongshan gas field is mixed source gas， which comes from Wujiaping Formation and Dalong Formation.
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