The generation mechanism of abiogenic alkane gas and whether the gas has industrial values are the focus of international attention. CH4 generated by Fischer-Tropsch synthesis is an important constitute of abiogenic alkane gas. The study on the components， isotopic compositions， and crust-mantle mixing model indicates that， the alkane gas from the Qingshen gas field in the Songliao Basin is contributed by abiogenic alkane gas from Fischer-Tropsch synthesis， besides the mantle-derived abiogenic alkane gas. The authors have selected the endmembers of typical biogenic and abiogenic gases and conducted mixed source calculation. The abiogenic alkane gas accounts for over 600×108 m3 in the proven reserves of the gas field， in which the amount of CH4 generated by Fischer-Tropsch synthesis exceeds 96×108 m3. The calculation results indicate that， abiogenic alkane gas under organic-inorganic interactions can form commercial accumulations. Since typical biogenic gas is predominant in conventional gas pools， and high- to over-mature coal-derived gas displays a certain similarity with abiogenic alkane gas， the typical characteristics of abiogenic gas is concealed. Therefore， abiogenic alkane gas generated by organic-inorganic interactions may exist widely in petroliferous basins， and its contribution to gas pools may be underestimated previously.
Tight gas exploration is an important part of China's unconventional energy strategy. The multi-stage channel three-dimensional exploration of Jurassic Shaximiao Formation tight gas in Jinqiu Gas Field in central Sichuan Basin has made continuous production breakthroughs. The evolution process analysis of hydrocarbon fluids in tight gas reservoirs is the key to study the accumulation mechanism of tight gas. Taking the sandstone core of Jurassic Shaximiao Formation in Qiulin and Jinhua gas fields in central Sichuan Basin as the research objects， based on petrological observation and in-situ micro-Raman spectroscopy observation， it is found that there are four types of reservoir diagenetic minerals： pure CH4 inclusions， pure CO2 inclusions， mixed CH4-CO2 gas inclusions and N2 rich gas inclusions. The pressure-temperature-time-composition （PVT-x） properties of CH4 and CO2 inclusions are obtained by Raman analysis and thermodynamic model of fluid inclusions. The density of pure CO2 inclusions is calculated by using the distance between Fermi peaks of carbon dioxide， two stages of CO2 fluid accumulation were found： primary CO2 accumulation stage （high density： 0.874-1.020 g/cm3； high homogenization temperature：>210 ℃） and secondary carbon dioxide accumulation period （high density： 0.514-0.715 g/cm3； low homogenization temperature： about 180-200 ℃）. CO2 fluids with abnormal high and uniform temperatures speculate that deep hydrothermal fluid activity and emplacement of reservoirs. The paleo-fluid pressure （44.0-58.5 MPa， mean paleo-pressure coefficient of 1.29） calculated by the methane Raman stretching vibration peak provides important evidence to recover the pressure evolution. The hydrocarbon accumulation was in the Late Cretaceous （about 75-65 Ma）， close to the early Himalayan uplift period （the deepest stage of stratigraphic burial）， and the late organic gas displacement replaced the early inorganic carbon dioxide accumulation. The paleo-pressure recovery of fluid inclusions indicates that the reservoir evolves from weak overpressure to atmospheric pressure， and the weak overpressure indicates that the reservoir has better preservation conditions in the early uplift.
There are many types of transitional facies shale， and the types of organic matter are complex. They are neither completely the same as continental nor completely the same as the characteristics of marine shale. Discussing transitional shale facies and their characteristics is a further understanding of shale gas storage space. This paper takes the Well M1 and Well DC1 shale of Permian Longtan Formation in the Sichuan Basin and its surrounding areas as the research objects ， combined with core observation， ordinary optical thin section observation， and argon ion polishing scanning electron microscope observation. The lithofacies are divided and compared and their characteristics are described. The results indicate that the shale lithofacies of the Longtan Formation in the typical areas of the Sichuan Basin and its periphery can be divided into five major categories according to the mineral content， which are clay shale facies， calcareous shale facies， siliceous shale facies， calcareous mixed shale facies， siliceous mixed shale facies. According to the abundance of organic matter， eight sub-categories are further divided into ultra-high organic matter siliceous shale facies， ultra-high organic matter siliceous mixed shale facies， ultra-high organic matter clay shale facies， high organic matter siliceous shale facies， high-organic clay shale facies， medium-organic calcareous shale facies， medium-organic clay shale facies and low-organic calcareous shale facies. Clay shale facies， siliceous shale facies and siliceous mixed shale lithofacies with high abundance of organic matter is favorable production zone， while calcareous and calcareous mixed shale facies with low abundance of organic matter is an unfavorable production zone. Clay shale facies is the most commonly developed lithofacies in shale facies. The abundance of organic matter in clay facies shale facies is higher than that of marine clay shale facies shale.
The Cambrian Xixiangchi Group in central Sichuan Basin has great exploration potential， and the intra-platform bank is the main reservoir， and its development characteristics are the focus of the study. Through the study of outcrop， drilling， logging and seismic data， the natural gamma-ray spectrum logging and wavelet transform are combined with conventional methods to study the sequence stratigraphic division and the development characteristics of the intra-platform bank. The following three new understandings are obtained. Firstly， the Xixiangchi Group is divided into four third-order sequences （SQ1-SQ4）， in which the transgressive systems tract and high-stand systems tract of SQ1， SQ2 and SQ3 are well developed， and the SQ4 denudation is more serious. Secondly， the Xixiangchi Group in central Sichuan Basin mainly develops limited platform facies， which can be further subdivided into three subfacies： platform interior flat， lagoon and intra-platform bank. Within the sequence stratigraphic framework， the sequence lithofacies palaeogeographic map is compiled. The intra-platform banks are mainly developed in four third-order sequences in the form of thinly laminated， among which the high-stand systems tract of sequence SQ2 and SQ3 are dominant. Thirdly， Leshan-Longnvsi paleo-uplift and the relative sea level change commonly control the vertical and horizontal distribution and superposed pattern of the intra-platform bank body， the paleo-uplift core and the underwater high point are the first to develop the intra-platform bank， while the wing part has a large accommodation space， the cumulative thickness of the bank body is larger than that of the core， and different secondary sea level change rates affect the monomer thickness. Favorable intra-platform bank reservoirs are mainly developed in the southwest， northeast and central areas of central Sichuan Basin， which is a favorable exploration zone. The results are of great significance for enriching thick single carbonate sequence stratigraphy and guiding oil and gas exploration in central Sichuan Basin.
The discovery of a series of natural gas in the Bongor Basin， Chad has opened up a new situation of simultaneous oil and gas development. There are obvious phenomena of oil and gas co-existence in D area of the southern depression. The study on the distribution characteristics and genesis of oil and gas in D area is of great significance to the natural gas prospect of Bongor Basin， and is conducive to the further understanding of the source rock potential of Bongor Basin. Taking Well D-1 as an example， the effect of natural gas on the accumulation of crude oil is analyzed by using the parameters such as natural gas composition， isotopes， light hydrocarbons， as well as the parameters such as crude oil chromatography and chromatism. Results show that crude oil at different depths of Well D-1 is the product of the same source in the same evolutionary stage. It was mainly affected by gas washing and migration fractionation in the later period. Natural gas brings the light components from deep reservoirs to shallow reservoirs， which enriches the light components in shallow reservoirs. The chromatogram shows the double peak feature， and the main peak of the front peak has obvious low carbonization. At the same time， natural gas mainly exists in shallow reservoirs， forming the coexistence of oil and gas.
The quality and production of shale gas are directly dependent on the development of fractures. Especially in the large-scale fracturing development， natural and hydraulic fractures play a significant role. Therefore， there is a growing demand for the quantitative characterization of shale gas reservoir fractures and the development of geological modeling technology. The natural fractures of shale gas reservoirs are mainly composed of structural fractures and non-structural fractures. The former is mainly controlled by external factors such as tectonic stress， while the latter is controlled by internal factors such as rock and mineral composition. The main characterization parameters and prediction techniques of fractures of different scales are various. Natural fracture models for shale gas reservoirs are mainly aimed at largescale and mesoscale fractures， which are respectively established. Hydraulic fracture modeling is more deeply studied in numerical simulation. Considering the current and future technology and production requirements， this paper puts forward and discusses the key directions of natural and hydraulic fracture modeling in shale gas reservoirs from the aspects of multi-information application， multi-technology integration and multi-discipline crossing. Natural fractures require multi-scale and multi-method fusion characterization and modeling. Hydraulic fracturing requires the fusion of multiple information constraints of forward and inverse simulation. Furthermore， combined with the concept of geology-engineering integrated modeling， this paper discusses the inner relationship and development of the geological model of shale gas reservoir from three aspects of model type， data integration and model fusion， so as to guide the efficient development of shale gas reservoirs in China.
At present， there are no reports about the organic-rich shale and its sedimentary environment of the Leikoupo Formation in Sichuan Basin. To this end， based on the latest drilling and coring data in the basin， through the observation of cores and thin sections， combined with X-ray diffraction of full-rock mineral analysis and trace element geochemical index identification， the organic-rich shale petrological characteristics of the Leikoupo Formation were developed. The study analyzed the paleoenvironment of organic-rich shale sediments， discussed the main controlling factors of organic matter enrichment， and pointed out the exploration potential of the sub-salt field of the Leikoupo Formation. The results show that： （1）The organic-rich shale in the Leikoupo Formation in the Sichuan Basin can be divided into two types of shale facies， clayey calcareous mixed shale and calcareous shale， with an organic carbon content of 0.58%-1.08% and RO of 2.36%-2.40%； （2） Sr/Cu， Sr/Ba， B/Ga， V/（V+Ni）， paleo-oxygen phase substitution coefficient and other indicators indicate that the Leikoupo Formation organic-rich shale is generally in a dry-hot， salt-water， and anoxic environment； （3）The evaluation of hydrocarbon generation potential shows that the clayey calcareous mixed shale in the sample is the best source rock， calcareous shale and limestone are the good or better source rocks. High paleo-productivity and anoxic environment are important conditions for the enrichment and preservation of organic matter； （4）The types of reservoir-forming combinations that may be formed by the organic-rich shale in the Leikoupo Formation in Sichuan Basin include three types： self-generation and self-storage type， side-generation and side-storage type， and lower generation and upper-storage type. It is concluded that the Meishan-Nanchong-Bazhong and Ya'an-Hongya areas are the most favorable areas for sub-salt exploration of the Leikoupo Formation in Sichuan Basin， Suining-Guang'an area is the most favorable area for salt exploration in the Leikoupo Formation.
In recent years， shale gas exploration and development of O3w-S1l1 in southern Sichuan Basin have gradually expanded to the deep reservoirs （vertical depth between 3 500 m and 4 500 m）. Preliminary progress have been made in two deep areas， including LZ block and DZ block， and the results of shale gas exploration and development are different （the former is obviously superior to the latter）， which is mainly related to the difference of enrichment conditions. This article takes Wufeng-Longmaxi formations of LZ block and DZ block as the research objects， and has comprehensively made use of the latest data of drilling， logging， well logging， seismic and experiments， and finally has clarified the characteristics， differences and main controlling factors of enrichment conditions in two blocks. The researches show that： （1）LZ block is superior to DZ block in shale gas enrichment elements， such as formation thickness， organic matter abundance， physical property characteristics， gas bearing property， as well as the thickness and quality of shale reservoir. （2）During the formation period of Wufeng-Longmaxi formations， LZ block was always located in the sedimentary center of deep-water continental shelf in southern Sichuan basin， and its paleosedimentary environment was superior to that of DZ block. And the difference of paleosedimentary environment resulted in the difference of organic matter accumulation and reservoir scale （thickness and quality） in the two blocks. （3）The preservation condition of DZ block is obviously controlled by fault-natural fracture system， while the shale gas leakage in LZ block is less affected by fault-natural fracture system. The pressure coefficient is the largest in southern Sichuan Basin， and the preservation condition is better. The comprehensive study suggests that， in the two deep areas of LZ block and DZ block in southern Sichuan Basin， the main controlling factors of shale gas enrichment difference are paleosedimentary environment and preservation conditions. The concept of "exploring deep-water shale reservoir in the deep area" should be upheld in the exploration and development of deep shale gas in southern Sichuan Basin. Several wide and slow syncline structures which developed among the fault-anticlines in the south side of southern Huayingshan fault-zones should be the potential favorable areas for shale gas exploration in the next stage. And the geological conditions of S1l1-1-4 in LZ block are superior， with the feasibility of three-dimensional development for double-layers.
Shale gas has become one of the most important unconventional energy sources in ultra-low permeability reservoir. Due to its unique reservoir properties and the combined action of multi-stage hydraulic fracturing， it forms a complex multi-scale flow mechanism. On the basis of flow material balance theory， using the simplify way of reservoir structural with Five-lines Flow Model， according to the relationship of reservoir capacity and seepage characteristics， the stratum can be divide into matrix pressure system and fracturing fracture pressure system. This article divided shale gas well SRV into matrix pressure system and fracturing fracture pressure system， built a new method of shale gas well productivity evaluation which is based on the flow material balance theory. This method can accurately and effectively evaluate the decrease in shale gas production and formation energy， and proved the fracture net reservoir capacity of the fractures in shale gas well is existed. Compared with HZ Multifrac model in RTA software， the model in this article has get more accurate evaluation result in typical production wells of Fuling shale gas field， exactly matches the measured flowing bottom hole pressure and shut-in static pressure in different production period， provided a theoretical basis for scientific formulation of reasonable production allocation system for Fuling shale gas wells.
The induced stress interference is serious under the condition of dense cutting fracturing， which makes it more necessary to optimize perforation cluster parameters to promote balanced propagation from multiple perforation clusters. The fracture propagation model is proposed by coupling rock deformation and fluid flow， and fracture morphology quantitative evaluation index is established to study the fracture propagation law under limit entry perforating parameters. The results show that the influence area of induced stress field becomes more complex due to the dense cutting fracturing. For the case of high density and uniform distribution of perforation holes， the heterogeneous propagation of multiple fractures is significant due to the induced shadow effect on the internal cracks. In the case of a fracturing interval with six perforating clusters， the strategy of small diameter and low perforating density can effectively promote the uniform propagation from multiple clusters. In the case of non-uniform limited perforating， keeping the total number of perforations unchanged， increasing the number of perforations in the middle two clusters， and reducing the perforation number at both ends of the same fracturing stage， the limited entry perforation strategy can effectively optimize the fracture morphology and improve the efficiency of multi cluster fracturing.
Productivity is the core index to evaluate the development effect of shale gas field， and the comprehensive nonlinear effect of high-pressure physical property， supercritical desorption， multiple micro-flow mechanism and stress sensitivity of shale gas can’t be ignored in its contribution to productivity. In this paper， based on the conventional five-zone compound flow model， a comprehensive consideration was given to gas nonlinearity and the stress sensitivity， an improved compound linear flow model was established and a semi-analytical solution for productivity was obtained. Then， the reliability of productivity solution was verified through examples and important influencing factors were clarified. The results indicated： （1）The model comprehensively considers the impact of nonlinear effects on gas production so as to predict the medium- and long-term productivity of gas wells accurately. （2）The greater the main fracture half-length， the greater the productivity， so long fractures should be designed as much as possible； too high or too low the cluster spacing and conductivity of the main fractures do not significantly change the extent of reservoir utilization， so the values of the cluster spacing and conductivity of the main fractures in this paper can be preferably around 10 m and around 4.0×10-15 m3； the greater the stress sensitivity coefficient， the lower the productivity， and the proppant with excellent performance should be selected or the fracture slip surface should be designed to improve the fracture conductivity. （3）The significance of horizontal well spacing and horizontal well length on improving the utilization of shale reservoirs is considerable. In actual production， the horizontal well spacing can be optimized to 500 m， and horizontal well length of 2 000-3 000 m can also be adopted for technical research.
Well deployment optimization is an important link to improve reservoir development effect， reduce risk and realize economic benefits. For shale gas wells， the initial gas production is high but the decline is fast， which requires a large number of continuous drillings to meet the demand of production， so it is impossible to achieve primary well pattern， mainly deploying wells in the preferred favorable area and using rolling production strategy. Taking Haynesville， the third largest shale gas play in the United States， as an example， this paper analyzes the well deployment process of about 3 500 gas wells from 2008 to 2017 through window analysis technology， and summarizes the method of well deployment and optimization strategy. The study results show that the development of Haynesville shale gas play is divided into three stages： experimental stage， regional expansion stage and maturity stage， and different well deployment strategies are adopted in different stages. Meanwhile， in each window single well is evaluated dispersedly in the early stage and single well or well group is deployed in the later stage rather than a “carpet type” continuous well distribution mode to reduce risk. The well deployment and optimization strategy of Haynesville shale gas play has certain guidance and reference for shale gas overall deployment planning， step-by-step implementation strategy and risk management in China.
High temperature and overpressure gas fields such as DFX-1， DFX-2 and LDX-1 have been discovered in Yinggehai Basin in recent years. Due to the high-temperature and overpressure conditions， low-speed mudstone is widely distributed. It is difficult to obtain accurate compaction correction factor （Cp） and the mudstone acoustic time （DTsh） in acoustic time difference analysis. The method of evaluating the primary porosity using the Wyllie formula encountered challenges. By analyzing the response characteristics and influencing factors of acoustic wave velocity of mudstone and sandstone in high temperature and overpressure strata， on the basis of clarifying the history of hydrocarbon accumulation， a large-scale statistical chart of formation pressure and gas saturation with Cp is established in different regions， which can obtain Cp under different pressures and different fluids. By analyzing the relationship between mud distribution in clastic reservoir and the history of accumulation， an accurate method for obtaining DTsh is established， which improved the evaluation accuracy of sonic porosity in high-temperature and overpressured formations， and lays a foundation for subsequent pore structure and classification evaluation. This method has achieved good results in the evaluation of related exploratory wells， and has the value of promotion and application.
The upper section of the Lower Ganchaigou Formation in the Yingxi area of the Qaidam Basin is a typical compact reservoir of mixed siliciclastic-carbonate lacustrine successions， which has tremendous potential for oil and gas resources. Classification characteristics of pore structure facies and logging characterization method are critical to economic recoverable reserves in this area. The main types of reservoir space are intercrystalline pores， intergranular pores， dissolution pores and microfractures. The distribution of aperture T2 spectrum is generally concentrated within 10 ms， and the pore and throat tend to be small and fine. According to the movable fluid effective porosity， the pore structures in the study area are divided into large pore-throat type， small pore fine throat type and tight type. Among them， the pore structure of small pore fine throat type and tight type is widely developed， and the large pore-throat type is good pore structure facies. Sandy or sandy limy dolostone are favorable for the development of microfracture. The dissolution facies is mostly developed in the high carbonate minerals. The commonly developed micritic carbonate minerals and high-content clay minerals are the reasons why the reservoir is tight. Core calibration log analysis shows that dolomite content tends to increase and clay content tends to decrease as the pore structure gets better. Based on the well logging parameters sensitive to pore structure facies， the log identification chart of pore structure facies in this area was established by principal component analysis， and the results of log identification chart were consistent with those of the NMR log results. The results of pore structure facies in single well showed that the large pore-throat type often developed in the strata with high carbonate mineral content， and the development of fractures had no obvious control on the types of pore structure facies. The economic-recoverable reserves of single wells are obviously constrained by the pore structure facies of the reservoir. Among them， the large pore-throat type facie reservoirs have good economic benefits from single wells， large pore-throat + small pore fine throat type facie reservoirs have certain economic benefits， but small pore fine throat type facie reservoirs are more difficult to get economic profit in the case of deep well depths.
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