Evolution Model Of Shale Gas Occurrence State And Gas-bearing Property Evaluation In Longmaxi Formation Of Sichuan Basin

Scientific issues:Shale gas becomes an important field of oil and gas exploration today.In the superimposed basin represented by the Sichuan Basin,the differential occurrence characteristics of shale gas are obvious.In this context,there are three key issues to be solved:（1）the main controlling factors of high-over matured shale gas occurrence state in complex superimposed basins;（2）the evolution law of occurrence state of high-over matured shale gas;（3）the evaluation method of gas-bearing property of high-over matured shale.Solving the above problems is not only of great theoretical significance for revealing the differential occurrence mechanism of high-over matured shale gas,but also of practical significance for the fine evaluation of shale gas resources and productivity evaluation.Research ideas and methods:The high-over matured shale of Longmaxi Formation in southern Sichuan Basin is taken as the main research object.Firstly,the gas generation and storage conditions of the Longmaxi shale are analyzed.Secondly,the shale gas in different occurrence states are quantitatively characterized.On this basis,the main controlling factors of shale gas occurrence state are determined and discussed by using the grey correlation method and single factor sensitivity analysis.Then,based on an improved research method of shale gas occurrence state evolution proposed in this paper,the evolution histories of shale gas occurrence state under different tectonic backgrounds are restored and compared,the differential occurrence mechanism of shale gas is revealed,and the evolution model of shale gas occurrence state is established.Finally,the gradient boosting decision tree algorithm is optimized to predict the shale gas content and obtain the spatial distribution of shale gas in different occurrence states.On this basis,the“sweet spot”of high-over matured shale gas is predicted and the corresponding shale gas resources is calculated.Results and discussion:（1）To solve the first scientific issue,the main controlling factors of the occurrence state of high-over matured shale gas are analyzed.The high-over matured shale gas occurrence state in Longmaxi Formation in southern Sichuan is mainly affected by temperature,pressure,organic matter content,pore structure and mineral composition.Low temperature,high pressure,high organic matter content and high porosity are all conducive to the occurrence and enrichment of shale gas,in which temperature and organic matter content have greater effects on adsorbed gas content,while pressure and porosity have a more significant impact on free gas content.（2）To solve the second scientific issue,the research on the evolution law of the occurrence state of high-over matured shale gas under different tectonic settings has been carried out,and the evolution model of high-over matured shale gas occurrence state during the burial process has been established,which is mainly divided into four stages:Stage I,only the adsorbed gas occurs,and the adsorbed gas content gradually increases;Stage II,both adsorbed gas and free gas occur,and free gas begins to account for the main proportion of shale gas,which is manifested in the evolution characteristics of decreased adsorbed gas content,increased free gas content,increased total gas content and increased free gas proportion.At this stage,the occurrence state of shale gas is mainly controlled by TOC content and formation pressure,followed by temperature;Stage III,both adsorbed gas and free gas occur,and free gas always accounts for the main proportion of shale gas.Specifically,the adsorbed gas content decreases,while the contents of free gas and total gas decrease first and then increase.The proportion of free gas increases gradually.The Shale reaches deep burial at this stage,and the shale gas occurrence state is mainly affected by formation temperature and shale porosity;Stage IV,both adsorbed gas and free gas exist.Whether free gas accounts for the main proportion of shale gas depends on the degree of decline in the proportion of free gas.This stage corresponds to the tectonic uplift process.The shale gas occurrence state is mainly affected by the formation temperature and pressure,showing the evolution characteristics of increased adsorbed gas content,decreased free gas content,decreased total gas content and decreased proportion of free gas.（3）To solve the third scientific issue,the research on the evaluation method of gas-bearing property of high-over matured shale was carried out.The prediction models for the adsorbed gas content and free gas content of high-over matured shale is established by the gradient boosting decision tree algorithm,with their linear regression coefficient（R²）reaching 0.65 and0.87 respectively,which shows good prediction effect.Application effect:Based on the prediction models of shale gas content,the spatial distribution characteristics of shale gas in different occurrence states in Longmaxi organic-rich shale in southern Sichuan are obtained.On this basis,the shale gas“sweet spot”of classⅠ-Ⅲin Longmaxi Formation is predicted according to the shale gas“sweet spot”evaluation scheme.The prediction results were verified to be reliable by the shale gas development situation and test production data of shale gas wells in the study area.And further using the volumetric method to calculate the realistic geological resources of shale gas of Class I“sweet spot”is 2.28×10¹²m³,with recoverable resources of 0.46×10¹²m³.The shale gas superseding geological resources corresponding to Class II“sweet spot”is 13.93×10¹²m³,with recoverable resources of2.09×10¹²m³.The potential geological resources of shale gas of Class III“sweet spot”is10.26×10¹²m³,with recoverable resources of 1.03×10¹²m³.