The Permian Shale Gas Occurrence Mechanism And Enrichment Pattern Of Typical Areas In China

The lithofacies present significant changes in the vertical and horizontal directions in Permian strata of China.Mudstone is interbedded with coal and sandstone,and leads to the accumulation of shale gas,coal bed gas and tight sandstone gas,which has important research significance.Therefore,taking the Lower Permian Shanxi and Taiyuan Formations in the northeastern Ordos Basin and the Upper Permian Longtan Formation in western Guizhou as examples,the experimental test and analysis,gas logging,gas desorption,well completion,and logging interpretation were used to study the geological characteristics.Then,shale gas evolution during geological history,occurrence characteristic and influential mechanism were discussed.Based on the lithological associations,sedimentary facies,gas generation thresholds,pore evolution and structural characteristics,shale gas enrichment factors and main influential factors of the study area were analysed,and then enrichment pattern was established.The research mainly achieved the following conclusions:?1?Based on thermal simulation,R_o measurement,Rock-Eval pyrolysis and logistic function,a thermogenic gas model was established;hydrocarbon expulsion model was established through hydrocarbon generation potential index;adsorbed gas content model was established based on temperature,pressure,and TOC.Adsorbed gas content is jointly controlled by temperature and pressure,increasing and then decreasing with depth,indicating that pressure is the main controlling factor at shallow depth and temperature is the main controlling factor at greater depth.Moreover,the depths corresponding to maximum adsorbed gas content increase with TOC,indicating that TOC inhibits the decrease of adsorbed gas content.Combining the characteristics of burial and thermal history,gas content evolution in the geological history period was analysed.Adsorbed gas and free gas contents had an increasing trend with formation subsidence and burial in the early period,and the later uplift of the formation led to the transformation of free gas to adsorbed gas.?2?Shale gas occurrence is controlled by hydrocarbon generation,hydrocarbon expulsion and adsorption capacity.Higher TOC and HI?S₂/TOC?values increase the hydrocarbon generation intensity;hydrocarbon expulsion is controlled by hydrocarbon generation potential index;the adsorption capacity increases with the increasing TOC of shale sample,while the sample source area has no significant effect on the adsorption capacity,and lower geothermal gradient and higher pressure gradient increase adsorbed gas content.When the TOC values are between 1-9 wt%,the depths corresponding to maximum adsorbed gas content are between 600-1100 m.Compared with the Shanxi and Taiyuan Formations in the northeastern Ordos Basin,the Longtan Formation in western Guizhou has a higher hydrocarbon generation strength and stronger adsorption capacity.Hydrocarbon expulsion has no significant difference in the two study areas.There are significant differences in free gas content at different locations in the study area,which are controlled by the TOC,HI?S₂/TOC?value,geothermal gradient,pressure gradient and depth.?3?Carbonaceous mudstone adjacent to coal seams presents a high gas content level,and is primarily developed in swamps in the delta plain environment,and swamps and lagoons in the barrier coastal environment.The gas generation threshold maturity?R_o?of shale?Kerogen type ??is 1.6%and the corresponding threshold depths of the northeastern Ordos Basin and southwestern Guizhou are estimated to be2265 m and 1050 m.Pore evolution is jointly controlled by hydrocarbon generation,clay minerals transformation,and compaction,and may have the tendency to decrease when R_o<1.6%or R_o>3.0%,but increase when R_o ranges between 1.6%and 3.0%,while the main influential factors of pore evolution differ in each period.Shale gas enrichment areas can be formed along the slope adjacent to coal seams?in swamps and lagoons?with moderate maturity?about 2.5%?in the northeastern Ordos Basin,and enriched in the areas adjacent to coal seams?in swamps and lagoons?with moderate maturity?about 2.5%?,abundant fractures,and favorable sealing faults in western Guizhou.