海陆过渡相页岩气地质甜点地球化学评价方法OA

[Background]China boasts abundant marine-continental transitional shale gas resources.However,marine-continental transitional shales in the country exhibit complex geological conditions characterized by rapid lateral facies transition,vertical superimposition of multiple lithologies,and small single-layer thickness.These characteristics pose a challenge to shale gas sweet spot prediction.Therefore,establishing a sweet spot grading and evaluation system tailored to these shales is of great significance for guiding decision-making in hydrocarbon exploration and supporting large-scale production.[Methods]Based on core observations,this study systematically evaluated the geochemical character-istics of marine-continental transitional shales in the 3rd submember of the 2nd member of the Permian Shanxi Forma-tion(also referred to as the Shan23 Sub-member)along the eastern margin of the Ordos Basin.The applied analyses and tests included total organic carbon(TOC)analysis,Rock-Eval pyrolysis,maceral identification,gold tube pyrolysis ex-periments on hydrocarbon generation,breakthrough pressure determination,and diffusion coefficient determination.Fur-thermore,a sweet spot evaluation method for marine-continental transitional shale gas was developed.[Results and Conclusions]The marine-continental transitional shales in the Shan23 Sub-member are characterized by the high abund-ance but unfavorable types of organic matter,with an average TOC content of 6.5％.Macerals in the shales are domin-ated by exinite and vitrinite,indicating that the shales are in the overmature stage.The shales in the Shan23 Sub-member are characterized by early and prolonged hydrocarbon generation,with a maximum hydrocarbon yield of 139.42 mg/gTOC.Experimental results indicate that the shales would experience peak oil generation at vitrinite reflect-ance(Easy％Ro)of 0.9％,large-scale gas generation at Easy％Ro of 1.5％,and a gradual termination of hydrocarbon gen-eration at Ro of 3.5％.Four lithologic assemblages are identified in the study area.Among these,the shale-limestone as-semblage exhibits the most favorable preservation conditions,followed by the shale-mudstone and shale-coal as-semblages,with the shale-sandstone assemblage demonstrating the poorest preservation conditions.A geochemical eval-uation method for the geological sweet spots of marine-continental transitional shale gas was established.Using this method,key geochemical parameters for Class Ⅰ favorable intervals were determined against the backdrop of the shale-limestone assemblage,including a TOC content＞4.0％,a sapropelinite content＞20％,and Easy％Ro ranging from 1.5％to 3.5％.The results of this study reveal key parameters for the geochemical evaluation of the geological sweet spots of marine-continental transitional shale gas,thereby providing a basis for play fairway prediction and exploration and de-velopment deployment of shale gas in the Shanxi Formation along the eastern margin of the Ordos Basin.