Geochemical characteristics and adsorption capacity of marine-continental transitional mudrock in eastern Sichuan Basin

The Longtan / Wujiaping Formation in eastern Sichuan basin is a set of marine-continental transitional shale. However,the research on its gas-bearing capacity and controlling factors is limited,which restricts the ex-ploration and research of shale gas. In this study,the geochemistry,reservoir pore type,pore structure parameters and methane sorption tests were carried out on the shale samples of the Longtan / Wujiaping Formation,and the effect of TOC content and other parameters on sorption capacity were then analyzed. The results show that from the Longtan formation in southeastern Sichuan basin to the Wujiaping Formation in northeastern Sichuan basin,the shales were formed in sedi- mentary environment,in order,the coastal swamp,the tidal flat lagoon,the shallow water shelf,and the deeper water shelf. The Longtan Formation is mainly consisted of mudstone,siltstone and coal seam. The TOC content is generally higher than 2% ,and the mineral composition is mainly clay minerals,with an average content of 42% -72% . With the change of sedimentary environment,the composition of clay minerals changes from I / S mixed layer +kaolinite +illite dominated to I / S mixed layer +illite dominated. The Wujiaping Formation is mainly consisted of siliceous shale and limestone,with their average TOC contents between 6. 56% and 7. 15% . The minerals are mainly quartz,followed by clay minerals and calcite. The Longtan Formation mudrocks is dominated by type III kerogen,and the organic matter pores are poorly developed in vitrinite and inertinite,and the clay mineral pores are dominant pore types. While the Wujiaping Formation mudrock is dominated by type II1 kerogen,and organic matter pores are the main pore type, mainly developing within sapropel and solid bitumen. After using the ternary Langmuir model to correct the excess ad- sorption,the absolute adsorption amount is significantly higher than the excess adsorption amount when the pressure is greater th3an 5 MPa. The Langmuir volume of the Longtan Formation shale is generally high,rangin3g from 2． 19 to 6． 98 cm / g,and the Wujiaping Formation shale has Langmuir volume between 2. 08 and 3. 96 cm / g. The higher Langmuir volume is associated with higher surface area for the Longtan Formation shales. TOC and clay mineral con- tents are the main controlling factors of the Langmuir volume of the Longtan / Wujiaping Formation shale. Due to the significant differences in the development of organic matter pores within the two sets of shales,the sorption of methane by organic matter may be significantly different. The organic matter in the Wujiaping shale promotes methane adsorp- tion capacity by providing a large amount of adsorption sites,while the surface adsorption and the linear distribution of long-chain structures of type III kerogen in the Longtan shale increase the methane adsorption capacity. The adsorption capacity of clay minerals in transition shale could significantly increase shale adsorption capacity,whereas the increase of terrigenous quartz would reduce the adsorption capacity of shale samples.