Study on formation conditions and reservoir characteristics of marine-terrigenous facies coal measures shale gas in western Guizhou

In order to intensive study on formation conditions and reservoir characteristics of marine-terrigenous facies coal measures shale gas,a variety of test and analysis were carried out on the shale of Longtan Formation in Dafang anticline area of Western Guizhou,including total organic carbon content(TOC),vitrinite reflectivity(Ro),rock-eval pyrolysis,X-ray diffraction,thin section identification,mercury intrusion porosimetry,analysis of surface area and pore size distribution. The results show that the thickness of the shale is thin (1.8～2.8 m on average),and it is frequently interbedded with the coal seam. But the cumulative thickness is large,and the mud land ratio is more than 70% on average. The shale has a high organic carbon content(0.23%～22.10%,3.51% on average,mainly distributed in 1%～7%),organic matter type is Ⅲ. Vitrinite reflectivity(Ro) is 2.67%～3.06%,2.85% on average,which mean they are in the over mature stage and organic matter gas is relatively sufficient. High organic matter abundance and sufficient gas generation process provide favorable basis for shale gas accumulation. Shale is mainly composed of clay minerals(48.4%～81.6%，66.3% on average),which are mainly composed of illite/smectite interstratified clay(73.0%～94.0%,85.1% on average),followed by chlorite(2.0%～23.0%,10.5% on average),with a small amount of illite(1.0%～7.0%,3.5% on average),and some samples contain only tiny amounts of kaolinite. Quartz content distribution is next(7.4%～27.2%,16.5% on average). The content of calcite,dolomite,siderite,pyrite and other minerals are less,which are generally in the range of 2%～7%.The average brittleness index is 27%.which mean the conditions of natural fracture development and artificial fracture expansion are worse than that of marine shale in this area. The shale of Longtan Formation in the area shows typical characteristics of low porosity and permeability. It’s average effective porosity is 3.66% and permeability is 0.005 6×10-3μm2.Porosity has a positive correlation with clay content,while brittle mineral content has a negative correlation. The pores of shale are open nanoscale pores,among which the mesopores provide the main pore space and surface area of shale,with an average pore volume of 0.0198 8 cm3/g and an average pore surface area of 10.861 31 m2/g. Mesopores provide favorable conditions for shale gas adsorption and storage.