Characteristics of terrestrial shale gas reservoir in Yanchang exploration area of Ordos basin

Taking Yanchang Formation in Yanchang exploration area as an example and combining with the distribution characteristics of continental shale in China, the authors investigated the accumulation conditions and the reservoir characteristics of continental shale gas using many experimental methods, such as organic geochemical analyses, X-ray diffraction(XRD), determination of porosity, scanning electron microscope (SEM), gas adsorption, etc. The results show that the thickness of Yanchang Formation shale in Yanchang exploration area is large (average 50m), the organic carbon content is relatively high(average TOC is 4.86%), the main types of organic matter are typeⅡ, and the thermal evolution degree of organic matter is of the mature stage(R_ran=0.84%~1.1%); the mass fraction of clay minerals is high(21.8%~71.5%, average 47.3%), and the mass fraction of quartz is low (10.8%~44.9%, average 22.3%), brittleness index is relatively small(27.1%~77.2%, average 49.6%), indicating that the shale reservoir is brittle and can be fractured. The porosity of Yanchang Formation shale in Yanchang exploration area is mainly between 1%~3%, and the pore types are diversiform. The intergranular pores and the inner pores are dominant, followed by intergranular pores, microcracks, dissolved pores and the organic pores, which provide reservoir space for the accumulation of shale gas. The gas content of Yanchang Formation shale is between 1.60~6.67 m³/t, showing a better gas generation capability.