Exploration potential of shale oil in Chang7 Member, Upper Triassic Yanchang Formation, Ordos Basin, NW China

Abstract

The geological conditions and exploration potential of shale oil in Chang7 Member, Upper Triassic Yanchang Formation, Ordos Basin, were studied from various aspects, including petrographic characteristics, storage ability, geochemical features, friability and mobility of hydrocarbon in the source rock, etc. A classification criterion of lithofacies for Upper Triassic Chang 7 source rock in Ordos Basin were established based on the correlation between lithology, organic carbon content and logging parameters, from which, the spatial distribution and development scale of two types of shale, black shale and dark massive mudstone, have been predicted. Qualitative and quantitative characterization of the micro-structures of Chang7 source rock using state-of-the-art microscopic facilities including argon ion milling – field emission scanning electron microscopy (FESEM), focused ion beam scanning electron microscopy (FIB-SEM) and Nano-CT reveal that the dominant pore types in Chang7 source rock are intra-granular pores and inter-granular pores; the pores and throats in the two kinds of lithofacies are both nano-scale, and the dark massive mudstone has better physical properties than the black shale. The Chang7 shale oil resources and mineability were evaluated based on the parameters from geochemical experiments on the source rock, including pyrolysis S1, chloroform bitumen ‘A’, TOC and thermal maturity, free hydrocarbon content, as well as geo-mechanical properties such as brittle mineral content and development of fractures. With large scale of favorable lithofacies, good storage ability and abundant hydrocarbon, Chang7 Member has the material basis for shale oil occurrence and accumulation, in addition, the shale oil there has accumulated greatly and has favorable properties for flowing in nano-scale pores and throats. All these show that Chang7 Member has high potential for shale oil exploration, in which, the dark massive mudstone is a more favorable target for shale oil exploration under the present technical conditions.