Micro fractures and pores in lacustrine shales of the Upper Triassic Yanchang Chang7 Member, Ordos Basin, China

Abstract

Following the successful exploration of shale oil in North America, China recently started to explore unconventional oil resources. Industrial shale oil flows were obtained from some strata in lacustrine basins, especially in the Chang7 Member of the Ordos Basin that may reach 150 barrels a day (e.g., in well Geng 295, 2655m). So one of the most significant questions now being asked focuses on the shale oil storage within the reservoir interval. In the present study, five organic rich shale samples were collected from the Chang 7 Member of the Yanchang Formation in the Ordos Basin and were analyzed by field emission scanning electron microscopy to characterize pores and fractures in the shales. The results show that microfractures are predominant storage space for shale oil produced from the Chang 7 Member. Three types of fractures and pores were recognized, including (i) fractures and pores associated with interstitial organic matter and organic lamina, (ii) fractures and pores occurring in minerals such as pyrite framboid, mica, and feldspar, and (iii) fractures and pores that are present along the contact interface of organic matter and minerals. These fractures and pores were mainly formed by oil generation, tectonic stress, or dissolution. The total organic carbon and brittle minerals are main factors controlling the abundance of pores and fractures. High total organic carbon and brittle minerals are favorable for the formation and preservation of pores and fractures in shale. These results may provide an insight that helps to locate sweet spots for shale oil exploration in Ordos Basin and affect oil exploration in shales in other lacustrine basins of China.