Hydrocarbon generation characteristics, reserving performance and preservation conditions of continental coal measure shale gas: A case study of Mid-Jurassic shale gas in the Yan’an Formation, Ordos Basin

Abstract

Shale gas in nonmarine facies has attracted less interest compared with the marine facies. The Ordos Basin is one of the Jurassic nonmarine continental coal basins in northern China. Several exploration wells have revealed the existence of shale gas resources in the Middle Jurassic Yan’an Formation in this basin. The Yan’an Formation is the coal-measures in this basin, dominated by an intercalation of siliciclasitcs and thick coals, and the shales were mainly developed in the fluvial, lacustrine and lacustrine delta environments. The shales are multi-inerlayered with sandstones and coals, and have an unstable horizontal distribution. characterized by a small thickness of individual layer (ranging between 0.1m and 34.77m), and large thickness of the total cumulative layers (up to 380m). The shales have intermediate to high contents of total organic carbon (ranging from 0.24% to 5.82%) and low thermal evolution degree in an immature-mature stage. The gas is mainly of biogenic origin, with relatively small fraction of thermogenic origin. The mineralogy investigation revealed that the shale have a lower content of brittle minerals and a high content clay minerals. The pore types are dominated by dissolution pores, intergranular pores, intragranular holes and micro cracks, and few organic holes can be seen. The porosity and permeability are low, with significant reservoir heterogeneity. The gas content in this nonmarine shale is low, dominated by adsorption phase. The shale gas in this basin were generally/enclosed within the groundwater flow closure zones, where the groundwater salinity is relatively low. The development characteristics of the shale and shale gas were also controlled by the depositional facies, and the conditions for hydrocarbon generation, reservoir performance, and gas preservation, can be favorably developed in the lacustrine facies, followed by the lacustrine delta facies, while the conditions in the fluvial facies are relatively poor. Although the resource potential of shale gas in the nonmarine continental coal-measures is lower than that of the marine facies, the shale gas, coal, coalbed methane, and tight sandstone gas often coexist in the continental coal basins which may support a more prospect energy scenario. Therefore, comprehensive exploration and collaborative development of the shale gas, coalbed methane and tight sandstone gas in the continental basin may be an ideal way to synthetically utilize the continental coal-measure gases.