Gas generation potential and pore characteristics of Jurassic lacustrine shale containing type Ⅱ kerogen: A case study of the Yabulai Basin, northwestern China

Abstract

Jurassic lacustrine shales containing kerogen types Ⅱ and Ⅲ are widely distributed in sedimentary basins around the world. Previous studies on gas generation potential and pore characteristics of Jurassic lacustrine shales mainly focused on type Ⅲ kerogen, but ignored the systematic study of the shales containing type Ⅱ kerogen. This led to a poor understanding of the gas generation potential and pore characteristics of Jurassic lacustrine shales, which limits the effective exploration and development of shale gas. Taking the Lower Xinhe (J2x1) shale of the Yabulai Basin in northwestern China as the research object, this paper examines the gas generation potential and pore characteristics of a Jurassic lacustrine shale containing type Ⅱ kerogen. The J2x1 shale is largely classified as a very good source rock, kerogen is predominantly type Ⅱ, with thermal maturity ranging from mature to highly over-mature. The J2x1 shale is characterized by low porosity and permeability, and high content of brittle minerals. Different from the Jurassic lacustrine shales containing type Ⅲ kerogen, the J2x1 shales have a large amount of interparticle and intraparticle dissolution pores in the clay minerals as well as organic pores. The porosity and pore structure of the J2x1 shale are strongly controlled by total organic carbon (TOC) content. The methane adsorption capacity of the J2x1 shale is in the range of 2.10–2.99 m3/t, which is controlled by TOC content, clay mineral content, temperature and pressure. The geochemical characteristics, mineral composition and adsorption capacity suggest that the J2x1 shale has good shale exploration and development potential. A mathematical model is established to describe the various characteristics of methane adsorption capacity with depth and TOC content. The methane adsorption capacity of the J2x1 shale first increases and then decreases with increasing depth, with the depth corresponding to the maximum methane adsorption increases with increasing TOC content. When the TOC values of the J2x1 shale are mainly between 1% and 10%, the corresponding depth of the maximum methane adsorption capacity is 400–800 m, and the adsorption amount of the shale tends to minimize at a depth range of 2600–5100 m. The favorable shale gas exploration depth of the J2x1 shale in the study area increases with increasing TOC content. This research provides a geological reference for the exploration of this Jurassic shale gas in the Yabulai Basin, and the relevant experimental data provide important parameters for the numerical simulation of shale gas reservoirs in the study area. Moreover, the results of this study also provide an important reference for other basins in northwestern China, as well as continental basins with similar geological conditions in other regions.