Closed-system pyrolysis-based hydrocarbon generation simulation and gas potential evaluation of the Shanxi Formation shales from the Ordos Basin, China

Abstract

The Shanxi Formation (Shan 1 and Shan 2 Members) shales show good prospects in shale gas development in the Yan'an area of Ordos Basin. Based on the simulation experiment of hydrocarbon generation of low maturity shale samples, the hydrocarbon generation characteristics of shale samples was studied systematically. Then, combined with the geochemical analysis of shale and gas generation simulation, shale gas potential was evaluated. The results reveal that Shan 1 and Shan 2 shale samples are favorable for shale gas enrichment by and large, with C1–C5 maximum yields of 146.96–160.83 mg/g TOC and 148.48–148.67 mg/g TOC respectively at a heat rate of 20 °C/h and 2 °C/h. The Shan 1 and Shan 2 shales are basically the same in terms of organic carbon production potential of each unit. The carbon isotopic composition of alkane gas reveals that heteroatomic compounds (NSOs) cracking is an important mechanism for shale gas generation of Shanxi Formation shales, and conducive to gas generation at high-to over-mature stages. Given thermal history and kinetic parameters of hydrocarbon generation, the shales of Shanxi Formation reached the maximum gas production potential in the Late Cretaceous, with a maximum yield of 160.3 mg/g TOC under present geological conditions. During geological history, the Shanxi Formation shales went through high-to over-maturity evolution, mainly producing dry gas, and their gas generation capacity was controlled by the organic matter abundance and cracking capacity. The gas generation potential of Shan 2 shale is higher than that of Shan 1, due to its higher TOC.