Determination method of shale gas content: A case study in the Ordos Basin, China

Abstract

Three shale samples, which were from MF-1 well (a) and MF-2 well (b) in the northern Ordos Basin of China and Yan'an Formation of Jurassic coal measure strata of J601 well (c) in the western edge of the Ordos Basin, were taken for thermal simulation and rock pyrolysis experiments. The rules of hydrocarbon generation production and S2 varied with the increase in thermal maturity (Ro), and the relationship between S2 and gaseous hydrocarbon generation production were analyzed. The analytical results indicated that as the Ro increased, the liquid hydrocarbon production of the shale samples first increased and then decreased, while the gaseous hydrocarbon production and total hydrocarbon production increased, and the S2gradually decreased. Gaseous hydrocarbon production increased as S2 decreased, and there was a two-stage negative linear correlation between S2 and gaseous hydrocarbon production. The gaseous hydrocarbon production increased slowly when the Ro value was below 0.8% and rapidly increased after this threshold was met. During the process of the hydrocarbon generation of the shale in the Ordos Basin, the evolution phase could be divided into three stages: the immature stage of producing liquid hydrocarbon, the mature stage of producing gaseous and liquid hydrocarbon, and the overmature stage of gaseous hydrocarbon thermal cracking. Considering the hydrocarbon expulsion efficiency of the shale samples, the total organic carbon (TOC) content had a positive logarithmic correlation with the shale gas content (the gaseous hydrocarbon production minus the expulsed gaseous hydrocarbon production). The hydrocarbon generation potential and gas content increased with the increase of TOC. With the knowledge of the shale samples' TOC content, the shale gas content of the sampling layer could be preliminarily determined based on the regression equations of TOC and gas content. The exploration potential of the shale layer could be evaluated according to the gas content calculation, after considering the standards for the shale gas favorable area and the target area gas content.