Later stage gas generation in shale gas systems based on pyrolysis in closed and semi-closed systems

Abstract

As a self-contained source-reservoir petroleum system, shale gas plays have different generation mechanisms from conventional natural gas plays, especially with respect to the effects of residual hydrocarbons on shale gas generation. In this study, closed-system pyrolysis experiments were conducted on a suite of residual shales obtained by the semi-closed pyrolysis of shales from the Neoproterozoic Xiamaling Formation under various conditions. The effects of oil expulsion on the geochemical characteristics of residual organic matter (both bitumen and kerogen) in shale and on shale gas generation in the high maturity stage were investigated. The results indicated that shales with high oil expulsion efficiencies should be depleted in the aliphatic fractions in residual shale. In contrast, most of the aliphatic fractions should remain in shales with low oil expulsion efficiencies. Additionally, oil expulsion can further affect the chemical and isotopic compositions of the gas generated at later stages of thermal maturation. With increasing oil expulsion efficiency, C1, C2, C3, C1–5, and C2–5 gas yields and gas wetness decrease, while C1, C2, and C3 carbon isotopic values become heavier.