Gaseous hydrocarbons cracking in shale: Mechanism, impact and resource significance

Abstract

Gaseous hydrocarbons (C₁ − C₅) are the primary components of shale gas. Their thermal degradation significantly impacts various aspects of shale gas reservoirs, including gaseous hydrocarbons composition and their carbon and hydrogen isotopes fractionation, H₂ generation, reservoir overpressure, mineral dissolution and brittle mineral formation, changes in reservoir properties and storage space, as well as the formation and identification of resource sweet spots. This study reviews the experimental simulations of gaseous hydrocarbon cracking and the geochemical research on major global shale gas reservoirs conducted over the past few decades. In shale gas reservoirs, C₅H₁₂, C₄H₁₀, C₃H₈, C₂H₆ and CH₄ initiate cracking at Ro values of approximately 1.0 %, 1.0 %, 1.3 %, 1.5 %, and 2.0 %, respectively, with main cracking stages occurring at Ro ranges of 1.7–2.4 %, 1.7–2.8 %, 1.8–3.2 %, 1.8–3.6 %, and 3.0–4.0 %. This cracking produces more gaseous molecules, significantly contributing to overpressure and H₂ in high-overmature reservoirs. Wet gas cracking contributes carbon isotope rollover but CH₄ cracking causes both carbon and hydrogen isotope reversals in shale gas. As CH₄ cracking onset marks peak gas generation of shale, carbon isotope reversal serves as a key sweet-spot indicator. Furthermore, hydrous pyrolysis of gaseous hydrocarbons generates formic and acetic acids that enhance porosity and permeability by dissolving carbonate and feldspar minerals. Concurrent silica precipitation during feldspar dissolution promotes quartz formation, improving reservoir fracturability. This study advances our understandings of the formation, evolution and associated geological and geochemical anomalies of shale gas reservoirs, which holds significant implications for the exploration and exploitation of shale gas resources.