A characteristic analysis between acidolysis gas and absorbed gas and its application to gas source correlation in Mao 1 member, Fuling area, Sichuan Basin

Abstract

The acidolysis gas in source rocks is the desorbed hydrocarbon gas in non-connected pores and carbonate crystal lattice, while the absorbed gas lies in the connected pores and is absorbed by surface. It is controversial about whether to use absorbed gas or acidolysis gas in gas source correlation. Using the absorbed gas and acidolysis gas in the first member of the Middle Permian Maokou Fm (hereinafter referred to as Mao 1 member) of Fuling area in the Sichuan Basin, the differences of components and carbon isotopes between absorbed gas and acidolysis gas and their applicable conditions in gas source correlation were discussed. The results show that: (1) there are larger differences of components and isotopes between absorbed gas and acidolysis gas. The absorbed gas has more heavy hydrocarbons and heavier carbon isotope. This difference can be attributed to the easier diffusion of methane and light hydrocarbons in absorbed gas, leading to relatively enriched heavy hydrocarbons and heavier carbon isotope in absorbed gas; (2) acidolysis gas is a mixture of hydrocarbon gas formed during different stages, while absorbed gas is the result of equilibrium between diffusion and supply of hydrocarbon gas, which can result in the differences on carbon isotope sequences between them; (3) the characteristics of natural gas produced after acid fracturing in the Mao 1 member in Fuling area resembles those of acidolysis gas in source rocks, while the characteristics of natural gas produced without acid fracturing are more similar to those of adsorbed gas; (4) the natural gas pool of the Mao 1 member is a carbonate gas pool with self-generation and self-storage. It is concluded that for the gas pool formed after migration, its producing pays have not been processed by acid fracturing, the produced gas should be correlated with the adsorbed gas in source rocks; for the carbonate gas pool with self-generation and self-storage, acid fracturing is usually needed, and the produced gas should be correlated with the acidolysis gas in source rocks. Keywords: Acidolysis gas, Adsorbed gas, Gas source correlation, Component, Carbon isotope, Sichuan basin, Fuling area, Middle permian, Gas pool in Mao 1 member, Self-generation and self-storage