Multiphase dolomitization of a microbialite-dominated gas reservoir, the middle Triassic Leikoupo Formation, Sichuan Basin, China

Abstract

Microbialites have great economic potentials in hosting hydrocarbons throughout the geological record. Diagenesis, particularly dolomitization, has been proven to be crucial in microbialite reservoir development, however, the impacts of dolomitization on microbialite reservoir quality were not yet clearly understood. In this study, core samples covering the Middle Triassic Leikoupo formation were collected, and combination of petrology, fluid-inclusion microthermometry, and stable and radiogenic isotopes analyses was conducted, aiming at unraveling the origin of the dolomites and their reservoir characteristics in the microbialites from the Sichuan Basin, China. Two types of microbial dolostone reservoir were determined and compared in terms of their dolomitization mechanisms and reservoir characteristics. Organogenic dolomitization mediated by microbial activities probably resulted in the initial biotic dolomicirte (D1) precipitation in sediment pile as a consequence of bacterial sulfate reduction (BSR), characterized by significant δ13C depletion compare to the middle Triassic seawater value. An early stage dolomitization, occurred in fluids dominated by seawater was responsible for the precipitation of very fine crystalline dolomite (D2). Burial dolomitization, precipitated medium-to coarsely crystalline dolomite cements (D3) from burial brine in temperatures between 120 °C and 160 °C and characterized by elevated 87Sr/86Sr values and slightly low δ13C values. Last hydrothermal dolomitization event, represented by saddle dolomite cement (D4), has probably occurred and is associated with deeply-seated hydrothermal fluids at temperatures >200 °C. Abiotic microbialite dolomudstone reservoir quality (D2) (porosity = 7.4%; permeability = 24.9 mD) superior to the biotic microbial-mediated dolomudstone reservoir (D1) (porosity = 1.4%; permeability = 0.6 mD). Microbialite dolomudstone reservoirs (D2) are microporous, with the presence of both primary porosity, e.g., framework and interparticle, and secondary porosity, e.g., solution-enlarged and micropores. The wide occurrence of intragranular and micro-fracture, together with the proximity of larger macropores, may increase connectivity and allow for a combined micropore to macropore transport process for natural gases. Micropores in these dolostones were interpreted to be formed by mole to mole replacement of calcite by dolomite. Hence, this study emphasizes the important role of dolomitization in the formation of ancient microporous microbialites reservoirs characterized by low porosity and low permeability.