Hydrothermal dolomitization in the Middle Permian in the Central Sichuan Basin, SW China: evidence from petrology, geochemistry, and fluid inclusions

Abstract

Limestone reservoirs formed by epigenetic karst and dolostone reservoirs formed by dolomitization are both developed in the Middle Permian of Central Sichuan Basin. In recent years, high-yield industrial gas flow has been obtained from these dolostone reservoirs, which makes the Middle Permian become the most realistic succeeding carbonate strata in the basin. Based on petrographic features, geochemical analyses (including C-O-Sr isotopes, trace elements, rare earth elements, and fluid-inclusions), seismic data, and previous studies, origin of dolomite in Middle Permian dolostone reservoirs in Central Sichuan Basin is studied. Three conditions of structurally controlled hydrothermal dolomitization (including two-stage strike-slip basement-rooted faults, underlying Cryogenian aquifers (or Pre-Sinian basement) and overlying Longtan Formation aquitard) are met. The Middle Permian dolostone reservoirs consist of saddle dolomites, fine-medium crystalline dolomites, and hydrothermal minerals, which are formed by hydrothermal dolomitization. The hydrothermal fluid replaces matrix limestone to form fine-medium crystalline dolomite, while saddle dolomite and other hydrothermal minerals are directly precipitated from hydrothermal fluid. Hydrothermal dolomitization fluid reforms matrix limestone in three ways, including dissolution and filling, replacement, and hydraulic fracturing, resulting to various types of reservoir spaces including dissolution vugs (or pores). Hydrothermal dolomitization of the Middle Permian in Central Sichuan Basin mainly occurs at the intersection of high-angle basement-rooted strike-slip faults, and the seismic section shows negative flower structure style.