Depositional framework and reservoir characteristics of microbial carbonates in the fourth member of the middle Triassic Leikoupo Formation, western Sichuan Basin, South China

Abstract

Microbial carbonate reservoirs are the main petroleum exploration targets in the upper submember of the fourth member of the Middle Triassic Leikoupo Formation in the western Sichuan Basin. To improve understanding of the distribution and reservoir characteristics of microbial carbonates, detailed facies descriptions and reservoir characterizations were performed under the framework of cyclostratigraphy. Ten facies can be identified and grouped into four facies associations in two transgressive-regressive cycles, within an epeiric marine carbonate platform in the western Sichuan Basin. In Cycle 1, Emei taphrogeny caused the formation of the Longmenshan subaqueous paleo-uplift, which formed a relatively restricted tidal flat environment and developed large-scale microbialites on its eastern side. In the central area, wavy to domal stromatolitic dolowackestone (F3) and thrombolitic dolomudstone (F4) are developed in the intertidal and subtidal of the central area with steep substrate gradients and restricted seawater circulation. Laminated microbial dolowackestone (F2) is mainly developed in the intertidal of the southern area with low substrate gradients and restricted seawater circulation. Intraclast dolograinstone (F6) are widely developed in the subtidal shoals of the northern area, which is frequently disturbed by tides and waves with better seawater circulation. In Cycle 2, increasing relative sea level weakened the barrier of Longmenshan subaqueous uplifts to waves, the restricted subtidal environment developed the intraclast dolograinstone (F6) and intraclast packstone-grainstone (F7), occupying the platform with better seawater circulation and higher energy. The framework or inter-clot pores of F2, F3 and F4, interparticle pores of F6, and vugs resulting from syn-depositional dissolution are the dominant pore types. High-quality reservoirs are mainly distributed in the regressive microbialites (F2–F4) of Cycle 1 in the central area and intraclast dolograinstone (F6) of Cycle 1 and Cycle 2 in the northern area. The porosity of microbialites and intraclast dolograinstone reservoirs mainly ranges from 2.26% to 10.00%, with a mean value of 4.96%. The permeability mainly ranges from 0.01mD to 13.98mD, with a mean value of 4.63mD. This study proposed a depositional framework of microbialites and evaluated its control on reservoir quality, which is of great significance for the further study of microbial carbonate reservoirs.