Facies model of a mixed siliciclastic-carbonate ramp of the Ma’antang Formation (Carnian period), Sichuan Basin, China

Abstract

The Ma’antang Formation of Late Triassic is of great importance in petroleum exploration in Sichuan Basin, China, but its sedimentary facies and thickness distribution need further specific investigation. In this paper, the concerning types, distribution, facies model, and controls of sedimentary facies of the Ma’antang Formation are thoroughly discussed, based on an analysis of 130 wells and 12 outcrops in the whole basin. As a result, it is proposed that the Ma’antang Formation is of siliciclastic-carbonate ramp facies model, including seven types of sedimentary facies: open sea shelf, organic reef, bioclastic bank, oolitic shoal, interbank sea, tidal flat, and delta. The organic reef, bioclastic bank, and oolitic shoal are distributed along the Longmen Mountain belt in the southwest part of the basin; the tidal flat, consisting of gray and black mudstone interbedded with thin siltstone and coal bed, is mainly located in the middle of the basin; the delta is distributed chiefly in the northwest of the basin. Three controls are contributed to the formation of the siliciclastic-carbonate ramp, including the flattish palaeotopography, the abundant supply of terrigenous clast and lower rate fluctuation of base level during Carnian period. Four types of reservoirs (organic reef, bioclastic bank, and oolitic shoal reservoir and deltaic reservoir) are developed in the Ma’antang Formation and two types of source rocks (the Ma’antang Formation shale and Xiao’tangzi Formation shale) can supply abundant hydrocarbon to it. The reservoirs and source rocks form efficient assemblages of source rock and reservoir vertically.