The fluvial and lacustrine sedimentary systems and stratigraphic correlation in the Upper Triassic Xujiahe Formation in Sichuan Basin, China

Abstract

The calcarenites and siliciclastic sandstones in the Upper Triassic Xujiahe Formation are key targets of the unconventional gas exploration in Sichuan Basin. To resolve the long-debated stratigraphic correlation of the interfingering lithostratigraphic members between the different exploration blocks, a facies classification scheme integrating sedimentary structures in borehole images (BHI) and lithology from wireline logs is calibrated in the cored wells and extrapolated into the noncored wells. Three major sedimentary systems are recognized in Xujiahe by mapping the facies trends and paleocurrent directions in the coarse-grained sediments. One system may dominate over the others at different times and/or locations in the foreland basin in response to the orogenic movements in the surrounding mountains. The Longmen Shan in the northwest of the study area is the main source provenance for the carbonate conglomerates and calcarenites. Meanwhile, the systems in the northeast and southeast deposited the sandstone wedges. Alternating lacustrine, swamp, and fluvial sediments are penetrated in both proximal and distal areas in the basin, although there is no indication of a basinwide lake in the Xujiahe period. Abundant sandy shale-breccia conglomerates containing large shale breccias, plant materials, and contorted shale blocks in association with thick foreset-bedded sandstones suggest subaqueous gravity flows possibly because of the collapse of a multistoried delta front. Characterization of these dynamic sedimentary systems through multidisciplinary interpretation of the seismic data, cores, BHI, and wireline logs is not only fundamental to the sedimentary analysis, but also the exploration strategy in the areas with optimal juxtaposition of reservoir sandstones and coaly source rocks.