Characterization of seismic information entropy attributes of braided river delta sedimentary microfacies for the upper Shaximiao formation in the Wubaochang area, northeastern Sichuan Basin, China

Abstract

The Wubaochang structure is located at the edge of the eastern Sichuan structural belt, at the intersection of the Dabashan and eastern Sichuan arcuate fold-and-thrust belts. The tectonic setting is complex and yields poor seismic imaging results. Within this structure, the Jurassic Shaximiao Formation is a low-efficiency tight sandstone gas reservoir with low single-well production. Currently, horizontal wells are installed in the main horizons to enhance productivity and efficiency. The drilled wells show that the reservoir consists of multiple horizons with rapid lateral facies changes and highly heterogeneous physical properties. Production well pressure patterns are segmented in striped zones and significantly controlled by the sedimentary microfacies. The microfacies, geometry, and distribution patterns of the sand beds are poorly understood. As a result, there have been few high-quality horizontal wells drilled in the reservoir, significantly restricting the exploration and development efficiency of the Shaximiao Formation in this area. In this study, we examined the regional sedimentary background and relative paleo topography of the study area and synthesized the results of core descriptions, rock composition analysis, and well log and seismic response feature evaluations. Finally, we determined that the sandstones in the study area are braided river delta deposits, mainly developed with the microfacies of braided channel, flood plain, mid-channel bar, mid-channel bar complex, underwater distributary channel, and interdistributary bay. We also examined the planar distribution pattern of the sand bodies with different microfacies, based on single-well microfacies division, paleo topographic feature evaluation, and seismic information entropy analysis. The results show that the mid-channel bar and mid-channel bar complex sands are the most favorable reservoir sands in this gas reservoir, followed by the braided channel sands and underwater distributary channel sands. The topography controlled the sand deposition distribution patterns and the development of the different microfacies. Seismic entropy reflected the amount of seismic signal information and degree of chaos. The examination of seismic entropy information provides a more stable and effective stratigraphic interpretation than the use of conventional seismic methods for identifying different microfacies of braided river deltas. Seismic entropy information can provide a reliable seismic and geological basis for optimizing microfacies targets and horizontal well placement.