Seismic sequence stratigraphy and sedimentary evolution of carbonate reservoirs in the Sinian Dengying Formation, Middle Sichuan Basin, Southwest China

The Sinian Dengying Formation in the Sichuan Basin, southwest China, mainly consisting of dolomites, is one of the most ancient gas-producing series in the world. During the past half-century, gas exploration in the formation has been largely based on the lithostratigraphic correlation, but a regional correlation scheme of time significance is usually insufficient, resulting in the difficulty of lateral correlation of strata between gas fields. Aiming to overcome the problem, we completed an interpretation of about 2500-km 2D regional seismic lines by using the seismic sequence analysis method. As a result, a sequence stratigraphic framework was successfully constructed, which consists of two sequences and five systems tracts. By integrating analysis of isopatch maps with stratal stacking patterns, we identify three depositional facies belts within the formation, which are a shallow-water platform facies belt in the eastern and southern regions, a relatively deep-water basin facies belt in the northwestern region, and a northwest-dipping slope facies belt between them. During the development of sequence one in the lower of the Dengying Formation, retrogradation and aggradation dominated in the eastern and southern platform region whereas depositional condensation prevailed in the northwestern basin region. At that time, the depocenter was located on the eastern and southern platform region. However, sequence two in the upper of the Dengying Formation is dominated by the northwest-dipping sigmoid, oblique and shingled prograding packages of the platform-margin slope facies belt, indicating that the depositional center was shifted to the previous basin region in the northwest. As a result, the basin was filled gradually, and the platform-slope-basin topography was finally evolved into a northwest-dipping ramp. Our study suggests that the Late Sinian Sichuan Basin would consist of a series of shallow-water platforms separated by relatively deep-water depressions or basins, which provides important clues for gas exploration.