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The dolomite reservoir of the Middle Permian Qixia Formation in the Shuangyushi structure in northwestern Sichuan Basin is deeply buried, thin, and highly heterogeneous. With low-resolution seismic data, it is not easy to identify the reservoir signal and it is difficult to accurately describe the plane distribution of thin reservoirs. Based on high-resolution processing of seismic data, well-seismic combination technology, forward simulation technology, and cross-well profiles, in this study, we defined the waveform characteristics that correspond to different modes of reservoir development, summarized the seismic response modes of reservoirs in the area, optimized sensitive attributes, and finely depicted their plane distribution. The ability of seismic data to identify thin reservoirs was significantly improved after high-resolution processing. According to a comprehensive analysis, the reservoirs of the Qixia Formation in the study area had two response modes: "weak peak" and "complex wave." The waveform clustering attribute was optimized to describe the plane distribution characteristics of the Qixia Formation reservoirs in the study area. Type-I and type-II waveforms were favorable waveforms for reservoir development, and they mainly were distributed in the west of the Devonian denudation line, in a strip along the north–south direction, and mostly along faults. The predictions made with this method agreed well with the actual reservoir development, which provided a reference for the prediction of deep and thin reservoirs in this area. [ABSTRACT FROM AUTHOR]

Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites. [ABSTRACT FROM AUTHOR]