Facies-controlled prediction of dolomite reservoirs in the Middle Permian Qixia Formation in Shuangyushi, northwestern Sichuan Basin

Abstract

The Middle Permian Qixia Formation in the Shuangyushi area, northwestern Sichuan Basin, develops shoal-facies dolomite reservoirs. To pinpoint promising reservoirs in the Qixia Formation, deep thin shoal-facies dolomite reservoirs were predicted using the techniques of pre-stack Kirchhoff-Q compensation for absorption, inverse Q filtering, low-to high-frequency compensation, forward modeling, and facies-controlled seismic meme inversion. The results are obtained in six aspects. First, the dolomite reservoirs mainly exist in the middle and lower parts of the second member of Qixia Formation (Qi2 Member), which coincide with the zones shoal cores are developed. Second, the forward modeling shows that the trough energy at the top and bottom of shoal core increases with increasing shoal-core thickness, and weak peak reflections are associated in the middle of shoal core. Third, five types of seismic waveform are identified through waveform analysis of seismic facies. Type-I and Type-II waveforms correspond to promising facies (shoal core microfacies). Fourth, vertically, two packages of thin dolomite reservoirs turn up in the sedimentary cycle of intraplatform shoal in the Qi2 Member, and the lower package is superior to the upper package in dolomite thickness, scale and lateral connectivity. Fifth, in plane, significantly controlled by sedimentary facies, dolomite reservoirs laterally distribute with consistent thickness in shoal cores at topographical highs and extend toward the break. Sixth, the promising prospects are the zones with thick dolomite reservoirs and superimposition of horst–graben structural traps.