Sequence stratigraphy and reservoir distribution of the Changxing Formation in northeastern Sichuan Basin

Abstract

The Permian Changxing Formation in northeastern Sichuan Province is characterised by a succession of carbonate rocks. A detailed subdivision of the third-order sequence in the Panlongdong section of Xuanhan County indicates that the Changxing Fm. can be divided into two third-order sequences. For the first third-order sequence, the transgressive system tract (TST) of the first sequence is made up of inter-bank deposits within open platform, and its highstand system tract (HST) comprises a set of reef-bank deposits belonging to platform-margin facies. The TST of the second sequence is characterized by a set of shallow-bank deposits, and its HST is mainly of a set of deposits belonging to both evaporite-flat facies and exposed shallow-bank facies. The sequence boundaries of the Changxing formation include an undersea gap unconformity boundary, and exposed-eroded unconformity boundary, corresponding to the type 2 sequence boundary of Peter Vail. Based on the study of sequence stratigraphy, the temporal and spatial distribution of reservoir in the Changxing formation can be inferred. The study demonstrates that there is a close relationship between reservoir distribution and sequence-strataigraphic division, with reservoirs mainly located in the HST of a sequence. The reservoir of the reef-bank type is mainly located in the HST unit of the first third-order sequence, and that of evaporite-flat facies and exposed shallow-bank facies is mainly developed in the HST unit of the second third-order sequence. Thus, a framework plot of sequence stratigraphy and a sedimentary facies ichnograph of third-order sequences-HST for the Changxing formation can form the basis to estimate the distribution of reservoirs in the two-dimensional space. The result shows that organic reef-bank reservoirs are located along the eastern and western margins of the Tongjiang-Kaixian carbonate platform, extending in a northwest-southeast direction. Similarly, the exposed-bank and evaporite-flat reservoirs are distributed along the Tongjiang-Kaixian carbonate platform and on its eastern and western sides, extending over a vast area. The reservoirs developed in the HST units of these two third-order sequences are characterized by abundant pore space, excellent reservoir quality and a widespread distribution. Therefore, these reservoirs are important target beds for oil-gas exploration.