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Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin, SW China, the controlling factors of reservoir development were analyzed, and the formation model of “intra-platform shoal thin-layer dolomite reservoir” was established. The Qixia Formation is a regressive cycle from bottom to top, in which the first member (Qi1 Member) develops low-energy open sea microfacies, and the second member (Qi2 Member) evolves into intra-platform shoal and inter-shoal sea with decreases in sea level. The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member. The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite, followed by vuggy dolomite. The semi-filled saddle dolomite is common in fracture-vug, and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network. Based on the coupling analysis of sedimentary and diagenesis characteristics, the reservoir formation model of “pre-depositional micro-palaeogeomorphology controlling shoal, sedimentary shoal controlling dolomite, penecontemporaneous dolomite benefiting preservation of pores, and late hydrothermal action effectively improving reservoir quality” was systematically established. The “first-order high zone” micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member. Shoal facies is the basic condition of early dolomitization, and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent. The grain limestone of shoal facies is transformed by two stages of dolomitization. The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores. The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event, makes it recrystallized into medium–coarse crystal dolomite. With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures, the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed. The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti-Moxi area of the Sichuan Basin, SW China, the controlling factors of reservoir development were analyzed, and the formation model of "intra-platform shoal thin-layer dolomite reservoir" was established. The Qixia Formation is a regressive cycle from bottom to top, in which the first member (Qi1 Member) develops low-energy open sea microfacies, and the second member (Qi2 Member) evolves into intra-platform shoal and inter-shoal sea with decreases in sea level. The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member. The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite, followed by vuggy dolomite. The semi-filled saddle dolomite is common in fracture-vug, and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network. Based on the coupling analysis of sedimentary and diagenesis characteristics, the reservoir formation model of "pre-depositional micro-palaeogeomorphology controlling shoal, sedimentary shoal controlling dolomite, penecontemporaneous dolomite benefiting preservation of pores, and late hydrothermal action effectively improving reservoir quality" was systematically established. The "first-order high zone" micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member. Shoal facies is the basic condition of early dolomitization, and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent. The grain limestone of shoal facies is transformed by two stages of dolomitization. The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores. The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event, makes it recrystallized into medium-coarse crystal dolomite. With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures, the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed. The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin. [ABSTRACT FROM AUTHOR]

Based on the comprehensive analysis of core, thin section, logging and seismic data, this study carried out the identification and comparison of Permian Changxing Formation sequences, clarified the typical sedimentary architectures of intra-platform shoal, investigated the vertical and horizontal development and distribution of intra-platform shoal in each sequence, and thus established the sedimentary evolution model of shoal body. The study results are reflected in four aspects. First, there are two complete third-order sequences (SQ1 and SQ2) in Changxing Formation in central Sichuan Basin. SQ1 is generally thick in the north and thin in the south, and SQ2 shows a thickness differentiation trend of “two thicknesses and three thinnesses”. Second, the Changxing Formation in central Sichuan Basin mainly develops intra-platform shoal, inter-shoal sea and intra-platform depression subfacies. In the vertical direction, the intra-platform shoal mainly presents two typical sedimentary sequences: stable superposed and high-frequency interbedded. Third, the stable superimposed sedimentary sequence is developed in the shoal belt at the edge of intra-platform depression, which is composed of two shoal-forming periods and located in the highstand systems tracts (HSTs) of SQ1 and SQ2. The high-frequency interbedded sedimentary sequence is developed in the southern shoal belt of intra-platform depression, which is composed of four shoal-forming periods and mainly located in the HST of SQ2. Fourth, during the SQ1 deposition, the intra-platform shoal was mainly developed at the edge of the intra-platform depression on the north side of the study area, and the inter-shoal sea subfacies was mainly developed on the south side. During the SQ2 deposition, the intra-platform shoal was widely developed in the area, forming two nearly parallel intra-platform shoal belts. The study results provide direction and ideas for exploration of Changxing Formation intra-platform shoal reservoirs in central Sichuan Basin.

Abstract: Through comprehensive research on drilling, logging and core data of T2l1 sub-member, which is the major pay of the Leikoupo Formation in the transitional zone of the central and southern Sichuan Basin, the structural characteristics, distribution regularity and its major controlling factors of intra-platform shoal are studied. The major rocks of intra-platform shoal in T1 sub-member, which is the major pay of the Leikoupo Formation in the transitional zone of the central and southern Sichuan Basin, the structural characteristics, distribution regularity and its major controlling factors of intra-platform shoal are studied. The major rocks of intra-platform shoal in T2 sub-member include oosparite, residual oolitic dolostone, sparrenite, residual doloarenite and sparry doloarenite, sparry bioclastic limestone and residual bioclastic dolostone, which constitute oolitic shoal, arene shoal and bioclastic shoal. The intra-platform shoal is characterized vertically by shallowing-upward sequences, and consists of three sedimentary units, namely shoal base, shoal core and shoal cap. Besides, three microfacies can be distinguished laterally, shoal core, shoal edge and intershoal sea. The variation of water depth and hydrodynamic conditions caused by sea level change control the characteristics and superposed pattern of shoal body. Meanwhile, the palaeogeomorphology with alternating uplifts and depressions defines the differential distribution and lateral combination pattern of intra-platform shoal body. [Copyright &y& Elsevier]1 1 sub-member include oosparite, residual oolitic dolostone, sparrenite, residual doloarenite and sparry doloarenite, sparry bioclastic limestone and residual bioclastic dolostone, which constitute oolitic shoal, arene shoal and bioclastic shoal. The intra-platform shoal is characterized vertically by shallowing-upward sequences, and consists of three sedimentary units, namely shoal base, shoal core and shoal cap. Besides, three microfacies can be distinguished laterally, shoal core, shoal edge and intershoal sea. The variation of water depth and hydrodynamic conditions caused by sea level change control the characteristics and superposed pattern of shoal body. Meanwhile, the palaeogeomorphology with alternating uplifts and depressions defines the differential distribution and lateral combination pattern of intra-platform shoal body. [Copyright &y& Elsevier]