Sedimentary facies analysis, palaeogeography, and reservoir quality of the Middle-Upper Cambrian Xixiangchi Formation in southeast Sichuan Basin, southwest China

Abstract

Various data were utilized to analyze the sedimentary facies, completely reconstruct the palaeogeographic maps, and evaluate the reservoir quality of the Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin, including evidence from field outcrops, drilling cores, microscopic thin sections, scanning electron microscopy (SEM), combined with experimental analysis data, such as petrophysical data (porosity (%) and permeability (mD)), and radioactive elements (uranium (²³⁸U), thorium (²³²Th), and potassium (⁴⁰K)), and isotopic data (δ¹³C)). In the Sichuan Basin, the Middle-Upper Cambrian Xixiangchi Formation was principally deposited in a restricted platform with a lithology predominantly composed of dolomite, with local occurrences of limestone and other rock types in small thicknesses. Graded beddings, cross beddings, horizontal beddings, storm depositions, and mud cracks locally developed in some samples. Four 3rd-order sequences (SQ1-SQ4) were identified within the Xixiangchi Formation in this study. Each sequence is subdivided into a highstand systems tract (HST) and a transgressive systems tract (TST). Reservoirs are principally developed in high-energy grain shoal deposits located in SQ2 and SQ3, with a minor occurrence in SQ1 and SQ4. The lithology of these shoal deposits is essentially composed of sandy dolomite and crystalline dolomite characterized by relatively low average porosity (2.61 %) and permeability (1.0073 mD) values. The increase of these values in several studied samples might be related to seepage and connecting dissolution pores and vugs through fractures which have a constructive effect on porosity and permeability. The occurrence of karst caves through superimposed supergene karstification and bedding karstification highly improved the reservoir's physical properties in some areas. The reservoir space is dominated by intergranular dissolution pores, karst caves, and fractures, and their formation was controlled by sedimentation, diagenesis, penecontemporaneous dissolution, and tectonic fractures, which can also be influenced by palaeogeomorphology and sea level fluctuation. Favorable hydrocarbon exploration areas with relatively good source conditions, high positions in the paleostructure, which are conducive to the migration and accumulations of hydrocarbon were identified in three areas of the basin: (1) on the slope around the Central Sichuan Uplift; (2) in the northeastern part, dolomite reservoirs developed with good physical properties and large cumulative thicknesses; and (3) in the southeastern part, shoal facies reservoirs were developed in high and steep structural settings with anticlinal traps. The insights gained in this study fill a significant gap in geological complexities related to sedimentology, palaeogeography, and hydrocarbon reservoir prediction.