Braided deltas and diagenetic control on tight sandstone reservoirs : a case study on the Permian Lower Shihezi Formation in the southern Ordos Basin (central China)

Abstract

The Lower Shihezi Formation, situated in the south of the Chinese Ordos Basin, is a tight sandstone reservoir that is generally influenced by its sedimentary facies and diagenesis, corresponding to distinct petrophysical reservoir properties for each lithofacies type. The existing lithofacies types within the Lower Shihezi Formation were determined through observations in field profiles and drill cores that were combined with the analysis of logging data. Diagenetic processes and their effects were analyzed by using different methods, such as conventional thin section, cast thin section, and scanning electron microscope (SEM) analyses. The results of this contribution indicate that the Lower Shihezi Formation in the southern Ordos Basin is dominated by braided delta facies and locally developed lacustrine facies. During diagenesis of this studied formation, compaction and cementation led to a reduction in porosity with compaction probably forming the main factor. Existing cementation is mainly manifested in the form of clay mineral cements, siliceous and carbonate cements, and induced a somewhat lower pore reduction effect on the reservoir compared to compaction. Minor dissolution led to the break-down of rock fragments, feldspars, and other soluble minerals, and thereby may have caused a subsequent formation of secondary pores, thus effectively improving the reservoir property. Consequently, due to the compositional differences and corresponding unique diagenetic effects, the distributary channels and underwater distributary channels display the highest reservoir properties. These are followed by the mouth bars and sand sheets. The lowest reservoir properties are shown by the natural levee microfacies.