Disparities in tight sandstone reservoirs in different source-reservoir assemblages and their effect on tight oil accumulation: Triassic Chang 7 member in the Qingcheng area, Ordos Basin

Abstract

Determining source-reservoir spatial assemblages and their tight reservoir disparities is essential for revealing the differential accumulation mechanism of continental tight sandstone oil, but systematic classification and case studies are rarely reported. In this article, based on lithological combination characteristics, the source-reservoir spatial assemblages of the Triassic Chang 7 Member in the Qingcheng area of the Ordos Basin are systematically divided into three categories, namely, reservoir-sandwiching source (RSS), source-reservoir interbed (SRI) and source-sandwiching reservoir (SSR). Moreover, the tight sandstone reservoirs in different assemblages were characterized by a combination of macro observations from cores and geophysical logs and microscopic analysis from cast thin sections, FE-SEM, HPMI and XRD data. The results demonstrate that the tight sandstone reservoirs within different source-reservoir assemblages have similar lithological properties, but vary greatly in reservoir quality and natural fracture networks. In general, the RSS sandstones have the broadest pore-throat size distribution and the best physical properties due to the prevalence of residual intergranular pores, and the reservoir quality is slightly better than that of the SRI sandstones and significantly better than that of the SSR sandstones. Additionally, natural fracture types and fracture intensities vary greatly in different source-reservoir assemblages. The RSS assemblage contains mainly high-dip angle fractures with relatively low fracture intensities, the SSR assemblage is dominated by low-dip angle bedding-parallel fractures with relatively high intensities, and the SRI assemblage is characterized by a combination of these two assemblages. Under the control of overpressure derived from hydrocarbon generation, tight reservoirs with different source-reservoir assemblages present different lower limits of petrophysical properties for tight oil charging, and the effective reservoir proportion leads to a decreasing enrichment degree in the RSS, SRI and SSR assemblages successively. Furthermore, the natural fractures in different source-reservoir assemblages enhance the storage capacity while contributing to disparities in the vertical migration of tight oil. The natural fracture network of the vertically stacked RSS assemblage allows tight oil to migrate upward for a relatively long distance, while the SSR assemblage can effectively prevent the upward migration of the tight oil, thus forming differential tight oil accumulation and distribution characteristics in the different source-reservoir assemblages.