Genesis of efficient reservoirs in mixed sedimentary setting: a case study of Benxi Formation in the Gaoqiao area, Ordos Basin.

The Benxi-Taiyuan Formation in the southern region of the Ordos Basin represents a typical mixed sedimentary system, which has witnessed recent breakthroughs in the exploration of tight sandstone gas within the Benxi Formation. However, the development patterns of reservoirs in this mixed sedimentary area remain unclear, influenced by the sedimentary model and the distribution scale of sedimentary sandstone bodies. This study investigates the paleogeomorphology, sedimentary model, sandstone body type, petrological characteristics, and pore structure of the Benxi Formation. Field profiles, drilling core observations, microscopic section analysis, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), and physical property analysis are employed. The findings indicate that the paleogeomorphology before the deposition of the Benxi Formation exhibits a west-to-east high-to-low trend, with a steeper gradient in the north and a gentler gradient in the south. This paleogeomorphology influences paleocurrent direction and the distribution of sandstone bodies. The sedimentary system, consisting of clastic tidal flats, mixed tidal flats, and mixed lagoons, develops from west to east on a plane, with the mixed lagoon being more prominent in the northeast of the study area. The sedimentary model determines the initial composition and structure of the reservoir, as well as subsequent diagenesis processes. Diagenetic reconstruction plays a crucial role in assessing the development degree of reservoir. Compaction and cementation result in the densification of the Benxi Formation sandstone. Dissolution and preservation of primary pores are critical for high-quality reservoir development. Among the various depositional models, the sand flat facies in the clastic tidal flat area of the Ben 2 Member and the tidal channel facies in the mixed tidal flat areas of the Ben 1 and Ben 2 members exhibit a high degree of pore development within the sandstone body, making them favorable locations for high-quality reservoir development. In summary, this study highlights the importance of paleogeomorphology, sedimentary models, and diagenetic reconstruction in understanding the controlling factors for efficient reservoir development in the mixed sedimentary system of the Benxi-Taiyuan Formation. Knowledge of these factors aids in the exploration of high-quality reservoirs within concealed and dispersed sandstone formations in the Ordos Basin. [ABSTRACT FROM AUTHOR]