Search

The western margin of the Ordos Basin and its adjacent regions have undergone a complex tectonic evolution from the Mesozoic to the Cenozoic era. However, the question of tectonic uplift since the Cenozoic era remains a topic of contention, and the regional thermal evolution history necessitates precise chronological evidence. Situated close to the thrust belt within the western margin of the Ordos Basin, the Niushoushan-Luoshan area holds pivotal significance in unraveling the Mesozoic tectonic events within the basin's confines. Through a meticulous exploration employing apatite fission track (AFT) analysis and thermal history simulation, this study delineates the Mesozoic uplift sequence and its temporal confines in the Niushoushan-Luoshan area. The results reveal that the Mesozoic uplift within this region predominantly occurred during the Middle Jurassic period (170 Ma) to the end of the Early Cretaceous (110 Ma). Furthermore, we observe a slightly earlier onset of uplift in the Luoshan area (170 Ma) compared to the Niushoushan area (160 Ma). This uplift is primarily attributed to the north-eastward extrusion of the Qilian orogenic belt. Combining our findings with existing research, we propose that the Cenozoic uplift in the western margin of the Ordos Basin and its adjacent areas started during the Late Triassic, comprising two distinct phases: the first phase unfolding from the Late Triassic (220 Ma) to the end of the Early Jurassic (185 Ma), and the second phase occurring from the Middle Jurassic (175 Ma) to the end of the Early Cretaceous (110 Ma); the uplift in the Niushoushan-Luoshan area is part of the second phase of uplift along the western margin of the Ordos Basin. The two Cenozoic tectonic uplift phases along the western margin of the Ordos Basin display characteristics of north-to-south and southwest-to-northeast propagation, respectively. It is inferred to be associated with the Late Triassic collision between the North China and South China blocks, as well as the movement of the Lhasa Block converging toward the northeast during the Middle to Late Jurassic.

The sandstone reservoirs in the Upper Paleozoic He 8 Member in the northern Tianhuan depression of the Ordos Basin are vastly different and feature particularly complex gas–water distributions. Scanning electron microscopy, fluorescence, Raman spectroscopy inclusions, relative permeability analysis, and nuclear magnetic resonance were utilized in this study based on core data, identification statistics, and various thin-section microscope measurements. Samples from the Upper Paleozoic He 8 Member in the northern Tianhuan depression were collected to study the characteristics of reservoir heterogeneity and gas–water distribution, which were controlled by differential diagenesis. The results indicate that compaction and dissolution are the two most important factors controlling reservoir heterogeneity. Large differences in diagenesis–accumulation sequences and pore structure characteristics affect reservoir wettability, irreducible water saturation, and gas displacement efficiency, thereby controlling the gas–water distribution. The He 8 Member is a gas reservoir that is densified because of accumulation. Reservoirs can be divided into three types based on the relationship between diagenetic facies and gas–water distribution. Type I is characterized by weak compaction, precipitate or altered kaolinite cementation, strong dissolution of diagenetic facies, and high porosity and permeability. This type is dominated by grain-mold pores and intergranular dissolution pores and produces gas reservoirs with high gas yield. Type II is characterized by medium-strength compaction, altered kaolinite or chlorite cementation, weak dissolution of diagenetic facies, and medium porosity and permeability. This type is dominated by residual intergranular pores, a few residual intergranular pores, and dispersed dissolution pores, producing gas reservoirs with low gas yield. Type III is characterized by medium-strength compaction, altered kaolinite cementation, and medium-strength dissolution of diagenetic facies. This type is dominated by kaolinite intercrystal pores and dispersed dissolution pores, producing gas reservoirs with high water yield. [ABSTRACT FROM AUTHOR]