Main controlling factors and genesis models of reservoir development in Lower Paleozoic of Qingyang paleo-uplift, Ordos Basin

In recent years, significant progress has been made in exploring the Lower Paleozoic deep natural gas of the Qingyang paleo-uplift in the southwestern Ordos Basin. However, the reservoirs in this region are generally tight, and the formation mechanisms of high-quality reservoirs are complex, making sweet spot prediction difficult. Based on drilling, logging, and seismic data, as well as experimental test data, this study analyzed the reservoir development types and main controlling factors of the gas-bearing layers, including the Cambrian Zhangxia Formation and Sanshanzi Formation, and the second and fourth members of the Ordovician Majiagou Formation (Ma 2 and Ma 4). A genesis model of the Lower Paleozoic reservoirs in the Qingyang paleo-uplift was established. Four types of reservoirs were developed in the region: dolomite reservoirs, shoal reservoirs, karst reservoirs (supergene karst and fault-karst reservoirs), and structural micro-fracture reservoirs. Among them, the Sanshanzi Formation as well as the Ma 2 and Ma 4 members mainly featured dolomite reservoirs, while shoal reservoirs dominated the Zhangxia Formation. Karst reservoirs and structural micro-fracture reservoirs were developed across all layers. The controlling effect of sedimentary facies belts, weathered crusts, and faults on the development of Lower Paleozoic reservoirs in the Qingyang paleo-uplift was significant. Sedimentary facies belts controlled the development of intergranular (dissolution) pores in shoals and intercrystalline (dissolution) pores in dolomite. Two phases of weathered crusts controlled the development of supergene karst reservoirs, with the weathered crusts at the top of the Ordovician being the most influential. Karst residual hills controlled the planar distribution of strong dissolution areas. The vertical dissolution intensity and gas content of reservoirs were closely related to their distance from the top of the weathered crust. Faults controlled the distribution of fault-karst reservoirs, and their associated micro-fractures effectively improved the reservoir properties. Based on these findings, four reservoir genesis models, including shoal-fault type, residual hill-fault type, dolomite-karst type, and dolomite-fault type, were established, providing insights into the exploration of the Lower Paleozoic natural gas in the Qingyang paleo-uplift.