Characteristics, main controlling factors, and formation of Ordovician karst reservoirs in the western slope of the Shulu Sag, North China.

The hydrocarbon exploration shows that the Ordovician karst reservoirs in the western slope of the Shulu Sag are of great significance, but there is a lack of systematic research. The characteristics, main controlling factors, formation, and evolution model of the karst reservoirs were discussed based on cores, thin sections, well drilling and logging, 3D seismic, and production data, in combination with petrophysical property analysis. The results show that the karst reservoirs include micrite, crystalline dolomite, karst breccia, and other lithologies. The storage space is dominated by dissolution pores, caves, and fractures (i.e., tectonic fractures, dissolution fractures, and tectonic-dissolution fractures). However, the matrix is characterized by low porosity and permeability with great heterogeneity, and fractures enhance the percolation capacity of the reservoirs. The formation and distribution of the karst reservoirs are dominantly controlled by integrating tectonic, paleogeomorphology, and karst cycles. Tectonic movements are the key to dissolution development, karst paleogeomorphology controls the planar distribution of high-quality reservoirs, and their vertical distribution is governed by karst cycles. The monoclinic structure formed by Indosinian to Yanshan fold is the prerequisite for the later development of multi-phase karst alteration. During the Himalayan period, the upper slope areas developed high-quality reservoirs due to the dual alteration of vertical vadose and horizontal underflow zones controlled by paleogeomorphology and seasonal paleowater table change. Thus, the formation and evolution of the karst reservoirs, including six stages, were established in this study. This work may provide theoretical guidance for future favorable oil and gas exploration. [ABSTRACT FROM AUTHOR]