New insights into the genetic types and characteristics of the Ordovician marine fault-karst carbonate reservoirs in the northern Tarim Basin

The previously coined fault-karst reservoirs have become new exploration and exploitation targets and types of marine carbonate hydrocarbon reservoirs. However, these reservoirs are significantly different from conventional weathering-crust karst reservoirs and also show diverse reservoir types and structural characteristics themselves. By delving into the differences in the dissolution and tectonic fracturing of fault-karst reservoirs, we categorize these reservoirs into three types: the over-dissolved residual type, the fractured and highly dissolved type, and the highly fractured and weakly dissolved type. Investigations into these types of fault-karst reservoirs reveal that these reservoirs significantly differ in dissolution intensity, fracture and vug sizes, cumulative oil production, formation energy, and fracture-vug connectivity. Furthermore, the distribution of over-dissolved residual fault-karst reservoirs is influenced by both weathering crust unconformities and the longitudinal dissolution of fault zones. In contrast, the formation and distribution of the remaining two types, manifesting a minimal association with unconformities and tectonic locations, are primarily governed by the fracturing and dissolution intensity of dissolved fault zones. Specifically, large-scale caves occur in the cores of dissolved fault zones, gradually transitioning to fractured-vuggy and fractured reservoirs towards their both sides.