Fault-karst systems in the deep Ordovician carbonate reservoirs in the Yingshan Formation of Tahe Oilfield Tarim Basin, China

Abstract

Fault-karst systems in deep and ultra-deep carbonate reservoirs have a huge potential for hydrocarbon exploration. However the characteristics of reservoir space and the main controlling factors of fault-karst reservoirs are unclear due to strong heterogeneity. Therefore, the exploration and development of deep and ultra-deep carbonate reservoirs are restricted. To fill the gap, core, thin section and well log (conventional logs and image logs) data are used to analyze the lithology and main reservoir space in the deep Ordovician reservoirs in the Tahe Oilfield, Tarim Basin, China. The lithologies are dominated by intraclastic limestone, dolomitic limestone and silt to fine-sized crystalline dolostone. The reservoir space mainly includes dissolution vug, fracture, fracture-vug and cave. Among them, caves and their surrounding fractures are the main storage spaces for hydrocarbons in the Ordovician reservoir. Four fault-karst systems are divided including “V” shape, “U” shape, “layer” shape and compound type according to the morphology of the seismic section. In addition, the reservoir space of different fault-karst systems is clarified using image logs. Combined with drilling data, it is found that the reservoir scale of “U” shape fault-karst system is larger, while that of “layer” shape fault-karst system is smaller. In addition, the direction of maximum horizontal in-situ stress is NE-SW determined by induced fractures picked up by image logs. The NE orientation faults and fractures are increased and the effect of karst fluid on reservoirs is enhanced. However, the NW direction faults and fractures are restricted. Therefore, the fault-karst system is mainly observed along the NE main fault. The fluid alteration has two influences on the reservoir quality. On the one hand, the fluid alteration can dissolve the carbonate rocks and expand the reservoir scale; on the other hand, the fluid will carry mud and sand, which will reduce the reservoir space scale. The distance from the fault will affect the types of reservoir space of the fault-karst system. Away from the fault, the reservoir space will change from caves and fractures to facture-vug, and finally into small-scale dissolution vugs. Therefore, it should not only pay attention to seismic beads features but also to the size of available reservoir space on logging during drilling. This study can provide insight into the different types of fault-karst system models, and help clarify the main controlling factors of the fault-karst system scale.