Storage space types and water-flooding efficiency for fault-controlled fractured oil reservoirs in Fuman oilfield, Tarim Basin

Ultra-deep hydrocarbon reservoirs in the Tarim Basin, governed by strike-slip faults, have become a pivotal target in China’s hydrocarbon exploration and exploitation efforts in recent years. However, the storage space types of these reservoirs, as well as their relationships with water-flooding efficiency, are yet to be clarified, presenting a significant challenge to the efficient development of the Fuman oilfield. Based on thorough analyses of outcrops, core samples, formation micro-imaging(FMI)logs, and dynamic monitoring data, we introduce the concept of faultcontrolled fractured reservoir to refer to the those in the Fuman oilfield, and systematically explore the three major storage space types and their relationships with the water-flooding efficiency. Key findings are as follows: (1)Fault cavity-type reservoirs, predominantly found in the core of the fault zones, feature hollow cavities created by adjustments of internal special volume after multi-stage structural activities on the faults’ sliding surfaces. The storage spaces of these reservoirs, relatively enclosed under burial conditions, are characterized by large pore volumes. This structural configuration enhances oil-water displacement efficiency, with some oil wells yielding a waterflooding recovery factor up to 93 % of the petroleum reserves in development;(2)Inter-breccia pore-type reservoirs, also primarily distributed inthe fault zones’ core, feature inter-breccia irregular storage spaces formed by the mutual support of adjacent breccias. These reservoirs exhibit a relatively uniform distribution, moderate porosity, and high liquid yield per unit pressure drop. However, the poor internal connectivity for the storage spaces leads to a low oil replacement rate by water injection. Therefore, it is necessary to explore the construction of a three-dimensional well pattern to improve development efficiency. (3)Structural fracture-type reservoirs are primarily distributed in the damage and process zones of the fault zones, with fracture zones with certain widths developing along the fault zones’ both sides and ends. In addition, a small number of pores are present around these fracture zones, with dominant seepage channels formed locally. This leads to a significant loss of injected water and, accordingly, lower oil displacement efficiency compared to the fault cavity-type. The study results can serve to support the production of 350×10⁴ tonnes in the Fuman oilfield. Furthermore, they prompt the optimization of schemes for waterflooding and enhanced oil recovery(EOR), having a significant referential value for the efficient exploitation of similar oil reservoirs.