Hydrocarbon accumulation mechanisms and industrial exploration depth of large-area fracture–cavity carbonates in the Tarim Basin, western China

Abstract

Abstract Chinese marine carbonate oil and gas exploration has shown rapid development momentum in recent years, and a number of large oil and gas fields have been discovered. These oil and gas fields have the following characteristics: the oil and gas are distributed over a large area and in quasi-layered form; reserves are of large scale but low abundance; the reservoir scale is greater than 1000 m; there is no unified gas–water interface, but a uniform temperature and pressure system; reservoirs are mainly low-porosity, permeable vuggy reservoirs with strong heterogeneity. The fracture–cavity carbonate reservoir space types include primary dissolved pores, incompletely filled underground rivers, unconformities, fractures formed by multi-stage faults, and others. The fluid states include Darcy flow, non-Darcy flow, pipe flow, and various forms of seepage. Due to poor connectivity between the carbonate spaces, oil and gas column heights are usually within 50 m, and their buoyancy is not sufficient to overcome capillary resistance, resulting in obvious oil, gas, and water gravity segregation and an inconsistent gas–water interface. These heterogeneous carbonate large-area and large-cavity oil and gas fields consist of a series of small composite contiguous oil and gas reservoirs, which means that the hydrocarbon–water distribution is not completely controlled by local structures and that reservoirs lack a clear boundary. Because of void layers formed by weathered dripping in syngenetic periods, most of the caves or holes in deeply buried layers are difficult to compact. In addition, early oil and gas injection has an important role in preserving reservoir effective porosity. Hence, an effective reservoir can be maintained at a depth of 10,000 m. Therefore, there may be great potential in uplift slope areas for exploration of vuggy carbonate oil and gas.