Characteristics and Accumulation Mechanism of Quasi-Layered Ordovician Carbonate Reservoirs in the Tazhong Area, Tarim Basin

Abstract

Weathered crust reservoirs of marine carbonate rocks and large-area quasi-layered carbonate reservoirs controlled by massive unconformity surfaces have become an important play for exploration of marine hydrocarbons across China. In the Tazhong area, oil and gas originating from the marine facies of the Ordovician Yingshan formation, present large-scale integrated continuous accumulation along massive structures. In particular, they are accumulated continuously in layered form in carbonate fracture-cavity reservoir system, and not controlled by tertiary structural belts. Moreover, the Yingshan reservoirs are complex in types, comprising cross distribution of high gas/oil ratio (GOR) condensate gas reservoirs, volatile oil reservoirs and normal oil reservoirs, and large burial depth span and high heterogeneity of quasi-layered reservoirs. The complexity in diagenesis and accumulation lead to the unique distribution and enrichment of carbonate reservoirs in the Tazhong area. The analysis of blow-down and leakage depths, heterogeneity, and physical properties of pores, caves and fractures of the Yingshan formation carbonate reservoirs indicates that the interstratal karsts have superimposed with faults and fractures to form fracture zones, so that the reservoir bodies present integrated continuous distribution characteristics spatially, providing favorable conditions for quasi-layered continuous enrichment of oil and gas. During late periods, a massive amount of natural gas had been filled along the gas-source faults. Based on the variations of relative contents of light hydrocarbons and aromatic naphthalene homologs in crude oil, it is found that the quasi-layered continuous carbonate reservoirs in the Tazhong area are different in terms of the properties of oil and gas and the types of oil reservoirs as a result of the formation of high GOR high-yield condensate gas reservoirs near the oil/gas-source faults due to gas washing, migration and fractionation. Late gas charge and transformation process acting on earlier oil reservoirs is an important geochemical action in the formation mechanism of carbonate reservoirs in the Tazhong area.