Accumulation Mechanism of Complicated Deep Carbonate Reservoir in the Tazhong Area, Tarim Basin

Abstract

The Tazhong area is located in the hinterland of the Taklimakan Desert, where abundant hydrocarbon resources are contained in the deep Ordovician carbonate rocks. Great exploration results have been obtained in the Middle and Lower Ordovician Yingshan formation over recent years, and a large layered oil/gas field has been proven. The fluid properties of this reservoir are complicated, and it can be divided into condensate gas reservoir, volatile oil reservoir and oil reservoir in accordance with fluid component and PVT phase behavior. These reservoirs distribute in alternation horizontally, without apparent boundaries vertically. Based on the study and analysis on test, exploration and production data, it is believed that the diversity of fluid properties results from the multi-phase uneven infusion action of oil and gas, especially the severity of gas cut at late stage: the reservoir suffered from severe gas cut became condensate gas reservoir, the one suffered from weak gas cut became volatile oil reservoir, and the one free from gas cut still keeps the original oil reservoir state. It is discovered through study that the strike-slip fault plays an important role in the occurrence of gas cut at late stage: it not only acted as the pathway of gas migration at late stage, but also had an important impact on the reformation of reservoir. The impact of reservoir nature on gas cut mainly finds expression in the difference of combination of reservoir space: the mutually connected large pore/cavity and fracture system was favorable for gas charge at late stage, where condensate gas reservoir was mostly formed; while the isolated pore/cavity where the tectoclase developed weakly was relatively sealed, was unfavorable for the sufficient infusion of natural gas at late stage, and usually keeps the original oil reservoir state or formed volatile oil reservoir. Based on the discussion to the factors that affect the severity of gas cut in the paleo-reservoir at late stage, this paper clarifies the accumulation mechanisms and controlling factors of different kinds of reservoirs, establishes the accumulation mode of 3 types of reservoirs, and this is very significant to the effective prediction of hydrocarbon distribution in the deep carbonate rocks of the Tazhong area.