Secondary alteration to ancient oil reservoirs by late gas filling in the Tazhong area, Tarim Basin

Ordovician carbonates have abundant resources of oil and gas in the Tazhong area of the Tarim Basin. Due to secondary alteration caused by multiple periods of oil and gas filling, especially late gas filling, fluids in ancient oil reservoirs are complex in nature, and multiple-phase oil and gas reservoirs, such as gas-condensate reservoirs, volatile-oil reservoirs, and normal oil reservoirs coexist horizontally. By examining the distribution, fluid physiochemical properties, biomarkers, gas isotopes, and fluid inclusions of these oil and gas reservoirs, this research has revealed that reservoir distribution is controlled by strike-slip faults, fracture-vuggy reservoir connections, and structural features, and that fluids of different-phase reservoirs are significantly different in physiochemical properties and in nature. Maturities of oil and gas are significantly different, suggesting that the area was filled at least twice by oil and gas of different maturities. Analysis of fluid inclusions shows that two types of inclusions are commonly developed: liquid-hydrocarbon inclusions in the first period and gaseous hydrocarbon inclusions in the second period, suggesting that oil filled at an earlier time and gas filled at a later time. It is proposed that gas-condensate reservoirs were formed after late gas invasion altered ancient oil reservoirs. Features of light hydrocarbons, such as wax and aromaticity, indicate the direction and intensity of gas-washing fractionation; biomarker parameters prove that, in terms of oil and gas maturity, gas-condensate reservoirs > volatile-oil reservoirs > normal oil reservoirs. The process of oil and gas accumulation in the Tazhong area is reproduced, and an accumulation model is proposed for multiple-phase oil and gas. In addition, a method is provided for evaluating the relative intensities of secondary alteration to early reservoirs by late gas filling, which is significant in exploration for oil and gas in deep layers and in prediction of hydrocarbon phases. (C) 2014 Elsevier B.V. All rights reserved.