Unraveling gas charging and leakage for oil reservoirs in the Mahu sag of the Junggar Basin, NW China using concentrations and ratios of biomarkers, light hydrocarbons and diamondoids

Abstract

It remains disputed why a large amount of oil but only a limited amount of gas has been discovered in the northwestern Junggar Basin of China, although most source rocks are post-mature. Quantitative GC, GC–MS and GC–IRMS analyses were performed on 92 oils from this region to investigate gas charging and leakage of the petroleum reservoirs in the Mahu sag and nearby areas of the northwestern portion of the Junggar Basin. The 92 oils have moderate to high concentrations of C30 hopane (18–3840 ppm) and ΣC29 regular steranes (38–6100 ppm), demonstrating that these oils have normal maturities, within the oil generation window. However, these oils have high heptane and isoheptane values in the ranges of 31.1–52.2 and 0.82–7.74, respectively, and diamondoid (4- + 3-methyldiamantanes) concentrations over a wide range of 1.07–22.0 ppm. These results demonstrate that the reservoirs for all the studied oils have multiple charging episodes: terpanes and steranes entered the reservoirs along with the initial oil charges from source rocks within the oil generation window while light hydrocarbons and diamondoids mainly entered the reservoirs along with the late gas and condensate charging from deep post-mature source rocks. The difference between the maximum and minimum 4- + 3-methyldiamantane (4 + 3MD) concentrations (Cmax – Cmin)/Cmax is equal to 0.95. A higher ratio of (Cmax – Cmin)/Cmax (> 0.50) can be indicative of late gas and condensate charging to the reservoirs. Lower gas/oil ratios (GOR) for the reservoirs of the studied oils can be mainly ascribed to gas leakage.