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

Tracing Oils and Gas Accumulation in Complex Petroleum Systems Based on Biomarkers, Light Hydrocarbons, and Diamondoids from Concomitant Crude Oils: A Case Study in the Shawan Sag of the Junggar Basin, NW China

“…Dahl et al (1999) introduced the concentration of C 29 ααα 20R sterane to assess oil maturities in the oil generation window prior to oil cracking . Tricyclic terpanes/(tricyclic terpanes + C 30 hopane) is mainly influenced by maturity at the late oil generation window. , Cai et al (2023) roughly classified oils into three maturity levels, i.e., higher, lower, and intermediate, based on the ratio of tricyclic terpanes/(tricyclic terpanes + C 30 hopane) in combination with the concentration of C 30 hopane and ΣC 29 sterane concentrations. C 30 hopane and ΣC 29 sterane concentrations of the studied oils are generally positively correlated, ranging from 13.22–2493.76 ppm and 8.65–2845.82 ppm, respectively (Figure b).…”

“…The typical 4 + 3-MD concentration of uncracked oil is 1−10 ppm, while the 4 + 3-MD concentration for highly cracked oil or mixed oil containing high-overmature components can reach 10 to 100 ppm. 47 Cai et al (2023) 6 reported that the 4 + 3-MD baseline for Permian source rocks in the Mahu Sag was less than 0.1 ppm and further proposed that (1-C min/Cmax) > 0.5 could be indicative of gas and condensate charging from postmature source rocks to reservoirs that contained normal uncracked 8a). There are three causes for the abnormally high concentration of 4 + 3-MD: (1) oil cracking or thermochemical sulfate reduction (TSR), (2) biodegradation of oils, and (3) mixing with high-maturity fluid.…”

“…According to the latest fourth-round assessment of hydrocarbon resources in the basin, the total gas reserve is approximately 3.2 × 10 12 m 3 , but proven gas reserves cover only approximately 1737.3 × 10 8 m 3 , nearly 5.4% of the total gas reserves . Therefore, natural gas exploration in the Junggar Basin has attracted increasing amounts of attention from scholars. − …”

“…The source rocks of the Fengcheng Formation in the northwestern border of the Mahu Sag are deposited in saline and alkaline lacustrine environments. , The average TOC value is 1.03%, with some up to 4.92%, and mainly contain Type I kerogen, which provided most of the oils to the Mahu Sag. ,− The source rocks of the Lower Wuerhe Formation mainly deposited in fresh lacustrine environment, containing gas-prone kerogen (Type III) in the southern area of the Mahu Sag, with TOC values in the range of 1.0%–3.5% and HI values <100 mg HC/g TOC for most samples, while the source rocks in the southern area of the Mahu Sag and in the central area of the basin (including the Shawan Sag) contain oil-prone kerogen; for example, a column of dark mudstone core obtained from the borehole of the Lower Wuerhe Formation in the JT1 well in the Zhongguai uplift showed a TOC value up to 4.15% and an HI value up to 3.37% with 550 mg/g TOC. ,,− Source rocks in Shawan Sag that are consistent with Mahu Sag, “Big Mahu Sag”, were put forward…”

Hydrocarbon Phase State Evolution and Accumulation Process of Ultradeep Permian Reservoirs in Shawan Sag, Junggar Basin, NW China

“…The phase states of hydrocarbons can be divided into the liquid phase, gas–liquid phase, condensate phase, and gas phase . They change with variations in temperature, pressure, and hydrocarbon composition. , In addition, multistage hydrocarbon charging, thermal sulfate reduction (TSR), gas invasion, and oil cracking change the phase states of hydrocarbons. − The oil and gas in ultradeep reservoirs in China commonly have high temperatures and pressures, usually originate from multiple sets of source rocks, and have undergone multistage hydrocarbon charging, − resulting in complex hydrocarbon phases.…”