Discovery of the high-yield well GT1 in the deep strata of the southern margin of the Junggar Basin, China: Implications for liquid petroleum potential in deep assemblage

The accumulation assemblages in the deep Jurassic and Cretaceous strata of the southern margin of the Junggar Basin (SMJB) have favorable petroleum accumulation conditions. Due to the huge burial depth, gas/condensate gas reservoirs are expected as the main targets, however, barely any exploration breakthrough has been achieved. The recently drilled well GT1 has obtained a high oil production of >1200 tons/day in the deep Cretaceous strata, which was classified as low gas-to-oil ratio (GOR) light oil. This is the first significant breakthrough in the deep assemblages in the SMJB which brings to the question of its genesis and preservation and the true petroleum potential in the area. In this study, we have presented integrated geochemical analyses including GC × GC-TOFMS, GC-MS and stable carbon isotopes and thus to conclude that the GT1 oil was sourced from Jurassic coal-measure source rocks with maturity of 0.7–0.9%Ro. As only a small amount of diamondoids and ethanodiamondoids were detected, the GT1 oil was classified as normal mature oil well preserved from thermal cracking despite its huge burial depth. The associated gas is coal-type gas with relatively high wetness and the similar maturity to that of the oil. According to the hydrocarbon accumulation analyses, the main source rocks were developed in the Lower-Middle Jurassic with present-day maturity of 1.0%–2.0%Ro. The sandstones developed in the transitional facies in the overlying Lower Cretaceous Qingshuihe Formation were impacted by the tectonic compression and preserved as belt-like distribution in anticline traps. Regional Cretaceous Tugulu Group also overlie these sandstone facies. This forms a favorable source-reservoir-seal assemblage for oil and gas accumulations in the area. Therefore, the deep assemblage in the SMJB is favorable for the accumulation and preservation of large-scale oil reservoirs and shows great potential for liquid petroleum exploration.