Geochemical Study On Hydrocarbon Accumulation In Deep Strata Of The Central Junggar Basin, Northwest China

With the advancement of global petroleum exploration and exploitation, a new trend is arising, i.e., to expand the field from in middle-shallow strata to deep strata. Thus, the study on hydrocarbon accumulation in deep strata has been one of the research frontier areas in petroleum geology and geochemistry. However, due to the relatively greater depth in comparison with middle-shallow strata, the hydrocarbon accumulation in deep strata has peculiarities, leading to the complexity and difficulty in the study. Here we conducted a case study in the central Junggar Basin, northwest China, to provide new fundamental data for regional petroleum exploration and broaden petroleum exploration fields. In the mean time, the results can also provide case and practical references for domestic and international petroleum accumulation in deep strata. Therefore, this study has significance of basic theory and practice.The deep strata in the central Junggar Basin refers to the strata below the Upper Triassic Baijiantan Formation. The Permian has been assumed to be the main source rock sequences in this assemblage. However, during the Triassic period, a flooding episode was believed to occur in the basin, which, thus, has been presumed to harbor significant hydrocarbon generation potential. This issue has not been well constrained due to the lack of samples. Based on a case study in the central basin that was a key target of the Triassic exploration, this study comprehensively analyzed the organic geochemistry of the Triassic dark mudstones. According to the analytical results, the abundance, types and maturity of organic matter were indicated, which, along with geological background, enabled us to evaluate hydrocarbon generation potential. The Triassic mudstones in the study area have a relatively high abundance of organic matter with total organic carbon content of approximately1.0%. The organic matter type is mostly kerogen type Ⅲ, with the bio-precursors mainly originating from terrestrial higher plants. The organic matter entered the maturation stage in the uplift areas, and reached the high to over maturation in the depression areas. These source rock characteristics showed few differences between different members. Given the wide distribution of the mudstone, it may have good hydrocarbon (especially gas) generation potential and, thus, they deserve a detailed and sophisticated assessment in the future exploration and research. During the source rock formation, plenty of terrestrial higher plants entered into the basin, augmenting the high abundance of organic matter with mostly kerogen type III. In addition, a detailed oil-source correlation study reveals that the Triassic oil-prone source rocks do contribute to the oil accumulation in the study area.Systematic geochemical analysis and oil-oil correlation in the study area showed that the crude oils and reservoir bitumens in the deep strata can be mainly divided into five types, including the mature and highly mature oils sourced from the Lower Permian Fengcheng Formation, the highly mature oils sourced from the Middle Permian Xiawuerhe Formation and mixed mature and highly mature oils sourced from the Fengcheng and Xiawuerhe formations. Among these, the oils in the Madong area are mainly sourced from the Mahu sag and the oils in the Shixi, Shidong, Mosuowan and Xiayan areas are mainly derived from the Penyijingxi sag.Geochemical analysis revealed that the natural gases in deep strata are mainly wet gases in mature to highly mature evolution stage. They are mostly coal-type gases and can be mainly divided into three types, including the coal-type gases sourced from the Carboniferous in the Dishuiquan sag, from the Jurassic in the Fukang sag and from the Permian Xiawuerhe Formation in the Penyijingxi sag (which are partially mixed with some oil-type gases). These three types of gases were mainly distributed in the central-eastern Mosuowan area, Shixi area and other areas in the central basin, respectively.Lastly, to restrict the process and model of hydrocarbon accumulation, we conducted a case study in the Shixi area, where is unique in that hydrocarbon reservoirs have been found in shallow, middle and deep strata. The results indicated that there are mainly four hydrocarbon filling stages, which are mainly sourced from the Permian in the Penyijingxi sag. The first stage is in the Late Triassic, when mature oils sourced from the Lower Permian entered into the deep strata. These oils were reallocated and remigrated to the middle strata in the Middle-Late Jurassic under the influence of the Yanshannian tectonic orogeny. In addition, highly mature Permian-sourced oils also accumulated during this stage. The mix of them results in the complexity of oil geochemistry. It is the second hydrocarbon accumulation stage. Then, during the Late Cretaceous, mature oils sourced from the Middle Permian Xiawuerhe Formation mainly filled Cretaceous reservoirs. This is the third hydrocarbon accumulation stage. In the last (fourth) stage occurred during the Neogene, faults and related migration conduit systems were mainly stabilized due to the relatively slim influence of the Himalayan orogeny. Thus, the highly mature oils mainly sourced from the Xiawuerhe Formation accumulated mostly in the deep Carboniferous strata. The hydrocarbons accumulation model can be summarized as "multi-stage hydrocarbon filling, lateral migration and vertical adjustment". There results have implications for the study of complex hydrocarbon migration and accumulation in deep strata with similar geological backgrounds worldwide.