Junggar Basin Chaiwopu Depression Petroleum Geological Conditions

Junggar basin was one of the three oil/gas enriched large multi-cycle superimposed basins in west of China. The sub-tectonic unit, Chaiwopu sag, which lies in the south margin of the basin, was a small intermountain superimposed sag developed on the fold basement of Upper Carboniferous. Research area not only has the typical features of petroleum geology of superimposed petroliferous basin, which characterized by multi-sets of source rock, multi-types of reservoir-seal assemblage, multi-times of hydrocarbon generation and expulsion, and multi-stages of accumulation, but has high organic source rock and appropriate souce-reservoir-caprock assemblage(SRCA). This paper took the theories of plat tectonics, continental sequence stratigraphy and modern petroliferous basin geology as the guidance, complied with the exploration methods of composite petroleum system of superimposed basins, from whole outline of the formation and evolution of the sag, integrated dynamic study with monographic study, and synthetic study, to discuss the characteristics and rules of reservoir formation and then to proceed to confirm the controlling factors of hydrocarbon enrichment.By identifying the main unconformity, calculating the erosion thickness of the research area, linking with structural evolution of Bogda orogen, Backstripping method was used to reconstruct Chaiwopu sag's subsidence history. The results indicated that since neopaleozoic as yet, Chaiwopu sag endured six stages of subsidence. They were fast subsiding stage in Early to Middle Permian, weak uplifting stage in Late Permian, weak subsiding and adjusting stage in Triassic to Middle Jurassic, uplifting stage in Late Jurassic, continues subsiding stage in Cretaceous and fast uplifting and fast subsiding stage in Tertiary and Quaternary. Corresponding with the subsidence history, Research area was superimposed by three proto-type basins, including Late Carboniferous to Middle Permian intracontinental rift basin, Late Permian to Cretaceous depression basin and Tertiary to Quaternary intermontain sag, and had the characteristics of complicated superimposed basins in west China.Applying the theory and methods of continental sequence stratigraphy, the sag's capping formation including Upper Carboniferous and Quaternary was divided into three basin-fill sequences, six first-grade sequences (tectonic sequences MS_{Ⅰ-Ⅴ}), nine secondary sequences (sub-tectonic sequences SS_1-9), and twenty-one third-grade sequences(SQ_1-21). Sequences boundaries of all grades were mainly composed of unconformity in different scales, depositional break interfaces, erosion surfaces, paleosol and lithology break interfaces et, ac. During rift basin, tectonic movement and sedimentary supply controlled the patterns of sequences. In depression basin phases, sedimentary supply and paleoclimate were the main controlling factors. From Tertiary to Quaternary, violent orogeny of Bogda controlled the development of sequences of the intermontain sag. Analyzed systematacially the distribution of sedimentary facies, which controlled by system tracts in isochronous strata frameworks. In rift basin period(first-grade sequences MS_{Ⅰ-Ⅱ}), north part of interest developed shallow to deep lake under the background of lake transgressive system tract, and formed the most valuable source rock, which composed of Lucaogou dark mudstone of deep lake facies in LTST. At the same time, south part of interest developed alluvial fan facies and fan delta faices, which consisted the most important reservoir of the area. The original source of sediments came from Mid-Tianshan orogen and Yilinhebilgen orogen in rift basin evolution. During depression basin evolution(first-grade sequences MS_{Ⅲ-Ⅴ}), the north part of interest developed shallow to half deep lake facies, braided river facies, braided delta facies, and the south part of interest developed alluvial fan facies and braided river facies. At the period of depression basin, the main original source of sediments still came from Mid-Tianshan orogen and Yilinhebilgen orogen, and subordinate original source of sediments came from Bogda orogen.On the basic of analyzing the factors which controlled both the type of reservoir and the quality of reservoir in burial diagenesis phases, in consideration of characteristics of compact lithology, poor property, abundant fractures of the sag's reservoir, the fractures origin, displacement experiments of fracture samples and the stochastic modeling results of the fractures were studied synthetically and systematacially to predict the possible factures belt.Chaiwopu superimposed sag developed multi-sets of source rocks and reservoir during its evolution history. The hydrocarbon reservoir characterized multiple periods of hydrocarbon generation, migration, accumulation and reconstruction, and complicated controlling factors of hydrocarbon. In accordance with the above complexity of the petroleum system in Chaiwopu, the paper defined it as Middle Permian(P2h, P21) and Middle to Upper Triassic(T2-3xq) gradually transformed composite petroleum system(·), which had a particular feature in west China superimposed basins, and indicated that the perspective position should be Xigada sub-tectonic belt in north part of the sag.