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Essential Elements for Hydrocarbon Accumulation in Lucaogou Formation of Jimusaer Sag, Junggar Basin


The Permian Lucaogou Formation of Jimusaer Sag in the southeast Junggar Basin is the focus for tight oil exploration and development in the basin. Commercial oils have been obtained from many wells, and important advances in tight oil exploration have been achieved. The Permian Lucaogou Formation in Jimusaer Sag contains fine-grained sediments dominated by mechanical and chemical sedimentation, which occurred in a saline lake depositional environment after the residual sea was closed. The lithologies are composed by sandstone, mudstone, argillaceous siltstone, silty mudstone, dolomitic siltstone, dolomite mudstone, silt dolomite, and argillaceous dolomite, with characteristics such as fine particle size, thin single layers, and frequently interbedded lithology. Core sample observation indicates that among all lithologies in the reservoir, silt dolomite and dolomitic siltstone contain most of the oil. The tight reservoir exhibits intensive heterogeneity and is characterized by medium porosity and low permeability. Microscopic observation reveals that the reservoir space mainly contains dissolved pores. The spatial distribution of reservoir sweet spots and source rocks is controlled by lake-level fluctuations. The sweet spots for siltstones in Jimusaer sag formed during the process of lake regression and composed a favorable space allocation with mudstones that formed during lake transgression. Mudstones in Lucaogou Formation has high hydrocarbon generating potential for its dominated sapropelic kerogen, the average TOC of about 5%, and Ro ranging in 0.7%-1.0%. Notably, the TOC of many siltstone and dolomite samples after extraction is more than 1%. Analysis of these siltstones and dolomites indicates that a significant percentage of clay minerals occur in them, because the Lucaogou Formation formed in a low- energy environment as a whole and the clay minerals have a strong ability for absorption and preservation of the organic matter during deposition. Therefore, the siltstones and dolomites in the Lucaogou Formation are capable to generate hydrocarbon. The organic acids discharged prior to hydrocarbon generation dissolve carbonate minerals in adjacent reservoirs to provide abundant reservoir spaces. The hydrocarbon entering into the adjacent reservoir without long-distance migration forms a tight oil accumulation. Such accumulation is actually in the tight reservoir retained in the hydrocarbon source rock series.