--> Source Rock Characteristics and Potential of the Lucaogou Organic-Rich Sediments in the Jimsar Sag, Junggar Basin, Northwestern China

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Source Rock Characteristics and Potential of the Lucaogou Organic-Rich Sediments in the Jimsar Sag, Junggar Basin, Northwestern China

Abstract

The Lucaogou organic-rich sediments composed of gray black mudstone, dolomitic mudstone, argillaceous siltstone and sedimentary tuff in the Jimsar Sag are world class source rocks. They are the main source rock for reservoirs in the Permian Lucaogou Formation, which is the primary exploratory stratum of tight oil reservoirs in the Jimsar Sag, Junggar basin, Northwestern China. Based on the Total Organic Carbon (TOC) and pyrolysis results of core samples, Lucaogou source rocks exhibit a wide range in TOC contents, laterally from 0.03 wt.% to 19.77 wt.%, and the chloroform bitumen “A” content is in the range of 0.0046–4.8648%, and the hydrocarbon generation index (S1+S2) is in the range of 0.11–79.4mg/g. It is also worth mentioning that the TOC of argillaceous siltstone samples after extraction is more than 1%. The high variation of organic matter abundance may result from mixed effects of the original depositional environment and progressive maturation. The biomarker and pyrolysis results provide evidence for a major contribution of aquatic algae and microorganisms with minor terrigenous organic matter input. The Lucaogou source rocks are dominated by Type I and II kerogen, thus considered to be oil-prone. The samples analyzed have vitrinite reflectance in the range of 0.72–1.05% Ro and pyrolysis Tmax in the range of 435–458oC, which indicates that the Lucaogou source rocks contain mature to late mature organic matter. One-dimensional basin modelling was performed to analysis the hydrocarbon generation and expulsion history based on the reconstruction of the burial/thermal maturity history. It is indicated that the Lucaogou source rocks began to enter the oil generation window at late stage of the Jurassic, and had been in oil generation and expulsion peak from the Cretaceous to present. The world class source rocks of Lucaogou Formation, with a thickness mostly greater than 200m, are widely distributed in the Jimsar Sag. It is one of the key factor to tight oil accumulation in the Jimsar Sag, Junggar Basin, and northwestern China.