Upper Permian Junggar and Upper Triassic Ordos lacustrine source rocks in Northwest and Central China: Organic geochemistry, petroleum potential and predicted organofacies

Two sets of lacustrine source rocks in the Junggar Basin and Ordos Basin have been analysed by organic geochemical approach and organic petrographic technique. One of these source rocks, the Junggar shales (Lucaogou Formation and Hongyanchi Formation), was deposited in a hydrologically closed, brackish lake under a sub-humid climate in higher latitude (39 degrees-43 degrees N), and consisted of organic-rich profundal lake shales (Lucaogou Formation) and lean fluvial-lacustrine lake margin mudstones. The other one, the Chang 7 Shale, was deposited in a hydrologically open, freshwater lake under a humid climate in a 31 degrees N palaeolatitude, and was characterized by high concentration of alginite-rich organic matter. The profundal Lucaogou Shale and Chang 7-3 Shale possess abundant total organic carbon, high generative potential S-2 and high Hydrogen Indices, which rank as the best prolific source rocks in both basins followed by Chang 7-1 Shale. Hongyanchi Formation and Chang 7-2 are much leaner than their counterparts, and contain relatively small portion of alginite but more terrigenous macerals. This nature of organic matter makes them less prolific and becoming gas-prone potential. These differences in petroleum potential mirror depositional environment changes in source rocks formation. Potential source rocks in both basins have the potential for generating high-wax oil but the brackish system clearly contains more PNA oil. The kerogen structures revealed by pyrolysis gas chromatography allowexplaining this trend. The Lucaogou, Chang 7-3 and Chang 7-1 shales enriching in long and moderate alkyl chains share a nature of high wax oil generation-trend. In, contrast, the Hongyanchi and Chang 7-2 shales appears to be dominated by short to moderated chain length distribution and higher overall aromaticity to generate more PNA oil, even gas and condensate. Thermal liability was shown to be linked to kerogen structure, in particular the presence of alicyclic and oxygen containing moieties, and these in turn are correlate with total organic carbon content. (C) 2016 Elsevier B.V. All rights reserved.National Basic Research Programme of China [2014CB239000]; Chinese Major National Scientific and Technological Programme [2011ZX05001]; China Scholarship Council (CSC)SCI(E)EIARTICLEpansongqi@pku.edu.cn90-10615