Material composition and hydrocarbon potential of the shale of WufengLongmaxi Formation in northwestern Hunan

In order to investigate source material and sedimentary environment of Lower Paleozoic Wufeng-Longmaxi Formations shale in northwestern Hunan, the authors studied the organic matters, biomarkers and mineral composition. The results show that they have the following characteristics:No odd-even carbon dominance appears; N-alkanes mostly have bimodal performance (C₁₈/C₂₅), and the ratios of ∑C₂₁-/∑C₂₂₊ are 0.45-1.11, indicating the dominant role of heavy hydrocarbon components; the average ratios of Pr/Ph vary from 0.27 to 0.44, showing reduction conditions. The tricyclic terpane values of C₂₁, C₂₃ and C₂₄ show inverted Vshaped distribution, and the ratios of Ts/Tm are between 0.39 and 0.66. The related content of C₂₇ sterane is slightly higher than that of C₂₉ sterane. Based on these features of biomarker, it is believed that the shales of Wufeng-Longmaxi Formation were formed in a reduction environment in shallow water in the Early Paleozoic, with high degree of thermal evolution of organic matter, and the main sources were bacteria, algae and other aquatic organisms. The vitrinite reflectance equivalent values vary from 2.57% to 3.01%, suggesting that all of the shale samples have reached the mature stage. The mineral compositions of the shale samples indicate that quartz and clay minerals dominate the mineral composition, indicating a good brittleness index and adsorption capability. It is concluded that exist favorable geological conditions for shale gas enrichment in the marine shale in the present study area:organic-rich intervals, moderate thermal evolution, good brittleness index and adsorption capability. The results obtained by the authors have laid an important basis for preference of shale gas exploration target area and further gas exploration in this area.