Black shales from the Longmaxi Formation in western Xikang-Yunnan ancient land:Geochemistry and geological implications

The detailed element geochemical studies are conducted to ascertain the geological genesis and sedimentary environments of the black shales from the Longmaxi Formation in the southeastern Ninglang Basin on western Xikang-Yunnan ancient land, and to provide helpful information for the shale gas exploration in this area. The black shales have higher SiO₂ contents ranging between 64.78% and 96.89%, positive correlation of Al₂O₃ and TiO₂ contents and negative correlation of SiO₂/Al₂O₃ ratios and Al₂O₃ contents, suggesting that the siliceous rocks in this area are not pure siliceous rocks but higher proportion of terrigenous argillaceous deposits. The Al/(Al + Fe + Mn)(0.53-0.75), Fe/Ti (6.38-14.96), (Fe + Mn)/Ti (6.40-18.20), Si/(Si + Al + Fe)(0.82-0.99)ratios and Al-Fe-Mn triangular diagram indicate that the siliceous rocks mentioned above are biogenetic. The MnO₂/TiO₂ ratios (0.005-1.20), δCe values (0.46-0.92), tectonic discrimination diagram and REE distribution patterns also indicate the continental-margin abyssal-bathyal sedimentary environments of the provenances. The salinity index of the sea water (m=4.96-36.88), CaO/(Fe + CaO)ratios (0.017-0.83), negative δCe anomalies and positive δEu anomalies, and Ce/La ratios (1.04-2.09)indicate the anoxic and low-to moderate-salinity marine sedimentary environments. There are slightly negative correlation between high organic carbon and high SiO₂ contents, indicating that the biogenetic siliceous roks should be responsible for the high organic carbon contents. The organic-rich source rocks from the Longmaxi Formation in the Lizihe region are interpreted to be favourable for shale gas exploration due to high organic carbon contents, greater shale thickness and abundant surface oil and gas showings.