Organic geochemical signatures and palaeo-environmental implications for the source rocks from the Lower Silurian Longmaxi Formation in northern Guizhou

The organic geochemical signatures such as organic carbon, isoprenoid alkanes, terpanes and steranes in the source rocks from the Lower Silurian Longmaxi Formation in northern Guizhou are generalized in the following, including (1) single-peak distribution of normal alkane, with the main peak carbon varying between nC₁₈ and nC₂₀; (2) Pr/Ph ratios ranging from 0.16 to 1.24, with a general advantage of phytane; (3) gammacerane index>0.10; (4) Ts/(Tm + Ts) ratios ranging from 0.42 to 0.61; (5) C₂₇/C₂₉ ratios ranging from 0.65 to 1.53; (6) organic carbon contents generally more than 0.5%, with a gradual decrease from bottom to top, and (7) δ¹³C_org <-28‰. The organic geochemical signatures outlined above indicate higher organic carbon productivity and reduction environments in the Longmaxi Formation. The source rocks in the Formation may have experienced three stages of evolution, including (1) higher organic carbon productivity and highly reduced environment in the basal part; (2) moderate organic carbon productivity and reduced environment in the middle part, and (3) lower organic carbon productivity and slight redox environment in the upper part.