The biomarkers in the source rocks from the Lower Silurian Longmaxi Formation in northern Guizhou

The steranes and terpanes in the source rocks from the Lower Silurian Longmaxi Formation in northern Guizhou are determined with the aid of gas chromatography and chromatography-mass spectrographic analysis.The analytical results suggest that the n-alkanes in the source rocks display a back single-peak pattern,with the main peak carbon values ranging from C₂₂ to C₃₀.The Pr/Ph ratios vary from 0.48 to 0.64;Pr/nC₁₇ ratios vary from 1.35 to 4.22,and Ph/nC₁₈ ratios vary from 0.92 to 0.97.The pregnane and homopregnane are very rich in steranes.C₂₇,C₂₈ and C₂₉ display a regular V-type distribution pattern(C₂₇ > C₂₈ > C₂₉),with a C₂₇ dominance.The sterane/aromadendrane ratios range between 0.57 and 0.71.The terpanes are characterized by high tricycloterpane values,low aromadendrane values and high gammacerane indexes.All the values for the maturity such as OPE<1,Ts/(Ts + Tm) ratios ranging between 0.42 and 0.48,C3122S/(S + R) ratios ranging between 0.6 and 0.66,and C₂₉ ββ/(ββ +αα) ratios ranging between 0.56 and 0.58 indicate the stratified water bodies and highly reduced conditions during the Longmaxian.The organic matter dominantly came from planktons and algae,and has been subjected to a certain degree of biodegradation.The thermal evolution of the source rocks has reached up to the stages of high maturation and overmaturation.