Abstract:

Organic-rich marine shales in the Lower Cambrian Shuijingtuo (Niutitang) Formation,the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation of western Hubei-eastern Chongqing area are very attractive targets for shale gas exploration in southern China.This research shows that the maximum paleogeotemperature experienced by the Shuijingtuo Formation ranges from 280℃ to 300℃,while the maximum paleogeotemperature of the Wufeng Formation and the Longmaxi Formation ranges from 200℃ to 220℃.The organic matter of the three formations is over-mature,with the vitrinite reflectance (R_o) for the Shuijingtuo Formation varying from 3.90% to 4.50%,while that for the Wufeng Formation and the Longmaxi Formation from 2.45% to 3.00%.However,nearly a complete series of n-alkanes mostly with bimodal distributions,isoprenoid alkenes,steranes,and terpanes have been observed in saturated hydrocarbons from the sampled shales' chloroform extract.This demonstrates that the conversion from liquid hydrocarbons to gaseous hydrocarbons (i.e. cyclo-polycondensation and hydrocracking) has been restricted within the shale strata,and thus the carbon skeleton of typical saturated hydrocarbon biomarkers could preserve at 200℃ to 300℃ for 50 to 180 Ma.What is also shown by the research is that some saturated hydrocarbon biomarkers in over-mature shales have not lost their ability to indicate maturity and biogenesis completely.Judging from the carbon number range and bimodal distributions of n-alkanes,and C₂₃ tricyclic terpane/C₃₀ hopance ratios in the sampled shales,the shales in the Shuijingtuo Formation have a higher level of organic maturity and the prokaryote is a major source of their organic matter,while the organic parent substance for Wufeng shales and Longmaxi shales are mainly derived from bacteria and alga.

Key words: organic matter, over-maturity, saturated hydrocarbon, shale, Lower Paleozoic, western Hubei-eastern Chongqing area