Elementary Geochemistry Of Middle-Upper Permian Shale From Well GD1 In Xuancheng Area

Organic matter-rich shale shows homogeneity macroscopically,but has strong heterogeneity in microcosmic aspects such as geochemical characteristics.Therefore,chemical stratigraphic division is gradually playing an important role in the evaluation of complex lithological strata and unconventional oil and gas reservoirs.Based on the analysis and test results of element geochemistry,this paper makes a chemical stratigraphic division and sedimentary environment restoration of the sedimentary strata of Dalong formation,Gufeng formation and Longtan formation in Gangdi 1 well,and analyzes its significance to shale gas exploration combined with the experimental test results of total organic carbon(TOC).The understanding of this paper is as follows:The different elements in the target interval are divided into three types according to their geological significance:Al,Si,Ti,Fe,K,Nb,Rb,Pb related to terrigenous debris;Ca,Mg,Pb related to carbonate Mn,Sr,Ba and V,Cr,Zn related to redox conditions.There is also a certain correlation between the content of some elements and the content of specific minerals,Al element content is related to clay mineral content;Si element content is related to detrital quartz and feldspar and other brittle minerals;Ca element content is related to calcite content;Mg element content is related to Baiyun Mountain content.According to the geological significance of different element ratios,the chemical stratigraphic unit division plan is determined,and the Dalong Formation and Gufeng Formation are divided into 9 chemical stratigraphic units,and the Longtan Formation is further divided into 7 chemical stratigraphic units.The Dalong formation is generally deposited in a warm,humid and oxygen-poor marine environment,the sedimentary water body changes from shallow to deep,and the change of water depth gradually changes from near-continental source to far-away continental source.Compared with the Dalong formation,the Longtan formation as a whole deposited in the hot and dry climate,belongs to the brackish sea-land transitional sedimentary environment,the overall sedimentary water body is relatively shallow,compared with the Dalong formation,the paleo-productivity is lower.Compared with Dalong formation and Longtan formation,Gufeng formation is mainly deposited in warm and humid climatic conditions similar to Dalong formation,but its paleosalinity is the highest of the three stratigraphic groups,and most of the chemical stratigraphic units belong to saltwater marine sedimentary environment.a few belong to brackish water marine transitional facies sedimentary environment,compared with Longtan formation,the sedimentary body is deeper and the paleoproductivity is higher.Based on the study of TOC content and geochemical characteristics of shale in Dalong formation,it is considered that the content of TOC in shale of Dalong formation is related to the ratio of paleo-productivity index Ba/Al,paleo-redox index V/Cr and terrigenous input index Si/Al.There is a good correlation between the content of Si element in sedimentary strata and brittle minerals in shale,so the content of Si element can reflect the compressibility of shale.Element geochemistry also has a good characteristic response to the gas-bearing property of organic shale.Based on the study of the geochemical characteristics of various elements,it is considered that the shale formation of Gufeng formation is the focus of shale gas exploration in the next step.