Constraints of Sedimentary Environment on Shale Organic Matter Enrichment: Insights from Elemental Geochemistry and Multiple Factor Analysis.

ABSTRACT

The organic-rich shale of the Wufeng-Longmaxi formation is an important section for shale gas exploration. The traditional univariate or bivariate analysis causes researchers to have great controversy about its enrichment mechanism. This study explores the combination of multiple factor analysis (MFA) and element geochemistry to calculate the contribution rate of a paleoenvironment to organic matter enrichment and clarify the main controlling factors of organic matter enrichment. Research has shown that there is generally high productivity from the Wufeng (O3w)-Longmaxi formation (S1l) deposition. The degree of terrigenous clastic input and weathering during the period of the O3w is relatively low, and sedimentary water restriction is strong, mainly developing an anoxic-dysoxic sedimentary environment. During the deposition of S1l1, the input intensity and weathering of terrigenous debris were slightly enhanced, and the increase of the water column led to the development of an anoxic environment at the bottom of the water layer. During the S1l2+3 period, the degree of terrigenous debris and weathering is the largest, and the high oxygen content of the water column is mainly a normal oxic environment. An MFA calculation shows that the paleoproductivity and paleoredox environment of the organic-rich shale section have the highest contribution rate of about 59.57% to the enrichment of organic matter, which is higher than that of paleoclimate conditions and terrigenous clastic input, indicating that the enrichment of organic matter is mainly controlled by paleoproductivity and the preservation environment. This study provides a basis for the application of MFA in element geochemistry and can serve as a model for other studies.