Insights of Big Data and Machine Learning on the Depositional Environment and Organic Matter Enrichment of the Wufeng-Longmaxi Shales

Shale organic matter (OM) enrichment plays a critical role in evaluating the future prospects of shale gas worldwide. The Ordovician-Silurian transition represents a critical interval notable for extensive deposits of organic-rich shale formations. Despite this, the underlying factors controlling OM enrichment during this interval, particularly in the Wufeng-Longmaxi (WF-LMX) shale, need comprehensive reevaluation. Our study compiles major and trace element concentrations for 1376 samples and TOC values for 3143 samples from the WF-LMX shale in South China to define quantitative thresholds for redox environment and paleo-productivity. Paleoenvironmental characteristics associated with the deposition of shales containing varying OM abundances (TOC <2%, 2–4%, and >4%) were clarified. Random Forest (RF) and Artificial Neural Network (ANN) models were applied to identify the primary factors controlling OM enrichment of the WF-LMX succession. Results of this work show that MoEF, UEF, VEF, and CrEF values of 10, 10, 1.5, and 1.5, respectively, represent the threshold for anoxic conditions during shale deposition. SiO2(exc), P/Ti, and Babio values of approximately 30%, 0.2, and 1000 ppm, respectively, signify high paleo-productivity. RF and ANN results suggest that redox environment and paleo-productivity as the foremost influences on OM accumulation, followed by paleoclimate. Organic-rich shales (Type-a, TOC > 4%) were primarily deposited under warm and humid climate conditions. Enhanced nutrient supply promoted elevated productivity and established anoxic bottom conditions that facilitated OM preservation. Type-c shales (TOC < 2%) accumulated under low-moderate productivity and oxygenated bottom conditions, leading to extensive degradation and dilution of organic matter. Type-b shales (TOC = 2–4%) exhibit intermediate characteristics between Type-a and Type-c. This study offers new perspectives on OM enrichment mechanisms in the WF-LMX shale succession and provides a theoretical basis for shale gas exploration within this stratigraphic interval.