Organic matter enrichment in shale deposited proximal to paleo-uplifts and its impact on shale gas exploration

Abstract

Much of the global exploration and development of shale gas is focused on organic-rich shale deposited in basin or slope/basin margin settings. There is a clear need to assess the degree to which organic-rich shale deposited in other settings, notably proximal to paleo-uplifts, are promising shale gas prospects. Organic-rich sediments that accumulated proximal to paleo-uplifts are recognized globally and include the Cambrian Niutitang Formation, Ordovician-Silurian transition “hot shales”, and modern sediments of the Black Sea. Though the subject of recent research efforts, much remains to be learned of the mechanisms of organic matter enrichment of fine-grained sediment deposited adjacent to paleo-uplifts. Our study focuses on the Ordovician Wufeng Formation that accumulated proximal to uplifted areas of the Upper Yangtze platform. Though the TOC content of the analyzed Wufeng Formation samples recovered from near the paleo-uplifts is less than that of Wufeng samples collected from basin or slope/basin margin settings, average TOC value of the former can still reach more than 2%. Wufeng Formation shale deposited proximal to paleo-uplifts accumulated under conditions of medium to high paleoproductivity, elevated terrigenous clastic input and sedimentation rate, and oxic-suboxic water column conditions. Principal component analysis suggests that the factor most critical to organic matter enrichment of Wufeng deposits proximal paleo-uplifts was paleo-redox conditions. The TOC abundance in Wufeng Formation shale deposited during late Katian stage is higher than that of shale deposited during Hirnantian stage. It was at the Hirnantian stage that the paleo-uplifts became most active in association with the Kwangsian Orogeny resulting in diminished paleoproductivity and preservation conditions as reflected in the reduced organic matter abundance of associated shale. Uplift of the paleo-uplift of the Yangtze Platform, a manifestation of the Kwangsian Orogeny during the Ordovician-Silurian transition, appears to have been largely responsible for spatial and temporal distribution differentiation of organic matter abundance of the Wufeng Formation of the Upper Yangtze Platform. Results of the present study contribute supplementary information regarding the mechanism of organic matter enrichment in shale and can provide guidance for shale gas exploration and development in geologically similar regions.