Unconventional Petroleum Sedimentology:A Key to Understanding Unconventional Hydrocarbon Accumulation

The commercial exploitation of unconventional petroleum resources(e.g.,shale oil/gas and tight oil/gas)has drastically changed the global energy structure within the past two decades.Sweet-spot intervals(areas),the most prolific unconventional hydrocarbon resources,generally consist of extraordinarily high organic matter(EHOM)deposits or closely associated sandstones/carbonate rocks.The formation of sweet-spot intervals(areas)is fundamentally controlled by their depositional and subsequent diagenetic settings,which result from the coupled sedimentation of global or regional geological events,such as tectonic activity,sea level(lake level)fluctuations,climate change,bottom water anoxia,volcanic activity,biotic mass extinction or radiation,and gravity flows during a certain geological period.Black shales with EHOM content and their associated high-quality reservoir rocks deposited by the coupling of major geological events provide not only a prerequisite for massive hydrocarbon generation but also abundant hydrocarbon storage space.The Ordovician-Silurian Wufeng-Longmaxi shale of the Sichuan Basin,Devonian Marcellus shale of the Appalachian Basin,Devonian-Carboniferous Bakken Formation of the Williston Basin,and Triassic Yanchang Formation of the Ordos Basin are four typical unconventional hydrocarbon systems selected as case studies herein.In each case,the formation of sweet-spot intervals for unconventional hydrocarbon resources was controlled by the coupled sedimentation of different global or regional geological events,collectively resulting in a favorable environment for the production,preservation,and accumulation of organic matter,as well as for the generation,migration,accumulation,and exploitation of hydrocarbons.Unconventional petroleum sedimentology,which focuses on coupled sedimentation during dramatic environmental changes driven by major geological events,is key to improve the understanding of the formation and distribution of sweet-spot intervals(areas)in unconventional petroleum systems.

Controlling Factors on Organic Matter Accumulation of Marine Shale across the Ordovician–Silurian Transition in South China: Constraints from Trace-Element Geochemistry

In recent years,significant progress in shale gas exploration has been achieved in the Upper Ordovician(Wufeng Formation)–Lower Silurian(Longmaxi Formation)shales in the Upper Yangtze area,South China.Although many studies have been carried out on the Upper Ordovician–Lower Silurian shales,the controlling factors causing organic matter accumulation of these shales remain controversial.This study uses trace-element geochemistry and sedimentological methods to evaluate terrigenous input,redox conditions and primary productivity to explore the mechanisms of organic matter accumulation.The variation of terrigenous fraction elements(Al,Th and Sc)concentrations reflect a mixed influence of sea-level change and weathering.The sea-level of the Upper Yangtze Sea went through two cycles of transgression to regression during the Ordovician–Silurian transition.The Linxiang Formation,Kuanyinchiao Bed and the upper part of Longmaxi Formation developed during the periods of regression,whereas the Wufeng Formation and the lower part of the Longmaxi Formation developed during the periods of transgression.The paleo-productivity indexes of TOC content,ratios of Ba/Al and P/Al,and redox conditions proxies of Mo concentration,ratios of U/Th and V/Cr generally display similar variation patterns with respect to the sea-level changes.High TOC contents and Ba/Al and P/Al ratios indicate the paleo-productivity was high on the sea surface,as shown by relatively good positive correlations between Th vs.TOC,and Sc vs.TOC.This indicates that the paleo-productivity was controlled by the nutrients input through weathering.The good positive correlations between redox conditions indexes(U/Th and V/Cr ratios)with TOC content reflects reductive preservation conditions(anoxic to euxinic),thus implying they were an important controlling factor for organic matter accumulation.Nevertheless,redox conditions were closely associated with sea level change and organic matter decomposition.Therefore,the sea-level change and weathering were the primary controlling factors for organic matter enrichment across the Ordovician to Silurian transition.