Characteristics and Origin of Source Rocks in the Small Faulted Lacustrine Basin

Abstract

Compared to large-scale lake basin, small faulted lacustrine basin is characterized by small area, shallow water, multi-and-near sources, oxidative sedimentary environment and high sedimentary rate. To recognize origin of the source rocks in small faulted lacustrine basin, four basins (Aer, Wuli, Saihan, Naoer) in the Erlian Basin Group, one of the most petroliferous basins in northern China, have been analyzed to characterize source rock potential, to reveal environmental and ecological changes and construct source rocks depositional models. These basins have dissimilar boundary fault activities and sedimentation rates, which are consistent with changes in clastic influx input, productivity and redox conditions. With more active boundary faults and higher sedimentation rates, the terrigenous organic matter inputs to Aer and Wuli basins are higher than Naoer and Saihan basins, which can be confirmed by relative steranes abundances. The carbon isotopic composition of carbonates suggests that the productivity of Aer and Wuli basins is higher than that of Naoer and Saihan basins. Total reduced sulfur and Pr/Ph ratios show an oxidizing environment in Aer and Wuli basins; while a reducing environment was present in Naoer and Saihan basins. Different tectonic conditions lead to variations in ecological communities and enable us to construct source rock deposition models. During source rock deposition in the small basin with active boundary fault and high sedimentary rate, the high influx of clastic sediments carrying high terrigenous organic matter and dissolved inorganic carbon and nitrate into the lake, might have sustained high productivity and oxidizing conditions. Due to high influx of clastic sediments and unstable water column stratification, the depositional environment is oxidizing. In the high productivity lake, there is still significant organic matter accumulation in oxidized environment after oxidative degradation. High productivity is the dominant factor controlling the formation of organic rich sediments and deposition of source rocks. During source rock deposition in the small basin with inactive boundary fault and low sedimentary rate, the depositional environment is reducing due to low influx of clastic sediments with low productivity. In the low productivity lake, reducing sedimentary environment substitutes for productivity and becomes the predominant factor controlling the formation of source rocks.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016