Controlling Factors and Formation Models of Organic Matter Accumulation for the Upper Permian Dalong Formation Black Shale in the Lower Yangtze Region, South China: Constraints from Geochemical Evidence

Abstract The black shale in the upper Permian Dalong Formation is considered as an excellent source rock in the Lower Yangtze region. However, mechanisms of organic matter (OM) accumulation in such a setting are scarcely understood. Here, the characteristics of total organic carbon (TOC) and elemental geochemistry of 33 rock samples from GD1 well are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that the lower and middle parts of Dalong Formation (section A) display high TOC contents ranging from 1.19 to 6.45% (average 3.19%), whereas the upper part (section B) exhibits medium TOC contents varying from 1.18 to 4.90% (average 2.09%). These data also elucidate that the target shales were deposited in a complex paleoenvironment with moderate to strong water-mass restriction that was characterized by warm and semiarid–semihumid paleoclimate, high biotic productivity, fluctuating plaeoredox conditions, and a relatively high sedimentary rate. Compared to the organic-rich shales from section A mainly developed under an anoxic condition, shales from section B formed in an oxic-to-dysoxic water environment exhibited a comparatively higher sedimentary rate. Moreover, among all these factors that might affect OM accumulation, the paleoredox conditions appear to be the dominant controlling factors for section A, whereas the biotic productivity, paleoredox conditions, and sedimentary rate are the main controlling factors for section B. Finally, two formation models for OM accumulation in Dalong Formation shale in the Lower Yangtze region are proposed. The “preservation model” for OM accumulation in section A emphasizes that the reducing deep-water environment, which was mainly caused by the regional sea level rise, is favorable for OM preservation. The “integrated model” for OM accumulation in section B stresses that greater biotic productivity intensifies respiratory oxygen consumption in a water column and a higher sedimentary rate can greatly shorten OM exposure time for respiration by oxygen, both of which cause OM accumulation under an oxidizing water environment. These findings also add to our knowledge that despite the oxygenated water environment during shale deposition, TOC contents are not necessarily lower.