Lithofacies and depositional settings of Mesoproterozoic shales: Insights from the ∼1.65 Ga Chuanlinggou Formation, Yanliao Basin, North China Craton

Abstract

Precambrian black shale has become a significant trend in petroleum exploration, and the Chuanlinggou Formation is the oldest potential hydrocarbon source rock deposit in the North China Craton. The Mesoproterozoic was a unique transitional stage in the evolution of the environment and life in Earth's history. Research on lithofacies and depositional settings in this stage has important scientific and oil and gas exploration significance. Outcrop observations, thin sections, and elemental and organic geochemical analyses were used to study the mineralogy, sedimentary structures, lithofacies assemblages and their relationships with the depositional environment. Three endmembers (silicate, carbonate and clay minerals) based on X-ray fluorescence were employed to establish the lithofacies classification suitable for the Chuanlinggou Formation in the Yanliao Basin. The lithofacies of the Chuanlinggou Formation were classified into six types, including calcareous mudstone, siliceous/calcareous mixed mudstone, argillaceous/siliceous mixed mudstone, silicate-rich argillaceous mudstone, clay-rich siliceous mudstone and siliceous mudstone. Four lithofacies assemblages (LAs) were identified according to the integrated lithofacies analysis from the bottom to the top of the section. Lithofacies assemblages 1 and 2 (LA1 and LA2), which were deposited in deep subtidal to shelf environments, have high petroleum potentials and are advantageous lithofacies assemblages for organic matter accumulation. The black shale developed early rift may not be as favorable as in the late rift, while the siliceous shale developed in the continental shelf-slope is a favorable lithofacies. A comprehensive cognition of the LA and depositional settings of the Chuanlinggou Formation might provide guidance for further petroleum exploration and sedimentary environments in the Mesoproterozoic.