Lithotypes, organic matter and paleoenvironment characteristics in the Chang73 submember of the Triassic Yanchang Formation, Ordos Basin, China: Implications for organic matter accumulation and favourable target lithotype

Abstract

Lithotypes, organic matter, and paleoenvironment characteristics are essential attributes in organic-rich lacustrine mudstone. Understanding the covariant relationship among them is significant not only for revealing the main controlling factors of organic matter accumulation but also for optimization of the favourable interval of unconventional petroleum resources.Herein, we present a systematic petrological and geochemical analysis of 24 samples collected from the Chang73 submember in the Triassic Yanchang Formation. Base on the texture, bedding, and composition of mudstone, the collected samples can be further divided into four lithotypes (Lithotype1 to Lithotype 4). Toc and rock pyrolysis data indicate that those four lithotypes mainly contain a high content of type I and II1 kerogen, good to excellent hydrocarbon generation potential, and are at the low mature to mature stage. Paleoenvironmental parameters, represented by paleosalinity (Sr/Ba, B/Ga), sediment provenance (Al2O3/TiO2 and Th/Sc–Zr/Sc), paleoclimate (C-value, Sr/Cu), paleoproductivity (P/Al, P/Ti, Ni/Al, Ni/Ti, Cu/Al, and Cu/Ti), bottom-water redox conditions (V/Cr, V/(V + Ni), and UEF-MoEF), and sedimentation rate ((La/Yb)N), indicate that the four lithotypes were all deposited in freshwater environments and sourced from felsic volcanic rocks but different in the paleoclimate, paleoproductivity, bottom-water redox conditions, and sedimentation rate. According to the ratios of C-value, B/Ga, Al2O3/TiO2, P/Ti V/Cr and (La/Yb)N with TOC, The degree of organic matter accumulation is positively correlated with the humidity of the paleoclimate, paleoproductivity, and reducibility of bottom water but negatively related to the sedimentation rate. Comparing the differences between the organic matter characteristics and paleoenvironment of the four lithotypes, we conclude that Lithotype1 is the most favourable target lithotype for developing unconventional oil resources.