New insights from geostatistics on the genetic relationship between shales, mudstones, and sandstones within a parasequence in the lower Chang7 Member of the Upper Triassic Yanchang Formation, Ordos Basin, Northern China

The Chang7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, North China, has received much attention in recent years due to its huge shale oil potential. However, few studies of the relationship between black shale and other lithofacies are available, and the origin of black shales in the parasequence is poorly understood. In this paper, we describe the detailed petrography of the Chang7 Member sampled by 578 drill holes in the southern Ordos Basin and present geochemical data to reveal the relationships between black shales, mudstones, and sandstones, aiming to understand the factors that controlled the formation of the black shales. Four sedimentary assemblages were identified within the Chang7_3 parasequence, using geostatistic cluster and correlation analyses. High total organic carbon (TOC) shale (HTS), HTS + medium-TOC shale (MTS), low-TOC mudstone (LTM), and fine sandstone (FS) facies were predominant in Assemblage 1, Assemblage 2, Assemblage 3, and Assemblage 4, respectively. The FS facies had almost no effect on the formation and preservation of organic carbon. The number of mudstone interbeds is sufficiently large to cause marked negative changes in the thickness and organic-carbon abundance of black shale. The sedimentary assemblage characteristics in a parasequence provide important insights into the link between black shale and other facies. This study has profound implications for understanding the formation mechanisms of black shale in lacustrine systems.