LAUSR

Abstract The original sediment composition is of great value to providing an understanding of depositional environment and hydrocarbon-generation potential, but it is easily altered by diagenesis and difficult to recognize. In this study, we examine carbonate-rich tight-oil mudstones of the Lower Cretaceous Arshan Formation in the Erennaoer sag of the Erlian Basin as an example. Based on analyses of mineralogy, stable carbon and oxygen isotopes, and trace/rare earth elements, the original composition of the Arshan mudstones has been reconstructed. Results show that the original Arshan mudstone composition was mainly siliciclastic detritus, tuffaceous and clay matrix, which has a major impact on diagenesis and organic matter content. The erupted volcanic ashes were beneficial to the introduction of nutrients, an increase in biomass and an associated increase in to the preservation of organic matter, which has been suggested by an improvement of TOC of Arshan tuffaceous mudstones, with average contents of 2.82%. The volcanic ashes in the studied mudstones had also enhanced methanogensis during the shallow burial. Additionally, the early alteration products of authigenic minerals include analcime or zeolite, most of which is entirely altered to analcime and chlorite. The formation of major eogenetic carbonate cements include early dolomite and calcite, which were related to volcanic materials and methanogenesis at shallow depths.