Sedimentary and sequence-stratigraphic characteristics of the lower second submember, Shahejie formation, M1 block, Wenmingzhai oilfield, Dongpu depression, China

Abstract

It is favorable that the sequence-stratigraphic characteristic and sedimentology analysis of Paleogene and the associated deposits of Shahejie formation (FM.) be allowed a number of sedimentary evolution stages to be characterized and a sedimentary model of the lower second submember of Shahejie FM. (ES2) to be established. An integration of drilling, logging, testing, core descriptions, and high-resolution sequence stratigraphy has been used to provide a new conceptual depositional model for the M1 block; the results show that the lower ES2 is deposited as terminal fan, occurring in areas where sediment-laden waters decrease in size and vanish because of evaporation. The depositional model is composed of proximal subfacies, middle subfacies, and remote subfacies according to its lithology, electricity, and sedimentary structures. The combined analysis of lithological and sedimentary sequences has come to the conclusion that there are three levels of base level cycles and three kinds of interfacies for the lower ES2. The former includes fifth-order short-term cycle sequence (V), fourth-order medium-term cycle sequence (IV), and third-order long-term cycle sequence (III), respectively. The latter includes the scoured surface, the bedding change interface, and the thickness’ cyclic changing interface of sandstone and mudstone. The conclusion has been drawn that the lower ES2 of the M1 block is determined as a transgression process with increasing water depth based on the analysis and correlation of vertical stacking pattern.