Depositional cycles in a rift lacustrine basin linked with tectonics, climate, and source rocks and reservoirs development: Lower Cretaceous in Naiman sag, Songliao Basin, Inner Monglia, Northeast China

Abstract

In order to improve understanding of the depositional evolution processes in rift lacustrine basins in Northeast China under Early Cretaceous greenhouse climate fluctuations, the roles of tectonics and climate along with the development of source rocks and reservoirs in the evolution of continental rift lacustrine basins in Naiman sag, Songliao Basin, Inner Monglia, were studied. Based on the detailed facies analysis of the subsurface boreholes, eight facies were identified in the Lower Cretaceous strata of Naiman sag and were grouped into three facies associations, namely fan delta, littoral to sublittoral lacustrine, and deep lacustrine. Based on the depositional characteristics, in combination with lacustrine evaporites and carbonate deposition, organic matter preservation, and lake depth, the Lower Cretaceous strata of the Naiman sag was further subdivided into four lake stages, namely initial formation of lake (Stage 1), sediment-fill shallow lake (Stage 2), rising to a high lake (Stage 3), and lake closure (Stage 4), respectively. The lake evolution in the Naiman sag was driven by both tectonics and climate changes. Fault-driven subsidence was likely contributed to episodic changes in accommodation and lake formation (Stage 1), expansion (Stage 3) and shrinkage (Stage 4). A combination of regional and global climate overprint controlled the sedimentary characteristics, depositional trends, and sediment and water supply. The global climate of ocean anoxic events 1a and 1b (OAE1a and OAE1b) may have effects on anoxic conditions of Stages 1 and 3, respectively. During regional climate changes between arid, hot and humid, seasonal climate tended to form evaporite deposits and multiple layers of oil shale deposits in Stage 1. During the more humid regional climate, steady runoff tended to form fan delta deposits prograding into the lake basin in Stages 2 and 4 and thick-bedded oil shale deposits in Stage 3. The development of source rocks is linked with lake formation and expansion stages (Stages 1 and 3), and the source rocks are mainly preserved in the deep lacustrine and littoral to sublittoral lacustrine environment. The development of reservoirs is associated with lake formation, sediment-fill, and shrinkage stages (Stages 1, 2, and 4), and the reservoirs are mainly preserved in the fan delta environment.