Sedimentary evolution of overlapped sand bodies in terrestrial faulted lacustrine basin: Insights from 3D stratigraphic forward modeling

Abstract

Terrestrial faulted lacustrine basins usually have large variabilities in structural subsidence and multiple river sources. The facies in such basins are thus characterised by frequently changed facies and complex relationship of overlapped sand bodies (e.g. Liaoxi Depression of Liaodong Bay, Bohai Sea, China). Therefore, it's a great challenge to quantitatively describe evolution of sand bodies and their overlap relationship. In this study a three-dimensional stratigraphic forward model is used to solve this problem, which is corrected according to well log data. After the parameter optimization was verified against borehole strata thickness and lithology, this model had a thickness error within 8%, and the lithology distribution agreed with the well data as a whole.Four evolution phases of the Shahejie 3 Formation were recognised through the modeling. In the first phase, the lake level rose, and the eastern fan delta sand developed. In the second phase, the fan delta sand scaled up, and the braided river delta front sand developed at a small scale. In the third phase, the lake level shifted from falling to rising, the fan delta sand further expanded the scale, and the braided river delta developed at a large scale. Sands on both sides overlapped at a large scale. In the fourth phase, the lake level rose, the lake basin expanded, and lake shale was the main lithology in the study area. From the sedimentary evolution of Es3 formation, sand overlapped in the third stage. In 39.75 Ma, two parts of the sand body began to overlap and reached largest scale in 39.6 Ma, which was approximately 10 km2. Both then retrograded to a separation in 39 Ma when the overlap ended.Coupling stratigraphic forward simulation with uncertainty quantification allowed us to estimate the influence of each stratigraphic parameter on sand overlap. Fluvial discharge appears to be the most influential parameter while the initial bathymetry and lake level variations exert a minor to moderate impact.