Sedimentary characteristics of hybrid event beds in deep-lacustrine basins and their impact on reservoir quality: Examples from the Bohai Bay Basin, Eastern China

Abstract

Hybrid event beds (HEBs) form important components of subaqueous sediment gravity flow models in deep-lacustrine sedimentary basins, largely due to their clay-rich nature meaning that they often form non-reservoir and/or baffles/barriers to fluid flow in the subsurface. Using examples from a typical deep-lacustrine basin, this study documents the sedimentary characteristics and distribution of HEBs and explores their effect on reservoir properties. To achieve this, a suite of drill cores and wireline data through a range of debrites, concentrated flow deposits, and turbidites are analyzed from Paleogene in the Bohai Bay Basin, China. The superposition and convergence of gravity flow deposits resulted in a complex spatial and temporal distribution and evolution of the HEBs in the study area. The majority of identified HEB types are interpreted to be associated with lobe deposition. However, slumping-induced HEBs are interpreted to be restricted to the proximal slump areas. Slump HEBs are interpreted to have formed through slumping and associated generation of sediment gravity flows, where HEBs formed through flow transformation of slumps into high-concentration flows (debris flows) and/or high-density turbidity currents in down-slope areas. Debris flow HEBs are interpreted to be formed by particle rearrangement through vertical settling during flow transformation within the subaqueous fans. The effects of relative buoyancy in debris flows, rearrangement of debris flow particles, muddy substrate erosion by turbidity currents, and slumping upon HEB development is discussed. The average porosity and permeability observed within reservoir intervals formed by gravity flow deposits is 17.8 % (ranging from 2 % to 25 %) and 126 mD (ranging from 3 mD to 816 mD), respectively. HEBs within the studied reservoir interval display low-porosity (<15 % on average) and low-permeability (<10 mD on average) values. The occurrence of HEBs within a reservoir increases the variation coefficient of permeability, quantity, and thickness of the interlayers and presents a significant heterogeneity. The results of this study are important to consider in the context of constructing reservoir models in deep-lacustrine reservoirs of the Bohai Bay Basin, and which can be applied to other lacustrine gravity flow deposits in sedimentary basins worldwide.