Geochemistry of an unconventional, saline lacustrine petroleum system: Dongpu depression, Bohai Bay Basin, China

Inter-salt shales are both important organic-rich sediments and shale oil exploration targets. This paper examines the controls on the composition of retained fluids in an inter-salt lacustrine shale oil system based on geochemical analysis of 125 samples with vitrinite reflectances of approximately 0.9% Ro from a 50 m shale-rich sequence between two evaporites. Core samples were immediately stored at −50 °C to reduce the evaporative loss of hydrocarbons. The sequence is highly heterogeneous and includes siltstone, argillaceous mudstone, calcareous mudstone, dolomitic mudstone, and argillaceous dolomite. Source rock samples with higher TOC contents (>1.0 wt%) and higher genetic potentials (S1 + S2 > 2.0 mg HC/g rock) are typically calcareous mudstone, dolomitic mudstone, and argillaceous dolomite, and contain predominantly type II kerogen. Salinity controlled both bottom water deoxygenation and the development of carbonate minerals, and the increase in aquatic organisms (including prokaryotes and eukaryotes) leads to a positive relationship between OM richness and quality. High TOC (>2.0 wt%) samples contain oil-prone kerogens and have high absolute contents of liquid hydrocarbons (S1-1 > 0.2 mg HC/g rock, S1-2 > 1.0 mg HC/g rock), and high relative contents of liquid hydrocarbons [(S1-1 + S1-2)/(S1-1 + S1-2 + S2-1) > 0.45]. The amount of oil retained in source rock horizons is mainly controlled by the sorptive properties of the solid phase organic matter. Oils (S1) within siltstones are derived from adjacent source intervals; multi-stage temperature pyrolysis shows that they are lighter – and thus more readily produced - than the oils retained within the source intervals. Although the absolute amounts of oil in siltstones are lower than those in the source intervals, a smaller fraction is sorbed on solid phase organic matter, so that there are similar amounts of moveable oil in siltstones and source rocks. Interbedded siltstones, organic-rich calcareous mudstones, dolomitic mudstones, and argillaceous dolomites are all favourable intervals for inter-salt shale oil development.