The sorting effect of hydrodynamics on the geochemical compositions of sedimentary organic matter in a lacustrine rift basin: Significance for hydrocarbon exploration on the Qibei slope, Bohai Bay Basin, China

Abstract

Hydrodynamic sorting exerts a first-order control on the quantity, type and molecular composition of sedimented organic matter. Although this physical mechanism has been invoked to explain the varying content and composition of organic matter in global continental margin sediments, it is rarely mentioned in the interpretation of the heterogeneity of source rocks in petroliferous basins. We investigate the controls of hydrodynamic sorting on the differentiation of organic matter content, type and molecular composition, and on the hydrocarbon generation capacity of source rocks in the lower part of the first member of the Eocene Shahejie Formation in the Qibei slope of the Bohai Bay Basin, by using the Rock-Eval pyrolysis, gas chromatography-mass spectrometry and logging interpretation tools. The source rock samples in the northeastern part of the slope generally present a relatively coarse grain size, less organic matter content, and are rich in humic material, whereas the source rocks in the southwestern slope area show opposite trends in these characteristics. Biomarker data demonstrate reducing sedimentary conditions and that the proportion of organic matter input from aquatic organisms versus terrestrial higher plants increases with a reduction in grain size. Pyrolysis data with a hydrogen index model shows that this organic matter-grain size relationship causes a difference in hydrocarbon generation capacity of the source rocks in various regions of the slope. This relationship also plays a role in a migration of the hydrocarbon generation center from a low-lying area in the northeastern slope to a relatively higher area in the southwest. In summary, hydrodynamic sorting is important in the distribution of organic matter pools in the slope area, causing changes in the abundance, type and composition of organic matter in the source rocks in the slope area and profoundly affecting the overall pattern of hydrocarbon generation.