Mixed siliciclastic-carbonate deposition and cyclical evolution of the Upper Shahejie Formation and its impact on reservoir development in the Eocene Huanghekou Sag, Bohai Bay Basin, East China

We have systematically investigated a typical lacustrine mixed siliciclastic-carbonate sedimentary system in this study, which primarily deposited in the Eocene Upper Shahejie Formation (Es 1 formation) of the Huanghekou Sag, Bohai Bay Basin. Using synthetic lithologic data sets, we summarized eight lithofacies from mixed siliciclastic-carbonate rocks and classified them into siliciclastic- and carbonate-dominated groups, indicating the variation of relative intensity between fluvial/deltaic process and carbonate productivity. Combining petrological and mineral component analysis, most lithofacies are compositionally replaced by micrite dolomite and show vertically different scales of cyclicity. The high-level cyclicity suggests the alternation between siliciclastic- and carbonate-dominated mixed rocks, whereas the lower level indicates interbedding of different dolomite-dominated lithofacies. The controlling factors on both levels are different. The higher level is probably controlled by climate changes, which finally resulted in the relative variations of siliciclastic supply, although the lower level of cyclicity changes more frequently and is considered to have been controlled by fluctuating hydrodynamic conditions. Typical mixed rock depositing patterns are summarized as the “sandwich type,” which are mainly distributed at siliciclastic-dominated sections and the “interbedded type” at carbonate-dominated sections. Besides, reservoir physical properties among lithofacies display significant differences as well, and the grain-supported framework of carbonate-dominated lithofacies suggests most constructive in keeping intergranular pores. The investigated lacustrine cyclical sedimentation in mixed siliciclastic-carbonate rocks has been less concerned in previous studies and may serve as a useful reference for lacustrine mixed rocks analysis in other rifted basins.