Sequence stratigraphic divisions and correlation of the middle sub-member of Eocene Shahejie Formation in the Bonan Sag of Bohai Bay Basin (China): Implication for facies and reservoir heterogeneities

Abstract

The middle sub-member of the Eocene Shahejie Formation is mainly associated with deltaic environment, and considered as hydrocarbon bearing zone in the southeast of the Bonan Sag, Bohai Bay Basin, China. Previous research and the exploration work in an area were carried out separately, and inconsistency in understanding of stratigraphic division, correlation, and facies was encountered. Furthermore, the facies evolution in the targeted formation is still questionable. To fill these short-comings, sequence stratigraphic correlation and interpretation were conducted on the middle sub-member (Es3z) of the third member (Es3) of Eocene Shahejie Formation in the study area. Integrative facies and paleo-environment analyses are carried out through the correlation study of wireline logs, seismic data and core sedimentology. The sedimentological interpretation reveals that sandstone groups in the Es3z show five different facies associations in both lateral and vertical successions developed in the inner and outer delta front. These depositional facies include deposits in distributary channels, interdistributary channels, mouth bars, natural levees, and sheet sandstones. The Es3z sandstone intervals are characterized by five depositional sequences in terms of the third-order sequence, including Es3z2+3, Es3z4, Es3z5, Es3z6+7, and Es3z8. They are found in all wells consisting of the lowstand, transgressive, and highstand system tracts. These system tracts are separated by the maximum flooding surfaces and correlation boundaries of sandstone groups with unique characteristics of well log motifs, calibrated with the cores by particular lithology. Facies and reservoir heterogeneities are defined within a sequence stratigraphic framework approach for hydrocarbon reservoir prediction. Our results show that sediments were sourced from the southeast toward the northwest. Due to tectonic activity and lake level's fluctuations, reservoir facies are prograding towards the basin side. The steepness of the region impacts the facies and reservoirs evolution together with tectonics and fluctuations in lake level, which turns from the sandy conglomeratic reservoirs into a thick succession of sandstone reservoirs towards the basin side in LST HST and TST system tracts. These reservoir facies are considered as good, and capped by grayish mudstone (inner delta front) to dark gray mudstone (outer delta front) in TST system tracts. The facies architecture of all sandstone groups possesses the hydrocarbons potential in the inner and outer delta front region in which interpreted oil, oil-water layer and water-oil layers show variation in progradation and retrogradation stacking pattern. Among them lower sandstone units of Es3z8 and Es3z6+7 show dominancy in oil rich reservoirs, followed by Es3z2+3, Es3z4 and Es3z5 in LST, HST and TST system tracts. These hydrocarbon units can be correlated laterally with exploration wells for getting new well sites in third order sequence stratigraphic framework. Our findings shed new light on the facies and reservoir heterogeneities, which provide a useful aid for hydrocarbon exploration in the study area.