Geochemical and palynological characteristics of the Paleogene source rocks in the Northeastern Laizhouwan Sag, Bohai Bay Basin, China: Hydrocarbon potential, depositional environment, and factors controlling organic matter enrichment

The Northeastern Laizhouwan Sag (NLS) is a marginal sag in the Bohai Bay Basin, China. Paleogene source rocks from 3 wells (K-a, K-b and K-c) in this region, i.e., the third to the first member of the Shahejie Formation (Es-3, Es-2, and Es-1), and the third member of the Dongying Formation (Ed(3)) were investigated for their petroleum potentials and depositional environments using geochemical and palynological methods. Among these members, the Es-3 and Es-1 members hold considerable hydrocarbon potential, while the rest are relatively fair. Although there are variations between the wells, the organic matter (OM) in the Es-3 member is primarily composed of low mature or mature type I-II1 kerogen, which were primarily contributed by lower aquatic organisms and prokaryotes. During the Es-3 stage, saline, stratified, and anoxic bottom water prevailed in the K-a well, while freshwater and weak reducing conditions dominated the K-b well. The OM in the Es-1 member was contributed more by lower aquatic organisms than that in the Es-2 member, but both are primarily composed of types II1-II2 kerogen; further, OM in the Es-2 member is low mature, while that for the Es-1 member ranges from immature to low mature, and higher maturity is expected in deeper buried places for the two members. Saline and anoxic bottom water prevailed during the both Es-2 and Es-1 members. Compared with other members, noticeable terrigenous OM was deposited during the Ed(3) stage, which is dominated by immature type II2 kerogen. Lower salinity and weak reducing depositional conditions occurred in the Ed(3) member. OM enrichment in the Es-3 member was mainly determined by primary productivity, while that of the K-a well was also enhanced by the anoxic bottom water condition. The accumulations of OM in other members are speculated to have been controlled by the deposition rate.