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

Abstract 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 (Es3, Es2, and Es1), and the third member of the Dongying Formation (Ed3) were investigated for their petroleum potentials and depositional environments using geochemical and palynological methods. Among these members, the Es3 and Es1 members hold considerable hydrocarbon potential, while the rest are relatively fair. Although there are variations between the wells, the organic matter (OM) in the Es3 member is primarily composed of low mature or mature type I and type II1 kerogen, which were primarily contributed by lower aquatic organisms and prokaryotes. During the Es3 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 Es1 member was contributed more by lower aquatic organisms than that in the Es2 member, but both are primarily composed of types II1 and II2 kerogen; further, OM in the Es2 member is low mature, while that for the Es1 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 Es2 and Es1 members. Compared with other members, noticeable terrigenous OM was deposited during the Ed3 stage, which is dominated by immature type II2 kerogen. Lower salinity and weak reducing depositional conditions occurred in the Ed3 member. OM enrichment in the Es3 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.