Geochemistry of Evaporites in Lacustrine Basin, Dongying Depression, Bohai Bay Basin, China

Abstract

The middle and lower parts of the Fourth Member of the Shahejie Formation in the Dongying Depression are representatives of evaporites and mud rocks in Eastern China. To investigate hydrocarbon potential, some deep wells, such as the Hk1 well and Fs2 well, were intensively cored and evaluated by geochemistry. The total salinity of the formation waters is very high and is dominated by CaCl2, suggesting favorable seal development and preservation of petroleum. The content of Al, Mn, and Ba and the ratios of V/Ni and (Fe+Al)/(Ca+Mg) were low, whereas the content of Na, Ca, and Mg and the ratios of Sr/Ba, Fe/Mn, and Sr/Ca were high in the evaporites and mudstones of the Fourth Member compared with those in the mudstones of the Third Member. Sr content was remarkably high. The co-sedimentary system of evaporites and mudstones displays a heavier isotope of carbon. The values of δ13 C for different group compositions and n-alkanes ranged from -28‰ to -21‰ and -18‰ to -32‰, the fractionation of δ13 C between group compositions was no greater than -2‰, and δ13 C of C17 and C18 significantly fluctuated. δ13 C for high-maturity evaporite source rocks was significantly increased by thermal evolution and thermochemical sulfate reduction. High content of gammacerane and low Pr/Ph was exhibited in the evaporite system compared with that in the non-evaporite system. Different responses of biomarker parameters for the different sedimentary systems were exhibited, such as C19 /(C19 +C23 ) terpanes, C29 /(C27 +C28 +C29 ) steranes, and C24 /C23 and C22 /C21 tricyclic terpanes. The reverse trend of the maturation parameters at depths corresponding to abundant carbonate minerals indicates that carbonate retards the thermal evolution of organic matter. Evaporites and mudstones can generate and expel hydrocarbons. The tested samples were mostly types I and III of organic matter, and their original generating capacity can reach 40 and 20 mg/g rock, respectively. The efficiency of hydrocarbon expulsion reached 60%, but the distribution of organic matter and its generative potential were highly variable. In general, the mudstones show greater generative potential than the evaporites.