--> Geochemical Characterization of the Eocene Organic-Rich Shales in Dongying Sag, Bohai Bay Basin, China

AAPG ACE 2018

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Geochemical Characterization of the Eocene Organic-Rich Shales in Dongying Sag, Bohai Bay Basin, China

Abstract

Dongying Sag, in which has huge petroleum reserves distributed, is an important hydrocarbon generation kitchen in Bohai Bay Basin. Ongoing exploration activities for shale oil call for further investigation on organic-rich shale intervals. In this study, bulk and molecular geochemical analyses of 29 shale core samples from lower part of the third member (Es3x) and the upper part of the fourth member (Es4s) of the Eocene Shahejie Formation were carried out to determine the characteristics and depositional environment of source rocks.

The TOC of Es3x and Es4s samples ranges between 1.98 to7.22% (average being 3.99%) and 0.51 to7.66% (average being 3.23%), respectively. Along with that, high values of bitumen contents and S1+S2 also suggest the vast majority of shales are good-excellent source rocks. Rock-Eval pyrolysis analysis indicates that kerogen compositions of samples are chiefly Type II and few of them are Type I. Tmax value classifies only 57% of samples as mature source rocks. Biomarker parameters assemblage (DBT/P vs. Pr/Ph ratios diagram) implies that the depositional environment was dominantly the sulfur-lean lacustrine. The Pr/n-C17 vs. Ph/n-C18 ratio diagram indicates that shales mainly contain Type II kerogen with the algal organic matter input deposited under a reducing condition. A higher proportion of C29 (41-54%) comparing to C27(26-36%) and C28 (18-28%) steranes reflects a relatively low contribution of aquatic algal-bacterial organic matter and a major terrigenous organic matter input, as indicated by regular steranes ternary diagram. Sterane isomerization parameters (C29αββ/(αββ+ααα) vs. C29ααα20S/(20S+20R) ratios diagram) reveal that samples are mature. Trace elements V/Ni ratio (1.47-2.69) shows that organic matter was deposited under suboxic to relatively anoxic condition. And Sr/Ba ratio vs. Gammacerane index diagram reflects an enhanced salinity water environment, especially for some samples of Es4s, which reflect hypersaline conditions and stable water column stratification.

The oil saturation index (S1×100/TOC) ranges between 63.43 to 295.10 mg/gTOC reveals that the majority of shales meet the commercial production requirement. The Es3x and Es4s are considered to be favorable intervals for shale oil from evaluations above, whereas the production effect is poor. With the low overall maturity, hydrocarbon generated with minor or no cracking is primary heavy oil, which may be the main cause.