GEOCHEMICAL ANALYSES OF EOCENE OILS IN DEEPLY BURIED SANDSTONE RESERVOIRS IN THE DONGYING DEPRESSION, BOHAI BAY BASIN, NE CHINA

Abstract

We report the results of organic geochemical analyses of 19 crude oil samples from reservoir sandstones in the 4th Member of the Eocene Shahejie Formation from wells in the Minfeng Sag, Dongying Depression, Bohai Bay Basin (NE China). In addition, 42 Shahejie Formation core samples of dark-coloured mudstones, including 28 extracts, were analysed. Geochemical data included Rock-Eval measurements, gas chromatography, GC-MS and diamondoid analyses.

Maceral analyses showed that mudstones in the 4th Member of the Shahejie Formation (“Es₄”) contain Types I and II₁ kerogen. The member can be divided into upper (Es₄s) and lower (Es₄x) intervals. Oil-prone Es₄s rock samples have good to excellent hydrocarbon-generating potential based on calculated initial TOC values; Rock-Eval T_max values indicate that they are sufficiently mature for hydrocarbon generation. Analytical results suggest that both Es₄s and Es₄x mudrocks are potential source rocks for oils produced at fields in the Minfeng Sag.

Analysed crude oils from the Minfeng Sag were classified into three genetic groups. Group I oils are mature to highly mature and have undergone a moderate degree of thermal cracking. They are characterized by a low β-carotane/nC₂₅ ratio and C₃₀ 4-methylsterane index (4MI); high values of oleanane index (oleanane /C₃₀-hopane), C₂₇ diasterane/C₂₇ regular sterane (C₂₇Dia/C₂₇), regular sterane/17α hopane and gammacerane/C₃₀ hopane (G/H); and medium pristane/phytane ratios (Pr/Ph). This suggests that Group I oils are mostly derived from source rocks in the upper part of the Es₄x unit which are interbedded with evaporites. Group II oils are mature and have high 4MI and Pr/Ph ratios, low oleanane index, regular sterane/17α hopane and C₂₇Dia/C₂₇ ratios, and mediumβ-carotane/nC₂₅ and G/H. These features are similar to those of Es₄s source rocks, indicating their genetic correlation. Group III oils show the lowest maturity and highβ-carotane/nC₂₅ and regular sterane/17α hopane, and low oleanane index, Pr/Ph and 4MI. Previously-published data indicates that oils similar to those in Group III were mainly sourced by Es₄s mudstones.