The Geochemical Characteristics of the Eocene Oils From the Rolling Anticline Belt of Chaheji in Baxian Sag, Bohai Bay Basin and Their Origins Analysis

Abstract

The Chaheji rolling anticline belt is the most petroliferous structural belt in Baxian Sag, nearly two fifths of proved reserves distributing here, and its oil pools mainly come from Eocene Shahejie (Es) and Dongying (Ed) Formations. Saturated hydrocarbons from 28 oil samples and extracts of 26 source rock samples are used for gas chromatography–mass spectrometry(GC–MS) analysis to finish oil-oil and oil-source correlations. Pristane/phytane(Pr/Ph) of these oil samples shows a wide range, from 0.66 to 2.81, but 4-methylsteranes/C29-steranes(4MS) and steranes/hopanes(S/H) have limited ranges, from 0.12 to 0.17 and from 0.13 to 0.85, respectively. The oil samples display C29-20S-sterane/C29-(20S+20R)-sterane(C2920S/(20S+20R)) between 0.26 and 0.44, reflecting low mature-mature stage of crude oils from Chaheji structural belt. In addition, the oil samples have gammacerane/C30-hopane(G/S) ranging from 0.04 to 0.25, C28-29TT/ (C28-29TT+Ts) (ETR; TT: tricyclic terpanes) from 0.3 to 0.48 and C27-diasteranes/C27-steranes(C27Dia/C27) from 0.19 to 0.42, displaying narrow variations. Dramatically, the biomarkers of the oil samples display relatively similar characteristics and have no essential distinctions, therefore, oils from Chaheji structural belt should be divided into a class, having a same origin. Source rock mainly developed in Es formation in Baxian Sag, from top to bottom, including lower Es1, medium and lower Es3, Es4 members. There are clear differences in geochemical parameters among lower Es1, medium and lower Es3 source rocks, for example, Pr/Ph, C2920S/(20S+20R) and C27Dia/C27 of superincumbent source rock showing a increasing trend, but with a decreasing trend for G/H, ETR and S/H. However, the biomarker parameters of Es4 source rock display highly similar characteristics with lower Es3 source rock except for C2920S/(20S+20R) and C27Dia/C27, which showing obviously higher than other source rocks. The characteristics of crude oil from Chaheji structural belt are most similar with medium and lower Es3 source rocks, showing a big difference with lower Es1 and Es4 source rocks, by GC-MS spectrum contrast and biomarkers quantitative analysis of saturated hydrocarbons from oil and source rock samples. So it is concluded that the crude oil from Chaheji structural belt is the mixed oil derived from medium and lower Es3 source rocks, with slightly or no contributions of lower Es1 and Es4 source rocks, based on the oil and source rock biomarkers analysis.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017