Study on Oil-source Correlation Methods of Severely Biodegraded Oils in Bozhong Subbasin, Bohai Bay Basin, China

Jun, Wang; Zhou, Xinhuai; Yonghua, Guo; Bo, Yang; Feilong, Wang; You, Zhang

There are three organic-rich, oil-prone source rock intervals in Bozhong Subbasin, Bohai Bay Basin: the third and the first members of Shahejie formation, and the Dongying Formation. Most oils with burial depths less than 2000m (6562ft) are severely biodegraded as evidenced by the occurrence of 25-norhopanes. To recognize the source of the biodegraded oils, biomarker characteristics of the three source rock intervals are concluded and several oil-source correlation methods are applied.

The upper Eocene third member of Shahejie Formation (E2s3) is dominated by semi-deep and deep lacustrine facies mudstone with highest TOC contents and hydrocarbon indices, and it has low gammacerane, C19 tricyclic terpane and C24 tetracyclic terpane contents, and high 4,23,24-trimethyl triaromatic dinosterane and 4-methyl-24-ethyl triaromatic sterane contents, indicating light saline water and blooming dinoflagellates paleoenvironment. The lower Oligocene first member of Shahejie Formation (E3s1) is saline lacustrine facies mudstone, lime mudstone and limestone with relatively high TOC contents and hydrocarbon indices, and it has high gammacerane and 4,23,24-trimethyl triaromatic dinosterane contents, low C19 tricyclic terpane, C24 tetracyclic terpane and 4-methyl-24-ethyl triaromatic sterane contents. The upper Oligocene Dongying Formation (E3d) is semi-deep and deep lacustrine facies mudstone in its lower part, with relatively high contents of vitrinite and nonfluorescent amorphous organic matter and classified as mixed type I and III kerogens, and it has low gammacerane and 4-methyl triaromatic sterane homologues contents, high C19 tricyclic terpane and C24 tetracyclic terpane contents, indicating fresh water, freshwater algae dominated and partially terrigenous organisms input paleoenvironment.

Carbon isotope of whole oil, 4-methyl triaromatic sterane homologues, gammacerane, C19 tricyclic terpane, C24 tetracyclic terpane, carbazole, and crude oil density and viscosity are used to determine the source of severely biodegraded oils. The biodegradation has little effect to carbon isotope of whole oil. The whole oil carbon isotopes from E2s3 and E3d source rock intervals are heavier than -27‰ and lighter than -28‰ respectively in east Bozhong Subbasin, and that from E3s1 source rock interval are intervenient. 4-methyl triaromatic sterane homologues discussed above can distinguish oils from the three source rock intervals, and they are seldom influenced by biodegradation.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013