Charging of Severely Biodegraded Oils Surrounding the Northern End of Liaodong Uplift from Multiple Lacustrine Source Rock Intervals, Bohai Bay Basin, China

Abstract

The northern end of Liaodong uplift has become a hot spot for exploration since the discovery of JZ23 oilfield, which is the first commercial oilfield on this uplift. The crude oils with shallow burial depth ranging from 773m (2536ft) to 992m (3255ft) are severely biodegraded as evidenced by the occurrence of 25-norhopanes. The severely biodegraded oils are characterized by high density ranging from 0.964 to 0.973 g/cm³ (20°C) and high viscosity ranging from 313.1 to 418.9 mPa s (50°C).

Biomarker associations of 37 crude oil samples and 125 source rock samples were analyzed to investigate the origin of the severely biodegraded oils on this uplift. Based on hierarchical cluster analysis of selected biomarker parameters, the oils can be identified into three groups. Group 1 oils display relatively high TA-dinosteroids/(TA-dinosteroids + C₂₈ TA-steroid 20S), medium C₂₈/(C₂₆+C₂₇) TAS and low (4-methyl-24-ethyl triaromatic sterane + 4-methyl triaromatic sterane)/(4,23,24-trimethyl triaromatic dinosterane + 3-methyl-24-ethyl triaromatic sterane), indicating that group 1 oils are interpreted to be a mixture of the first and the third member of Shahejie Formation (Es¹, Es³). Group 2 oils display high C₂₇-20R/C₂₈-20R TAS, low C₂₈/(C₂₆+C₂₇) TAS and medium-high (4-methyl-24-ethyl triaromatic sterane + 4-methyl triaromatic sterane)/(4,23,24-trimethyl triaromatic dinosterane + 3-methyl-24-ethyl triaromatic sterane), indicating that group 2 oils mainly come from the third member of Dongying Formation (Ed³). Group 3 oils display medium-high C₂₇-20R/C₂₈-20R TAS, low-medium C₂₈/(C₂₆+C₂₇) TAS and medium (4-methyl-24-ethyl triaromatic sterane + 4-methyl triaromatic sterane)/(4,23,24-trimethyl triaromatic dinosterane + 3-methyl-24-ethyl triaromatic sterane), indicating that group 3 oils mainly come from the Es³ and Ed³ source rocks. Moreover, these three group oils can be clearly separated by δ¹³C values of whole oils.

The triaromatic steroids can be used to distinguish oil source of the severely biodegraded oils from lacustrine source rocks, as they are seldom influenced by biodegradation. The overall oil migration orientation on the northern end of Liaodong uplift is generally from west to east. Therefore, the source kitchen for the northern end of Liaodong uplift is predicted to be the Liaozhong depression. The eastern steep slope of the Liaozhong depression, along the oil charging pathways, is identified as the most promising region for oil exploration.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019