Source and accumulation process of Jurassic biodegraded oil in the Eastern Junggar Basin, NW China

Abstract

Biodegradation usually obscures or even radically alters the original characteristics of oil biomarkers. The mixing of oil from multiple sources makes each source difficult to trace. Identifying the source of biodegraded oil from multiple sources has always been a hard nut to crack. Rising to this challenge, in this study-we carried out a comprehensive investigation of biodegradation impacts, oil-source correlation, and oil charging history to trace the source and reveal the mixing process of biodegraded oil in the Toutunhe Formation (J 2 t ) in the eastern Junggar Basin, NW China. The oil of this area was biodegraded to different extent, consequently, many commonly used biomarker parameters (e.g. Pr/Ph, Pr/ n C 17 ) became less powerful for oil-source correlation. To address this problem, the resistance of many biomarkers to biodegradation was analyzed, and those of high bio resistance were selected to generate a more reliable oil-source correlation. The results revealed that biodegraded oil was a mixture of oil sourced from Lucaogou Formation (P 2 l ) and Xiaoquangou Formation (T 2-3 xq ). Core sample observation, microscopic fluorescent analysis and fluid inclusion analysis were combined to analyze comprehensively oil charging history. The analysis of accumulation process exhibited that the existing oil in J 2 t was a mixture originated from the P 2 l and T 2-3 xq source rocks in two separate charging stages when it underwent a complicated process of charging, biodegradation, recharging and mixing.