Molecular composition assessment of biodegradation influence at extreme levels – A case study from oilsand bitumen in the Junggar Basin, NW China

Abstract

In order to understand the effects of extreme biodegradation on petroleum molecular compositions, a series of shallow buried oilsands from the northwestern margin of the Junggar Basin, NW China has been investigated using geochemical methods. Most samples have experienced at least level 8 biodegradation on the scale of Peters and Moldowan, but their molecular compositions still show dramatic variations. The bi- and tricyclic aromatic compounds are almost completely removed with aromatic steroid hydrocarbons being the only visible molecules in the aromatic hydrocarbon fraction. However, aromatic steroids are not sensitive to the assessment of biodegradation extent possibly due to mixed effects with water washing. At an extreme level of biodegradation after diasteranes were attacked, pregnanes, tri- and tetracyclic terpanes, non-hopane pentacyclic terpanes such as 18α-30-norneohopane, C30 diahopane and gammacerane remain prominent. The post extreme level of biodegradation can be further differentiated on the basis of the presence and absence of these ‘refractory’ components together with 25-norhopanes (NHs), 17-nortricyclic terpanes (NTTs) and C23 demethylated tetracyclic terpane (C23NTeT). These NHs, NTTs and C23NTeT are produced from corresponding hopanes, tri- and tetracyclic terpanes during biodegradation but they are biodegradable as well. Extreme plus 1 level of biodegradation is characterized by the attack of pregnanes and tricyclic terpanes. Extreme plus 2 level of biodegradation is characterized by the attack of tetracyclic terpanes and non-hopane pentacyclic terpanes. When biodegradation level reaches to extreme plus 3, NHs and NTTs are largely depleted. At extreme plus 4 level, the only traceable components are C23NTeT and C29NH. While different basins or reservoirs may have their unique features of the biodegradation process, relative rates of biodegradation of the different hydrocarbon classes that occur in the Junggar Basin may provide a protocol for intensity assessment after the extreme level of biodegradation is reached.