A GC×GC-ToFMS Investigation of the Unresolved Complex Mixture and Associated Biomarkers in Biodegraded Petroleum

Heavy biodegraded crude oils have larger numbers of coeluting compounds than nonbiodegraded oils, and they are typically not resolved with conventional gas chromatography（GC）. This unresolved complex mixture（UCM） has been investigated using comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry（GC×GC-To FMS） within a set of biodegraded petroleums derived from distinct sedimentary basins, including northwestern Sichuan（Neoproterozoic, marine）, Tarim（Early Paleozoic, marine）, Bohai Bay（Eocene, saline/brackish） and Pearl River Mouth（Eocene, freshwater）. In general, the hydrocarbons that constitute the UCM in petroleum saturate fractions can be classified into three catalogues based on the distributions of resolved compounds on two dimensional chromatograms. Group 1 is composed mainly of normal and branched alkanes, isoprenoid alkanes and monocyclic alkanes; Group 2 comprises primarily terpanes ranging from two to five rings, and Group 3 is dominated by monoaromatic hydrocarbons such as tetralins and monoaromatic steranes. In addition, the UCM is source dependent and varies between oil populations. i.e., the UCM of petroleum derived from Precambrian and Early Paleozoic marine, Eocene saline/brackish and freshwater source rocks is specifically rich in higher homologues of A-norsteranes, series of 1,1,3-trimethyl-2-alkylcyclohexanes（carotenoid-derived alkanes）, and tetralin and indane compounds, respectively.

A new analytical method for quantifying of sterane and hopane biomarkers

An analytical method for quantifying biomarker compounds of the sterane and the hopane existing in saturated hydrocarbons has been established by using comprehensive two-dimensional gas chromatography-flame ionization detector(GC×GC-FID) with optimized operating parameters. The new method achieves the quantification by using a GC×GC-FID system which is able to completely separate steranes from hopanes. The data obtained by the new method are of good repeatability and reliability. Compared with the original data, the relative standard deviations(RSDs) of 12 reference compounds are less than 5%. The RSDs of the quantitative results of the biomarkers based on seven separate analyses are also less than 5%. Compared with the traditional method of gas chromatography-mass spectrometry(GC-MS), the new method has a number of advantages, such as common internal standards(ISs), high resolution, no co-eluting peak, and no interference caused by diagnostic ion peaks. The new method provides petroleum geologists with an effective and scientific means in future researches.