Characterization of unusual tetracyclic compounds and possible novel maturity parameters for Brazilian crude oils using comprehensive two-dimensional gas chromatography-time of flight mass spectrometry

Abstract

Eleven Brazilian oil samples from different sources, biodegradation and maturity levels were analyzed by comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GC×GC-TOFMS). Geochemical characterization associated with principal component analysis (PCA) allowed the identification of the three most mature oils. The GC×GC chromatographic conditions used in this study allowed us to perform group-type (or chemical class) separation in the following second dimension (2D) order: (i) tricyclic terpanes; followed by (ii) steranes with 3 rings containing 6 carbon atoms and 1 ring containing 5 carbon atoms; (iii) tetracyclic terpanes with 4 rings containing 6 carbon atoms; (iv) pentacyclic terpanes with 4 rings containing 6 carbon atoms and 1 ring containing 5 carbon atoms; and finally (v) pentacyclic terpanes with 5 rings containing 6 carbon atoms. This information was used to tentatively identify the ring structure of eight unusual compounds found in the saturated fraction of the oils. These compounds had the double bond equivalent (DBE) value four, which was calculated based on molecular ions M+ at m/z 274, 288 and 316, indicating possible tetracyclic structures. Two of them presented a diagnostic peak at m/z 191 and six at m/z 203. A trend between the ratios of two of these unusual compounds and the C24 tetracyclic terpane versus 4,6/1,4 dimethyldibenzothiophene ratio shows promise as a maturation parameter. Thus, these unusual compounds are prone to respond to the maturation on a scale from mature to overmature, when compared with traditional maturation parameters.