Prediction of source rock characteristics based on terpane biomarkers in crude oils: A multivariate statistical approach

Abstract

The distributions of eight tricyclic and eight pentacyclic terpanes were determined for 216 crude oils located worldwide with subsequent simultaneous RQ-mode factor analysis and stepwise discriminate analysis for the purpose of predicting source rock features or depositional environments. Five categories of source rocks are evident: nearshore marine ( i.e., paralic/deltaic); deeper-water marine; lacustrine; phosphaticrich source beds; and Ordovician age source rocks. The first two factors of the RQ-mode factor analysis describe 45 percent of the variation in the data set; the tricyclic terpanes appear to be twice as significant as pentacyclic terpanes in determining the variation among samples. Lacustrine oils are characterized by greater relative abundances of C 21 diterpane and gammacerane; nearshore marine sources by C 19 and C 20 diterpanes and oleanane; deeper-water marine facies by C 24 and C 25 tricyclic and C 31 plus C 32 extended hopanes; and Ordovician age oils by C 27 and C 29 pentacyclic terpanes. Although thermal maturity trends can be observed in factor space, the trends do not necessarily obscure the source rock interpretations. Also, since bacterial degradation of crude oils rarely affects tricyclic terpanes, biodegraded oils can be used in predicting source rock features. The precision to which source rock depositional environments are determined might be increased with the addition of other biomarker ( e.g., steranes) and stable isotope data using multivariate statistical techniques.