The Application of Tricyclic and Tetracyclic Biomarker Profiles to Infer the Organofacies of Source Rocks and Oils Using a Global Geochemistry Database
Tricyclics and tetracyclic terpanes have previously been used for source facies evaluation and their use has been documented in the literature (see Zumberge, 1987 or Samuel et al., 2010). The tricyclic signature provides an excellent screening tool for source facies assessment from a limited molecular weight range that is highly resistant to alteration, thermally stable to elevated maturities and not susceptible to overprint by recent organic matter in seeps. Further evaluation using isotopic composition and full biomarker evaluation often provides only minor uplift, especially in larger regional or screening studies. Profiles of the C19 – C26 tricyclic compounds, including the C24 tetracyclic compounds, were created to classify the organofacies of source rocks and oils into 4 categories following the nomenclature of Pepper and Corvi (1995). Organofacies A constitutes a marine, sulfur‐rich (>1%), clay‐poor, carbonate to siliceous source rock that has relatively low C19 – C20, high C23 tricyclics and in some cases high C24 tetracyclic compounds. Organofacies B is a marine, low‐sulfur (<1%), clay‐rich, source rock that has relatively low C19 – C20, high C23 – C25 triyclics and low C24 tetracyclic relative to C26 tricyclic. Organofacies C represents non‐marine, lacustrine (freshwater to saline) source rocks that are differentiated from B by having higher C21 relative to C23 tricyclic and C26 tricyclic higher than C25 tricyclic. Organofacies D/E represents non‐marine, fluvial to paralic, clay‐rich, low sulfur source rocks characterized by high C19 – C20 tricyclics with high C24 tetracyclic compounds. This study demonstrates the utility of the method by applying it to a global geochemical database containing more than 14,500 gas chromatography/mass spectrometry (GC/MS) analyses of source rocks and oils ranging in age from Precambrian to Quaternary. A pattern recognition calculator was developed to allow for easy assignment of tricyclic‐tetracyclic profiles to organofacies. The organofacies assignments were supported using multivariate statistical approaches of hierarchical cluster analysis (HCA) and principal component analysis (PCA) on a calibration dataset. Clusters differentiated by multivariate analysis are in good agreement with the visual analysis of tricyclic‐tetracyclic profiles and support the organofacies model. Tricyclic‐tetracyclic profiles that are not easily classified represent oils with a more complicated history of hydrocarbon generation from multiple source rocks and possible oil mixing. The analysis resulted in delineation of areas of major effective source rocks and their respective facies. Furthermore, through geologic time, the organofacies of source rocks deposited in different basins correlates with known global sea level cycles and oceanic anoxic events. Examples from basin‐specific case studies illustrate the application of the methodology.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019