Solid Bitumen Occurrences in the Pyrenean Basin (Southern France): A Case Study across the Pliensbachian–Toarcian Boundary

Abstract

The study across the Pliensbachian–Toarcian boundary sedimentary record in the Bizanet section of the Pyrenean Basin (southern France) revealed the presence of solid bitumen. This secondary organic matter was characterized using petrographic (transmitted and reflected white lights, incident blue light, and scanning electron microscopy) and geochemical (total organic carbon, total sulfur, and insoluble residue) techniques. The spore coloration index (SCI) was also determined. With the characterization of the optical properties and reflectance of the solid bitumen, it was possible to distinguish four different families (A–D) that display a wide range of reflectance values, from 0.21% to 2.64% BRr, i.e., from glance pitch to meso-impsonite. SCI values were higher than 9–9.5 (%Req > 1.50%). The comparison between the equivalent vitrinite reflectance values of the solid bitumen and SCI showed that this index and the solid bitumen D values are concordant, indicating that solid bitumen D can be considered an indigenous bitumen. The other three families of solid bitumen (A–C) are considered as having migrated. The laterally equivalent Pont de Suert section (South Pyrenean Zone) displays no trace of solid bitumen which points to the important role of the morphotectonic context of the Bizanet section in the migration of these hydrocarbons, namely, the presence of a major thrust fault in the eastern Corbières close to the section’s location.

Insights on Using Solid Bitumen Reflectance as a Thermal Maturity Proxy in the Bakken Formation, Williston Basin, USA

“…The term “solid bitumen” is used to describe a solid or semisolid viscous petroleum phase which is observed petrographically and has well-defined optical and spectroscopic characteristics that allow it to be distinguished from other types of sedimentary organic matter. − The measured mean reflectance of solid bitumen (symbolized BR o , in %) is increasingly used as a thermal maturity proxy in source-rock reservoirs since the advent of the shale revolution in petroleum production. − This is particularly the case in North America, where source-rock reservoir production predominantly occurs from Paleozoic marine facies in which vitrinite is rare or absent . However, despite its now widespread use, many workers still attempt to link measured BR o values to an equivalent vitrinite reflectance value (symbolized VR e in %). − Vitrinite reflectance (symbolized VR o in %) is determined by measuring light reflectance from the woody remains of vascular land plants. The conversion of measured BR o values to a calculated VR e value is performed using one of multiple published empirical expressions which relate BR o to VR o , as summarized in a recent study on the Bakken Formation .…”