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.

Highlights

  • In source-rock reservoirs, it is possible to find two types of organic matter (OM): primary and secondary

  • Insoluble residue (IR) values vary between 12 and 82 wt.%, with the highest average value observed in the Schistes carton Mb. (51 wt.%)

  • Toarcian carbonate deposits from the Bizanet section (Pyrenean Basin) using optical microscopy techniques. These bitumen families differ in terms of their optical properties and present a large range of reflectance

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Summary

Introduction

In source-rock reservoirs, it is possible to find two types of organic matter (OM): primary and secondary. Secondary OM is divided by organic geochemists into extractable and non-extractable materials, the so-called bitumen and pyrobitumen, respectively. International nomenclature classifies oil, solid bitumen, and pyrobitumen as secondary OM. These components are usually dominant in sedimentary sequences with maturities in the late oil and dry gas windows. This heavily influences the reservoir quality as certain types of solid bitumen behave as a cement, reducing reservoir porosity and permeability e.g., [6,7,8,9,10,11,12]. Solid bitumen reflectance can be used to assess maturity in the absence of vitrinite and provide information on the migration pathway e.g., [13,14,15]

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