Properties of solid bitumen and other organic matter from Oligocene shales of the Fore-Magura Unit in Polish Outer Carpathians: Microscopic and geochemical approach

Abstract

In this study, we apply microscopic and geochemical (Rock-Eval) methods to evaluation of both the origin and maturity of solid bitumen and other organic matter dispersed in Oligocene shales from the Skrzydlna Tectonic Bay and the Mszana Dolna Tectonic Window of the Fore Magura Unit, Outer Carpathians, Poland. Homogenous solid bitumen occurs in early mature and mature samples of rocks from the Skrzydlna Tectonic Bay (vitrinite reflectance VR from 0.60 to 0.80%). It reaches reflectance values (BR) from 0.31 to 0.61% and is interpreted as the intermediate product between kerogen and oil originated during the oil generation process. In the late mature and post-mature samples from the Mszana Dolna Tectonic Window (VR from 1.10 to 1.49%), homogenous solid bitumen reflectance varies from 1.45 to 1.76%. This is an indigenous, thermally altered solid bitumen or residue after hydrocarbon generation, hence, it should be called the pyrobitumen. In samples from both locations, granular solid bitumen was found, as well, of reflectance values (GBR) from 0.25 to 0.92%.A very strong correlation of homogenous solid bitumen, pyrobitumen and granular solid bitumen reflectances with vitrinite reflectance was found in studied samples. When measured in the same sample, the solid bitumen from the Skrzydlna Tectonic Bay always showed lower reflectance values than the coexisting vitrinite. In samples from the Mszana Dolna Tectonic Window, in which measured vitrinite reflectance VR was always above 1,0%, the, observed pyrobitumen invariably showed the higher reflectance values whereas the granular solid bitumen consistently revealed lower reflectance than vitrinite, homogenous solid bitumen or pyrobitumen coexisting in the same sample. The Rock-Eval analyses confirmed that bitumen is indigenous in all studied samples. The degree of thermal maturation of organic matter based on vitrinite reflectance measurements was equivalent to organic matter maturity determined by the Rock-Eval Tmax values, which could be confirmation that our microscopic observations and selection of vitrinite fragments were correct.We were able to perform more individual measurements of homogenous solid bitumen particles and we obtained lower average standard deviation for BR values than for VR values. Due to the dark vitrinite abundant in samples taken in the Skrzydlna Tectonic Bay and the small-size vitrinite particles seen in samples from the Mszana Dolna Tectonic Window we found it difficult to select proper vitrinite particles for microscopic examinations. By contrast, both the solid bitumen and the pyrobitumen particles were often easier to recognize due to their characteristic shape and larger size than those of vitrinite fragments. For these set of samples we conclude that reflectance measurements run on homogenous solid bitumen are more reliable in comparison with those performed on vitrinite.