The organic petrology of vitrinite-like maceral in the Lower Paleozoic shales: Implications for the thermal maturity evaluation

Abstract

Vitrinite-like maceral (VLM) is a type of autochthonous, synsedimentary, dispersed organic matter, which is ubiquitous in many Lower Paleozoic marine shales. The VLM reflectance has been used as an alternative tool to evaluate the thermal maturity of pre-Devonian marine sediments lacking a classical vitrinite, however, the existing knowledge about its optical properties and reflectance characteristics is still relatively limited. In this study, the optical properties of VLM were examined by means of organic-petrological methods applied to a series of naturally and artificially matured VLM-rich shales samples and their reflectance crossplots. The strong positive correlation between the true maximum reflectance (Rmax) and bireflectance of VLM indicates the increasing anisotropy of VLM with increasing thermal maturity. VLM shows a linear coalification path in the studied shales. It displays biaxial neutral features in the samples with Rmax ≤ 2%, but shows obvious optically biaxial negative characteristics in the samples with Rmax > 2%. The shape of VLM reflectance indicating surface (VLMRIS) gradually becomes oblate with increasing maturity. These results indicate that VLM exhibits appearance and optical properties similar to vitrinite, and its reflectance can be used as a reliable indicator for thermal maturity evaluation. The equivalent vitrinite reflectance (EqVRo) values, which were calculated based on the mean measured maximum reflectance of VLM (VLMR¯max), evidence that the Niutitang Formation shales from China and Alum shales from Denmark are post-over mature, and the Yurtus Formation shales in Tarim Basin and the Alum shales from Sweden are still within the oil window.