Maturation and migration processes in intact source rock micro plugs induced by chemical and thermal treatment: A new approach combining Rock-Eval pyrolysis and organic petrography

Abstract

This study proposes a new approach to investigate hydrocarbon generation and migration processes in petroleum source rocks under laboratory conditions. For the first time, programmed open-system pyrolysis using a Rock-Eval 6 instrument is applied on small, polished source rock plugs to induce changes in the type and distribution of solid bitumen, which are qualitatively investigated by means of organic petrography before and after different pyrolysis experiments. This approach allows us to directly visualize the evidence of solid bitumen (SB) formation and distribution after the following treatments: (1) thermal extraction at 300 °C for 3 min, (2) chemical extraction of the surface using dichloromethane (DCM), and subsequently (3) artificial maturation by applying the standard procedure of Rock-Eval programmed pyrolysis up to a temperature of about 400 to 500 °C (depending on the initial thermal maturity of the sample). After each step, the surface of the treated micro plugs was digitally scanned to allow a comparison of specific areas of interest before and after the respective treatments and alterations. For this study, core samples of the Eagle Ford Shale Formation are used, which are suitable for such investigation, as the Eagle Ford Shale exhibits a variety of different initial thermal maturities from immature to overmature. Furthermore, the presence of large intraparticle porosities provides suitable voids where changes in SB distribution can easily be observed. After the thermal extraction, the volume of observed SB in the early- and moderately mature samples slightly increased, while most of the thermally mobilized bitumen from inside the plug was evaporated and thus lost from the sample. The initial solid bitumen was not or hardly affected by thermal extraction at 300 °C for 3 min, i.e. it was thermally stable at this temperature. Solid bitumen reflectance was slightly higher than in the initial sample showing a minor maturation. In contrast to thermal extraction, solid bitumen in the early mature samples was almost completely dissolved after DCM treatment. These petrographic observations on micro plugs are in agreement with the pyrolysis results of powdered fractions, where the initial Rock-Eval S1 and to a much lesser extent also the S2 signal decreases after the chemical extraction in DCM. While the SB in early mature sample was almost entirely dissolved by DCM, only about half of it is was dissolved in the moderately mature sample showing more mature and more stable SB. After the artificial maturation, the volume of SB in early- as well as in moderately mature samples clearly increased within the intraparticle pores, indicating presence of new SB due to bitumen migration from the deeper part of the micro plug to the surface. In contrast, SB in initially overmature sample is mainly present as thermally-highly stable pyrobitumen, indicated by no change in the SB distribution after chemical and thermal treatments representing a stable type of SB at the very end of petroleum generation.