Hydrous pyrolysis maturation of vitrinite-like and humic vitrinite macerals: Implications for thermal maturity analysis

• We submitted samples to various temperatures and time lengths in hydrous pyrolysis assays.
• Vitrinite-like reflectance presented lower values in comparison with humic vitrinite.
• We present a set of equations to correct Vitrinite-like reflectance to an equivalent humic vitrinite reflectance.
• When studying Lower Paleozoic source rocks these corrections may be helpful to allow delimitation of thermal maturity.

The evaluation of thermal maturity of Lower Paleozoic sedimentary rocks is somewhat complex, mainly because of the absence of humic vitrinite maceral material. Various sedimentary basins of this age contain a syndepositional vitrinite-like maceral that resembles vitrinite but has a lower reflectance. Samples of shales from outcrops and boreholes from the Ponta Grossa Formation (Middle Devonian or older) in the Paraná Basin were studied for maturation assessment. Hydrous pyrolysis assays were conducted for 27 combinations of time and temperature to compare the reflectances of the vitrinite-like maceral and humic vitrinite in response to increasing temperature and time. This data confirmed that the maturation pathway of the vitrinite-like maceral is lower than that of humic vitrinite. The Spore Coloration Index for the outcrop samples was also evaluated for natural series studies. The combination of the natural and artificial series led to the development of two-stage linear regression equations that estimate the equivalent vitrinite reflectance values from measured vitrinite-like maceral reflectances. To validate the proposed equations, reported measurements of vitrinite-like maceral reflectances from sedimentary basins in China, North America and Europe were selected from the literature for comparison. The validations showed that the values obtained from the proposed equations correlated well with other maturity parameters.