Assessment of Thermal Maturity Using Palynomorph Darkness Index (PDI) in the Early Paleozoic of Saudi Arabia

Abstract Palynomorph Darkness Index (PDI) is a fully quantitative optical method for assessing thermal maturity which has been established as a rapid and low-cost ‘add-on’ to palynostratigraphic investigations, especially in pre-Devonian sediments. Here, we applied the PDI method to Silurian strata in Saudi Arabia to enhance calibration of Acritarch PDI to vitrinite reflectance (VRo) via Vitrinite Reflectance Equivalents (VREs) such as pyrolysis-based Tmax determination, and graptolite and chitinozoan reflectance. Unoxidised palynological concentrates were obtained from core samples from various sections in Saudi Arabia from the early Silurian (Qalibah Formation) and a maturity range from immature through the oil, condensate and gas windows to the postmature stage. PDI measurements were taken on suitable acritarch taxa using a transmitted light optical microscope and a digital camera. The PDI values obtained were correlated with VREs derived from Tmax, graptolite reflectance and chitinozoan reflectance. Published PDI – VRo correlations established in Devonian-Carboniferous sections in the eastern USA were also considered. The present results confirm that acritarch PDI can be successfully applied for thermal maturity estimation, especially in pre-Devonian age, organic-rich sediments, where most of the available alternative methods show weak correlation toVRo. The most suitable acritarchs for PDI determination are the Veryhachium / Neoveryhachium Group. Preliminary results suggest that four distinct stages can be recognized in the VRo/VRE: PDI correlation trendline. Data gathered so far allowed a preliminary correlation trendline to be established between PDI and VRo/VRE values which can be used for determining thermal maturity ranges based on simple quantitative measurements of the degree of palynomorph darkness under optical microscopy during routine palynological analyses, though further work is needed to refine PDI vs. VRo/VRE in rocks in the upper range of thermal maturity.