Acritarch Palynomorph Darkness Index (PDI) as an indicator of thermal maturity in Silurian sections from Saudi Arabia

Palynomorph Darkness Index (PDI) measurements on acritarchs from Silurian strata in Saudi Arabia exhibit an irregular but potentially useful relationship to other vitrinite reflectance proxies. Four distinct stages in PDI evolution can be recognized with thermal maturity. In Stage 1, acritarch PDI is <10% and shows no increase with increasing maturity. A very rapid increase in PDI towards the end of the oil window from <10% to ca. 25% characterises Stage 2, which is followed by a gradual increase in Stage 3 from ca. 25% to approximately 80%. PDI Stage 4 is represented by postmature rocks, with acritarch PDI >82% and showing little change with increasing maturity. Acritarch fluorescence is totally extinguished within the oil window. • Acritarch Palynomorph Darkness Index (PDI) from early Silurian “hot shales” in Saudi Arabiawas determined. • Calibration of acritarch PDI to Tmax- and “vitrinite reflectance equivalents” reveals a complex but useful relationship. • Acritarch PDI is a simple optical method for estimating the thermal maturity of pre-Late Devonian rocks. • Acritarch fluorescence appears less intense and is extinguished at a lower level of thermal maturity than in other studies.