Palynofacies, paleoenvironment and thermal maturity of early Silurian shales in Saudi Arabia (Qusaiba Member of Qalibah Formation)

The early Silurian Qusaiba Shales are proven source rocks of the many Paleozoic petroleum systems of Saudi Arabia. In this study, a new, simple and cost-effective method for semi-quantitative analysis and graphical representation of palynological assemblage composition (PZC Phytoplankton–Zooplankton–Cryptospore ternary diagram) and a new visual thermal alteration index (IPA–TAI Integrated Palynomorph–Amorphous organic matter Thermal Alteration Index) integrating amorphous organic matter and palynomorph color, were developed based on the palynological/palynofacies analysis of some selected wells penetrating the early Silurian Qusaiba Shales throughout Saudi Arabia. The methods were tested against independent paleoenvironmental and thermal maturity indicators mainly based on geochemical parameters, confirming their validity as effective alternative methods for optical kerogen analysis and interpretation. The thermal maturity values estimated for core samples in the Qusaiba Shales from 13 wells in Saudi Arabia showed a strong correlation with Vitrinite Reflectance Equivalent (VRE%) maturity values determined from graptolite reflectance and pyrolysis Tmax measurements in the entire range of maturities (immature to overmature stages), proving that the present visual thermal alteration index can be successfully employed as a time efficient and reliable method for estimating thermal maturity in the Qusaiba Shales of Saudi Arabia. The present results show that the Qusaiba Shales were deposited in mostly mid- to outer shelf, and occasionally nearshore marine environments under prevailing dysoxic and anoxic water conditions, resulting in accumulation and preservation of organic matter.