Differential Maturation, Generation, and Expulsion of Petroleum: Causes and Effects on Jurassic Unconventional Resources in Saudi Arabia

摘要

The sequential relationship between the Jurassic Tuwaiq Mountain source-rock maturity, petroleum generation, over-pressuring, and expulsion-fracturing was evaluated over the Arabian Intrashelf Basin. The evaluation is part of the ongoing assessment of the Jurassic unconventional resources in Saudi Arabia. It revealed the existence of differential thermal maturation with different rates of petroleum generation and expulsion. The differences are related to the variations in the source rock organo-facies, thermal conductivity, and heat flow. This differential behavior of maturation, generation, and expulsion of petroleum has resulted in forming separate Tuwaiq Mountain mature source rock play fairways in the Arabian Intrashelf Basin. API gravity, organic-sulfur content, and pyrolysis data for Jurassic source rocks in the Arabian Intrashelf Basin were analyzed. The main findings from this analysis explained puzzling thermal maturity behaviors at different depths. Transformation ratios and petroleum-generation pressures were mapped and correlated with models for high-sulfur and low-sulfur Tuwaiq Mountain source rocks. The transformation ratio models were generated by using imperially-derived sulfur-estimated kinetics (activation energies Ea and frequency factors Ao) for petroleum-generation in the Tuwaiq Mountain source rocks. They were also compared to various published source rock models that range from low-sulfur Type II to high-sulfur Type IIS. The evaluation also included a derived map of petroleum-expulsion fracturing in Tuwaiq Mountain source rocks. The maps generated in this study were shown to be a promising tool to define the limits of the unconventional mature source rock play fairways and possible areas of higher productivity. The definition of such differential maturation, generation and expulsion of petroleum significantly impact the prospecting strategy for Jurassic unconventional resource in Saudi Arabia. Insights on such cause-and-effect relationships can guide exploration for unconventional resources existing in extremely low-permeability, self-sourced, fractured organic-rich mud rocks in the deep parts of basins where they are thermally mature.