Microscopic characterization and modelling of pore structure heterogeneity in Upper Cretaceous immature, organic-rich carbonate source rocks, central Jordan

Carbonate mudrocks contain substantial reserves of organic matter, but their exploitation is hindered by the complexities of their pore networks. This study explores and models the heterogeneity of pore structures in immature, organic-rich carbonate source rocks. To achieve this, we employed digital rock analysis and nuclear magnetic resonance (NMR) techniques to gain a deeper understanding of the pore structures and their spatial distribution. Representative samples were collected carefully from a vertically cored well drilled through the Upper Cretaceous source-rock succession in central Jordan. Results show that the studied interval is divided into six distinct microfacies: one in the top unit of the Al-Hisa Phosphorite (AHP) Formation, and five within the overlying Muwaqqar Chalk Marl (MCM) Formation. Each microfacies displays unique porosity and kerogen content characteristics, with higher total organic carbon (TOC) values within the MCM Formation (18 wt% on average) compared with the samples collected from the AHP Formation (TOC of 4.34 wt%). NMR analysis revealed a trend where the formations shifted vertically from mud-dominated microfacies of lower porosity and lower pore sizes in the middle of the MCM Formation, to a transitional zone at the base of the MCM Formation and then to grain-dominated microfacies of higher porosity and pore sizes in the AHP Formation. Micro-computed tomography (micro-CT) imaging provided a deeper insight into the structure and distribution of pore/kerogen bodies in the MCM Formation. The findings emphasize the significant variations in the porosity and kerogen distribution across the different microfacies, highlighting the impact of both the depositional environment and diagenetic changes on the pore structures.