Diagenetic Control on Rock-Type Heterogeneity in Dolomitized Upper Triassic Jilh Formation (Saudi Arabia) Revealed by In-Situ LA-ICP-MS U/Pb Dating

The Upper Jilh Formation was deposited during the Late Triassic (~238Ma - ~228Ma) within a broad peritidal to shallow marine carbonate platform. Facies are pervasively dolomitized and locally associated with good development of intercrystalline macropores, mainly in initially grainy facies. This study aims to provide new geochronology insights into multiple dolomitization processes affecting the Upper Jilh Formation and to establish connections between diagenetic overprinting and large-scale rock-type variability. Core and petrographic observations of 580thin-sections from 12 wells enable the reconstruction of the paragenetic sequence. Stable isotopes (δ18O & δ13C) were carried out on more than 850 samples and coupled with LA-ICP-MS in-situ U/Pb chronometry on 10 samples, along with one sample analyzed for fluid inclusion microthermometry. This compelling geochemical dataset was also integrated with plug measurements, including 53 MICP points, to define core-based rock types (RRTs) characterized by distinct petrographic, chronographic, diagenetic, and pore system attributes. Analyses indicate at least three phases of dolomites. Planar-s to non-planar dolomites (rD1) are usually medium crystalline and δ18O composition between -4‰ and 0‰. U/Pb chronometry indicates a precipitation within a narrow timeframe during the Late Triassic (214 ± 7.6 Ma, n = 7). rD1 is usually associated mud-dominated fabrics and show limited small intercrystalline macropores. Porous planar-e dolomites (rD2) are more common within grainstones and display coarser crystals with rare locally ferroan overgrowths. This phase has δ18O values between -6‰ and -1‰, and preserved a LA-ICP-MS age of 171± 7.8 Ma (n=5) indicating a broad Early Jurassic precipitation. These dolomites are associated with larger intercrsytalline macropores with larger pore throat diameters (RRT3). Burial cements include large mm-scale non-ferroan saddle dolomites with very negative oxygen isotopes (average -9.5‰) precipitated by hot brines (~160°C, fluid salinity of 23.2 wt.% NaCl eq.) likely circulating along faults. This phase was not directly datable by U-Pb geochronology. Based on its paragenetic relationship with other phases, its precipitation age is estimated between 171Ma and 135 Ma. Indeed, the last dated diagenetic event is an Early Cretaceous (~135 ± 20 Ma) blocky calcite associated with pore-filling anhydrite, locally replaced by calcite due to limited TSR. Rock-typing analysis reveals that pore systems are influenced by both sedimentological and diagenetic factors and can be reliably predicted in uncored sections. For the first time on the Arabian Platform, in-situ U/Pb chronometry has been used to date the key pore enhancing and pore occluding diagenetic phases. These findings provide valuable data for integration with basin modeling to improve reservoir characterization and predictive model accuracy.