New evidence of regressing and transgressing Jurassic siliciclastic coastlines within the Dhruma Formation in Northern Central Arabia, Saudi Arabia

The Middle Jurassic Dhruma carbonates have long been identified as a major hydrocarbon reservoir with massive proven reserves in Saudi Arabia and other Arabian Gulf countries. Regional studies indicate that these hydrocarbon-rich carbonates are replaced laterally by siliciclastic deposits in northern and southern central Arabia. However, none of those studies have clarified how these siliciclastic deposits were formed or how they can be correlated with their carbonate counterparts. Recently drilled shallow stratigraphic wells near the outcrops of the Dhruma Formation (approximately 250 km NW of Riyadh) provide an opportunity to analyze these siliciclastic sediments. Around 1500 m of cores from six shallow stratigraphic wells are described and interpreted, providing an understanding of the transgressive and regressive Middle Jurassic siliciclastic Dhruma shorelines. An unusual aspect of the Dhruma shorelines was the long-lived transgressions (greater than half the thickness of the succession) that initially created embayed estuarine coastlines. Rivers coming into the estuaries were backfilled and strongly impacted by tidal currents entering the embayments from the sea. Fluvio-tidal and tidal sandbars filled 11 m deep inner estuarine channels and were eventually overridden by muddy central basin deposits and then by the remnants of coastal barriers with tidal inlets. Higher frequency transgressions during the middle part of the Dhruma interval left an unusual record of stacked shelf ridges, until widespread shelf carbonates developed when the clastic supply system had back-stepped far landward. In the upper third of the Dhruma Formation, the clastic supply system was re-established, with river- and tide-influenced deltas regressing across the preexisting shelf area. Five thick transgressive-regressive sequences, in addition to a series of low-supply, high-frequency mid-Bajocian transgressive events, provide the details of the evolution of this early Middle Jurassic coastline in central Arabia. Given a likely total time span of 5-7 Ma (early Aalenian to Bathonian), the Dhruma siliciclastic sequences have a duration of similar to 1 Ma and an average thickness of 30-80 m. The existing Middle Jurassic paleogeography map in central Saudi Arabia has been enhanced, and a correlation has been made with the Dhruma reservoir-equivalent outcrops in southern central Arabia. (C) 2018 Elsevier B.V. All rights reserved.