New evidence of regressing and transgressing Jurassic siliciclastic coastlines : Dhruma Formation, northern central Arabia

The Middle Jurassic Dhruma carbonates are long known 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 provided knowledge on how these siliciclastic deposits were formed or can be correlated with their carbonate counterparts. Recently drilled shallow stratigraphic wells on and near the outcrops of the Dhruma Formation (about 250 km NW of Riyadh) provide an extraordinary opportunity to analyze the sedimentological and stratigraphic characteristics of these siliciclastic sediments. Around 1500 meters of cores from six shallow stratigraphic wells are described, providing an understanding of the transgressive and regressive siliciclastic Dhruma shorelines, and how these fit with their better-known carbonate counterparts. Thirteen facies associations were identified from the analyzed Dhruma cores, and they were classified into transgressive and regressive (or both) associations. In addition, five transgressive-regressive sequences were identified, each with an estimated age duration of 1.2-1.6 My, and an average thickness of 32- 78 meters. This study is the first to document the sedimentology and stratigraphy of the Dhruma siliciclastic succession in northern central Arabia, and highlights a complex mixed system of deltaic, estuarine and shelf-ridge deposits that interfinger with carbonate deposits. Also, it contributes to the reconstruction of the existing Middle Jurassic paleogeographic map in Saudi Arabia, and to a correlation with the recently discovered age-equivalent siliciclastic reservoir in the Rub Al-Khali basin.