Late Jurassic biofacies characteristics of Jubaila-Arab formational contact, Saudi Arabia

The lithostratigraphic relationship between the Jubaila and Arab Formations is not fully understood and is often misinterpreted in the subsurface. Since the type section of the Jubaila is in the outcrop and that of the Arab is subsurface, there is no common feature to easily define the contact in subsurface. The highly productive Arab-D reservoir spans the Jubaila and Arab Formations. One reservoir layer in Ghawar field (at the top of reservoir zone 2B) is considered in previous studies to possibly equate with the Jubaila-Arab formational boundary, and it is confirmed in this study (Fig. 1). Using semi-quantitative micropalaeontological analysis of thin sections from three localities in Riyadh area where the Jubaila and lower Arab Formations are exposed enables recognition of this boundary in eight wells from eight fields in the east of Saudi Arabia. In this study, the Arab-D reservoir has been divided into nine biofacies that characterize the Jubaila and Arab Formations and the nature of the Jubaila-Arab formational contact. Calibration of the wireline logs with these biofacies will assist in the recognition of this event in uncored wells, and assist in regional Arab-D reservoir layering. Such information will also contribute to improve the understanding of intra-Arab-D reservoir sequence stratigraphy, palaeoenvironmental and sedimentological variations and unexpected production behavior of the reservoir.