Recent exploration activity by Sino Saudi Gas Ltd (SSG, a joint venture between Sinopec and Saudi Aramco) in southern Rub'al-Khali has resulted in the identification of a Rhuddanian-aged sandstone directly above the Base Qusaiba Hot Shale and interbedded with the cooler shales. While not as extensive and prominent as the well known mid-Qusaiba Sandstone, this lower sandstone has great potential as a hydrocarbon reservoir given its location directly above and interbedded within one Saudi Arabia's principal source rocks.
A sudden unexpected increase in mud gas and sandstone cuttings while drilling the Qusaiba shale in an SSG exploration well, led SSG to cut a 30 ft conventional core. The core contained interbedded sandstone and shale. FMI logs also clearly showed interbedded sandstone and shales, which allowed the very precise location (to within less than 3″) of rotary sidewall cores to complement the conventional core. Analysis of the conventional cores indicates that the sandstones were deposited by mass-flow mechanisms, including turbidity flows. The depositional system is interpreted as the lateral edges of a basin floor fan.
While routine porosity and permeability analysis of the conventional core plugs and rotary SWCs indicates that the Rhuddanian Sands are a very low permeability reservoir at this location, it did flow hydrocarbons on a cased hole test.