oa Late Ordovician Clastic Reservoirs in Northwest Saudi Arabia - Gas Exploration Potential and Challenges

The Nafud Basin of Northwest Saudi Arabia is in the early phases of hydrocarbon exploration. The sparse well control indicates a working petroleum system in this thick Paleozoic Basin with predominantly gas prone, under pressured tight sandstones found in the late Ordovician Quwarah member of the Qasim Formation and the overlying Sarah Formation. Outcrop of the lower Paleozoic sediments to the southwest show the effects of the late Ordovician (Ashgillian) glaciation with multiple deeply incised sub-glacial tunnel valleys under the African polar ice sheet. Reservoir quality is controlled by the original depositional fabric, burial diagenesis, burial history, and present depth. The tight Ordovician reservoirs drilled to date show the diagenetic effects of early deep burial and subsequent uplift with quartz overgrowth, and grain coating chlorite which influence pore connectivity. Hydraulic fracture stimulation has shown success to enhance productivity of these tight reservoirs. Recent exploration efforts include; extensive 2D and 3D seismic to map the distribution of Sarah glacial valleys, 3D structural restorations to address the stress history and potential fracture trends, and capillary pressure estimation of the gas saturated core which can be used to estimate column heights and address the issue of conventional traps versus basin center accumulations.