Continental successions of alternating fluvial reservoir sandstones and fine-grained interfluvial deposits make hydrocarbon production planning and reservoir management difficult. The vertical and lateral distribution of fluvial sandstones within the surrounding nonreservoir rocks, in particular, is a critical factor that needs to be investigated before production or injection wells can be successfully drilled.
In the Oseberg field of the Norwegian North Sea, about 21.7 million Sm3 (standard cubic meters) of recoverable oil is stored within fluvial channel sandstones of the Middle Jurassic Ness Formation. The Ness Formation reservoir is characterized by a relatively low proportion of good-quality reservoir rocks (approximately 30% of the total rock volume). Sedimentological studies indicate that reservoir sandstone bodies are developed mainly as elongate, laterally restricted sandstone bodies concentrated within certain stratigraphic levels and geographic areas. Identifying and accurately mapping these sandstonerich intervals are important in defining possible drilling targets.
The distribution of reservoir rock within the upper Ness Formation has been the focus of detailed geological and three-dimensional (3-D) seismic mapping. Inverted seismic data, integrated with geological well data, give an empirical correlation between acoustic impedance and sandstone proportion. In turn, these relationships provide the basis for defining areas with a higher probability of encountering reservoir sandstone. Subsequent production drilling into the Ness Formation has confirmed the existence of reservoir sandstones within areas defined by the integration of high-resolution stratigraphic mapping and the geophysical analysis. The integration of sedimentological and 3-D seismic techniques presently offers the best potential for defining drilling targets within the Ness Formation in the Oseberg field.