ABSTRACT

The Sherwood beds of the Mission Canyon Formation (Madison) in Renville County, North Dakota, are part of an overall shallowing-upward regressive carbonate and evaporite sequence. Four Sherwood depositional trends are aligned parallel to depositional strike. From east to west these trends are: salina-sabkha, shoreline-lagoon, island-shoal, and open shelf. Productive lithofacies in the shoreline-lagoonal trend are peloidal and intraclastic wackestones and packstones with secondary vuggy porosity. The trap is an updip facies change to impermeable carbonates and anhydrites. Two productive lithofacies characterize the island-shoal trend: algal wackestones and packstones with fenestral and vuggy porosity, and coated-grain packstones and grainstones with interparticle and vuggy porosity. Lateral and updip traps are formed by carbonate mudstones deposited in lagoons and in very low-energy, mud-dominated "channels".

An increase in depositional slope just basinward of the facies change from anhydrite to carbonate favored development of shoals and islands. A general thickening of Sherwood beds accompanies this break in slope.

The modern Trucial Coast in Abu Dhabi resembles the ancient Sherwood depositional system. However, the orientation and composition of Sherwood islands differ from the Abu Dhabi islands. Sherwood islands are highly elongate in a dip direction because of the influence of northeast-southwest oriented paleostructures, very low wave and tidal currents, and the direction of the prevailing winds. Compositionally, Sherwood islands contain more carbonate grains and less mud than their Abu Dhabi counterparts.

Sherwood production can be prolific as demonstrated by individual wells in the Sherwood Field. High production rates in the Smith Field should stimulate more Sherwood exploration along the northeast flank of the basin.