Southwest Section AAPG Annual Convention

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This paper gives a brief history of the Late Ordovician Viola/Simpson dolomite play in Clay and Wichita counties of North Texas, and its possible potential for future exploration. Regionally, the remnants of the Viola/Simpson dolomite zone are present in this area within a narrow northwest to southeast trending trough located between the Red River Uplift and the Bend Arch. The trough formed during the Strawn when an arm-like extension of the Fort Worth Basin formed from its previous limits in Montague and Clay Counties into Wichita County. The Viola/Simpson interval was removed by erosion from the Wichita Uplift and preserved in the trough and later covered by thick basal sand and conglomerate units (Cate, 1960).

The play began in 1948 in eastern Clay County, Texas, when Phillips Petroleum discovered good oil production from a porous dolomite at the pre-Penn erosional unconformity. Phillips considered the dolomite to be Simpson in age and the name was given to several other dolomite discoveries in the area. Confusion began after 1955 when some operators began using the younger Viola for the lower dolomite zone. There are currently eight lower Viola/Simpson oil fields near Henrietta, Texas, that have produced 2.8 MMBO along the up-dip erosional edge of the Viola/Simpson dolomite. Correlation of the lower dolomite zone from the oil fields in the Henrietta area to wells 25 miles to the east and west indicate that the zone is most likely Viola in age. This has not been proven with subsurface well control. Correlation of the Viola/Simpson from North Texas to the main depocenter in the Southern Oklahoma Aulacogen is difficult because it is eroded from the intervening Wichita and Muenster Uplifts. In this study, a green shale interval below the base of the massive Viola lime is used to separate the zones.

The dolomite pay occurs within 20 to 40 feet of the base of a massive dense limestone that ranges in thickness from zero to over 300 feet in thickness. The base of the massive limestone rests on the Simpson shale and is an excellent structural and seismic marker. The best lower Viola reservoirs are located along the northeast and up-dip side of the Henrietta fields where the dolomite is located at the unconformity. The zone grades into non-reservoir limestone to the southwest. Little or no work has been published on the Viola/Simpson lower dolomite environment of deposition.

The Viola/Simpson production was originally found in North Texas by drilling singlefold seismic structures. 2D seismic was used during the 1980's and most recently 3D seismic has been found to be successful in locating the erosional edge of the Viola dolomite.