Speaker: Jerry Gilbert, Gilbert Exploration and Consulting
Subject: LOWER VIOLA/SIMPSON DOLOMITE PLAY, NORTH TEXAS: PAST HISTORY AND FUTURE POTENTIAL
Abstract: This paper gives a brief history of the Late Ordovician Viola/Simpson dolomite play in Clay, Montague, 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 Red River Uplift and preserved on the south side of the adjacent trough in North Texas. It was later covered by thick basal Pennsylvanian sand and conglomerate units (Cate, 1960).
The play began in 1948 in eastern Clay County, Texas, and very little has been published since that time. Phillips Petroleum discovered good oil production on small structures from porous dolomite encountered 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 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. Correlation of the Viola/Simpson from North Texas to the main depocenter in the Southern Oklahoma Aulacogen is difficult because the zone is eroded from the intervening Red River and Muenster Uplifts.
The gross Viola formation thickness ranges from zero to over 300 feet, and the dolomite pay zone occurs 20 to 40 feet above the base. The base of the Viola limestone rests on the Simpson shale which is an excellent structural and seismic marker. The best lower Viola reservoirs are located along the up-dip erosional edge where the dolomite is in contact with the Mississippian Barnett shale source rock. Recent thin section work by Allison (2015) indicates that the lower Viola pay zone was deposited as a thin bedded slope facies in an anoxic environment. In general, it is composed of 40% carbonate and 60% silica. Porosity is secondary due to solution enlarged fractures and vugs.
The Viola/Simpson production was originally found in North Texas by drilling single-fold seismic structures. 2D seismic was used during the 1980’s and most recently 3D seismic has been successful in finding Viola production.
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