Seismic Support Leading to Discovery and Development of Strahm South Field, Forest City Basin, Northeastern Kansas
Abstract
For the past five or six decades, the Forest City basin has been subject to sporadic exploration activity. Prior to the advent of multifold seismic data acquisition, many significant fields (e.g., Davis Ranch in Wabaunsee County) were discovered by integrating subsurface well control with shallow core-hole data. In some cases (Strahm field, Nemaha County), spot correlation seismic data were utilized in conjunction with shallow surface/outcrop structure mapping.
At Strahm South field, east-central Nemaha County, Kansas, an initial subsurface lead was investigated by acquiring 6.25 mi (10.0 km) of nominal 12-fold CDP data in early 1984. This initial survey resulted in a discovery and was augmented by the phased acquisition of 11.4 mi (18.25 km) of "development" data. Pre-drilling seismic mapping indicated significant thinning over a key Lansing-Viola interval; this feature generally indicates deep subsurface structure, particularly at the Ordovician. The survey data also indicated critical west dip of more than 100 ft (30 m) at the top of the Viola and probable closure.
Drilling has confirmed this 100 ft (30 m) of dip and that the Strahm South field is an elongated, faulted, and closed feature. It is slightly over 1 mi (1.6 km) long, with a productive width apparently less than 1,300 ft (390 m). Estimated ultimate recovery from six Viola wells will likely exceed 275,000 bbls. Untested Hunton and Simpson pay remains behind pipe.
Although other geophysical and geochemical techniques can be of assistance prior to drilling untested structures in this area, multifold seismic results are especially encouraging. Both lateral and vertical resolution, particularly with modem recording geometries and processing, confirm good agreement with actual subsurface data. When measured against the cost of a typical dry hole in the area, the CDP surveys are also very cost effective if designed with appropriate constraint.
