A case history of petroleum exploration in the southern Forest City basin using gravity and magnetic surveys
Abstract
Three geophysical surveys were conducted in Johnson and Douglas counties in Kansas. The first two projects were gravity surveys each covering nearly 50 mi2 (130 km2). In all three surveys, readings of the gravity or magnetic fields were taken along county roads at 1/4-mi (.4-km) intervals. Normal data reductions and corrections were applied to the data. The data were displayed as contoured Bouguer gravity maps and a diurnal tide-corrected magnetic map. The Bouguer gravity maps indicated gravity anomalies with 0.20-1.20 mgal closures. Many of the closures were elliptical with their long axes paralleling local structural trends. Some of the gravity anomalies corresponded to known oil and gas fields. In particular, the Gardner oil dome near Gardner, Kansas, is associated with a 0.80-mgal gravity anomaly. The magnetic map indicated closures of 60-80 gammas. Some of the magnetic data corresponded well with the gravity data. Like the gravity data, some of the magnetic anomalies corresponded to known oil and gas fields. Several wildcat wells were drilled on or near gravity or magnetic highs. The first notable find was the discovery well of the Little Wakarusa Creek oil field. Oil is now being produced from 14 wells in this field from a 15-ft (4.5-m)-thick Cherokee sandstone at a depth of about 700 ft (210 m). The oil field is located in southeastern Douglas County nearly 5 mi (8 km) south of Eudora, Kansas. Initial flush productions varied between 10 and 40 bbls of oil per day. The magnetic and gravity data were interpreted to be related to deep basement structures. The deeper structures (.5-3 mi [.8-4.8 km] in depth) influence near-surface structures in the Cherokee Group of Pennsylvanian rocks. The structures provided trapping mechanisms for hydrocarbon accumulation in the Cherokee sandstones.
