September 2001 Supplement  Case Study

Small-scale 3-D seismic shoot adds new oil reserves

Dale Helpingstine [dale@shakespeare-oil.com]; and Don Williams, Shakespeare Oil Company; Phil Caserotti [pcaserotti@juno.com]; Geophysical and Geological Consultant; and Steve Gustison [gustison@isgs.uiuc.edu], Midwest Region PTTC

Bottom line. Shakespeare Oil redefined the structural interpretation of the Devonian Geneva dolomite reservoir in Tonti field, Marion County, Ill., using data from a 0.375-sq-mi 3-D seismic survey. The limited-scale survey, costing only $30,200, indicated that the central well, completed in 1940, was not on the true structural high.

A new well, drilled in March 2000 and located as a result of the 3-D survey, came in 12 ft high to the old central well. Initial potential was 58 bopd and no water. In June 2001, production was still averaging 10 bopd and 40 bwpd. Incremental attic oil reserves developed by using the limited-scale 3-D seismic survey are estimated at 50,000 to 60,000 bbl.

Background. Shakespeare, established in 1950, is a small independent, operating primarily in the mature Illinois basin with additional holdings in Wyoming, Montana and North Dakota. Operating in this environment, upside potentials for development activities are not huge, so risks must be well-defined. While attending a PTTC Midwest Region 3-D seismic workshop, Shakespeare personnel were exposed to the methodologies for implementing 3-D seismic, its potential in the Illinois basin and a case study over a Silurian reef.

Discovered in 1938, Tonti field has produced over 14 million bbl of oil from Mississippian and Devonian sediments draped over a Silurian reef. The reservoir is the Geneva dolomite, which is a brown, crystalline, pure sucrosic dolomite. There are no apparent fractures in the drill cuttings. The reservoir has good porosity (approximately 20%) and good permeability. Wells at Tonti are completed open hole in the top 5 to 10 ft of the reservoir. The new well was also completed open hole and received no acid or frac stimulation.

Completed in 1940, the McMackin 6 initially flowed 2,396 bopd from the Geneva dolomite and still produces 8 bopd and 1,300 bwpd. As operator, Shakespeare was experiencing production difficulties in this well associated with its aged casing. With existing knowledge placing the structural crest at this location, initial response was to plug the old well and drill a twin.

The 3-D seismic survey. Knowledge gained at a PTTC workshop helped Shakespeare to take a different approach utilizing 3-D seismic to select the optimal drilling location. Shakespeare contacted Phil Caserotti, a local geophysical consultant and presenter at the PTTC workshop, to help them design and interpret a low-cost, 3-D seismic survey that would yield the needed information. Great Lakes Geophysical from Williamsburg, Michigan, shot the survey and Exploration Development, Inc., from Parker, Colorado, processed the data.

With the reef measuring less than 1/2 mi across, along with added lease constraints, a limited scale, 0.375-sq-mi (20-ac) 3-D survey was designed that included six N-S receiver lines and seven E-W shot lines spaced 110 ft apart. Vibroseis was used rather than explosives, since a major state highway was within the shoot area.

A sequence of four separate maps that were interpreted from the 3-D seismic data are shown in Fig. 1. The first is a contoured, two-way time map (C.I. = 2 mil) to the top of the Devonian. This map was the first indication that the reef structure had two separate closures, thus altering the earlier interpretation.

Fig. 1. Sequence of four separate maps that were interpreted from 3-D seismic data. A) Contoured, two-way time map (C.I. = 2 mil) to the top of the Devonian. B) Range and distribution of the average velocity to the Devonian. C) Top of Devonian structure (C.I. = 10 ft) shows the two separated closures, each indicating previously undrilled potential. D) Time slice taken at 569 milliseconds, shows distribution of the closure.

From well control, the second map, showing the range and distribution of the average velocity to the Devonian, was constructed and then used to convert the time map to depth. The third map of the top Devonian structure (C.I. = 10 ft) shows the two separated closures, each indicating previously undrilled potential. The last map is a time slice taken at 569 milliseconds, which shows the distribution of the closure.

Development well results. Based on the revised 3-D interpretation, Shakespeare drilled a new well in March 2000 at the optimum seismic location. Drilled to a TD of 3,531 ft, the well encountered the pay zone 12 ft higher than the existing central well, as opposed to the predicted 15 ft. Initial potential was 58 bopd and no water. In June 2001, nearly a year later, production was still averaging 10 bopd and 40 bwpd.

Incremental attic oil reserves are estimated at 50,000 bbl to 60,000 bbl. To recover these reserves, Shakespeare invested $180,000, including $30,200 for the 3-D seismic shoot. The 3-D seismic costs, which represent about 17% of total cost, range from $0.50 to $0.60 per bbl. The 3-D survey enabled Shakespeare to drill a high-potential new well, rather than a twin well, with confidence.

Extended benefits. Success in Tonti field has led Shakespeare to use 3-D seismic for delineating new reef structures. To date, the new shooting has indicated only areas of no reef development, thus avoiding dry hole costs. With the enhanced resolution of 3-D seismic, Caserotti believes this technology should be used more in this basin to explore and develop deeper pay zones, as well as to delineate stratigraphic traps. PTD