Abstract

Navigator (Tannehill "B" Sandstone) Field is 9-1/2 miles southeast of Spur, Dickens County, Texas, in the northeastern end of the Eastern Shelf of the Midland Basin. This paper describes the integrated exploration processes that discovered the complex structural-stratigraphic reservoir.

The discovery well, Eastern Energy Corporation's Neeley A-1, was drilled based on subsurface geological mapping of the Tannehill "B" Sandstone. Subsurface geology was supported by geochemical techniques and dipmeter (FMS) data from a down-dip show well drilled by Eastern Energy 2 months earlier. The Neeley A-1 well encountered 28 feet of Tannehill sandstone at 4408 feet depth and potentiated 167 BOPD + 16 BWPD (pumping).

The Tannehill "B" Sandstone is the most pervasive of three sandstone units between the base of the Stockwether Limestone and Croton Creek Limestone. Data from samples, sidewall cores, dipmeters (FMS/FMI), and electric logs indicate Tannehill "B" Sandstone was deposited during a regressive cycle of fluvial-deltaic sedimentation and is encased between two transgressive cycles of limestone deposition. The sandstone represents both fluvial bars and distributary channel bars.

The author's awareness of the structural implications caused by differential compaction of the expected reservoir sand and the very important impact on the subtle structure (less than 30 feet of closure in an average field) was critical before deciding to drill the well. The depositional environment, as seen by studying the adjoining Sage Draw Field, was important to duplicate, if possible, to make drilling a wildcat an attractive financial venture for investors. Exploration efforts in this particular area were no accident.

An integrated exploration approach for Tannehill reservoirs is absolutely essential if one is to be successful. The author's success in the adjoining four county area (some 17 new field wildcat discoveries and 2 field extensions) is not due to any one exploration technique, but rather several blended tools, the application of which varies as one explores in different areas. No one tool can be given sole credit for the discovery of the Navigator Field, but rather an ongoing integration of primarily subsurface geology aided by surface geochemical techniques, geomorphology, micromagnetics, magnetic susceptibility, etc. Development of the field incorporated a continuing review of newly acquired subsurface data combined with FMI dipmeter data with some use of magneto-tellurics. The FMI dipmeter data was extremely critical to the initial success and subsequent speed of development drilling.

As of August 1998, Eastern Energy drilled 24 producing wells in the field. The field has produced more than 3/4 MMBO and is expected to yield 3-4 MMBO after secondary recovery.

A thorough understanding of the geology and continued review of all data sets found significant reserves compared with other surface exploration techniques employed in the area. The discovery showed how an integrated exploration program, using area-proven tools and experience, reduces risk and finds reserves cost effectively. Navigator Field has heightened interest in the area. Burk-Gunn Oil Co. recently purchased the field and is currently unitizing acreage for pressure maintenance and eventual secondary recovery operations.