ABSTRACT: Tumbleweed Delta-Lower Tannehill Point Bars, Geochemistry and Dipmeter Logs of Dickens and King Counties, Texas

Michael J. Usseglio

The oil-prone Tannehill sandstone trend is an extensive Eastern Shelf clastic depositional system derived from an ancient uplift located several counties away to the east-northeast of Dickens and King counties. The older 100-mi-long lower Tannehill system, preceding the upper Tannehill (Frye) system, trended east-west into King and Dickens counties, where it lies in the domain of the shallow-water-shelf delta distributary, situated between the well-documented prolific point-bar (fluvial) deposits of Baylor, Knox, and eastern King counties, and the massive nonproductive slope-mouth bar (deeper water) deposits of western Kent and western Dickens counties.

The individual prolific and shallow sand bodies in the shelf distributary of Dickens and King counties are shelf delta point-bars, with some reworking. Geologists for years have erroneously played the sands and interpreted dipmeter logs in this area as fluvial point-bars. Case histories illustrate the complex stratigraphic traps of varying sizes that are formed and the problems with dipmeter interpretations.

Prospecting techniques involving subsurface geology and soil-gas geochemistry have resulted in wildcat success ratios in excess of 25% and development well success in excess of 85%. Numerous fields with proven total recoverable reserves in excess of 800,000 bbl of oil have been found over the last 10 yr. The premise central to modern-day geochemical soil-gas prospecting is that very small amounts (parts per million) of light hydrocarbons move upward continuously (but not always vertically) over time from subsurface hydrocarbon reservoirs toward the ground surface, and such microseepage is detectable by modern instruments sensitive to parts per billion. Case histories illustrate that such unconventional techniques are very successful only in certain geologic provinces, and are to be in egrated with subsurface geology and other conventional methods.

AAPG Search and Discovery Article #90096©1990 AAPG Southwest Section Meeting, Wichita Falls, Texas, March 11-13, 1990