ASQUITH, G. B., Dept. of Geosciences, Texas Tech University; HARDAGE, B. A., Bureau of Economic Geology at The University of Texas at Austin; LANCASTER, D. E., S.A. Holditch and Associates, College Station, Texas; and PENDLETON, V.M., Integrity Geophysics, Tulsa, Oklahoma

Abstract: Potential Low-Porosity Pay Zone in the Montoya Limestone Waha Area, Reeves County, Texas

In the Waha Field in Reeves County, Texas, the Ordovician Montoya Limestone has produced 43.8 BCF and 42,900 BC from seven wells. In these wells, the pay zones are all located in the upper 250 feet of the Montoya and are correlative. The lithology, based on lithodensity lithology plots, is either limestone or cherty limestone, and based on petrophysics, the pore type appears to be matrix intergranular porosity. Thin sections of well cuttings reveal that the limestone is a carbonate grainstone with coated grains (superficial ooids).

An 80 foot low porosity (maximum porosity 4%) zone that exhibits an excellent hydrocarbon invasion profile (LLD > LLS > MSFL), indicating a permeable formation with moved hydrocarbons, is located in the lower Montoya in Waha well no. 57. The lithology of this zone, based on a lithodensity lithology plot, is limestone with minor amounts of chert. Petrophysics suggests the pore type to be matrix intergranular porosity with fractures.

The lateral extent of this Montoya low porosity zone can not be delineated due to the lack of sufficient dual laterolog-Rxo logging suites in adjacent wells. The one exception is well no. 77 located 2 miles to the south where this low porosity is present but is only 25 feet thick.

This low porosity but permeable zone may represent a Montoya reservoir that could be exploited with horizontal drilling.

AAPG Search and Discovery Article #90918©1999 AAPG Southwest Section Meeting, Abilene, Texas