Joint Meeting of the Rocky Mountain Petroleum Sections of AIME, 3–4 March, Denver


This paper illustrates the use of well logs in evaluation of Wyoming carbonate reservoirs, particularly the Phosphoria formation. The Phosphoria is a stratigraphic reservoir of Permian Age and includes primary and secondary porosity patterns. Procedures of calculating pore space water saturations are explained and illustrated. Log approaches to physical characteristics of the carbonate reservoirs are described, such as the presence of shale, formation fractures, relative permeability of intervals, etc.

The importance of high formation water resistivities in these reservoirs is well demonstrated. Calculations of recoverable reserves are demonstrated in one well and three fields.

Irreducible water saturations and "critical water saturations" as encountered in this formation are presented and discussed.

Methods and results, as reported herein, represent a practicable method of evaluating wildcat carbonate tests or carbonate fields in the Big Horn Basin.


The author has applied standard log interpretation techniques to Wyoming Big Horn Basin carbonates [Fig. 1] with reasonable success since 1953. Experience has shown that the main stigma in analyzing carbonates and successfully completing them for production is the effect of fracturing systems which are very often present.

The Phosphoria dolomite is the most difficult producing Big Horn Basin reservoir in these respects and is therefore concentrated upon in this report. Known carbonate reservoirs present in this basin are shown in Fig. 2, the most important of which are Madison limestone end Phosphoria dolomite. Productive stature of Phosphoria can be easily compared by cumulative production to Jan.1, 1956. There is no intent to discount Madison lime and the deeper Big Horndolomite for they are excellent reservoirs. It is most likely, though, that oil accumulation in Phosphoria has been found in more areas because of the stratigraphic nature of its deposition.

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