Clay Mineralogy and Its Controls on Production, Pennsylvanian Upper Morrow Sandstone, Farnsworth Field, Ochiltree County, Texas

Tim W. Munson

Farnsworth field in Ochiltree County, Texas, is the most prolific upper Morrow oil field in the Anadarko basin, producing more than 36 million bbl of oil and 27 billion ft³ of gas since its discovery in 1955. The bulk of the production comes from an upper Morrow-aged sandstone locally referred to as the Buckhaults sandstone. The Buckhaults sandstone is a coarse to very coarse-grained, arkose to arkosic wacke.

Grain-size distributions, sedimentary structure analysis, and sand-body geometry indicate that the Buckhaults was deposited in a fluvial-deltaic environment as distributary channel and distributary mouth-bar sands. Depositional strike is northwest to southeast.

The source area for the Buckhaults sediments was primarily a plutonic igneous terrane, with a minor contribution from volcanic and reworked sedimentary rocks. The proposed source area is the Amarillo-Wichita uplift to the south. In addition, the Cimarron arch and/or Keyes dome to the west-northwest may also have contributed sediment to the study area.

The large (average) grain size, the amount of feldspar present, and the overall immaturity of the Buckhaults sediments indicate a relatively short distance of transport.

After deposition, the Buckhaults sediments underwent a series of diagenetic alterations including (1) dissolutions of silica from volcanic quartz and detrital chert fragments, (2) alteration of detrital feldspar and formation of authigenic clays, (3) precipitation of calcite cement, and (4) development of authigenic pyrite. The dissolution of silica and alteration of feldspars served to enhance primary porosity while the authigenic clay formation and calcite cementation were deleterious to reservoir porosity and permeability. Average values for porosity and permeability in the Buckhaults sandstone at Farnsworth field are 14.53% and 45.2 md, respectively.

The field was produced under primary production conditions until 1965, at which time a secondary waterflood was initiated. The field is presently under study by Union Oil of California for the possible implementation of a tertiary CO₂ injection program.

Detailed scanning electron microscopy and x-ray diffraction analysis of cores from the productive interval coupled with comparisons of varying completion practices across the field indicate a significant correlation between individual well performance, clay mineralogy, and completion technique.

AAPG Search and Discovery Article #91025©1989 AAPG Midcontinent, Sept. 24-26, 1989, Oklahoma City, Oklahoma.