Abstract

Clearfork cherty limestones interbedded with fine to very fine-grained sandstones and shales are the basin and slope facies of the Clearfork restricted platform dolomites and evaporites of the Central Basin Platform. Porosity is sometimes developed within these basinal Clearfork limestones. Porosities, of up to 13%, are developed within a 50 ft thick (6292’-6342′) cherty dolomitic limestone, within our example well. This porosity was described on the mud log, as chalky and finely crystalline with minor fluorescence.

Petrophysical cross plots of sonic porosity versus total porosity, M versus N and Rt/Rw versus Rxo/Rmf (Dew Plot) indicate that the pore type to be either matrix intergranular or intercrystalline porosity. Petrophysical cross plots, however, of Ratio (Swr) versus Archie (Swa) water saturations and Rxo porosity versus total porosity indicate vuggy porosity. The presence of high residual oil saturations (ROS) in the flushed zone will cause Swr > Swa and total porosity > Rxo porosity. High ROS values can be the result of 1.) dead, immovable oil, 2.) an oil-wet reservoir, or 3.) low reservoir permeability. In this Clearfork example the lack of moved hydrocarbons (High ROS) is probably the result of the very fine pores and low permeability associated with the chalky porosity.

A Production Ratio Index (PRI) Plot and a net pay summary indicate that both porosity zones should be oil productive. Plots of Rxo porosity versus Rt porosity and Rt/Rw versus Rxo/Rmf both, however, indicate a lack of moved hydrocarbons. When these intervals were perforated, formation water with 0% to 3% oil-cut was swabbed before frac. After frac and after frac water was recovered, only formation water was swabbed. The swab recovery rates varied from 2 to 6 barrels of fluid/swab run. The poor oil recovery from the Clearfork is consistent with the lack of moved hydrocarbons indicated by the petrophysical cross plots.