Petrophysics of a Pennsylvanian Virgilian Limestone in the Oklahoma Panhandle: an Interesting Study in Carbonate Petrophysics

ASQUITH, G.B., The Center for Applied Petrophysical and Reservoir Studies, Texas Tech University, Lubbock Texas

An examination of the Pennsylvanian Virgilian limestones in a well presently producing from a Pennsylvanian Morrow sandstone, revealed an interesting porous limestone. The lower part of this porous interval (4396’ to 4430’) has neutron-density porosities (?nd) from 6% to 16% with deep resistivity (ILD) values from 1.9 to 13 ohm-m. In addition this interval has a wet resistivity invasion profile (FL>ILM>ILD). The upper part of this porous interval (4387’ to 4394’) has neutron-density porosities (?nd) from 18% to 28% and deep resistivities up to 19 ohm-m.

Conventional log analysis of the upper interval at a depth of 4388 feet resulted in a neutron-density porosity(?nd) of 28%, an Archie Water Saturation (Swa) of 14.8% (a=1 and m=n=2), and a bulk volume water (BVW) of 0.041. Using conventional analysis the porous interval (4387’ to 4394’) looks like behind-pipe pay. However, additional log analysis at 4388 feet resulted in a sonic porosity (?s) of 16%, a Moveable Hydrocarbon Index (Sw/Sxo) of 0.53, a Ratio Water Saturation (Swr) of 45.2%, and a resistivity porosity (?rxo) of 8%. The additional log analysis indicates that the upper porous interval has vuggy porosity, and because ?s>>?rxo, the porosity is probably oomoldic.

Examination of the mud log indicates that the upper porous interval (4387’ to 4394’) is a limestone with oomoldic porosity, some speckled yellow fluorescence (no cut) and no gas shows. Therefore, the upper porous interval is not a behind-pipe pay, because an oomoldic carbonate with no hydrocarbon shows and a BVW value of 0.041 should produce only water.