Abstract

The Permian (Wolfcamp) Tannehill Sandstone of Central Texas was deposited in a fluvial-deltaic environment. After deposition, authigenic illite-smectite mixed-layered clays and chlorite were precipitated in the pores. The presence of dispersed authigenic clays results in low resistivity (<2 Ohm-m) in the Tannehill oil productive sands and in a low resistivity contrast between oil and water productive sands.

A series of cross-plots were used to determine productive from non-productive Tannehill zones in two wells. These cross-plots include: 1) Q-Plot, 2) Rt Porosity versus Total Porosity, 3) Pickett, 4) Sw (Archie) versus Sw (Ratio) and 5) Rt/Rw versus Rsl/Rz (Dew Plot). Productive versus non-productive Tannehill sandstones could be differentiated from the cross-plots before shaly sand analysis was attempted.

The Dual Water Method, using an adjacent shale resistivity of 2.5 Ohm-m, resulted in effective water saturations of 65% to 85% even in the oil productive zones. The Fertl Method resulted in effective water saturations of 55% to 70% in productive zones. The over-estimation of effective water saturation by the Dual Water Method versus the Fertl Method results from the erroneous use of adjacent shale resistivity for the authigenic clay in the Tannehill Sandstone.

With net pay cut-offs of V_cl=20%, effective porosity=10%, and effective water saturation (Fertl)=60%, the oil productive Tannehill sand had 8 feet on net pay and the water productive Tannehill sand had no net pay. When tested, the oil productive zone IP’d for 85 BOPD and no water; the non-productive zone tested water in the top 22 feet of sand with a slight show of oil.