Abstract: Productive, Low-Contrast (Resistivity) Frontier Formation Sandstones, Southwestern Wyoming

James C. Mullarkey

Recent completions in pay sands exhibiting 7 to 11 ohm-meters of resistivity measured by the deep induction log flowed at rates of 1 to 3 million cubic feet of gas per day (MMCF/D). The pay sands are low-contrast (LC), relative to the shale baseline deep induction resistivity of 6 to 7 ohm-meters, and therefore are 'non-conventional.' The productive intervals are marine sandstones on the Moxa Arch in the lower Second Bench of the Second Frontier Formation (Turonian). Productive sands can be easily bypassed if perforation intervals are selected using only indirect well log measurements. A completion process that incorporated wellsite information including the direct observation of the reservoir rock in cuttings and mud log information has resulted in high rate completions.

The productive intervals were initially identified with mud log hydrocarbon shows. The intervals of maximum gas show were perforated and a correlation with highly productive zones was observed. Four wells in the Lincoln Road Field area will be used to demonstrate the character of these LC pay sands and compare them to 'conventional' resistivity pays directly overlying the LC pay zones. The depositional environment is marine middle shoreface and intense bioturbation has resulted in well connected intergranular porosity. After fracture stimulation, several of these completions were connected to pipeline flowing over 10 MMCF/D. The LC pay sands exhibit good reservoir characteristics, evident in well log, core and petrographic data, typically more than 16% porosity with permeability rangi g from 0.08 to 0.12 millidarcies.

The causes of the LC resistivity log response are multiple and not fully understood. Identified causes in the Frontier Formation include illite and pyrite content and the very fine grain size of the well sorted sandstones.

These observations are significant because there are overlooked reserves in other low-contrast pay sands in the Rockies. There is a vast amount of available rock and well log data that can be used to identify similar depositional settings with bypassed reserves. The deliverability from these Frontier Formation sandstones shows that low-contrast pays can be more lucrative than 'conventional' thinking or completions would indicate.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994