The addition of a third detector to pulsed-neutron tools (PNT) enabled a new measurement, saturation gate (SATG), which is sensitive to gas vs. fluid. The saturation calculation was originally created using a single mineral model of clean sandstone in which it worked well. Sandstone formations typically have some shale content and as the shale content increased, the gas saturation algorithm undercalled gas saturation. An updated two mineral model, taking into account both sandstone and shale, is needed to accurately calculate gas saturation in shaley-sandstone environments.

Two methods are being introduced to correct for the shaley-sand effect. The first method uses a crossplot technique of sandstone to shale SATG data vs. the volume of shale and corrects the shale affected points back to the sandstone fan. The second method creates two fans: one for 100% sandstone and the other for 50% shale, and the fans are moved to fit the data. The first method results in a straightforward ratio of the change in SATG to the change in shale volume to plug into the correction while the second method is both more intuitive in context and interpretative in practice. The shale is modeled using a mixture of 50% illite clay and 50% quartz. Depending on the mineralogical composition of the shale, other fans could be constructed.

Two tight-gas shaley-sand examples are used to compare the gas saturation response using both the single (no shale correction) and shaley-sand mineral models. Results show the updated model enhances accuracy of gas saturation in shaley-sand environments.

Previous pulsed-neutron sigma and carbon-oxygen saturations had limitations. Sigma-based saturation can calculate both gas vs. water and oil vs. water saturation but is limited to high salinity formations in which the dynamic range is large. Carbon-oxygen-based saturations are not affected by salinity but only work for oil vs. water saturation. SATG saturation fills a void for gas vs. fluid saturation in low, mixed, or unknown formation water salinities.

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