The initial openhole neutron density log shows steam- and air-filled
sands above X,500 ft. Pulsar service's sigma, thermal neutron porosity (TPHI),
and FNXS logged in cased hole all also show gas (steam or air) in the same
interval. In this situation, where openhole porosity logs are available, they
can be used to compute gas saturation in conjunction with any of these
gas-responding measurements, usually with the deeper-reading sigma or TPHI.
However, if openhole logs are not available, the difference between the
crossplotted FNXS and TPHI responses can be used to not only solve for the gas
saturation but also the total porosity after gas correction. The overlay on the
crossplot of FNXS and TPHI shows the expected response of various lithologies.
The subhorizontal upper boundaries are where 100% waterfilled porosity plots,
and the subvertical boundaries to the left represent where 100% gas-filled
porosity plots. The responses of TPHI and FNXS significantly differ because
TPHI is a hydrogen-dominated measurement, whereas FNXS is not.
Without the new FNXS measurement or openhole logs, solving both gas
saturation and porosity from pulsed neutron logs is underdetermined. In
openhole, an accurate formation porosity in gas-filled formations is usually
computed from a combination of density and neutron porosities. In cased hole,
FNXS plays a role similar to that of density because its response contrasts
with the traditional neutron porosity–type response, which is dominated
by hydrogen. As a result, the response for air- and steam-filled sands is in
the gas region of the crossplot and the fluid-filled sands and siltstones plot
along the 100% fluid line.