SPWLA 36th Annual Logging Symposium, June 26-29,1995 A New Quantitative Methodology For Gas Saturation Determination And Its Applications In Prudhoe Bay Moustafa E. Oraby AEPT : ARC0 Exploration & Production Technology Jerry Brady AAI : ARC0 Alaska Incorporation Thu-Thuy Dang AEPT : ARC0 Exploration & Production Technology’ ABSTRACT Gas saturation, Sg, is one of the most difficult petrophysical parameters to interpret. Also, most of the well known gas detection methods e.g neutronldensity and neutron/sonic are qualitative and can not be easily used to quantify the gas saturation. In this paper, a new methodology is introduced that can be used to quantitatively provide gas saturation in gas or gas/oil reservoirs using the neutron porosity. The new methodology uses the open-hole calculated lithology for a reservoir to forward model the neutron tool response for diffezent assumed gas saturation varying from O-100%. The number of the forward thermal neutron responses generated at each depth depends on the gas saturation increment. The forward modeling is based on calculating the formation neutron parameters using any of the available neutron diffusion codes which are always fast in computing time and reliable in their results. The calculated formation neutron parameters are then transformed into neutron porosity responses using transformations that most of the service companies have already derived for their tools. To calculate gas saturation, the forward thermal neutron responses anz then overlaid with the actual tool response and the intersection at each depth determines the gas saturation. The accumcy of that methodology depends on the accuracy of the open hole formation lithology that is used to forward model the thermal neutron responses, the accuracy of the neutron parameters to the tool response transformation, the quality of the neutron tool data acquisition, and the gas increment used to generate the forward modeled neutron responses. This methodology has been tested in many wells in Prudhoe Bay. The results are more accurate and representative than the conventional gas saturation models. Results of some of these applications and the accuxacy of the methodology are presented here. Introduction: Gas and gas/oil reservoirs are very hard to interpret and calculate their formation petrophysical properties. Gas has its unique finger prints on many of the logs through its low hydrogen index and low density. Some of the logs in gas and gas/oil reservoirs can be only used to identify the existence of gas but they can not be used to calculate some of the important petrophysical parameters like formation porosity. In gas and gas/oil reservoirs, it is more important to calculate the amount of gas in the reservoir than just detecting it. Determining the gas saturation on a foot-by-foot basis is essential in perforating decisions and production. Also, through the reservoir life time, quantitative gas monitoring will show the amount of gas movement, the resaturated zones and the reservoir depletion efficiency. For these reasons, a new quantitative approach for gas saturation determination, Sg, in gas and gas/oil reservoirs is introduced. It is also applied to wells in RR ’ Now at Pennzoil E&p