Determining connate-water saturations is a key factor in calculating original hydrocarbon pore volume (HCPV) for the Ivishak reservoir, the largest productive horizon in the Prudhoe Bay field. As-received water saturation, S-w, measurements on oil-based-mud cores (OBC＇s) from 27 wells were used as the core standard for S-w determination. Measured values ranged from < 1 to > 56% PV, which raised questions as to the validity of these values as being representative of in-situ conditions. Studies of the effects of the cutting, surfacing, and handling of these cores were conducted. Some cores were recovered with centers uninvaded by mud filtrate. S-w values in these sections were the same as those in mud-filtrate-invaded sections of other portions of the same core. Pressure and conventional cores had similar S-w values in adjacent intervals and to nearby oil-based-mud-cored wells. Rigsite S-w were measured and found to be the same as those determined routinely by Core Laboratories in Anchorage. These data offered convincing proof that, above the oil/water transition zone, water was not flushed from the core during the coring process, expelled or evaporated during surfacing, or evaporated while in transit to Anchorage for routine measurements. A large, alternative, centrifuge capillary pressure database was developed to provide an independent check of the OBC results. Development of this database required selection of samples statistically representative of rock types in the reservoir and use of laboratory techniques that produced reliable results. Comparing S-wPc with S-wobc values required the consideration of the burial and hydrocarbon accumulation histories and the variations in hydrocarbon properties within the reservoir.