Abstract

The Tertiary Wasatch Formation and Upper Cretaceous Mesaverde Group in the Natural Buttes area of the Uinta Basin are productive of natural gas and condensate from what appear to be basin-centered tight gas sands. Operating companies usually log their boreholes using the Dual Laterolog for resistivity measurements and the Density/Neutron combination log for porosity determination. Cores for measurement of porosity and permeability are not common. By using a key well with core data, a methodology is presented in this paper for determination of: a) zones of water-free hydrocarbon production, b) the relative productivity of zones, and c) the potential length of the hydrocarbon column. Formation water resistivity (Rw = 0.2 ohm-meters @ 60 degrees Fahrenheit) is computed from the Spontaneous Potential (S.P.) log. The few S.P. curves available from the area indicate a resistivity similar to that of the mud filtrate (Rmf) used to drill the wellbore and is similar to values measured from produced water. Cross-plot of log porosity with deep resistivity (Pickett plot) for potentially productive zones show variations in the water saturation (Sw) from 35% to 85%.

Porosity and air permeability, measured from routine core analysis are crossplotted to reveal a pattern of data by significant clusters representing the mean pore throat radius (port size) of the samples. Port size is used to discriminate capillary pressure type curves, from which one can estimate the height of the gas column required to achieve a range of Sw. In addition, port size, when posted onto a plot of Sw (from the Pickett plot) and depth, is an excellent tool to predict water-free hydrocarbon production and relative producing rate of a zone of interest.