ABSTRACT

A new model has been derived for calculation of water saturation in shaly reservoir sandstones. The model is based on effective medium theory (EMT) and has been used successfully on a wide range of clayey, low-resistivity reservoirs. A further, independent test of saturations calculated by the model was made for a deep Wilcox shaly reservoir at a depth of 13,176 ft (4016 m) in Fordoche field. The reservoir (W8) has a low, average resistivity of about 2.5 ohm-m and an estimated 18% clay as compared to an average resistivity of 1.5 ohm-m in an underlying wet sandstone (W-10) of similar clay content. For the reservoir, calculated water saturations are in the range of 35 to 50%, increasing downward, and suggest that the shaly sandstone should produce water-free oil. Other shaly sand models generally give water saturations in excess of 50% for the reservoir. Water saturations calculated by the EMT model agree well with other parameters for sandstones: reported production, core analysis, and synthetic capillary pressures. The reservoir (W8) had an initial potential of 213 bbl of oil per day, confirming water-free oil production which could have been predicted by the EMT model.