EFFECT OF WATER SATURATION ON SEISMIC REFLECTIVITY OF SAND RESERVOIRS ENCASED IN SHALE

Abstract

Recent discoveries of the correspondence between seismic reflectivity and the presence of hydrocarbons in reservoirs have had a profound effect on petroleum exploration. This correspondence appears most pronounced in poorly consolidated sand reservoirs encased in shale. Reflectivity, defined as the absolute value of the reflection coefficient at the shale/sand‐reservoir interface, is determined by the product of the longitudinal velocity and bulk density of the shale and, separately, of the sand reservoir. The formula for velocity taken from Geertsma shows that the square of the velocity in a sand reservoir varies inversely with bulk density and also inversely with fluid compressibility. As water saturation (fractional value of pore space occupied by water) increases in an oil sand, decrease in velocity caused by increasing bulk density is more than compensated by increase in velocity due to decreasing fluid compressibility. On the contrary, in a gas sand, the decrease in velocity due to increasing bulk density with increasing water saturation is not completely compensated by the increase in velocity due to decreasing fluid compressibility. Appropriate values for shale density and velocity, for reservoir porosity, and for reservoir constituent densities and compressibilities are used to determine the reflectivity of a shale/oil‐sand and, separately, a shale/gas‐sand interface as a function of water saturation at depths of 2000, 6000, and 10,000 ft. At each depth, the reflectivity of a shale/oil‐sand interface decreases moderately with increasing water saturation; whereas, the reflectivity of a shale/gas‐sand interface decreases moderately from a completely gas‐saturated reservoir to a water saturation of approximately 0.95, after which the reflectivity decreases abruptly and appreciably to the reflectivity of a completely water‐saturated sand reservoir. Thus, a small quantity of gas (5 percent or less) increases reflection amplitude significantly, and reflection amplitude is not a simple linear measure of the amount of gas in the reservoir.