4. Illinois Basin, Ste. Genevieve Limestone, oolitic carbonate reservoir with microporosity

Source: Asquith, G. B., 1986, Microporosity in the O’Hare Oolite Zone of the Mississippian Ste. Genevieve Limestone, Hopkins County, Kentucky, and its implications for formation evaluation: Carbonates and Evaporites, v. 1, no. 1, p. 7-12.

The problem as stated by Asquith is that the bound water (BVW) in this reservoir is 0.053 which the author indicates is too high for produce hydrocarbon. However, the well produced 560 BOPD without water. Examination of the core data indicates a bimodal porosity -- 1) microporosity within micritized oolites and 2) intergranular porosity between ooids. The grain size ranges from 0.35 to 0.5 millimeters. The ratio resistivity method indicated BVW of 0.034, indicative according to Asquith of a good reservoir.

PfEFFER analysis included Pickett cross plots annotated with depth and gamma ray. Two intervals of interest were identified where points form “elbow” bends. The upper interval, 2030 to 2034 feet, has a maximum porosity of 7.5%, Sw of 32% and BVW of 0.024. The porosity is probably too low in this zone for an economic reservoir. The lower zone, from 2036 to 2044 feet, has a maximum porosity of 17%, Sw is around 30%, and BVW of 0.05. This lower interval is interpreted as the good reservoir rock. The micro pores in this lower interval probably led to an increase in the BVW. However, the elbow bend and associated clustering of points suggest that these BVW values are nearly at irreducible values.