5. Illinois Basin, Aux Vases Sandstone

Source: Meunier, J., 1996, Aux Vases Sandstone, Illinois Basin, Franklin County, Illinois, Hydrocarbon production from low contrast, low resistivity reservoirs: Rocky Mountain and Mid-Continent regions, Log Examples of Subtle Pays, Rocky Mountain Association of Geologists.

The subtle pay in this example is a low-resistivity sandstone reservoir in the Farrar #1 McCreary well. The well logs did not satisfy the operator’s criteria for pay. The low resistivity is attributed to the abundance of clay and associated large fraction of bound water. The average horizontal permeability from a core from this sandstone is 45.23 millidarcies (md) with a vertical permeability of 4.3 md and porosity of 20.6%.

The PfEFFER analysis was used to generate Pickett cross plots annotated in color of depth and gamma ray. The data were entered manually onto the spreadsheet at 1-foot increments. The points form a cluster in a cleaner, mid-portion of the sandstone. The cross plots indicate that the reservoir has a high BVW of 14%, high porosity of 20 to 23% and Sw of 60%, respectively. The resistivity is 2 ohm-m or less. Even without the core information, a shaly or fine grained sandstone is suggested containing a portion of fine pores and a large fraction of bound water. However, the clustering of points is indicates nearly irreducible water saturation conditions in a hydrocarbon-bearing reservoir. The well was completed for 21 BOPD and 21 BWPD.