1. Texas Gulf Coast, Miocene Upper Frio Sandstone

Source: Coates, G., 1992, Core data and the MRIL show a new approach to formation factor: SPWLA Transactions.

Coates describes the Texas Gulf Coast Miocene Upper Frio Sandstone example as typifying high porosity, shaly sandstones. An Magnetic Resonance Imaging Log (MRIL) was run through this sandstone. Coates interprets the MBVI to be 12% at the top of the sandstone, decreasing to about 6% in the basal 25 feet of the unit. The entire sandstone is interpreted as wet and non productive since the upper portion, while at irreducible water saturation, has relatively high bound water.

The PfEFFER analysis consisted of preparing Pickett cross plots with points annotated by depth, gamma ray, and MBVI. Data in this example and the others that are part of the “BEYOND OZ” workbook were entered manually into the spreadsheet at 1-foot increments. The results show BVW is about 12 to 13% at the top of the sand which agrees with the MBVI value. However, several feet of pay are suggested by the clustering of points between 5198 and 5202 feet measured depth, suggesting irreducible water and hydrocarbon saturation. Sw is around 35% in this cluster and is at acceptable levels for pay. In addition, PfEFFER was used to recognize a typical pattern attributed to a transition zone between 5198 and 5206 feet. This is underlain by a water-wet interval below 5206 feet. PfEFFER analysis can compliment the use of the MRIL confirming or enhancing the interpretation.