Cotton Valley Sand Play
Analogue
Wilson and Hensel (The Cotton Valley Sand of East Texas: A Log-Core Study, 1982, 23rd Annual Symposium of the SPWLA) documented some of the problems of evaluating the Cotton Valley Sand with logs, noting some peculiarities and deviations from the normal formation evaluation. For example, on average, the cementation factor m falls in the 1.69 to 1.94 range. The normal m used in the Archie equation calls for a number of 2. The n, or saturation exponent, ranges from 1.65 to 2.44. Average irreducible water saturations range from 18 percent to as high as 90 percent. There is not a good correlation of porosity to permeability; for example, they found that a 9 to 10 percent porosity sand may have permeabilities between 0.05 and 3 millidarcies. The industry is still able to make good wells in this low grade rock because of the ability to stack over 500 feet of net pay in each well.
Hall, Janks, and Ladle (Sedimentology and Diagenesis of the Cotton Valley Sandstone in the Amoco No. 1 Cullers Well, Rusk County, Texas, 1984, East Texas Geological Society Publication) explain some of the reasons why there are high apparent water saturations in rock that produces good gas wells. The cause is three factors: (1) the high clay content, the small pore throat sizes, and the occurrence of pyrite. They found mixed layer illite-smectite, (2) chlorite, and (3) discrete illite. Mixed layer clays have high amounts of “bound” water in their crystal structure, and chlorite has high iron content – both qualities contribute to low resistivity sands.
The authors also found the depositional environment to be analogous to Galveston Island, a large barrier island off the coast of Texas. These islands are composed of long strike oriented bars cut by tidal inlet channels and the associated ebb and flood deltas on either side. As these islands migrate through time, they can distribute sand over a wide area of both space and time. The Cotton Valley Sand is often called a blanket sand, because it of its widespread occurrence throughout east Texas, but there are ways to track the heart of the barrier island bars, namely by using formation micro-imaging logs.
The Amoco Cullers well has produced about 1.4 BCFG and 3900 barrels of oil from a single completion zone at 10,168 feet to 10,186 feet. This interval was cored and the rock shows observable fractures. These fractures contribute to the permeability of the rock and influence the trend of “sweet-spots” in the fields.
Both northwest and northeast fracture trends are observed when mapping production trends by hand. Energy Frontiers Partners uses other methods to map these permeability trends, using fractal statistics.