Cotton Valley Sand Play
Geology
The Cotton Valley Sand is a fairly well documented play. The sands are often barrier complexes, with corresponding barrier bar shoals, cut by tidal channels. Some sands are deposited under wave-dominated deltaic conditions and corresponding feeder channels.
Just considering the common barrier bar systems, the most common decline-type curves can be correlated to three different rock facies within this environment. William Wescott, in the 1984 publication Jurassic of East Texas, noticed that a cross-plot of porosity versus water saturation (Sw) produced three groups of rock type, which he called Factor I, II, and III. The differences between these Factors depend on the interplay of the porosity and Sw.
Energy Frontiers Partners (EF) has broadly correlated the three rock Factors of William Wescott to diagenetic differences seen in the barrier bar system. Figure 8 shows how the three Factors correlate to amount of rock type in the bar, and also the type decline seen in the bar. As end members, Factor I corresponds to the poorest producers, while Factor III corresponds to the best quality of production having a clean period and a dual dog-leg exponential decline curve. The Factor II rock type corresponds to the most common decline curve seen in the trend – one with a decent Initial Potential and a hyperbolic decline curve shape with long life. Natural fractures also play a role in the production quality. Clay genesis is also important, because the clays have a tendency to preserve porosity, even as the sands were buried deeper. If the clays were not present, the cleanest sandstone with high quartz content will often form quartz overgrowth crystals around each rounded quartz grain, creating a high degree of cementation.
The three Factors of rock type and decline curve shape also correlate to the three different trends seen in the rescaled range of probability distribution (Figure 3).
Once these correlations are identified, exploitation fairways can be defined by mapping various production parameters, such as Estimated Ultimate Recovery, Initial Potential, and decline curve type. Trends should be mapped by hand in order to bring out the fracture and diagenetic influence on the production and rock quality. EFP is also investigating other fractal-based statistics to help identify key areas to exploit, and explain both good and poor producing areas. These mapping techniques are unique in the industry, which considers the play a pure statistical play with no obvious or discernable trends.