The Upper Jurassic / Lower Cretaceous Cotton Valley tight gas sand formation has made a significant contribution to North American gas production over the past decades. It is the first significant siliciclastic sediment deposited in the East Texas Basin, and dozens of tight gas fields exist in the Sabine Uplift of East Texas and Louisiana. Stratigraphically compartmentalized fluvial deposits, which are prone to high water cut, dominate the Upper Cotton Valley sandstone. However, laterally continuous, marginal marine lithofacies, confined by two major shales that provide good hydraulic fracture containment, dominate the Lower Cotton Valley sandstones. We acquired a comprehensive dataset of core and log data in a key study well. We conducted detailed routine and special core analyses, including high-pressure mercury injection capillary pressure and benchtop NMR measurements. We have developed a log-based, core-calibrated rock tying method for the Cotton Valley tight gas sand. The study suggests that the rocks have low porosity and low permeability overall, but are not tight enough to hold most of the formation water in the reservoir. Primary depositional facies, clay content, and subsequent diagenesis created complex pore geometry, which is the dominating factor in formation fluid flow. This paper explores the details of data integration of various measurements and demonstrates their reasonable consistency, and explains the petrophysical rock type model. Specifically, it demonstrates the complexities of pore body and pore throat network features of different rock types revealed by NMR data and mercury injection data. By using the rock-type based formation evaluation method, we have established a new definition of "producible pay" zones in this tight gas sand reservoir.
The Cotton Valley Sandstone in East Texas (Figure 1) consists of tightly cemented, very fine- to fine-grained sandstone interbedded with mudstone, siltstone, and carbonate.
