Queen City Sand Play
Geology
Figure 3 shows a schematic cross-section of how the Queen City fields are configured, in most cases throughout the Gulf Coast region of south Texas. The fields are usually situated over four-way closures that set up older Wilcox Sand fields. Down-to-the-coast listric normal faults create these four-way closures. Traps form in the sandstone intervals sandwiched between thick shale intervals.
The depositional environment for the Queen City sands was along a mud rich shelf some distance away from the sands being deposited in the deltas (Figure 1). Sand shoals, or sands bars, developed along this flat shelf. The shoals are rich in clay, and tend to have thin laminations of sands, making pay difficult to identify on older petrophysical logs. Because of high clay content, the resistivity of the sands can be quite low – often below 4 ohms – making then appear to be saturated with salt water. The water is bound within the crystal structure of the clay, and is largely irreducible, or bound within the clay structure at a microscopic scale.
High clay content does pose a problem for permeability, and many of these reservoirs must be fracture stimulated. This is the strategy taken at Mestena Grande field. Other successful completions have been made along the trend.
Three dimensional seismic data can be used to identify amplitudes (Figure 4). Earlier work by Crimson Energy shows that these amplitudes correlate very well to better well performance.