Structure of the Hambert-Aristocrat area
In the Latham Bar Trend, just to the northeast of the mapped area, Al-Raisi et al. (1996) recognized a dominant northeasterly-oriented fault pattern, which coincides with the wrench fault pattern mapped by Weimer (1996) in the Denver Basin. They also found that the northeasterly faults compartmentalize the Terry Sandstone into a number of fault blocks. Within any fault block, normalized GOR values for individual wells increase up the structure, suggesting that each block is mutually isolated. Weimer (1996) has also identified structural compartments with widely varying GOR values for the underlying Muddy (J) Sandstone in another part of the Denver Basin.
The original computer-generated structure-contour map of the Hambert-Aristocrat area, datumed on the 'D2 Bentonite' beneath the Terry Sandstone, revealed no meaningful geologic pattern. The concept of a dominant northeasterly wrench fault pattern and subordinate northwesterly fault pattern (Weimer, 1996) was integrated into this map to develop a more geologically realistic, though complex, structure-contour map (Fig. 3) and cross section (Fig. 4). Normalized GOR values exhibit the same relation to structure within fault blocks as that of the Latham Bar Trend (Al-Raisi et al., 1996), so GOR's provided a secondary guide to refining the structure-contour map (Fig. 4). A 3D seismic survey shot within Sections 17, 20, and 29 (VanKirk, 1996) revealed faults which are coincident with those mapped from well control, providing further support to our structural interpretation. However, it is worth noting that not all small faults mapped from well control were visible from the 3D survey.
Although partially masked by the northeasterly fault pattern and secondary northwesterly pattern, regional structural dip is about 4ft./mile toward the northwest (Fig. 3). In the northwest, D2 Bentonite elevations are generally at 240-280 ft. a.s.l., while toward the southeast they are in the range 280-320 ft. a.s.l. Local variations are a result of faulting and folding in the area.
It has not been possible to determine the amount of lateral offset on these faults, but where faults cut the Terry Sandstone at wells, vertical offset is on the order of 10-160ft. (Fig. 4). Fault cuts through the Terry Sandstone were interpreted in 46 wells in our database (2.3% of total wells), but it is very likely that there are more small faults. Repeat sections were not identified, indicating the vertical component of faulting is normal. M. W. Decker (pers. comm., 1996) suggests that fault plane dips are about 50-60 degrees in this interval. Also, because of the abundance of well tops data, it was possible to interpret folds within individual fault blocks (Figs. 3 and 4).
The sense of motion on Denver Basin wrench faults is thought to be right lateral (Weimer, 1996). These faults developed over pre-existing basement shear zones that were reactivated during the Laramide Orogeny. The compressive forces that uplifted the Rockies were translated into lateral slip in the Denver Basin. Considering compressive stress directions, the fault patterns mapped in Figure 3 are reasonable. Some of the small structural lows and highs mapped in the area might represent 'pull-apart' grabens and 'pop-up' blocks, respectively, which accomodate changes in the volume of rock as it is deformed by shearing.
Within the center of the Hambert-Aristocrat Field area, well log mapping revealed a local structural low (Figs. 3 and 4) which also appears on the 3D seismic survey (VanKirk, 1996). Oil wells (GOR's generally <5,000) are concentrated in this low area, while gas wells (GOR's generally >10,000) are concentrated in adjacent structurally high blocks. This distribution of oil and gas wells attests to the primary structural control on production. However, GOR distributions also appear to be related to structural position within individual fault blocks (Fig. 4), suggesting that fault blocks are mutually isolated by sealing faults.
Unfortunately, no pressure data were available to test pressure differentials across faults. However, there is some evidence that calcite-fill may provide the seal along faults. In a Terry core (Eckhardt # 1; Fig. 3), vertical crystals of calcite fill a fracture that dips 45 degrees relative to the long axis of the core. Sandstones immediately above and below this fracture exhibit matrix permeabilities which are considerably lower than those in sandstones farther from the fracture. Also, most density logs in the area exhibit a computed density of about 2.5g/cc However, a well (HSR Salisbury 6-29; Fig. 3) with an an anomalously high density (>2.6g/cc) plots on the structure contour map (Fig. 3) within a highly faulted, small compartment , suggesting that a fault cuts the well and the fault is sealed with higher density material (calcite?).
