Conclusions and Applications
1. A methodology is described for downloading a large database into PC format. Although the methodology is time-consuming to complete, once the database is built and purged of any well data deemed to be erroneous, it is possible to quickly generate multiple iterations of useful maps. The main utility of routine computer-generated maps is to provide a starting point for geologists to input geologic knowledge and concepts into final maps.
2. In the Hambert-Aristocrat region, structural dip is gently toward the northwest at a gradient of 4ft./mile. However, there is considerable local structural variability owing to complex fault and associated fold patterns. The dominant fault orientation is northeast, with a secondary northwest orientation. The same structural complexities have recently been mapped for the entire Denver Basin by Weimer (1996), who considers structure to be dominated by a regional northeast-trending, basement-controlled, wrench-fault system. Lateral offset of faults in the Terry Sandstone have not been determined, but vertical throw is in the range 10-160ft. in individual wells. Over 2% of the wells in the area exhibit fault cutouts within the Terry Sandstone. 3D seismic over a limited area has confirmed larger faults.
3. Within the Denver Basin, as well as other Rocky Mountain basins, a common observation has been the anomalous distribution of oil and gas wells producing within a given formation (i.e. an oil well might occur structurally higher than an adjacent gas well in close proximity). These anomalies, expressed by normaliized GOR's for individual wells, can be explained by considering the complexity of faulting, and resultant sandstone compartments bounded by faults. This GOR distribution, coupled with density log and core data, suggest at least some of the faults are sealing, and reservoir compartments are mutually isolated.
4. The Terry Sandstone is comprised of at least seven parasequences. The base of Parasequence B, which overlies shelf mudstone, is a major sequence boundary, and each succeeding parasequence records minor relative sea level stillstand within an overall transgressive systems tract. Each parasequence is comprised mainly of upper shoreface strata toward the southwest (paleolandward) which grade laterally into lower shoreface/offshore strata toward the northeast (paleoseaward). Gross interval isopach maps of individual parasequences reveal complex geometries, coinciding with blocky/thinning-upward, upper shoreface strata toward the southwest, but more simplistic thickness trends, coinciding with thickening/cleaning-upward strata, toward the northeast. Such trends reflect variations in degree of marine reworking.
5. Complex structural compartmentalization provides the primary control on reservoir fluid distribution and oil and gas production in the Hambert-Aristocrat area. Secondary, but important stratigraphic controls include: (1) the distribution of upper shoreface sandstones immediately overlying a sequence boundary, and (2) transgressive marine shales which, when not faulted, probably horizontally compartmentalize the Terry Sandstone.
6. Additional potential pay types that have not been perforated or cored include: (1) thin, relatively good permeability lower shoreface sandstone interbeds which will appear on well logs as low contrast-low resistivity intervals and (2) upper parts of thickening/cleaning-upward, lower shoreface/offshore strata. Additionally, complex structural compartmentalization may have resulted in some untapped oil or gas which may be obtainable through targeted infill, or horizontal drilling.