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?).
Based upon core and well log patterns, Siemers and Ristow (1986) recognized two types of vertical stratigraphic successions comprising the entire Terry Sandstone interval in the Antelope-LaPoudre Fields (Fig. 1). One type thickens and cleans (becomes sandier)-upward and the other type exhibits a blocky/thinning and fining (becomes shalier)-upward pattern. The thickening/cleaning-upward succession is mapped as a northwesterly-oriented, linear sandstone body which lies to the northeast of a more areally-extensive succession characterized by a blocky/thinning-upward pattern. The blocky/thinning-upward succession is oil-productive in the area while the thickening/cleaning-upward succession contains gas in non-commercial quantities. Siemers and Ristow (1986) interpreted the thickening/cleaning-upward succession as a marine offshore bar and the blocky/thinning-upward succession as a marine sediment-fill within a protected area landward of the offshore bar. These same two patterns occur along trend in the Hambert-Aristocrat area (Fig. 5). However, Siemers and Ristow (1986) ignored sea level fluctuations in their interpretation, which we feel is incorrect for the Hambert-Aristocrat area, and in the area they described.
Five cores from the Terry Sandstone in the Hambert-Aristocrat area (Fig. 3) were described and interpreted. Four of the five cores represent the blocky/thinning-upward succession and one cored well represents the thickening/cleaning-upward succession (Fig. 5). For each core, sedimentary textures, structures, trace fossils and stratification successions were described and facies were identified. Trace fossils in these cores include Diplocraterion, Skolithos, Terebellina, Planolites, Chondrites, Ophiomorpha, Zoophycos, and Asterosoma. Inoceramus fragments are also present. In these cores, and in general, Skolithos is typical of shoreface strata and Zoophycos and Inoceramus most diagnostic of open shelf deposits (Pemberton and MacEachern, 1995, Fig. 5). Facies discussed here are similar to those described by Bergman (1994) for the Shannon Sandstone shoreface deposits at Hartzog Draw Field and by Siemers and Ristow (1986) for the Terry Sandstone.
On the basis of the core descriptions, the following facies were defined (Fig. 6a):
Facies 1 = Bioturbated mudstone (to sandy mudstone); Facies 2 = Burrowed to bioturbated sandy mudstone (more sand and remnant laminations than Facies 1); Facies 3 = Burrowed to bioturbated muddy sandstone; Facies 4 = Planar to slightly cross-bedded sandstone without burrows; Facies 5 = Ripple-bedded sandstone with minor burrows; Facies 6 = Mudstone/sandstone clasts in a mudstone/sandstone matrix
The Mach II Rossi 14-4 Aristocrat (Figs. 3 and 6a) typifies the sedimentology observed through the blocky/thinning upward strata. The lowermost interval from the base of the core to 4520ft. (Fig. 6a) is comprised of Facies 1 (about 95% mud). Inoceramus fragments and Terebellina (?) are present, suggesting an open shelf environment of deposition.
From 4520ft. to 4503ft., sediment is dominantly cross-bedded, fine-grained sandstone (Facies 5) with occasional thin interbeds of burrowed to bioturbated muddy sandstone (Facies 3). Skolithos and Chondrites trace fossils are present. The contact between this sandstone and the underlying shelf mudstone is sharp, burrowed, and erosional (Figs. 6a). This interval is interpreted as an upper shoreface succession.
From 4503 ft to the top of the core at 4470ft., the sediments are thin interbeds of burrowed to bioturbated muddy sandstone (Facies 3) and sandy mudstone (Facies 2) containing Skolithos and Terebellina (Fig. 6a). This entire interval is interpreted as mainly lower shoreface deposits, although thin transgressive shales are interpreted to be present. Thin parallel- to cross-bedded-, clean, fine-grained, storm-deposited sandstones (Facies 4) occur sporadically throughout this interval. These sandstones exhibit sharp bases and somewhat more gradational, finer-grained tops. The upper 3 ft. of core contains Zoophycos, suggesting more open shelf conditions. About 10 ft. above the top of the core there is a distinctive gamma-ray log marker interpreted as a transgressive marine shale. The Amoco Prod. 1 Vern Marshall (Fig. 3) core penetrated a similar interval which is comprised of laminated (not highly bioturbated) shale.
The Sotexco #1 Kinsman (Fig. 3) core contains a series of unusual shale- and sandstone-clast breccias (Facies 6). Such features are not present in the other cores. The base of each breccia is sharp, and clast sizes decrease upward indicating depositional size-grading during waning flow. Possibly this deposit is a debris flow or slump generated off the side of a shoreface (tidal??) channel.
The Amoco Prod. Donald J. Moser No. 1 (Fig. 3 and 7) well is the only one to have cored the thickening/cleaning upward interval. The lower part of this core is comprised of bioturbated shelf mudstone (Facies 1) containing Zoophycos. This mudstone is gradational upward into bioturbated, Skolithos-bearing, thinly-interbedded sandy mudstone (Facies 2) to muddy sandstone (Facies 3). The cored interval is interpreted as open shelf grading upward to lower shoreface deposits. The D2 bentonite was penetrated in this core, which allowed verification that the 'hot' gamma-ray kick used to correlate well logs (see below) is the expression of a bentonite bed.
The entire Terry Sandstone interval, from the top to the base of the D2 Bentonite (the D2 bentonite is actually stratigraphically beneath the 'base Terry Sandstone') does not dramatically change in thickness (140 - 160ft.) across the Hambert-Aristocrat area (Fig. 8). However, the internal stratigraphy is quite complex. Applying sequence stratigraphic principles to the cores and to well log correlations, the Terry Sandstone can be subdivided into at least seven parasequences (Figs. 5, 9 and 10, labelled A-G). Individual strata within parasequences generally grade laterally seaward from blocky/thinning-upward to thickening/cleaning-upward successions (Fig. 5); this gradation is typical of shoreface parasequences (VanWagoner et al., 1990, their Fig. 8). With one exception, transgressive marine shales separate each parasequence.
Depositional history of the seven parasequences within the Hambert-Aristocrat area is undoubtedly complex. Numerous recent studies of superbly exposed outcrops of shoreface sequences, published in Van Wagoner and Bertram (1995), attest to their complexity and the numerous significant erosional surfaces that will not usually be detectable or correlatable on subsurface well logs alone. Thus, the interpretation put forth below (Fig. 8) may even be too simplistic, particularly for parasequences B - D.
Parasequence A (Figs. 5, 9 and 10) is the open shelf mudstone beneath the base of the Terry Sandstone. In much of the area, its top is erosionally truncated.
Parasequence B (Figs. 5, 9, 10, and 11) is up to 30ft. thick and overlies the basal shelf mudstone of Parasequence A. In core and on well logs, this parasequence exhibits a sharp, erosional base toward the west and northwest (paleolandward) which becomes a gradational contact toward the east and northeast (paleoseaward). There is a corresponding change in well log pattern from blocky/thinning- to thickening/cleaning-upward in the same direction. The sharp, erosional base is interpreted to have resulted from 'forced regression' (Posamentier, et. al., 1992) during sea level lowering, so it is a depositional sequence boundary. Based upon well log patterns, strata grade seaward from upper shoreface to lower shoreface/offshore. A gross interval isopach map is shown on Figure 11. Since the basal contact of this parasequence is gradational toward the northeast, isopach thicknesses could not be accurately determined there. However, the contact between the sharp-based, blocky/thinning-upward succession and the gradationally-based, thickening/cleaning-upward succession (Fig. 11; just paleoseaward of the 5ft. isopach) lies along the same trend as that discussed by Siemers and Ristow in the Antelope-LaPoudre area (1986).
Parasequence C (Figs. 5, 9, 10, and 12) is 15-35ft. thick. The base of this (and overlying) parasequences is marked by a distinct shale marker on the gamma-ray log interpreted to be a transgressive marine shale. A gross interval isopach map reveals complex stratal geometry in the paleolandward direction, with interval thicks oriented both northwesterly and northeasterly; corresponding well log patterns are predominantly blocky/thinning-upward. Toward the northeast, a clear northwesterly isopach trend prevails which corresponds to a thickening/cleaning-upward log pattern.
Parasequence D (Figs. 5, 9, 10, and 13) is 15-35ft. thick. A gross interval isopach map shows a similar geometry to that of the underlying parasequence, with mutually perpendicular thickness orientations in the paleolandward position corresponding to a blocky/fining- upward log pattern, and a northwesterly thickness orientation in the paleoseaward direction which corresponds to a thickening/cleaning-upward log pattern. The position of the boundary between wells with the two log patterns, relative to the boundary position within underlying Parasequence C, varies from somewhat paleolandward to somewhat paleoseaward.
Parasequence E (Figs. 5, 9, 10, and 14) is 15-30ft. thick. A gross interval isopach map shows that within the mapped area, there is a predominant northwestly isopach orientation which corresponds to a thickening/cleaning-upward log pattern, except along the west-southwest edge of the area, where a blocky/thinning-upward log pattern is present, and to the northeast, where a north-south thickness orientation dominates. Toward the southwest, the boundary between blocky/thinning- and thickening/cleaning-upward patterns is considerably more paleolandward than is the case with underlying parasequences.
Parasequence F (Figs. 5, 9, 10, and 15) is 10-25ft. thick. A gross interval isopach map reveals a dominant northwesterly orientation. Over most of the mapped area, this interval exhibits a /thickening/cleaning-upward log pattern except in the southwest corner where the blocky/thinning-upward log pattern prevails. The boundary between blocky/thinning- and thickening/cleaning-upward patterns is slightly more paleolandward than is the case for parasequence E.
Parasequence G (Figs. 5, 9, 10, and 16) is 15-25ft. thick. A gross interval isopach map shows an overall northwesterly thickness orientation across the entire area. Well logs of this interval exhibit a thickening/cleaning-upward log pattern over the entire area.
The base of Parasequence B is a sequence boundary (Figs. 6a and 10). This boundary is sharp-based toward the west and southwest where there has been erosion into underlying shelf shale, but is gradational and conformable toward the east and northeast (Fig. 11). Maximum lowering of relative sea level at this time resulted in the farthest seaward extent of any of the six parasequences which overlie the boundary. Each succeeding parasequence records a period of increase in rate of rise of relative sea level, followed by relative stillstand (i.e. reduction in rate of sea level rise or rate of sediment supply exceeded/equaled the rise of sea level; Kamola and VanWagoner, 1995) (Fig. 11). The backstepping nature of Parasequences B-G, as determined by successively paleolandward positions of boundaries between the blocky/thinning- and thickening/cleaning-upward strata (compare Figs. 11, 12, 13, 14, 15, and 16) indicate the parasequences record periodic stillstands within an overall transgressive systems tract. (Fig. 11).
Most of the parasequences, particularly C and D (Figs. 12 and 13), show gross interval isopach trends and corresponding well log patterns which suggest two distinctly different depositional geometries. Upper shoreface strata in the paleolandward direction exhibit complex geometries and northeasterly to northwesterly trends. Lower shoreface/offshore strata in the paleoseaward direction are characterized by less complexity and more uniform northwesterly trend. These trends reflect variations in marine reworking.
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