Sedimentology
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.
