John C. Hopkins

Channel-fill Deposits Formed by Aggradation in Deeply Scoured, Superimposed Distributaries of the Lower Kootenai Formation (Cretaceous)

ABSTRACT Three large channels of the lower Kootenai Formation are exposed in the walls of the Missouri River valley east of Great Falls, Montana. Although the channels occur in two different stratigraphic units, they have several features in common. Each channel is contained within crevasse and bay-fill sequences, but the contacts between channel-fill deposits and laterally adjacent strata are erosional. The channels have a broad U-shape, range up to 300 m wide and 35 m deep, and exhibit a distinctive style of fill. Channel-filling occurred in increments by accretion from the bottom up and sides in, to form a concave layering which is more or less symmetrical about the axis of each channel. Lithology of the fill of each channel is quite different, however, and ranges from mudstone, to interbed ed sandstone and mudstone, to sandstone. The channels are interpreted as superimposed distributaries formed by avulsion when the locus of sedimentation shifted from one lobe to another. The lithology of the channel-fill deposits appears to be a function of the abandonment rate. A mudstone-filled channel results where abandonment is rapid, as is the case with upstream diversion of a trunk river system. Sandstone and mixed sandstone-mudstone fills predominate where a distributary is progressively abandoned, for example, where the discharge is diverted into an alternately favored distributary. Superimposed channels are difficult to map in the subsurface by geologic means. They cut across the trend of adjacent facies, and so their presence cannot be predicted from analysis of the containing strata.

Paleozoic stromatactis and zebra carbonate mud-mounds: Global abundance and paleogeographic distribution

Carbonate mud-mounds with zebra and stromatactis structures are present in every Paleozoic system and series, but are more common in Devonian and Carboniferous deposits, reaching their acme in Mississippian System (lower Carboniferous) rocks. Global distributions illustrate that mud-mounds spanned the planet ranging from tropical to polar circles. Such a wide latitudinal span signifies that they not only grew in and occupied warm depositional environments, but also in settings where oceanic waters were cold and seasonally light limited. Moreover, their proliferation during the Devonian and Carboniferous was at a time when planet-wide climatic ice-house conditions are thought to have prevailed. Mud-mounds, therefore, may also be products of cool and cold-water carbonate sedimentation.

Traps associated with paleovalleys and interfluves in an unconformity bounded sequence; Lower Cretaceous Glauconitic Member, southern Alberta, Canada

The Glauconitic member in Badger, Little Bow, Retlaw, and Turin fields is an unconformity bounded sequence that formed on an ancient coastal plain in response to relative sea level fluctuations. The member consists of valley-fill and inter-valley strata. Valley-fill sandstone bodies are thick elongate pods that formed from inner estuarine bars when sedimentation was laterally confined between valley margins. Inter-valley sandstone bodies are thin discontinuous sheets that accumulated during highstands when outer estuarine embayments covered interfluvial areas adjacent to associated valleys. Numerous oil pools are stratigraphically trapped within quartzose sandstones in valley-fill and inter-valley strata of the Glauconitic member in the study area. Common updip seals for these reservoirs are (1) intra-sequence facies changes from sandstone to shale, and (2) low-permeability lithic sandstones that fill the cross-cutting paleovalleys of a younger sequence. Traps associated with many valley-fill pools are enhanced by differential-compaction anticlines. Several oil pools in the study area are hosted by discrete quartzose sandstone bodies that lie beneath a valley filled with low-permeability lithic sandstone. These quartzose sandstone bodies are interpreted to be remnants of older Glauconitic deposits that escaped erosion when a younger valley incised into, and followed the trend of, one or more older Glauconitic valleys.

Tectonic wedging beneath the Rocky Mountain foreland basin, Alberta, Canada

High-quality seismic reflection data reveal the geometry of a blind, thinly tapered wedge of allochthonous rocks inserted into autochthonous foreland basin strata for >8 km east of the previously recognized deformation front (triangle zone) of the Canadian Rocky Mountain foothills west of Calgary, Alberta. Upper and lower detachment surfaces have been identified as boundaries between continuous and discontinuous reflection patterns over the length of the wedge. Coherent reflections above the upper and below the lower detachment show that strata outside the wedge are essentially undeformed. The upper detachment is parallel to bedding for at least 7 km, with a dip that decreases gradually from west to east. At its distal limit, the upper detachment lies 350 m above the lower detachment and does not merge with it. The internal reflection geometry of the wedge changes with position. We interpret the thickest part of the wedge to be dominated by thrust slices up to 500 m thick, whereas the toe of the wedge is folded and faulted internally.

Characterization of Reservoir Lithologies within Subunconformity Pools: Pekisko Formation, Medicine River Field, Alberta, Canada

The Pekisko Formation in western Canada is a third-order sequence comprised of an open-marine grainstone through restricted carbonate mudstone succession. Truncation of the Pekisko along an unconformity edge prior to the Jurassic and several periods of incision from Early Jurassic-Early Cretaceous have formed an intricately sculptured subcrop belt. In the Medicine River field, oil is trapped in discrete pools close to the unconformity edge. Reservoir facies comprise three lithologies. Medium-crystalline dolostone with intercrystalline and vuggy porosity forms the reservoir in an elongate dolostone body that has replaced grainstone. Fine-crystalline dolostone is facies selective within a lime-mudstone unit in which the common reservoir rock is bioturbated dolomudstone with intercrystalline porosity. Grainstones have secondary porosity formed though leaching of microcrystalline calcite allochems and intergranular cements. Reservoir quality is assessed from consideration of orthogonal permeability values (Kmax = maximum horizontal permeability, K90 =horizontal permeability at 90° to the maximum, and Kv = vertical permeability) obtained from conventional whole-core analyses. Sedimentary lamination primarily affects the Kv, and the presence of fine-grained lamination dictates that Kv 3K90 and Kv > K90. From this relationship, the fracture density in all three lithologies appears to be similar. Low and variable porosity and permeability values in grainstones indicate that the fracture system is poorly connected to the matrix. Despite leaching and fracturing at the unconformity edge, reservoir distribution closely follows patterns of facies that were susceptible to dolomitization. Future exploration potential lies in fine-crystalline dolostone bodies that may form stratigraphic traps downdip from the unconformity edge.

Contemporaneous Subsidence and Fluvial Channel Sedimentation: Upper Mannville C Pool, Berry Field, Lower Cretaceous of Alberta

Upper Mannville C pool is a small oil pool which produces from Lower Cretaceous fluvial sandstones in Berry field, southeastern Alberta. Ultimate oil reserves of the pool are 5.8 million m3 of oil contained within a sandstone body about 7.5 km long and 2 km wide. The pool is unusual for its occurrence on the flank of a local structural high, within a large, regional structural depression. Sandstone distribution, rather than present-day structure, dictates the occurrence of hydrocarbons, yet the distribution of sandstones was controlled by structure. A fluvial channel was diverted into an area of synsedimentary faulting or downwarping, which then controlled the depositional form of an asymmetric wedge of channel, levee, and crevasse-splay sands. The most likely cause of subsidence was differential salt solution. Regionally, the Berry area lies along the margin of several Devonian salt formations which are known to have undergone partial solution. Lateral migration of the locus of salt solution (and hence, subsidence) through time produced both the facies and structural configurations observed in this oil pool.

Sedimentology of Quartzose Sandstones of Lower Mannville and Associated Units, Medicine River Area, Central Alberta

ABSTRACT Quartzose sandstones which overlie Lower Jurassic strata in the Medicine River area can be divided into three different units, each separated by an unconformity. The two lower units (J2 and J3) fill a deep valley cut into older strata; the upper unit (Ellerslie) covers the area. Uniform, porous, fine, supermature quartzarenite up to 50 m thick comprises the J2, the oldest of the three quartzose sandstones units. The high degree of mineralogic and textural maturity and associated small-scale cross stratification suggest that the sand was transported to a local watershed by eolian processes, then concentrated in the valley by fluvial action. Sediments of the J3 unit are immature and submature, kaolinitic chertarenites and sublitharenites derived by humid weathering of Paleozoic carbonates. The unit as a whole coarsens upward, indicating valley-fill by progradation into a standing body of water. Ellerslie sediments are principally mature and supermature quartzarenites and interbedded mudstones whose constituents were derived from the Shield to the east. Extensive bioturbation of sands, the presence of limestone beds and, toward the top of the formation, glauconite pellets, suggest that deposition took place in a brackish-to-marine standing body of water. The three quartzose sandstone units are not unique to the Medicine River area and can be compared with similar units elsewhere in western Canada. J2 and J3 sandstones are the respective lithologic counterparts of the Roseray and Success Formations of southwestern Saskatchewan. Ellerslie sediments are the marine equivalents of widespread continental lower Mannville deposits in eastern Alberta and Saskatchewan.

Reservoir Sandstone Bodies in Estuarine Valley Fill: Lower Cretaceous Glauconitic Member, Little Bow Field, Alberta, Canada

Over 50% of the established oil reserves (1.325 million m3) of the Little Bow field in southern Alberta are stratigraphically trapped within upper Mannville G, U, and W pools. These pools are hosted by three parallel, elongate sandstone bodies within an estuarine valley fill, which lies at the base of the Lower Cretaceous Glauconitic member. The paleovalley is 2.0-2.5 km wide, 4-30 m thick, and has been mapped over a length of 55 km. The three reservoir sandstone bodies are 3-4 km long, 300-500 m wide, and up to 22 m thick. Reservoir pressure histories and original fluid contacts indicate hydrodynamic isolation of the three pools. Examination of well logs and seismic sections indicates the three sandstone bodies are separated by 100 to 200-m wide shale units. T ese shale units are interpreted as the fills of mud-prone estuarine channels. The sandstone bodies of G, U, and W pools are interpreted as deposits of longitudinal sand bars that lay between the muddy channels in a configuration similar to that found within the modern Gironde estuary. The data and depositional interpretation presented in this paper provide new information that should help improve modeling of reservoirs within estuarine valley fills of the Glauconitic member.

Fabric alteration and isotopic signature of carbonates in a recharge area of the Madison Aquifer, Montana, USA

Abstract The Mississippian Mission Canyon Formation (Madison Group) is a shallow-water carbonate unit that has intermittently acted as an aquifer over geologic time. Outcrops in a modern recharge area provide physical evidence of conduit flow in the form of paleocaves, bedding plane conduits, and solution channels. Principal alterations of the rock matrix are microspar neomorphism, leaching, and dedolomitization of carbonate mudstones. Petrographic and isotopic data indicate that the modern aquifer is equilibrating with ground water and over printing characteristics that may have been inherited from paleoaquifers.

Waterflood response of reservoirs in an estuarine valley fill; upper Mannville G, U, and W pools, Little Bow Field, Alberta, Canada

Lower Cretaceous upper Mannville G, U, and W pools in Little Bow field are hosted by separate, parallel, elongate estuarine sandstone bodies within an incised valley fill. Each sandstone body is 3-4 km long, 300-500 m wide, and up to 22 m thick. G pool was discovered in 1972 and placed on primary production; oil production declined gradually and was accompanied by modestly increasing gas-to-oil ratios (GOR) and water-to-oil ratios (WOR). U and W pools were discovered in 1982 and 1983, respectively, and were produced by primary methods until initiation of waterflooding in 1985. Response to waterflooding these two pools has been a rise, then decline, in the GOR, followed by rapidly rising WOR, which is currently up to 10:1 in wells adjacent to water injectors. Production re ponse indicates control by mesoscale and microscale reservoir heterogeneities. Mesoscale heterogeneities include permeable sandstone beds several meters thick, that are continuous between adjacent wells, and stochastic shale beds up to 80 cm thick, which are not yet correlatable between wells. Rapid breakthrough of water occurred in producing wells adjacent to injectors due to channeling in thick permeable sandstone beds between shale beds. Microscale heterogeneities are principally mineralogically segregated laminae of cross-stratified sandstones. Permeability values from cores indicate variations of one order of magnitude among laminae. It appears that water passes preferentially through low-permeability sandstone laminae due to the higher mobility of water compared to oil.