The Kipnik Formation, named herein, is defined from the Late Hauterivian to Barremian Kugmallit Trough in the Mackenzie Delta, Arctic Canada. The trough was formed by extensional tectonics associated with the opening of the Canada Basin and was infilled by kilometre-thick accumulations of clastic sediments. Samples from the Kugmallit Trough are known only from two exploration wells – Shell Kipnik O-20 and Gulf Mobil Ogruknang M-31. Examination of palynomorphs, foraminifera, and ostracods from cuttings of these wells, integrated with regional subsurface and outcrop correlations, indicated that the existing subsurface stratigraphic interpretations [Upper Jurassic to Barremian] of the Kugmallit Trough were in need of revision. The revised Upper Hauterivian–Barremian succession thus consists of the Siku, Kipnik (new), and Mount Goodenough formations. The Siku and Kipnik formations are known only from the subsurface, but the Mt. Goodenough Formation is widespread and was deposited over a regional unconformity. The Siku to Kipnik deposition is a large-scale transgressive-regressive succession that represents deposition during a period of initial subsidence (transgression) followed by uplift and erosion (regression). The shale dominant Siku Formation contains a distinctive unnamed foraminiferal assemblage that consists of agglutinated species typical of offshore or deeper water. Ostracods of the Siku Formation are contained in the informal Galliaecytheridea postsinuata zone, which is confined to the Siku Formation. Ostracods of the G. postsinuata zone suggest shelf environments. The Kipnik Formation is sand-dominant with thin beds of shale, siltstone and coal. Agglutinated foraminifera occur sparsely because of coarse, rapid sedimentation. Inner shelf environments are suggested by the foraminifera. The lower half of the Mount Goodenough Formation is shale-dominant and the upper half consists of intercalated shale and sandstone. Foraminifera and ostracods occur abundantly in the Mt. Goodenough subsurface and outcrops of the Richardson Mountains. The foraminiferal Convallina mcneili Zone of Barremian age occurs in the Mt. -Goodenough Formation and its composition of agglutinated and calcareous benthic foraminifera suggests outer shelf or deeper environments. Ostracods of the Mt. Goodenough Formation in subsurface and outcrop are assigned to the informal Clithrocytheridea spp. zone. Ostracods suggest an outer shelf or deeper-water environment. Palynomorphs indicate that the Siku Formation is Late Hauterivian, the Kipnik Formation is probably latest Hauterivian to Early Barremian, and the Mount Goodenough Formation is Barremian.
Lithostratigraphic correlation and mapping of formations and units within the Upper Elk Point subgroup provide updated information on their extent and distribution within the province of Alberta. Together with detailed bed-scale evaporite mapping of three evaporite minerals — halite, anhydrite, and gypsum — within evaporitic successions for net-thickness maps, these data allow new representations of the paleogeography of these units across the province. Paleogeographic maps of the Keg River, Prairie Evaporite and Muskeg formations reveal new details on the location of the La Crete sub-basin in northern Alberta, and the distribution and nature of Keg River Formation buildups and the overlying evaporite strata within this depositional realm. Net-evaporite mapping gives a robust picture of the distribution of Upper Elk Point subgroup evaporites, and allows for a detailed characterization of heterogeneities, halite dissolution, and sulphate karstification. Mapping of gypsum reveals that rehydration of anhydrite to gypsum (gypsification) through meteoric inflow is most pronounced within the La Crete sub-basin in northeastern Alberta, particularly where thick anhydrite deposits are associated with interbuildup basinal areas east of the Prairie Evaporite halite dissolution scarp. This association provides an explanation for the location of where active gypsification, dissolution and associated karstification is occurring and where it can be expected to occur. The process of gypsification, and ultimately sulphate dissolution, is requisite for the formation of porous dedolomite zones within the carbonates of the Prairie Evaporite Formation. Dedolomitized beds are recognized as aquifer units that are known to have contributed to Devonian-sourced, high-salinity water inflows to mine pits in the mineable oil sands area. Evidence is provided for a top-down advancement of halite and sulphate dissolution in all evaporites in northeastern Alberta. Circular, chain-like karst lakes are likely surficial expressions of the meteoric conduits for top-down karstification of sulphates east of the Prairie Evaporite halite dissolution scarp, similar to that observed in the well-documented sulphate karst district of Wood Buffalo National Park.
Quantitative palynostratigraphy of the Middle to Upper Jurassic strata of the Sverdrup Basin is used to calibrate palynomorph signatures to Boreal ammonite occurrences to improve high-latitude chronostratigraphy and supplement a paucity of Upper Jurassic paleoclimatic data in Arctic Canada. Terrestrial palynological signatures of Middle and Upper Jurassic strata of the Sverdrup Basin, Arctic Canada, are assessed with associated Boreal ammonite occurrences to generate a multi-taxon biostratigraphic scheme to improve inter- and intra-basin correlations. Palynological analyses of two stratigraphic sections exposed on Axel Heiberg Island, Arctic Canada, generated a succession of six informal palynoassemblages related to closely co-occurring ammonite assemblages. The ammonite assemblages are a result of both endemic evolution of Boreal lineages within this relatively isolated Arctic basin (i.e. Middle and Upper Jurassic Cardioceratidae) and replacement at the superfamily level due to periodic incursions of southerly faunas into the basin. These migrations imply periods of connectivity with more southern basins that may be related to transgressive flooding, tectonic opening of north-south seaways and associated climate variation. Evidence for regional climate variation is supported by the taxonomic composition of terrestrial palynomorph groupings. Changes in the relative abundance of ecologically important spore and pollen taxa are consistent with a climatic shift from warm-temperate in the Late Toarcian-Early Bajocian to more arid and cooler conditions during the Middle Jurassic. Communities of Cupressaceae-Taxaceae and parent plants of Laricoidites magnus with an understory of ferns characterized the Late lbarcian-Early Bajocian, whereas the cooler conditions of the Middle Jurassic promoted expansion of coniferous plant communities in the hinterland. The latest Jurassic was characterized by the return of temperate conditions and Cupressaceae-Taxaceae conifer communities.
The Canol Formation is only 24.5 m thick at its historic type section at Powell Creek, northern Mackenzie Mountains, whereas in the off-bank sections of the Mackenzie Plain subsurface, where it is considered a high-quality shale hydrocarbon prospect, it thickens to 60-120 m. This paper reviews available lithological and conodont biostratigraphic information from the type section, discusses choices of contacts and subdivisions, and explores the limits of regional correlation using gamma spectrometry proxies. We position the base of the Canol Formation at the top of the lower resistant unit of the "allochthonous limestone beds", the thick offreef debris package present in this outcrop but absent in other well-known Canol sections. "The base of the formation can be of a latest Givetian age as suggested by the norrisi zone conodont fauna from the "allochthonous limestone". The top of the Canol Formation is placed at the base of a distinct, 2.1 m thick horizon with concretionary carbonate beds within the thick shale transition between the Canol and the Imperial formations. Limestone nodules from this horizon produced a conodont fauna that can occur in the jamieae to Upper rhenana zones (Frasnian zones 11-12) thereby suggesting a middle to earliest Late Frasnian age for the Canol top. The cross-section tying several outcrop and well sections across the regional facies zonation reveals that the Dodo Canyon Member, a unit erected in thick off-bank Canol sections, is traceable at Powell Creek. In this correlation, the Vermillion Creek Member, which is the lower portion of the Canol Formation in thick off-bank sections, finds its counterpart in the allochthonous limestone beds sensu MacKenzie (1970). This cross-section is the first correlation of the Canol stratotype at member level available in published sources. Thinness of the Canol Formation at Powell Creek, as well as its location in the carbonate bank toe-of-slope setting, are factors impairing its reference value and calling for more representative sections to act as reference sections and constitute a composite-stratotype for the Canol Formation.
The Campanian Belly River Group (BRG) is a nonmarine elastic cycle in the Western Canada Sedimentary Basin. In southern and central Alberta, the BRG has been subdivided in ascending order into the Foremost, Oldman, and Dinosaur Park formations based on distinctive lithologic, petrographic, and geometric characteristics. Regional surface and subsurface correlation of the BRG reveals the three formations are discernable in southwestern Saskatchewan. The BRG and its associated formations are formally recognized for the first time in Saskatchewan with facies, depositional environments, and a stratigraphic framework interpreted to provide a concise treatment of the Group in southwestern Saskatchewan. A new lithostratigraphic unit within the uppermost Dinosaur Park Formation is recognized based on laterally extensive barrier island, lagoon, and estuary basin deposits. The Neekaneet Member is established as a means to aide in discussing the transition from nonmarine elastics of the BRG to marine shales of the overlying Bearpaw Formation.
The Series 2 and Miaolingian (Lower and Middle) Cambrian succession in the Colville Hills region, Northwest Territories consists (ascending order) of the Mount Clark, Mount Cap and Saline River formations, all of which were deposited in an epicontinental basin, herein named the Colville Basin. The Mount Clark Formation is sandstone-dominated with a shale package near the middle of the unit. The Mount Cap Formation is divided into an informal lower member (shale with dolostone and sandy dolostone beds) and upper member (shale with mixed limestone and dolostone). A high-resolution study of all ten available industry cores used process sedimentology and ichnology to delineate fifteen lithofacies in the Mount Clark and Mount Cap formations: six sandstone dominated; three mudstone dominated; four of heterolithic sandstone and mudstone; and one each of dolostone and glaucony. The facies can be grouped into four facies associations (FAs). FA1 consists of sandstone with wellpreserved, wave-formed sedimentary structures, recording deposition in storminfluenced shoreface settings. Bioturbation is minimal to absent, suggesting that wave energy exerted a major stress on burrowing organisms. FA2 consists of sandstone in which bioturbation is extensively developed and trace-fossil diversity is high, including local development of Skolithos "pipe-rock", but with poor preservation of physical sedimentary structures. Deposition is interpreted to have been in fairweather shoreface settings. FA3 contains varying amounts of mudstone, generally recording deposition in more distal environments. Absence of bioturbation in some successions of FA3 may reflect stresses due to seawater chemistry, possibly low dissolved oxygen contents. FA4 encompasses bioturbated, locally sandy carbonates deposited in relatively shallow water. Three transgressive-regressive (T-R) sequences in the Mount Clark and lower Mount Cap Formation can be traced over an area of approximately 300,000 km(2). The base of the Cambrian is a regional unconformity and sequence boundary. T-R Sequence 1 is entirely within the Mount Clark Formation. Based on archival trilobite collections, it is of Bonnia-Olenellus Zone age or older, but unlikely to be older than early Cambrian. T-R Sequence 2 spans the Mount Clark-Mount Cap Formation contact and has Bonnia-Olenellus Zone trilobites in its uppermost part, though its base may be older. T-R Sequence 3 is entirely within the Mount Cap Formation and its top corresponds to the boundary between the informal lower and upper members of the formation. The base of the sequence is within the Bonnie-Olenellus Zone, its maximum flooding surface is within the PlagiuraPoliella Zone, and the lowest trilobites of the Glossopleura Zone are present just above its top. The upper member of the Mount Cap Formation could not be subdivided into sequences, but contains a prominent maximum transgressive surface near its base and is capped by an unconformity at the base of the Saline River Formation. The upper member is mainly of Glossopleura Zone age. As noted by previous workers, syndepositional extension during deposition of the Mount Cap Formation was pronounced in the Mackenzie Trough but more subdued beneath the Colville Hills.
The great Late Cenomanian transgression of the Greenhorn Sea is recorded in western Alberta and adjacacent British Columbia by shallow-marine deposits of the lower Kaskapau Formation that blanket underlying deltaic deposits of the Dunvegan Formation. Overlying the basal 'A -X unit' of the Kaskapau are the allostratigraphically-defined Doe Creek and Pouce Coupe units, each of which forms a SW-thickening wedge up to about 100 m thick. The Doe Creek and Pouce Coupe units comprise, respectively, 10 and 8 allomembers, each defined by a marine flooding surface. Doe Creek facies are organized in upward-shoaling successions less than about 10 m thick. Shallow-water marginal-marine facies prevail in the west and north, with transgressive mudstones overlain by thin, sharp-based shoreface sandstones that, in places, contain roots and dinosaur tracks. In some instances, marine mudstone grades directly upward into a rooted, muddy paleosol. Mapped regionally, these facies define a broad, shallow, SE-facing embayment approximately 200 x 200 km. 'Trace and molluscan fossils suggest that salinity was below normal marine level, and benthic fauna were, at times, stressed by high turbidity and sedimentation rate. The central part of the embayment is dominated by offshore mudstone that encloses isolated, sharp-based lenticular bodies of clean sandstone up to 9 m thick. Sandstones are ovoid to elongate in plan view, and commonly extend many tens of km N-S or NE-SW. These sandstones are interpreted as top-truncated lowstand delta and strandplain deposits that became isolated on the outer ramp following marine transgression. The sharp base of most shoreface sandstones implies that deposition took place during relative sea-level fall. All Doe Creek allomembers thicken into the foredeep, with the tectonically-flexed surface inclined to the SW. Palaeogeographic maps show, however, that lowstand shorelines were oriented approximately NE-SW, and that the depositional surface sloped to the SE. This geometric relationship implies that, despite tectonic subsidence increasing to the SW, the rate of sediment supply and efficiency of redistribution were able to maintain a wave-graded surface sloping to the SE. Because shorelines migrated across, rather than up and down tectonic dip, allomembers are interpreted to have been controlled primarily by high-rate, high-frequency eustatic cycles, rather than by tectonic movements. The Pouce Coupe unit represents a complete reversal of basin paleogeography relative to the Doe Creek. Pouce Coupe shoreface sandstones are stacked in a linear belt just west of 120 degrees W and grade westward into offshore muddy facies that thicken to the WSW. The entire unit is truncated eastward by the K1 unconformity and is absent over most of Alberta. It is postulated that the Proterozoic Kiskatinaw domain acted as a crustal weak zone, forming a hinge that defined the eastern margin of the Pouce Coupe flexural depocentre.
The practice of building analog models and training images from outcrop exposures is an important tool in better predicting subsurface facies distribution in the petroleum industry. As with subsurface data, however, incomplete information and data bias can lead to inaccurate characterization of outcrop geology at multiple scales. Cretaceous fluvial strata of Wyoming offers excellent exposure of two systems - the sand-rich and highly amalgamated Trail Member of the Ericson Sandstone and the sand-poor, isolated channels of the Dry Hollow Member of the Frontier Formation. For each system, multiple outcrops were characterized through the traditional means of stratigraphic column measurement, as well as through photogrammetric survey acquisition and interpretation. We saw in both studies that, despite an effort to measure sections that were representative of the entire outcrop, measured sections consistently overestimated the reservoir proportions. Ten measured sections within the Trail Member show a Net-toGross (NTG) ranging from 50-80% sandstone, with an average of 72%. A more complete spatial characterization of the entire outcrop through photogrammetric interpretation suggests a much lower NTG of 53%. Similarly, for the Dry Hollow Member fluvial strata, measured sections show NTG ranges of 8-50% with an average of 37% sandstone, while the photogrammetric model shows a NTG of only 16%. These differences are significant and lead to very different reservoir models. Further, the assumption is commonly made that the outcrop, if well characterized, is representative of the formation at a larger scale. Models of the Dry Hollow Member at Cumberland Gap show that this is a tenuous assumption and can lead to models that are not representative of the system. Outcrops of the Dry Hollow are sparse and often discontinuous, and extrapolation of calculated facies proportions between two well-exposed outcrops at Cumberland Gap led to significant placement of sands between the outcrops, where the lack of exposure leads to a lack of control data in the model. This resulted in increased reservoir connectivity that is not representative of the system, and shows that even on a sub-kilometer scale, the extrapolation of detailed, quantitative facies proportions can be inappropriate, and if done blindly can lead to an inaccurate characterization of the system. Through detailed characterization of the Trail and Dry Hollow fluvial systems, it is shown that building quantitative geomodels from outcrop exposures, even using modern techniques such as photogrammetric analysis, can be subject to significant bias and mischaracterization at multiple scales and for multiple reasons if care is not taken.
The Upper Triassic (Carnian) Charlie Lake Formation in northeastern British Columbia, Canada is a heterolithic mixed siliciclastic-carbonate succession of dolomite dolomitic siltstone, well-sorted sandstone, bioclastic sandstone, and anhydrite. This succession is unusual in that it records the only significant non-marine interval in the Triassic of the Western Canadian Sedimentary Basin. In the Kobes-Altares-Blueberry area, moderately abundant core are available through the middle members of the Charlie Lake Formation. All available core were analyzed in order to interpret the depositional setting of constituent members and identify reservoir quality lithofacies and horizons. The middle Charlie Lake Formation in the Kobes-Altares-Blueberry area was deposited in an arid coastal setting. Sixteen lithofacies were identified and placed in three lithofacies assemblages and the latter characterize three depositional settings: coastal salina/playa, aeolian dune, and proximal marine ramps. The coastal salina/playa lithofacies assemblage is heterolithic, and is dominated by dolomitic and anhydritic facies. Depositional subenvironments include intertidal flat, ephemeral lagoon, shore proximal ephemeral lake, sabkha and supratidal paleosol. Potential reservoir lithofacies in this assemblage includes peloidal to stromatolitic dolomitic siltstone deposited in lagoonal and intertidal flat settings. The aeolian dune lithofacies assemblage preserves the movement of small, shore-proximal dunes and associated interdune successions. Well-sorted, fine-grained sandstone beds comprise the best reservoir lithofacies in this assemblage. The proximal marine ramp facies assemblage records several marine transgressions that punctuate the dominantly nonmarinc/marginal marine Charlie Lake Formation. Bioclastic packstone, bioclastic sandstone and peloidal dolomitic siltstone to sandstone beds, all exhibit good porosity and are potential hydrocarbon reservoir units within this lithofacies assemblage.
New methods are required to support unconventional reservoir uncertainty modeling. Unconventional plays add additional complexity with greater uncertainty in reservoir measures (e.g. unreliable permeability measures in low permeability rock) and weakened relationships between measurable reservoir properties and production results (production mechanisms may not be well understood). As a result, unconventional plays are often referred to as "statistical plays", suggesting the reliance on statistical characterization of production spatial distributions. Various methods have been proposed for probabilistic modeling with statistical plays. Regardless of workflow, it is critical to account for spatial context, including: production spatial continuity; local conditioning from well-based production; local secondary information; and boundaries of the area of interest. Methods that are insensitive to spatial context are unreliable for decision-making. To these methods, the uncertainty in the aggregate production over the next set of wells is the same regardless of their respective locations. Even in unconventional reservoirs, the spatial context still matters. There are theoretical methods to explore the uncertainty in the production rates of wells within a pad and of the aggregate production of wells over pads within a development block. Yet, the most flexible methods are based on empirical model resampling. These methods extract multiple samples from actual reservoir models to simulate the drilling strategy over multiple realizations and scenarios of the subsurface uncertainty model, an ensemble of possible models. These methods integrate all available information sources while further leveraging the uncertainty model that is routinely built for reservoir forecasting. The aim of this paper is to demonstrate the ability of the resampling method to answer reservoir development questions, such as: how much variability in well production is predicted between wells in a single pad; how much variability in well aggregate production is predicted between pads; how much information does the first well's production provide about the total pad production; and when is it best to abandon a pad? Knowing the answer to these questions improves reservoir development decision-making. This paper advocates for new data analytics, geostatistical methods and workflows to support the best use of geoscience and engineering practice.
Despite of high cost, seismic data have routinely been collected for oil sands development. While these data can be extremely valuable for the whole array of applications, including reservoir characterization, they are still, for the most part, largely underutilized. The main reason for the limited use of seismic in oil sands is the subtle sandstone-shale elastic differences and the lack of practical methods and techniques that make efficient use of the seismic information and mimic geophysical interpretation. In this paper, we present two novel approaches to deal with this challenge. The first approach works with 3D post-stack inverted seismic acoustic impedance data to derive facies trend models based on the local analysis of impedance geobodies. Too much impedance overlap between different facies that is observed globally and prevents efficient facies differentiation is resolved by extracting and analyzing objects from the 3D seismic volume that have local impedance contrasts. The second approach presents an optimization of empirical differential compaction calculations for the use in probabilistic 2D mapping of continuous/SAGD-able pay and its quality characteristics. Both approaches are shown to be straightforward and easy to implement into any software of choice. They are proven to lead to significant improvements in oil sands reservoir characterization based on a study of the Christina Lake and Kirby East leases of Cenovus Energy Inc.
Geostatistical facies modeling algorithms are used in reservoir modeling workflows to create geological models which improve the predictive power of the flow simulation models. In heterogeneous reservoirs, it is of key importance to not only apply statistical techniques, but also incorporate prior geological knowledge. Fluvial dominated deltaic deposits can show a high degree of heterogeneity arising from the interaction of stacking of lobate deposits and the continuous erosion and deposition of the distributary channels while building the delta. To simulate these depositional structures, honouring the physical laws of nature, process-based models can be used to generate synthetic deposits. However, such results are driven by physics and therefore cannot be steered to honour exact well data. We address this challenge by integrating physics-driven process-based models with statistical techniques from MPS. Combining these two different methods is MPS relies on discreet geometric patterns. This is addressed by classifying the process-based model results into discreet facies. A major advantage of this integrated technique is the potential to generate multiple MPS training images through simulation of additional process-based model realizations and we also analyze the effect of using one versus multiple process-based models as input. In this work, we show how the best aspects of both process-based models and MPS modeling can be combined to create improved geological models.
A Bayesian Belief Network (BN) has been developed to predict fractures in the subsurface during the early stages of oil and gas exploration. The probability of fractures provides a first-order proxy for spatial variations in fracture intensity at a regional scale. Nodes in the BN, representing geologic variables, were linked in a directed acyclic graph to capture key parameters influencing fracture generation over geologic time. The states of the nodes were defined by expert judgment and conditioned by available datasets. Using regional maps with public data from the Horn River Basin in British Columbia, Canada, predictions for spatial variations in the probability of fractures were generated for the Devonian Muskwa shale. The resulting BN analysis was linked to map-based predictions via a geographic information system. The automated process captures human reasoning and improves this through conditional probability calculations for a complex array of geologic influences. A comparison between inferred high fracture intensities and the locations of wells with high production rates suggests a close correspondence. While several factors could account for variations in production rates from the Muskwa shale, higher fracture densities are a likely influence. The process of constructing and cross-validating the BN supports a consistent approach to predict fracture intensities early in exploration and to prioritize data needed to improve the prediction. As such, BNs provide a mechanism to support alignment within exploration groups. As exploration proceeds, the BN can be used to rapidly update predictions. While the BN does not currently represent time-dependent processes and cannot be applied without adjustment to other regions, it offers a fast and flexible approach for fracture prediction in situations characterized by sparse data.
Paleozoic rocks occur in seven wells in the Hopedale Basin, offshore Labrador (Hopedale E-33, South Hopedale L-39, Tyrk P-100, Gudrid H-55, Roberval K-92, Indian Harbour M-52, and Freydis B-87), where they represent erosional remnants primarily associated with Cretaceous syn-rift half grabens. Previous palynological studies have reported a range of ages for the Paleozoic in these wells, including Ordovician, Devonian, Carboniferous, and undifferentiated Paleozoic. Palynological analyses in the present study confirm that recovery is limited, primarily due to the predominance of lithologies that hinder palynomorph preservation, specifically dolostones. Palynomorphs (predominantly acritarchs and chitinozoans) have been analyzed from new and archived samples from the seven wells and have, for the first time, produced consistent age determinations for the Paleozoic strata in all wells: palynomorph assemblages considered in place indicate a Middle to Late Ordovician age. Thermal maturity was determined from Ordovician acritarchs and leiospheres observed in the Gudrid H-55, Roberval K-92, Indian Harbour M-52, and Freydis B-87 wells, with thermal alteration indices (TAI) ranging from approximately 5+ to 6+ (dry gas). Carboniferous miospores are present from Paleozoic strata in the southern Hopedale Basin wells, where their TM values range from 3+ to 4. These miospores are interpreted as contaminants introduced into the samples via cavings or as drilling mud contamination. The data presented here provides new biostratigraphic constraints for the Paleozoic of the Hopedale Basin (Middle to Late Ordovician), facilitates the correlation of the Paleozoic intervals in these wells, and enables the consideration of Labrador margin Lower Paleozoic strata in regional studies as well as in paleogeographic and paleoenvironmental reconstructions.
This work examines the impact of forward stratigraphic modeling on lithofacies representation in a 3-D volume by integrating data from geological process simulations and actual well logs. The complex depositional architecture and property variation in basin floor fan systems highlights the need for modeling techniques that capture heterogeneity to improve subsurface property prediction away from known data. Geological process simulation enhances our understanding of sediment distribution in a defined 3-D geological system. The workflow involves: 1) replication of depositional patterns in a basinal fan setting using the Geological Process Modeling (GPM) software; 2) lithofacies classification from sediment grain distribution, and turbid water velocity in the geological process model; and 3) generation of lithofacies proportion maps and vertical trends from simulated geobodies, to serve as an additional conditioning parameter in facies modeling. The simulation produced sediment distribution patterns, lobe switching and stacking features that are characteristic in turbidite settings. The impact on lithofacies representation was assessed using a Net-to-Gross analysis of actual and process-based sedimentological logs, with a good match found between process-based and actual-data facies models in several well locations. These results lead us to suggest that the geological process simulation approach can improve inter-well facies property prediction in a basin floor fan setting.
The late Cenomanian Kaskapau Formation records transgression of the Greenhorn Sea over deltaic strata of the Dunvegan Formation. However, stratigraphic and paleogeographic details of the initial stages of this profound reorganization of basin geography have not previously been determined. The basal Kaskapau strata above the Dunvegan Formation are assigned to the informal allostratigraphic 'A-X unit'. Subsurface and outcrop correlation shows that five regionally-mappable allomembers, bounded by marine flooding surfaces, can be mapped within the A-X unit which, as a whole, forms a wedge that thickens south-westward from < 5 to 60 m over approximately 300 km. Within each allomember, three main depositional environments can be distinguished. Sandy heterolithic facies in the north and west form few-metre scale, upward-shoaling successions that contain abundant brackish-water molluscs and are capped by paleosols and dinosaur-trampled surfaces. These rocks represent river-dominated deltas that prograded into a low-energy embayment, about 200 x 200 km, and open to the SE. The central part of the embayment accumulated mud-dominated heterolithic successions with a restricted fauna of lingulid brachiopods and inoceramids, suggestive of turbid, low-energy and reduced salinity conditions. The muddy facies enclose isolated units of well-sorted fine- to very fine-grained sandstone that form NE-SW elongate bodies up to 170 km long, 50 km wide and 11 m thick. Each sandstone body is interpreted to represent the shoreface of one or more strandplains and/or wave-dominated deltas that developed across the mouth of the embayment at sea-level lowstand. Isopach mapping shows that the A-X unit, as a whole, forms a prismatic wedge, thickest adjacent to the fold and thrust belt in the SW. This geometry indicates that accommodation was created by spatially uniform flexural subsidence in response to a linear tectonic load striking NW-SE. Isopach maps of individual allomembers, however, show that subsidence took place in a more complex pattern, with localized depocentres of approximately 100 km radius that suggest brief periods of subsidence before being superseded by a new depocentre along-strike. This pattern may reflect temporally and spatially discontinuous deformation in the adjacent orogenic wedge. During each relative sea-level cycle, the shoreline oscillated by about 150-200 km in a NW-SE direction. Shoreline movement was perpendicular to the direction that would be expected (i.e. SW-NE), if flexural subsidence had been the dominant control on relative sea-level change. This geometric relationship suggests that allomembers were generated by high-frequency, high rate eustatic changes that were superimposed on a lower rate of flexural subsidence. Isopach maps spanning Dunvegan allomembers C, A+ B, and the Kaskapau A-X unit show a near 90 degrees anticlockwise rotation of isopleths. This abrupt re-orientation of the axis of flexure marks a new phase of subsidence linked to the onset of NE-directed thrusting that was driven by dextral transpression across a restraining bend in the Northern Rocky Mountain Trench fault. This new stress regime was in turn linked to a change in the convergence direction between the North American and Farallon plates.
Recently, most researchers have argued for a deep-water origin for black shale in the Bakken Formation of the Williston basin based on sedimentology, paleontology and geochemistry characteristics that place black, laminated, sparsely fossiliferous, radiolarian, pyritic shale as the seaward lithology in an assemblage that originally transitioned landward into shallow-water facies. The shallow-water interpretation advocated in this paper is based on stratigraphic characteristics that include the absence of strata that represent a landward equivalent to the shale, gradual landward shale pinchouts, the restriction of Bakken strata to basinal areas, the Bakken onlap depositional style, and placement of a major maximum flooding surface near the base of the overlying Lodgepole. Black mud deposition in the Bakken is interpreted to have occurred in a low-relief, semi-enclosed, epeiric-lagoon environment with typical water depths of 0–30 m. It is theorized that a high-rainfall climate caused salinity stratification that produced bottom-water anoxia, which preserved organic material. Perhumid climate conditions (year-round rainfall) fostered thick soils and dense vegetation that limited sediment release and induced mud-dominated, sand-poor deposition. During middle Bakken deposition, an arid to semi-arid climate eliminated the bottom-water anoxic conditions and caused carbonate-siliciclastic deposition. The abrupt vertical transitions from black shale (lower Bakken), to carbonate-siliciclastic lithologies (middle Bakken), to black shale (upper Bakken) were caused by paleoclimate change.
Stratigraphic subdivisions and sequences established for the Lower Triassic Montney Formation based on extensive core control in west-central Alberta have been extended to the northern sector of northeastern British Columbia (NE BC). The paper draws extensively on unpublished data from multiple industry sources. The correlations are based on six (6) cross-sections incorporating 63 wells with a cumulative section length of over 1000 kms. The cross-sections are presented in two formats: proportionally-spaced LAS log sections, included as text figures; equally-spaced raster log sections with extensive labels (including selected core summaries) in an appendix. The Montney in this paper is divided into three unconformity-bounded, third-order sequences. The divisions and their internal subdivisions (from base up) are: the Griesbachian-Dienerian Lower Montney, built up by transgressive and highstand systems tracts; the Smithian Middle Montney, with a basal lower Smithian lowstand wedge, a 'mid' Smithian transgressive systems tract, and a thick mid to upper Smithian highstand systems tract; the Spathian Upper Montney, with a basal lower Spathian lowstand wedge, and an overlying mid to upper Spathian siltstone unit (the former Lower Doig Siltstone of earlier GDGC reports) built up by a basal transgressive systems tract and an upper highstand systems tract. The bounding unconformities for these Montney divisions (from the base up) are: the global first-order Permian-Griesbachian unconformity and sequence boundary (SB), a complex, multi-event, paleokarst boundary correlative with Permo-Triassic mass extinction event/s (base Lower Montney); the global third-order Dienerian-Smithian unconformity and SB (Lower to Middle Montney boundary); the global third-order Smithian-Spathian unconformity and SB (Middle to Upper Montney boundary); and the global second-order Spathian-Anisian unconformity and SB (Upper Montney to Doig boundary). Paleostructure on the Paleozoic unconformity below the Montney is a key control on the thickness of the total Montney, and of thicknesses of internal subdivisions, of facies, and of depositional trends. Upper Devonian Leduc reef and platform margins are the principal controls on Montney paleostructure from the southern Peace River arch area southeastward into south-central Alberta, while reactivated basement faults, including the Dawson Creek Graben Complex (DCGC) and the Hay River Fault Zone (HRFZ), are the major paleostructural controls northward into NE BC. Paleostructure is expressed best by third-order residual structure mapping of the top-Paleozoic unconformity, with three residual structure map-figures incorporating cross-section overlays included in this paper. A paleostructural location summary is included below each well log on the raster cross-sections. In the mid 1990s, preliminary biostratigraphic dating of the Montney in west-central Alberta was based on palynology. Within the last five years or so, new conodont biostratigraphic dating of the Montney in NE BC, from one published source but mainly from data released for this paper by Progress Energy Canada, has provided strong support for the proposed ages of subdivisions in the mid to upper Middle Montney (mid to late Smithian) and throughout the Upper Montney (early, mid, late Spathian). The only apparent discrepancy between conodont age dates and log-based lithostratigraphic picks and correlations in this paper centre around the placement of the Dienerian-Smithian (Lower to Middle Montney) boundary. As all conodont-dated wells lie within or peripheral to the HRFZ or other basement faults, structural movement may be a factor in this apparent discrepancy, but it is more likely due to uncertainties concerning the age range of key conodont species around or across the Dienerian-Smithian boundary, and of some stratigraphic picks. Some of the most intense bioturbation observed by the first author in any Montney cores occurs in the updip, shallower part of the lower Spathian lowstand wedge in westernmost Alberta and NE BC (where it is productive). A similar bioturbated interval also is recognized now in the updip, shallower part of the lower Smithian lowstand wedge in west-central Alberta. Ichnological analysis included in this paper demonstrates that the ichnogenera in both wedges essentially are the same, albeit more robust in the Spathian wedge. Extensive cryptobioturbation in HCS-dominant event beds in the upper Middle Montney enhances reservoir properties. The subdivisions, correlations and nomenclature for the Montney defined in this paper may stir some disagreement, but should serve as a base for an eventual industry-wide nomenclature and subdivision framework for the Montney.
This study presents the first published subsidence analysis during the deposition of the Montney Formation. It was deposited during the Early Triassic (ca. 252.2-245 Ma) in the Western Canadian Sedimentary Basin (WCSB) located along the western margin of the North American craton. Subsidence analyses of six representative wells and two outcrop sections along a proximal to distal transect are presented using a backstripping method integrating recent high-resolution stratigraphic correlations for the Montney Formation. The entire Paleozoic to Cenozoic sedimentary column of the WCSB was backstripped to put the deposition of the Montney Formation into a broader context and provide results regarding the type of subsidence and geodynamic setting for the Montney Formation. The spatial and temporal evolution of the subsidence during the deposition of the Montney Formation indicates that the most likely basin setting is a foreland. The tectonic subsidence during the Triassic is herein interpreted as a combination of the topographic loading of the orogenic wedge (flexure) and the sublithospheric "loading" caused by slab load-driven subsidence (dynamic subsidence). This suggests that the retro-foreland basin setting was associated with an eastward dipping subduction during the deposition of the Montney Fm. Three foreland stages are thus recorded in the whole WCSB, with evidence for: 1) a Late Permian (fore-arc) pro-foreland setting; then 2) a Triassic collisional retro-foreland basin prior to the well-known; 3) Jurassic-Cenozoic collisional retro-foreland.
