Ireton Formation Research Articles

Lateral fluid propagation and strike slip fault reactivation related to hydraulic fracturing and induced seismicity in the Duvernay Formation, Fox Creek area, Alberta

SUMMARY Fault shear slip potential is analysed in the area where induced earthquakes (up to 3.9 Mw) occurred in May–June 2015 approximately 30 km south of Fox Creek, Western Canada Sedimentary Basin, Canada. The induced earthquakes were generated by the hydraulic fracturing of the Upper Devonian Duvernay Formation. Interpretation of a 3-D seismic survey and analysis of the ant tracking attribute identifies a linear discontinuity that likely represents a subvertical fault with strike length of 1.4 km, which is aligned with the zone of induced earthquake hypocentres. 1-D–3-D geomechanical modelling is conducted to characterize mechanical rock properties, initial reservoir pressure and stress field. Hydraulic fracture propagation and reservoir pressure buildup simulations are run to analyse lateral fluid pressure diffusion during well treatment. The interaction of natural fractures introduced as Discrete Fracture Network and hydraulic fractures is tested. 3-D poroelastic reservoir geomechanical modelling is completed to simulate slip reactivation of the identified fault zone. The obtained results support that additional pressure buildup of 20 MPa in treatment wells can propagate laterally along hydraulic fractures (and potentially natural fracture network) for about 550 m and reach the fault zone. The increase of fluid pressure by 20 MPa in the fault zone results in dextral slip along the fault, mostly in the interval of the Duvernay and overlying Ireton Formations, corroborating prior focal mechanism results and hypocentral depths. The simulations indicate that lateral transmission of additional fluid pressure from the fracturing stimulation area to the fault zone could happen in a few days after the treatment of lateral wells that is supported by the observed induced earthquakes. This study helps to quantify changes in fluid pressure and stresses that may result in fault shear slip during hydraulic fracturing and predict the potential of induced seismicity connected to hydrocarbon production from the Duvernay Play.

Geologic and resource assessment of the Upper Devonian Grosmont and upper Ireton Formations, central Grosmont shelf complex, Alberta, Canada

The Devonian Grosmont and upper Ireton (GUI) Formations of northeastern Alberta contain between 300 and 574 billion bbl of in situ bitumen. The GUI stratal succession shallows upward and is partitioned into five progradational, shallow marine sequences. Although matrix porosity is highest within intertidal deposits (commonly 20%–30%), matrix permeability is similar for subtidal and intertidal facies (commonly 10–3000 md). Reservoir zones are 29–98 ft (9–30 m) thick and from oldest to youngest include the A1, A2, B, C1, C2, D1, D2, D3, and upper Ireton. Because of truncation beneath the sub-Cretaceous unconformity, the GUI thins from 656 ft (200 m) within the western part of the study area to an erosional edge within the easternmost part. Intertidal facies most commonly underlie the sub-Cretaceous unconformity and are commonly karsted. Karst breccia and associated fractures occur within thin (3–20 ft [1–6 m]) intervals that are parallel to bedding and most abundant in proximity (164–328 ft [50–100 m]) to the unconformity. Within the study area bitumen is concentrated above 475 ft (145 m) above sea level, and in-place bitumen is estimated at 58 billion bbl (9.2 × 106 m3); the greatest in-place bitumen volume (66%) is associated with the D2, D3, and upper Ireton intervals. A proof-of-concept numerical model evaluates the potential effectiveness of steam-assisted gravity drainage enhanced oil recovery within the GUI. The model suggests bitumen recovery as high as 189,000 bbl over a 20-yr period with an injector–producer spacing between 33 and 41 ft (10 and 12 m).

Influence of geological setting on stress released by hydraulic fracture-induced earthquakes

Recent studies of injection-induced seismicity suggest that induced events have lower stress drops than tectonic earthquakes, suggesting stress drop can potentially be used as a discriminant parameter between natural and anthropogenic earthquakes. The main physical mechanisms responsible for low stress drop earthquakes, that is earthquakes that release less stress during failure than the average natural earthquake in a similar tectonic setting, are related to the increase in pore pressure and fault lubrication that result from the presence of fluids on the fault surface. Fluids reduce the effective normal stress applied on a fault allowing it to slip under lower stress conditions. Hough (2014) found that induced earthquakes have a factor of 2 to 10 lower stress drops relative to natural earthquakes, for 11 injection-induced moderate earthquakes in central and eastern United States based on their intensity reports. Sumy et al. (2016) also found a factor of 10 difference between the stress drops of natural and induced earthquakes when studying 87 Mw1.8 to Mw3.5 induced earthquakes in Oklahoma. A few other studies found opposing results. Huang et al. (2016) found the stress drops of 25 small earthquakes in central Arkansas to be comparable to Californian tectonic earthquake stress drops, and Viegas and Abercrombie (2011) also found the stress drops of a sequence of moderate earthquakes in Colorado to be comparable to the stress drops of Californian earthquakes. However, Viegas and Abercrombie (2011) noted that Central and North America intraplate earthquakes are expected to have higher stress drops than plate boundary California earthquakes (Viegas et al., 2010) making the comparison a false equivalency. Small earthquakes recorded during two hydraulic fracturing treatment programmes targeting stratigraphically equivalent formations in two regions of the Western Canadian Sedimentary Basin (Muskwa in Horn River Basin and Duvernay in Fox Creek), show very different stress drop estimates. Earthquakes in the Horn River Basin show lower stress drops by a factor of 10 to 20 than earthquakes in Fox Creek (Duvernay Basin). While the seismic events from both sites are generated during similar injection programsme, we propose that the differences in the observed stress drops are caused by different regional stress characteristics, as reflected by the differences in geology. The Horn River Basin stratigraphy is dominantly horizontal, whereas the Fox Creek region is characterized by the presence of the Leduc Formation reefs. These dolomitized reef complexes cross-cut the Duvernay shale formation, where both the Duvernay and overlying Ireton formations drape over the reef facies (Stoakes, 1980). We propose that earthquakes occurring in higher stressed regions have higher stress drops, as these regions are subjected to higher strain rates and store larger amounts of elastic strain.

Hydrocarbon breaching of a regional aquitard: The Devonian Ireton Formation, Bashaw area, Alberta, Canada

This study is an investigation of aquitard characteristics and hydrocarbon entrapment in the Upper Devonian strata of the Bashaw area of Alberta, Canada. Oil and gas are trapped at two stratigraphic levels, the Leduc-aged Bashaw Reef Complex (D-3) and the Camrose Member–Nisku Formation (D-2), which are separated by a low-permeability aquitard of the Ireton Formation, a marl with variable carbonate content. The Ireton aquitard provides the principal control to cross-formational fluid flow in the area. Over much of the Bashaw Reef Complex, the Ireton aquitard ranges from approximately 25 m (82 ft) thick to less than 1 m (3.3 ft) over paleotopographic highs. Associated with thinning is a change in lithofacies and carbonate content from approximately 50% carbonate in the thicker, more basinal facies to more than 80% carbonate and shallower-water facies over the paleotopographic highs of the Leduc. Early replacement dolomitization, probably by density-driven reflux, generated intercrystal porosity on the order of 3 to 4% in the more carbonate-rich facies, which rendered these parts of the aquitard less effective to later hydrocarbon retention. Subsequent differential compaction of the underlying reef complex led to a raised rim morphology and, in places, to a thickness inversion of the Ireton aquitard, that is, a thicker Ireton aquitard draped over the Leduc raised rims. As a result, petroleum migration occurred primarily along the raised rims and was trapped within the present-day highs that are draped by more than 10 m (32.8 ft) of Ireton aquitard. Where the overlying Ireton aquitard drape is less than 10 m (32.8 ft) in thickness, breaching and remigration from Leduc traps into the overlying Nisku-Camrose traps may have occurred and/or can be suspected. Invariably, the Ireton aquitard was breached where its carbonate content exceeded approximately 80% and where it is less than 4 m (13.1 ft) thick. The critical controls to hydrocarbon breaching are pervasive dolomitization of the Ireton aquitard with high carbonate content, which enhanced porosity and permeability, and aquitard thickness. Many Nisku and Camrose pools are situated either directly above or updip from such breaches.

Numerical modeling of reflux dolomitization in the Grosmont platform complex (Upper Devonian), Western Canada sedimentary basin

The Upper Devonian Grosmont platform in the Western Canada sedimentary basin is a pervasively dolomitized giant heavy-oil reservoir with reserves of 317 billion bbl of bitumen. The principal type of Grosmont platform dolomite formed early and on the basis of stratigraphic and geochemical evidence is interpreted as early diagenetic reflux dolomite. We use a numerical ground-water flow model to investigate the viability of reflux to dolomitize the Grosmont platform. We simulate reflux at four key stages of platform evolution, incorporating the transient effects of changes in platform architecture, rock properties, and the salinity of platform-top waters. The pattern and magnitude of reflux is critically controlled by permeability and the distribution of platform-top brines, which are concentrated up to gypsum saturation. Reflux flow is focused in the relatively permeable carbonates of the Grosmont Formation and is from the platform interior toward the platform margin. The 120-m-thick shales of the Ireton Formation that separate the Grosmont and Cooking Lake formations restrict cross-formational flow and brine transport. During a 100-k.y. period of relative sea level rise and platform-top drowning, brines of reflux origin continue to sink and entrain platform-top waters (latent reflux). Where the intervening aquitards are thin or absent, reefs of the Leduc Formation capture reflux brines from the overlying Grosmont platform and focus cross-formational brine transport. Lateral contrasts in salinity are sufficient to drive a series of free convection cells in the relatively permeable reefs of the Leduc Formation. Computed distributions of fluid flux in conjunction with magnesium mass-balance calculations that incorporate the range of uncertainty, particularly in permeability, support the suggestion that the reflux of gypsum-saturated brines could have formed much if not most of the dolomite in the Grosmont Formation in the 1.6 m.y. available.

Dolomitization of the Leduc Formation (Upper Devonian), Southern Rimbey-Meadowbrook Reef Trend, Alberta

ABSTRACT Upper Devonian Leduc carbonate buildups along the southern part of the Rimbey-Meadowbrook reef trend in Alberta (buried at depths of 2300-3500 m) have been extensively dolomitized. Dolomitization occurred first by pervasive replacement and later by minor cementation. In general, replacement dolomites are composed of fabric-destructive, fine to medium (60-250 µm) planar subhedral to anhedral crystals. Replacement dolomitization postdates submarine cementation and deposition of shales and carbonates from the overlying Ireton Formation, and overlaps stylolitization. It predates dolomite cementation that is interpreted to have originated from hydrothermal fluids in a shallow burial environment prior to basin tilting. This, combined with the d18O (mean = -4.5 ± 0.4 PDB) and 87Sr/86Sr (mean = 0.70853) values in the replacement dolomites, suggests that replacement dolomitization occurred also prior to basin tilting, probably during the Antler Orogeny in the subsurface at a temperature of about 45°C and a depth of about 500 m, from slightly modified Late Devonian seawater. Two generations of dolomite cements postdate replacement dolomites: a coarse and planar-e(s) cement (Dolomite-C1), and a later coarser nonplanar cement (Dolomite-C2). Dolomite-C1 has oxygen (mean d18O = -4.73 PDB) and strontium (mean 87Sr/86Sr = 0.70855) isotopic compositions similar to replacement dolomites. These similarities, combined with the local association of Dolomite-C1 with stylolites, suggest that some Dolomite-C1 precipitated from fluids whose composition reflects pressure solution of the preexisting replacement dolomites. The composition of fluid inclusions in Dolomite-C1 (mean Th = 117°C; mean salinity = 19 wt % aCl equivalent) indicates that most Dolomite-C1 precipitated from warm hydrothermal brines. Little variation in fluid-inclusion homogenization temperatures occur with burial depth, suggesting that Dolomite-C1 precipitated prior to basin tilting, and may be related to Antler orogenic events. Dolomite-C2 postdates Dolomite-C1, is relatively more depleted in 18O (mean d18O = -9.06 PDB), and has a more radiogenic 87Sr/86Sr ratio (0.71087). Preliminary fluid-inclusion data from the Crimson Field suggest precipitation temperatures of about 150°C. Thus, Dolomite-C2 originated from fluids of higher temperatures and different composition than Do omite-C1. The petrographic, paragenetic, and geochemical characteristics of replacement dolomites in this study are similar to those of other Leduc buildups along the reef trend and of other Devonian dolostones of western Canada, supporting a model of large-scale basinwide fluid flow. The pervasive distribution of replacement dolomites along the Rimbey-Meadowbrook reef trend, and the similar petrographic and geochemical character of dolomites from both the Leduc and Cooking Lake Formations, is consistent with the hypothesis that the underlying Cooking Lake platform acted as a subsurface conduit system for the dolomitizing fluids. A potential regional fluid source may have been formation fluids (including residual evaporite brines) expelled tectonically during the Antler orogeny.

Low‐ and high‐relief Leduc formation reefs: A seismic analysis

Leduc reefs have grown to widely varying heights and areal extents along the Rimbey‐Meadowbrook trend of central Alberta, resulting in significantly different seismic signatures. Three examples considered in this paper include two high‐relief or full reefs from the Leduc‐Woodbend field, an atoll and a pinnacle, each around 200 m in height but differing greatly in areal extent, about [Formula: see text] for the atoll and [Formula: see text] for the pinnacle. The third example, a low‐relief or basal reef from the Morinville field, is about 100 m high and [Formula: see text] in areal extent. The Leduc‐Woodbend and Morinville reefs exhibit quite different seismic signatures. For example, 25 ms of time‐structural drape along the top of the Devonian is observed across the Leduc‐Woodbend atoll but only 15 ms across the Morinville reef. There is 30 ms of pullup at the Beaverhill Lake level beneath the Leduc‐Woodbend atoll, 15 ms for the Morinville reef. Also, it is very difficult to differentiate the Leduc reflection from the Duvernay reflection, with which it merges, on the Morinville (basal‐reef) section. In contrast, the Leduc reflection can be correlated readily on the Leduc‐Woodbend atoll section; and reflections from the offreef shales (Duvernay and Ireton formations) terminate abruptly against the reef flank. In addition, the amplitude of the underlying Cooking Lake platform reflection varies laterally, depending on the velocity of the overlying formation (Duvernay shale or Leduc reef) and, to a lesser extent, the thickness of the overlying reef. This variation is not as useful in distinguishing between low‐relief and high‐relief reefs as it is in indicating the presence or absence of reef.

Leduc, A New Life At 38

Abstract Discovery of oil in well Imperial Leduc No.2 on May 10, 1947, marked not only a long-awaited success for Imperial Oil Limited in western Canada, but also provided a major impetus for the subsequent explosive growth of the Alberta petroleum industry. Unique among its Devonian neighbours on the active Cooking Lake Aquifer, the Leduc D-3A Pool had a thin, areally-extensive oil leg, and a large gas cap. Balancing the thinning oil zone between the gas cap and aquifer has been an unrelenting challenge of reservoir management. Reducing the oil zone from its original 11.6 metres to 1.5 metres has been a 37-year success story. However, the operator is about to lose the upper hand. Two years of appraisal and planning have focussed on the future of the remaining hydrocarbon reserve. This paper summarizes the past and present of the Leduc pool, and looks futuristically toward the day of final depletion. Introduction The Leduc D-3A Pool is located 20 kilometres southwest of Edmonton (Fig. 1). Discovered in May 1947, It is one of a series of reef structures located on the Rimbey-Meadowbrook geologic trend which extends some 480 kilometres in a southwest-northeast direction across central Alberta. Leduc is located on the southern portion of the trend, which also includes the major hydrocarbon-bearing pools of Homeglen Rimbey. Westerose South, Westerose, Bonnie Glen, Wizard Lake, Glen Park, Acheson, Big Lake and St. Albert (Fig. 2). All pools receive strong pressure support from the Cooking Lake Aquifer. Based on geological data, the Leduc D-3A Pool is divided into three distinct areas (Fig. 3):the Main Pool, the focus of this paper;the Saddle Area; andthe Southeast Extension The three areas of the Leduc D-3A Pool originally contained die hydrocarbon accumulations shown in Table 1. Since discovery, 63 per cent of the 50.4 × 106m3 of original oil-in-place, or 98 per cent of estimated reserves have been recovered from the Main Pool. At present, it is estimated that a thin 1.5 metre oil leg remains. While the challenge of managing oil production from a thin oil sandwich has been successful, the continually diminishing oil zone and near-depletion of reserves necessitates planning and executing the next phase of Main Pool depletion-a rapid gas cap production, commonly referred to as a blowdown. This paper familiarizes the reader with the past and present of the Leduc D-3A Pool, and provides forward-looking insight into the reservoir at times referred to as the genesis of the Canadian oil industry. Reservoir Geology General The Leduc D-3A Pool is one of the many Devonian reefs (Fig.2) enclosed in basinal shales of the Ireton and Duvernay Formations that grew upon the extensive Cooking Lake carbonate platform (Fig. 4). Fluids derived from compaction of the adjacent basinal sediments are thought to be the source of the dolomitization of all the bioherms or pools along the chain, as well as the western margin of the Cooking Lake platform.

Nature and Control of Shale Basin Fill and its Effect on Reef Growth and Termination: Upper Devonian Duvernay and Ireton Formations of Alberta, Canada

ABSTRACT Within basin-filling shales of the Frasnian Duvernay and Ireton Formations a number of inclined electric log markers indicate the presence of significant submarine topography during deposition. Log markers reflect submarine hardgrounds on the platform and in the basin, and may be traced laterally into thin carbonate-rich layers on the slope. Markers formed in response to a relaxation in terrigenous sedimentation against a background of continuous carbonate deposition, and may be considered essentially synchronous surfaces. The presence of synchronous stratigraphic markers allows precise quantitative depth estimates of lithofacies and biofacies ranges to be made. Log markers may be used to divide the Duvernay and Ireton Formations into a total seven informal chronostratigraphic units termed depo-units. Depo-units are broadly sigmoidal in cross section and imbricated basinward, their platform portions forming a stacked sequence of upward-shoaling units in the east of the basin. They indicate that deposition took place during a number of episodic rises in sea level, each of the order of 20 m. A reciprocal pattern of sedimentation involving rise and stillstand characterizes and explains depositional patterns in this stratigraphic interval. Mapping of the upper surfaces of individual depo-units allows the paleogeography to be reconstructed at a number of stages during basin-filling. Formation of log markers and depositional episodes in basin-filling shales have been used to explain growth stages and eventual termination of hydrocarbon-producing Leduc reef buildups within and surrounding the Shale Basin.

Differential Compaction Within the Woodbend Group of Central Alberta

ABSTRACT Upper Devonian Leduc reefs found in the subsurface of central Alberta are generally buried by more compactable shales of the Ireton Formation. The compactability contrast between the shales and the carbonates is sufficient to produce significant structural deformation of the overlying horizons. Overburden pressures can also result in differential collapse and/or compaction of the reef itself. These reductions in bulk volume can be analysed by several different methods: constructing a compaction curve; measuring the amount of pressure solution that has taken place; or preparing isopach and structure-contour maps. Application of the above methods and comparison with model studies carried out on similar sediments leads to the conclusion that differential compaction within the Woodbend Group of central Alberta has affected the structure of all but the shallowest of beds. There is also strong evidence that collapse of the reef core itself has been an important mechanism in the development of reef morphology, and that rebound and undercompaction may have been active in the past. These studies indicate that approximately 2000 ft of sediment have been eroded at the Paleozoic unconformity.

Brachiopoda of the Melville Island Group (Upper Devonian), Banks Island, N.W.T.

ABSTRACT On northeastern Banks Island, N.W.T., a total of 3700 feet of late Devonian clastics are exposed. In the lower 2600 feet of section occur rich benthonic faunas consisting primarily of brachiopods, pelecypods and corals. Thirteen brachiopod species are illustrated. One genus, Retichonetes, has not been previously reported from the Arctic Islands. Of particular importance is the presence of the rhynchonelloid zone fossils, Ladogioides pax and Calvinaria albertensis, and recognition of the overlap in stratigraphic ranges of Eleutherokomma and Cyrtospirifer. Analysis of the brachiopod faunas yields an age of from earliest Frasnian (zone of Ladogioides pax = Waterways and basal Hay River Formations) to late Frasnian (zone of Calvinaria albertensis = lower Mount Hawk and Ireton formations).

Low-Angle Regional Unconformities: ABSTRACT

Brief periods of erosion without concurrent structural deformation have produced low-angle regional unconformities. These unconformities may provide the environment for stratigraphic traps where hydrocarbons accumulate within the truncated beds. However, the slight amount of erosional stripping (a few feet per mile) and the absence of significant changes in the lithology make the detection of these low-angle regional unconformities difficult. The chances of recognizing and delineating them seem to depend, at least in part, on the selection of a datum. Low-angle regional unconformities are common in the Western Canada basin. Only three were selected as examples, because they occur in a sequence generally considered to be one continuous depositional unit: (1) beneath the Mississippian Debolt, (2) beneath the Devonian Calmar, and (3) beneath the Devonian Ireton Formations. Slight variations in local rates of subsidence effect continuous changes in the direction of the depositional dip. These changes from formation to formation generally are minor. When sedimentation resumes after an hiatus, its new depositional dip represents the cumulative shift in regional tilt gradually introduced during the absence of deposition. The new dip direction in many cases relocated individual members of a sedimentary cycle, more especially the occurrence of sand lenses in a clastic province or of reef build-ups in a carbonate-shale sequence. End_of_Article - Last_Page 635------------

Depositional Environments of Ireton Formation, Central Alberta

In the Woodbend Group of central Alberta, shales of the Ireton Formation occupy the basinal areas between the Leduc reefs. Within the shales are thin limestone beds which can be traced as electric-log markers over large areas. These markers probably approximate time lines, and they show a distinct westward divergence from the Ireton-Nisku contact. Their stratigraphic behavior has allowed subdivision of the Ireton Formation into rocks typical of the three critical environments of deposition established by Rich (1951a). Studies of cores and well cuttings corroborated the conclusions about environment which were reached by the study of the stratigraphic behavior of the marker beds. The lithologies described by Rich for the unda, clino, and fondo environments are present in the Ireton. The sediments of the Ireton are composed of terrigenous clay transported over a shelf area in the east, combined with fine carbonate derived from the scattered reefs throughout the area. The carbonates of the Upper Ireton are indigenous to the shelf environment. The pattern of shifting environments indicates that the inter-reef area was filled progressively from east to west. The paleogeography of a restricted area during Woodbend time has been deduced in a generalized manner by the application of Rich's concepts.

Stratigraphic Evidence for Tectonic and Current Control of Upper Devonian Reef Sedimentation, Duhamel Area, Alberta, Canada

In the Duhamel area of central Alberta, the Upper Devonian section under study comprises in ascending order: (1) Cooking Lake formation, (2) basal calcilutite of the Duvernay formation, (3) Leduc reef, and (4) Ireton formation. In off-reef sections, the Duvernay consists of basal calcilutites and argillaceous beds overlain by bioclastic limestones and an upper sequence of brown and black shales and calcilutites, overlain by the Ireton formation. The Cooking Lake formation is composed mainly of pelletoid (pseudo-oolitic) calcarenites and calcilutites formed probably by precipitation on a widespread bank. Calcarenites were concentrated along the western side of an interpreted shoal, the margin of which trended north to north-northeast approximately along the locus of later Leduc reef growth at Duhamel. The basal Duvernay calcilutite unit is less than 10 feet thick above the Cooking Lake shoal, but increases in thickness abruptly to 50 feet directly west of the shoal margin. It is interpreted that these facies and thickness changes occurred along a tectonic hinge line caused by possible fault movement in the Precambrian basement that determined the locus of later Leduc reef growth. Leduc reef growth at Duhamel commenced essentially after the basal Duvernay calcilutite was deposited. Earliest or incipient patch-reef growth was confined to the relatively positive eastern side of the hinge line during latest Cooking Lake sedimentation. However, with differential subsidence along the aforementioned hinge line, the reefs grew laterally westward, overlapping the thickest section of basal Duvernay calcilutites, and the subsequent main reef buildup occurred on the interpreted downwarped, western side of the hinge line. The bioclastic limestone unit of the Duvernay increases in thickness toward reefs, suggesting a derivasion by erosion of actively growing reefs. The thickness distribution of bioclastic deposits indicates dominant currents from the northeast that transported a greater amount of debris to the southwestern (leeward) side of the reefs; however, thicker deposits at the southeast may have resulted from longshore current transport southward along the fore-reef side. During upper Duvernay deposition, increased subsidence resulted in mainly vertical reef growth and reduction in amount of reef erosion; only calcium carbonate and argillaceous muds accumulated more than one mile away from the reef locus. The muds were deposited in relatively deep water, resulting in dark bituminous and partly laminated rocks with a sparse benthonic fauna. Brown and black shales accumulated in greater amounts in the more protected and stagnant waters southwest (leeward) of the reefs, and only in minor amounts as far as 10 miles north and east of the Duhamel reef front. In lower Ireton time, further increase in subsidence rate removed the restricting effects of the biohermal reefs and gray-brown and green-gray calcareous shales and calcilutites were deposited in off-reef areas. Reef growth was probably terminated during lower or middle Ireton deposition. Facies and isolith maps of pertinent units in the Cooking Lake and Duvernay formations may be used as an effective new tool in the search for petroleum-bearing Leduc reefs in central Alberta.

Resistivity Mapping and Petrophysical Study of Upper Devonian Inter-reef Calcareous Shales of Central Alberta, Canada

The Woodbend group of Upper Devonian age in central Alberta is composed of a reef complex characterized by large-scale facies changes. The reefs, which grew in a subsiding basin, were initiated in restricted areas of suitable water depth. They are surrounded by the calcareous shales and argillaceous limestones of the Duvernay and Ireton formations. Correlation sections and isopach maps indicate a slightly greater subsidence on the eastern side of the basin during Duvernay deposition, after which the basin tilted so that the Ireton sea floor sloped gently west. Very fine carbonate clastics derived from the reefs were spread throughout the basin during Duvernay and lower Ireton time by a current probably flowing southeast. The distribution of these carbonates was detected b mapping the average apparent resistivity of a stratigraphic interval. The basin reached its maximum depth when the calcareous shales of the middle Ireton were deposited. It then began to shallow and eventually the thinly interbedded coquinoid limestone and shale of the upper Ireton were laid down around and over the reefs. Fine laminations indicate fairly quiet-water deposition of the Ireton and Duvernay sediments. Limestone nodules in these rocks are probably pull-apart structures. The pore volume of these rocks decreases with increasing depth and carbonate content and resistivity increases correspondingly. The straight-line relation of carbonate content and porosity suggests that reduction of porosity is directly proportional to the volume of carbonate grains present. Other factors affecting porosity aside from carbonate content and depth are small by comparison. Internal redeposition of calcite has been unimportant. Resistivity mapping in the subsurface shows promise of being a valuable exploration tool for determining the relative amount of coarser sedimentary grains in shale.

Application of Resistivity Mapping to Upper Devonian Interreef Ireton Formation of Alberta: ABSTRACT

The Woodbend group of Upper Devonian age in central Alberta is a reef complex characterized by large-scale facies changes. The reefs, which grew in a subsiding basin and were initiated in restricted areas of suitable depth, are surrounded by the calcareous shales and argillaceous limestones of the Duvernay and Ireton formations. Isopach maps indicate relative movements of the basin during deposition. Very fine carbonate clastics derived from the reefs were spread throughout the basin during Duvernay and lower Ireton time. The distribution of these carbonates was detected by mapping the average apparent resistivity of a stratigraphic interval from borehole measurements. The pore volume of these rocks decreases with increasing depth and carbonate content, and resistivity increases correspondingly. The straight-line relation of carbonate content and porosity suggests that reduction of porosity is directly proportional to the volume of calcite grains present. Other factors affecting porosity aside from carbonate content and depth of burial are small by comparison. Internal redeposition of calcium carbonate has been unimportant. Resistivity mapping in the subsurface shows promise of being a useful exploration tool for determining the relative amount of coarser grains in shale. End_of_Article - Last_Page 1101------------

Sedimentary "boudinage" structures in the Upper Devonian Ireton Formation of Alberta

The inter-reef Ireton Formation of the Upper Devonian Woodbend Group of Alberta is essentially a calcareous shale with variable amounts of inter-bedded nodular limestones. Structures formed in limestone of the Ireton Formation vary from thin relatively undeformed beds to pinching and swelling ones, and finally to completely isolated lenses or nodules that are variously oriented. The nodules consist of finely granular lime (fine sand to very fine silt size) with minor quartz in the same grain size range as the calcite, and variable amounts of clay. All nodules are surrounded by finer grained less limey rock and display fine, peripheral, calcite-filled cracks. These structures are thought to be sedimentary boudinage formed by compaction. Thin limey beds sandwiched between more clayey and hence more plastic beds, are pulled apart by the laterally moving plastic beds when subjected to unconfined compaction near the depositional interface. Continued compaction under hydrostatic conditions further modifies these structures.

Late Devonian Geologic History in Stettler Area, Alberta, Canada

Late Devonian history in the Stettler area is divisible into four sequences of deposition comprising a major evaporite cycle. Unconformities mark the base and top of this cycle. The sequences in increasing age are: (1) final normal marine, (2) biostromal and evaporitic, (3) marine-reefoid, and (4) initial normal marine. The initial normal marine sequence is represented by normal, fragmental or oolitic, locally dolomitized limestones and shaly limestones and calcareous shales constituting the Beaverhill Lake group. The southward thinning of this group and increasing import of fossiliferous-fragmental limestones and secondary dolomites south and southeast indicate decreasing negative tectonism from a northern basinal province to an unstable shelf at the south. The marine-reefoid sequence, represented by the Woodbend group, commenced with a marine limestone episode followed by a reef-growing episode. The marine limestone episode was characterized by deposition under shelf conditions of normal marine, fossiliferous-fragmental, partly dolomitized limestones, locally reefoid. Leduc, Duvernay, and Ireton formations (of the Woodbend group) were deposited during the reef-growing episode. Three phases of development of reef-growth are recognized: early, medial, and late phase. Increased subsidence in central and south-central Alberta occurred during the early phase of reef growth. Biohermal reef-growth commenced where organic shoals had become prolific near the end of Cooking Lake time. These biohermal structures had considerable effect on off-reef sedimentation. The subsidence rate was not sufficient to keep pace with reef-growth. Thus they acted as sills, impeding effective circulation of marine waters in the inter-reef areas where dark-colored bituminous shales and limestones were deposited. Lighter-colored limestones and shales formed in localities of relatively free circulation. The rate of subsidence in the basinal area increased during the medial phase, and biohermal growth continued. Marine conditions varying from nearly normal to restricted were established throughout most of the inter-reef areas. Deposition of clay from northwest of the Central Alberta basin into inter-reef areas reached an approximate maximum during this medial phase. The minor importance of carbonates and absence of agitated-water limestones and seconda y dolomites in off-reef areas are suggestive of contemporary subsidence such that reefs grew partly below effective wave-base. An approach to shelf deposition in the late phase of reef-growth is indicated by an increase of dolomites, with shaly beds, and normal marine to fragmental limestones, in the inter-reef areas. The arrival of relatively more stable conditions was partly instrumental in causing cessation of biohermal growth. The biostromal episode represents a continuation in the progressive reduction of rate of subsidence and tectonic differentiation initiated in the late phase of biohermal reef-growth. Biostromal structures End_Page 2500------------------------------ flourished in relatively shallow water depths. These organic shoals became instrumental in controlling the physical, chemical, and organic factors of sedimentation. Progressive shoaling of waters formed restrictive barriers, and evaporitic precipitation commenced in the inter-shoal areas. The precipitation of evaporites commenced earliest in the inter-shoal areas and later on-lapped the shoals. The biostromal episode was terminated by an approach to subaerial conditions. The evaporitic episode represented continuation of restricted conditions throughout most of the area. Evaporitic sedimentation extended southward into southern Alberta, but intensity of restriction was less severe. The return to normal marine conditions of deposition toward the end of Wabamun time marks completion of the major cycle initiated by Beaverhill Lake sedimentation.