Devonian Grosmont Formation Research Articles

A Study on the Formation Environment of Dolomite within the Devonian Grosmont Formation, Alberta, Canada

A Study on the Formation Environment of Dolomite within the Devonian Grosmont Formation, Alberta, Canada

A random Cantor set as a measure of the statistical homogeneity of fractured rock masses

This study addresses the applicability of the exponent of a power-law scaling model, as an index of the statistical homogeneity of fractured rock masses. We systematically examine the capability of a random Cantor set, Dc, to quantify the combined effect of fracture density and size distribution on fracture networks. Total 129 two-dimensional fracture configurations were generated, based on different combinations of fracture size distribution and fracture density. The Dc for each generated fracture network system was calculated using the random Cantor dust method. We found that the standard deviation of trace length has no effect on the calculated Dc, and the estimated Dc increases with the total density and/or mean trace length. To explore the field applicability of this study, we investigated the statistical homogeneity of a fractured rock mass of the Upper Devonian Grosmont Formation in Alberta, Canada. The results clearly show that the methodologies given in this study are capable of determining the statistical homogeneity of fractured rock masses.

Paleokarst in the Grosmont Formation and reservoir implications, Saleski, Alberta, Canada

We delineated a bitumen-rich paleokarsted carbonate reservoir of the Upper Devonian (Frasnian) Grosmont Formation with a high-resolution 3D seismic survey tied to core and petrophysical log data from 35 wells within a [Formula: see text] study area in northern Alberta, Canada. There were two laterally continuous karst facies: a solution-enhanced vuggy dolostone that resulted from the carbonate dissolution of body fossils and a stratiform breccia that resulted from the dissolution of interbedded evaporites. Three laterally discontinuous karst facies were identified: sinkhole fills, collapsed paleocaves, and solution valley fills. We measured 368 subcircular features (sinkholes and collapsed paleocaves) having a median circle-equivalent diameter of 69 m and representing 5.5% of the total study area. Sinkhole fills include Cretaceous-aged sandstone, mudstone, and coal. Collapsed paleocaves were filled with matrix-supported breccia that had clasts of disoriented blocks of dolomite and a matrix of disaggregated dolomite and Cretaceous-aged mudstone. The paleocaves and sinkholes formed in the solution-enhanced karst facies of the Grosmont C at the interface of an interpreted ancient vadose-phreatic mixing zone. The marine deepwater deposition of the Clearwater Formation during the Albian filled the depressions created by the mechanical collapse of the paleocaves and provided a seal for thermal operations. The fracture density inferred from seismic amplitude variation with angle and azimuth analysis and corroborated by well data showed that fractures are ubiquitous and were enhanced during meteoric karst. The high-vertical permeability resulting from solution-enhanced fractures, the laterally predictable flow units, and a competent seal make this an ideal reservoir for thermal bitumen recovery.

Detailed topography of the Devonian Grosmont Formation surface from legacy high-resolution seismic profiles, northeast Alberta

The Devonian Grosmont Formation in northeastern Alberta, Canada, is the world’s largest accumulation of heavy oil in carbonate rock with estimated bitumen in place of [Formula: see text]. Much of the reservoir unconformably subcrops beneath Cretaceous sediments. This is an eroded surface modified by kartstification known as the Sub-Mannville Unconformity (SMU). We studied the reanalysis and integration of legacy seismic data sets obtained in the mid-1980s to investigate the structure of this surface. Standard data processing was carried out supplemented by some more modern approaches to noise reduction. The interpretation of these reprocessed data resulted in some key structural maps above and below the SMU. These seismic maps revealed substantially more detail than those constructed solely on the basis of well-log data; in fact, the use of only well-log information would likely result in erroneous interpretations. Although features smaller than about 40 m in radius cannot be easily discerned at the SMU due to wavefield and data sampling limits, the data did reveal the existence of a roughly east–west-trending ridge-valley system. A more minor northeast–southwest-trending linear valley also was apparent. These observations are all consistent with the model of a karsted/eroded carbonate surface. Comparison of the maps for the differing horizons further suggested that deeper horizons may influence the structure of the SMU and even the overlying Mesozoic formations. This suggested that some displacements due to karst cavity collapse or minor faulting within the Grosmont occurred during or after deposition of the younger Mesozoic sediments on top of the Grosmont surface.

캐나다 북부 알버타주 데본기 후기 탄산염암 지역의 탄성파 층서

캐나다 북부 알버타주의 데본기 후기 Grosmont층은 중생대 백악기와 데본기 후기 사이 형성된 침식 부정합면 하부에 위치하며 Ireton층의 셰일에 의해 4개의 단위로 구분되고, 상향 천해화(shallowing-upward)를 보이는 전형적인 대륙붕 환경이다. Grosmont층은 탄성파의 극성, 연속성, 주파수/간격, 진폭 등의 해석요소를 고려하여 4개의 단위(LG, UG1, UG2, UG3)로 구분할 수 있었고 반사파는 중-고진폭, 중-저주파수의 특징을 보이고 반사면은 연속성이 좋으며 서로 평행한 형태로 나타났다. 시추공의 암상자료를 바탕으로 대륙붕 또는 플랫폼(platform) 환경으로 해석할 수 있지만 반사파의 특성이나 형태만으로 순차층의 경계면과 퇴적환경을 인지하기란 쉽지 않기 때문에 이 연구에서는 부순차층세트(parasequence set)로 하여 층서 해석을 시도하였다. 침식 부정합면에 의해 퇴적계 연합체와 함께 부순차층세트가 형성되는데, 침식부정합면은 암상자료와 탄성파상의 카르스트화작용 및 침식면에 의해 그 인지가 가능하였다. 연구지역에 분포하는 Grosmont 탄산염암층은 플랫폼 및 대륙붕 환경으로부터 분지 방향의 플랫폼 주변부을 향하면서 층후가 점차 감소하는 쐐기형태로 발달하며 전진하는 시그모이드-오블리크(sigmoid-oblique) 형태의 음향특성을 갖는 것이 특징적이며, 이는 해수면 변동과 연계되어 퇴적작용이 진행되었음을 지시해주고 있다. 퇴적단위의 대부분은 주로 해퇴 및 저해수면 환경 하에서 형성된 것으로 해석되어 플랫폼 환경을 뒷받침해주는 증거가 된다. 특히, 고해수면 환경 하에서 형성된 퇴적체 하부의 셰일층은 반복되는 해퇴기간 동안 대부분이 박층으로 분포하는 것으로 해석된다. The Upper Devonian Grosmont Formation in northern Alberta, Canada, underlies the erosion unconformity that formed between the Cretaceous and Upper Devonian. The formation is divided into four units on the basis of intercalated shales and showing a typical shelf environment of shallowing-upward. It was possible to separate four units(LG~UG3), considering the seismic interpretation attributes of polarity, continuity, frequency/spacing and amplitude and showing the reflection characteristics of the medium-high amplitude, medium-low frequency, good continuity, and subparallel reflection events. The formation can be interpreted as shelf or platform, based on in-situ core data. However, it is difficult, only with reflection attributes and features, to recognize the boundaries and sedimentary environment of parasequence. Therefore, we try to interprete by parasequence set in this study. The parasequence set was formed by erosion unconformity with systems tracts. The erosion unconformity can be recognized by facies data and karst, erosional surface. Grosmont carbonate deposits ranging from platform and shelf to shelf slope are; by wedge-shaped strata of characterized by complex sigmoid-oblique progradational configurations, reflecting a depositional history of upbuilding and outbuilding in response to sea-level changes. Most of the sedimentary units is interpreted as platforms under regression and lowstand environments that support is evidences. In particular, shale layer at the basal part of the highstand systems tracts represents the regressive to lowstand of sea level.

Drilling Challenges in the Bitumen-Saturated Grosmont Formation

Summary The Upper Devonian Grosmont formation, located in the West Athabasca oil sands deposit, contains an estimated 318 billion barrels of bitumen. The reservoir is a heavily karsted and fractured, bitumen-saturated dolomite with up to 38% porosity and permeabilities commonly measured in the darcies. These properties make the Grosmont an excellent candidate for steam assisted gravity drainage (SAGD). Grosmont SAGD development will be exposed to similar operating conditions as McMurray SAGD projects, and will have similar logistics, well construction and materials challenges. However, there are many additional challenges, including severe loss circulation, that further complicate well design. This paper presents the challenges experienced by Laricina Energy in drilling Grosmont wells at Saleski and Burnt Lakes over the last three seasons. It will also address additional areas for future research and development in the drilling of vertical and horizontal wells.

Evaluation of Recovery Technologies for the Grosmont Carbonate Reservoirs

Summary The Upper Devonian Grosmont formation is a bitumen-saturated, carbonate unit located in Northern Alberta. It is considered to be among the world?s next largest unconventional oil resource plays. Since early 2006, there has been an increased interest in Grosmont resources exhibited by a range of companies, including super-majors. Several in-situ pilot tests were conducted in the central portion of this area in the 1970s and 1980s, using steam and in-situ combustion processes. Similar to field tests in the McMurray formation oil sands before invention of the Steam-Assisted Gravity Drainage (SAGD) process, none of the early recovery technologies tested proved to be economic. Because the "gravity" drainage process has proved successful in commercial development of the McMurray formation oil sands since the mid- to late-1990s, the recovery potential for the Grosmont formation should be re-evaluated, based on improved recovery techniques. Results from cyclic steam stimulation (CSS) field tests are compared and analyzed to understand the similarity and fundamental differences in reservoir properties between the McMurray formation oil sands and the Grosmont formation carbonate rocks. A preliminary interpretation is provided for laboratory test results for solvent processes applied to Grosmont carbonate cores. The scaling considerations from the laboratory results to field expectations are discussed. The paper also provides a direction for future studies and optimization opportunities for reservoir recovery leading to the commercial development of Grosmont carbonate reservoirs.

An Overview of the Geology of the Upper Devonian Grosmont Carbonate Bitumen Deposit, Northern Alberta, Canada

The platform carbonates of the Upper Devonian Grosmont Formation hold an estimated 50 billion m3 of bitumen in place and therefore host a significant amount (one-sixth) of the total bitumen resources in northern Alberta. However, unlike the overlying Athabasca oil sands, the bitumen resource has not been tapped as yet. The present study is an overview and integration of recent subsurface studies, largely focused along the eastern subsurface erosional edge of the Winterburn Group in northern Alberta. The basis of the regional geologic framework includes a detailed study of 42 cores with an emphasis on facies description of both reservoir and nonreservoir facies, and the summary of a diagenetic sequence model to demonstrate the major influences on bitumen accumulation. The Grosmont Formation carbonates have been subject to various stages of diagenesis, including dolomitization and karstification with a profound effect on the distribution of porosity and permeability, which resulted in highly heterogeneous reservoirs. In addition, low effective permeabilities because of high bitumen saturation, the high viscosity of the heavily biodegraded bitumen, and the continuity and integrity of shales capping bitumen-bearing reservoir facies are further factors that must be considered in applying future in-situ recovery schemes to the Grosmont carbonate bitumen deposit.

Abstract: Paleokarst in the Upper Devonian Grosmont Formation, NE Alberta, Canada: Recognition, Distribution, Origin

Abstract: Paleokarst in the Upper Devonian Grosmont Formation, NE Alberta, Canada: Recognition, Distribution, Origin

Recognition and Delineation of Paleokarst Zones by the Use of Wireline Logs in the Bitumen-Saturated Upper Devonian Grosmont Formation of Northeastern Alberta, Canada

The Upper Devonian Grosmont Formation in northeastern Alberta, Canada, is a shallow-marine carbonate platform complex that was subaerially exposed for hundreds of millions of years between the Mississippian(?) and Cretaceous. During this lengthy exposure period, an extensive karst system developed that is characterized by an irregular erosional surface, meter-size (several feet) dissolution cavities, collapse breccias, sinkholes, paleosols, and fractures. The karsted Grosmont Formation, which contains giant reserves of bitumen, subcrops beneath Cretaceous clastic sediments of the giant Athabasca tar sands deposit. The paleokarst in the Grosmont Formation can be recognized on wireline logs in relatively nonargillaceous carbonate intervals (<30 API units on the gamma-ray log) as excursions of the caliper log, off-scale neutron-density porosity readings, and severe cycle skipping of the acoustic log. The paleokarst is more prevalent in the upper units of the Grosmont Formation, and the effects of karstification decrease toward stratigraphically older and deeper units. The paleokarst usually occurs within 35 m (115 ft) of the erosional surface. The reservoir properties of the Grosmont Formation (e.g., thickness, porosity, permeability, and seal effectiveness) are significantly influenced by karstification. Depending upon the location, karstification has either benefited or degraded the reservoir characteristics. Benefits include porosity values greater than 40% (up to 100% in caverns) and permeability values of 30,000 md in severely fractured intervals. Detrimental reservoir characteristics include erosion, porosity and permeability reduction, and seal ineffectiveness.

Evolution of Steam-based Technology For the Recovery of Canadian Heavy Oil Reservoirs

Abstract Heavy and extra-heavy oil reservoirs located in Western Canada (Alberta and Saskatchewan provinces) collectively represent one of the largest accumulations of hydrocarbons in the world. Over the past 20 years, a sustained field and laboratory research program aim at economic recovery of Canadian heavy oil and bitumen has taken place. This has resulted in the development and commercialization of many recovery processes using steam. Current production from active thermal projects is about 18,000 m3 per day. To cope with the large variation of geological and petrophysical conditions found in such oil reservoirs, a wide variety of successful steam-based recovery processes have evolved. These include steam stimulation above and below fracture pressure, pressure cycle steam drive, steam drive, and steam-assisted gravity drainage from both underground and surface access. The principal mechanisms believed to be taking place in' each case will be outlined. Pertinent applications of each steam process in relationship to the micro and macro description of the reservoir essential to economic recovery are discussed. Introduction Alberta and Saskatchewan have vast heavy oil and bitumen reserves. They are part of a variety of deposits ranging from Upper Devonian to Mississippian limestones and dolomites to Lower Cretaceous sand deposits of continental to marine shoreline origin. Each deposit has distinct characteristics which require unique recovery technologies. This wide variety of recovery technologies, with emphasis on the use of steam will be discussed in this paper. The heavy oil and bitumen deposits have been known for centuries, but research began in the late 1940's when large amounts of conventional oil were discovered in Alberta. Commercial production of heavy oil began in the late 1960's. In the early 1970's, when the oil price increased and Canada's conventional 011 production and reserves were already declining heavy oil production and research expanded rapidly. To stimulate this development, the Alberta Government established the Alberta Oil Sands Technology and Research Authority (AOSTRA) in 1974. Since then, AOSTRA, by collaborating with the Canadian petroleum industry, has been a major player in the development of the current technology. This paper describes briefly the geology of the main deposits and the steam-based technology which has been most effective for each geological setting. Geological Features Figure 1 shows the deposits in five major geographic locations: Athabasca Oil Sands Area, Cold Lake Oil Sands Area, Peace River Oil Sands Area, Carbonate Trend (Triangle) underlying the Athabasca and Peace River Oil Sands Areas and Heavy Oil Belt. The largest of these deposits may be the Devonian Grosmont Formation found in the Carbonate Trend. Because of low porosity high variability and poor reservoir definition this formation is most difficult to exploit. Although bitumen-bearing carbonates have been encountered throughout northeastern Alberta, only limited drilling has been done and reserves are poorly defined. The deposits occurring in unconsolidated Cretaceous sands have been well delineated. The most recent oil-in-place and initial established reserve figures are shown in Tables 1 and 2.

Pore Size and Pore Throat Types in a Heterogeneous Dolostone Reservoir, Devonian Grosmont Formation, Western Canada Sedimentary Basin

The Devonian Grosmont Formation in northeastern Alberta, Canada, is a giant heavy-oil reservoir. The main reservoir rocks are dolomitized and karstified platform and ramp carbonates, and the best reservoir facies occur in the upper Grosmont (UGM) units 3 and 2. In these units, reservoir properties are highly heterogeneous. Hand specimen, thin section, UV, and SEM petrography, as well as grading scales, mercury capillary pressure curve analysis, and statistics, have been used to characterize reservoir heterogeneity. Our investigation led to a new pore size classification for carbonate reservoirs; this new classification has four pore sizes: microporosity (pore diameters 256 mm). A combination of microscopic observations and capillary pressure curve characteristics led to the recognition of four pore throat texture types on the microporosity scale, and to five types on the mesoporosity scale. Microporosity pore types include (1) intracrystal dissolution porosity, (2) pervasive intercrystal and intracrystal dissolution porosity, (3) intergranular and/or intercrystal porosity in grainstones, and (4) primary or solution microporosity in mud matrix (only in limestones). Mesoporosity pore types include (1) intercrystal porosity, (2) solution-enhanced intercrystal porosity, (3) oversized porosity, (4) intragranular solution porosity, and (5) intergranular solution porosity. Some of these types are homogeneous (e.g., non-fabric selective dissolution porosity and inte crystal primary porosity), whereas others are heterogeneous. Generally, hydrocarbon recovery efficiency is good in the homogeneous pore throat types, but poor in the heterogeneous types. Reservoir heterogeneity has further been characterized by comparison and statistical analysis of plug and full-diameter core (FDC) data that indicate (1) plug porosities, on average, are higher than FDC porosities; (2) plug permeabilities, on average, are higher than FDC permeabilities; (3) plug porosity and permeability show relatively good linear relationships in certain core intervals, but FDC data do not; and (4) generally, the more homogeneous reservoir rocks have relatively high matrix porosity. Heterogeneity is common in Grosmont dolostone reservoir rocks, and affects overall reservoir quality. At the megascopic scale, the reservoir properties can be classified as homogeneous, dual-porosity, and multilayered. The dual-porosity model has three subcategories: fractured, channeled, and pressure solution derived.

Oil Sand Strength Parameters At Low Effective Stress: Its Effects On Sand Production

Abstract Heavy and extra-heavy oil reservoirs located in Western Canada (Alberta and Saskatchewan provinces) collectively represent one of the largest accumulations of hydrocarbons in the world. Over the past 20 years, a sustained field and laboratory research program aim at economic recovery of Canadian heavy oil and bitumen has taken place. This has resulted in the development and commercialization of many recovery processes using steam. Current production from active thermal projects is about 18,000 m3 per day. To cope with the large variation of geological and petrophysical conditions found in such oil reservoirs, a wide variety of successful steam-based recovery processes have evolved. These include steam stimulation above and below fracture pressure, pressure cycle steam drive, steam drive, and steam-assisted gravity drainage from both underground and surface access. The principal mechanisms believed to be taking place in' each case will be outlined. Pertinent applications of each steam process in relationship to the micro and macro description of the reservoir essential to economic recovery are discussed. Introduction Alberta and Saskatchewan have vast heavy oil and bitumen reserves. They are part of a variety of deposits ranging from Upper Devonian to Mississippian limestones and dolomites to Lower Cretaceous sand deposits of continental to marine shoreline origin. Each deposit has distinct characteristics which require unique recovery technologies. This wide variety of recovery technologies, with emphasis on the use of steam will be discussed in this paper. The heavy oil and bitumen deposits have been known for centuries, but research began in the late 1940's when large amounts of conventional oil were discovered in Alberta. Commercial production of heavy oil began in the late 1960's. In the early 1970's, when the oil price increased and Canada's conventional 011 production and reserves were already declining heavy oil production and research expanded rapidly. To stimulate this development, the Alberta Government established the Alberta Oil Sands Technology and Research Authority (AOSTRA) in 1974. Since then, AOSTRA, by collaborating with the Canadian petroleum industry, has been a major player in the development of the current technology. This paper describes briefly the geology of the main deposits and the steam-based technology which has been most effective for each geological setting. Geological Features Figure 1 shows the deposits in five major geographic locations: Athabasca Oil Sands Area, Cold Lake Oil Sands Area, Peace River Oil Sands Area, Carbonate Trend (Triangle) underlying the Athabasca and Peace River Oil Sands Areas and Heavy Oil Belt. The largest of these deposits may be the Devonian Grosmont Formation found in the Carbonate Trend. Because of low porosity high variability and poor reservoir definition this formation is most difficult to exploit. Although bitumen-bearing carbonates have been encountered throughout northeastern Alberta, only limited drilling has been done and reserves are poorly defined. The deposits occurring in unconsolidated Cretaceous sands have been well delineated. The most recent oil-in-place and initial established reserve figures are shown in Tables 1 and 2.nalysis in which kinematic mechanisms beyond plasticity equilibrium could not be handled.

A Unique Dolomite Reservoir: The Upper Devonian Grosmont Formation in Northeastern Alberta, Canada

A Unique Dolomite Reservoir: The Upper Devonian Grosmont Formation in Northeastern Alberta, Canada

Lithofacies, Diagenesis and Related Reservoir Properties of the Upper Devonian Grosmont Formation, Northern Alberta

ABSTRACT A detailed study of the Upper Devonian Grosmont Formation was undertaken in order to assess reservoir development. An understanding of the original depositional facies, the subsequent diagenetic history and the resultant pore structure allows for a more effective reservoir assessment in this bitumen-bearing reservoir. Environments of deposition ranging from subtidal to intertidal to supratidal are interpreted, based on lithofacies description, stratigraphic succession, and spatial distribution. Primary porosity was occluded early in the diagenetic history of the Grosmont by calcite cements. Secondary porosity resulted from dolomitization. Highest porosity, permeability and bitumen saturation occur in units of the Upper Grosmont 2 and 3, near the erosional edge in the northeastern part of the study area. The massive vuggy dolomite (lithofacies E) appears suitable for the application of EOR methods because of the megascopic properties of the pore system. The finely laminated dolomite lithofacies (F) and the dolomite-breccia lithofacies (G) appear favourable because of the microscopic properties of their pore systems. The network of large vugs and fractures observed in lithofacies E, and the extensive intercrystalline porosity of the sucrosic dolomite in lithofacies F and G, would allow a more efficient steam sweep and hence better bitumen recoveries. RESUME Une etude detaillee de la Formation Grosmont (Devonien Superieur) a permit de documenter le developpement de ce reservoir d'hydrocarbures. Une interpretation des facies de deposition sedimentaire, de la diagenese qui a subsequemment affectee ces sediments et de la structure des pores permet une meilleure comprehension de ce reservoir contenant du bitume. Des milieux de deposition variant de zone infracotidale, zone intercotidale, et zone supracotidale ont ete interpretes d'apres la description des lithofacies, leur succession stratigraphique et leur distribution laterale. La porosite primaire fut obstruee rapidement dans l'histoire diagenetique du Grosmont par des ciments de calcite. La porosite secondaire fut produite par dolomitisation. Les plus hautes valeurs de porosite, de permeabilite et de saturation en hydrocarbures se retrouvent dans les unites du Grosmont Superieur 2 et 3, pres de la discordance d'erosion, dans le nord-est de la region d'etude. La dolomite massive (lithofacies E) avec des cavites de dissolution se presente comme etant favorable a l'application de methodes de recuperation assistee du petrole (EOR) a cause des proprietes megascopiques du systeme poreux. La dolomite finement litee (lithofacies F) et la breche dolomitique (lithofacies G) semblent favorable aux methodes EOR a cause des proprietes microscopiques de leur systeme poreux. Le systeme de cavites et de fractures observees dans le lithofacies E et l'excellente porosite de type intercristalline des lithofacies F et G font qu'une plus grande partie du systeme poreux entre en contact avec la vapeur et ainsi en decoule un plus grand recouvrement du bitume. Traduit par l'auteur

Biological marker and conventional organic geochemistry of oil sands/heavy oils, Western Canada basin

Forty-three samples of oil sands/heavy oils from most of the major Cretaceous deposits and the Upper Devonian Grosmont Formation of the underlying “carbonate trend”, have been examined by gas chromatography and gas chromatography-mass spectrometry. Major organic geochemical differences observed between samples/deposits include the presence or absence of n-alkanes and isoprenoid alkanes, together with changes in the distributions of biological marker compounds. These differences reflect the degree of biodegradation suffered by the deposits. As noted earlier by others, there is a trend toward increased biodegradation from west to east in the basin. Three geochemical factors demonstrate that at least the Cretaceous samples are strikingly similar to one another, once the effects of biodegradation are discounted. These factors are the carbon number distribution of steroidal alkanes (C 27, C 28, and C 29 diasteranes); the presence of 28,30-bisnorhopanes; and the relative abundance of 28,30-bisnorhopanes and gammacerane as compared with the ubiquitous 17α(H)-hopanes. These distinctive biomarker compositions are ratios indicate that the same or very similar sources generated the Cretaceous oil sands/heavy oils, despite the enormous volumes and their widespread geographic and stratigraphic distribution. The extent of isomerization of regular steranes and hopanes indicates that the bitumens show the same general level of maturity. All these data suggest that the Cretaceous bitumens were derived from a mature, conventional oil which was in turn derived from a presently unknown source fecies. This oil must have migrated over large distances, suffering extensive biodegradation in place and possibly during migration. There appear to be at least two mechanisms of biodegradation that the biomarkers have undergone in the Cretaceous oil sands/heavy oils. Most samples from which regular steranes have been removed by extensive biodegradation do not show 25-norhopanes. In four Wabasca Deposit Grand Rapids A sand samples, however, 25-norhopanes are present in variable amounts relative to regular hopanes, despite the fact that three of these four samples still contain regular steranes. Possible candidates for source facies may be present in the Mesozoic clastic succession as suggested by others, but systematic work at all stratigraphic levels is required to discover a source facies with the requisite geochemical properties and the potential of having generated the enormous volumes of oil contained within the oil sands and heavy oil reservoirs of the Western Canada Basin.

Dolomitization and Calcitization of the Devonian Grosmont Formation, Northern Alberta

ABSTRACT The Grosmont Formation (Upper Devonian) comprises a platform carbonate sequence which extends over 100,000 sq. km in the subsurface of Alberta and contains extensive areas of calcitized (dedolomitized) dolomite reservoir rocks. Hydrocarbon content of the Grosmont Formation is estimated to be 50 109 m3 of 7 API gravity oil, with bitumen recovery rates as high as 70 percent using thermally enhanced recovery. Environments of deposition in the Grosmont Formation range from subtidal and intertidal to supratidal. Early, stratigraphically controlled and later, cross-cutting dolomites are observed. Dissolution fabrics are generally related to the uppermost dolomitized units of the Grosmont Formation, below an unconformity which overlies Paleozoic rocks. Calcitization textures are found in limestone and dolomite units and do not bear any relationship to the unconformity. Petrographic examination shows that calcitization of dolomite occurs in different forms. Large (0.25-0.5 mm) dolomite rhombs, observed floating in micrite or at calcitic micrite-dolomite contacts, have cores replaced by calcite. Dolomite rhombs that are not calcitized contain cloudy cores with abundant calcite inclusions. In some samples, fragments of dolomite rhombs are observed with the cores dissolved away, indicating that part of the calcitization process involves dissolution. A complete range, from dolomitic rocks that show no evidence of calcitization to calcitic rocks with the textures of sucrosic dolomites, are observed. Late calcite cements, filling the remaining available pore space, are also related to the calcitization process. There is a strong tendency for dolomites from stratigraphic units to be grouped by their isotopic compositions. Three phases of dolomitization are interpreted in the Grosmont Formation: an early sabkha dolomite (Upper Grosmont 3); dolomites formed during reflux of hypersaline brines (Upper Grosmont 2); and dolomites related to waters expelled from shales during burial (Lower Grosmont). Further, dolomites in each unit tend to be depleted in the heavy isotopes of both carbon and oxygen with increasing degree of dissolution and/or calcitization. The geological and geochemical evidence indicates that calcitization was early and related to dolomitization, probably with a later stage that took place during uplift and the entry of meteoric water along the unconformity.

Bitumen Accumulation in Grosmont Platform Complex, Upper Devonian, Alberta, Canada

The Upper Devonian Grosmont Formation, a broad carbonate platform complex in Alberta, Canada, contains an estimated 300 billion bbl of bitumen. It has been suggested that these vast reserves are related to Lower Cretaceous Athabasca oil sands. Detailed gas chromatographic-mass spectrometric studies of a wide range of biologic marker compounds confirm this suggestion. The Grosmont Formation contains bitumen of similar maturity and source to the Athabasca deposit, but it has been subjected to a greater degree of biodegradation and water washing, possibly as a result of its reservoir rock characteristics. The difference in the degree of biodegradation is manifested by the absence of bicyclic terpanes and by the reduced concentrations of the C30 and the 22R epimers of the extended hopanes in the Grosmont bitumen. Also, the greater degree of water washing of the Grosmont bitumen is inferred from the observed distribution of the bicyclic, tricyclic, and tetracyclic terpenoid sulfides, which shows a characteristic loss of the lower molecular weight members in the carbonate bitumen. The correlation established here between the two deposits suggests that if the precursor oil has indeed undergone long-distance migration, the Paleozoic carbonates could have acted as a path for migration. Finally, the observed distribution of steranes in the Grosmont bitumen corresponds to the suggestion that the Mannville Group shales were not the major source rocks of the oil-sand and carbonate bitumen accumulations of northern Alberta.

Geological Setting and Production History, Grosmont Pilot Project, Northern Alberta: ABSTRACT

Enormous quantities of bitumen are contained along the eastern margin of the Upper Devonian Grosmont Formation in northern Alberta. This formation can be readily subdivided into four informal stratigraphic units which represent, along with several smaller scale internal depositional cycles, successive stages in the evolution of a hugh carbonate complex: argillaceous slope sequences pass upward to a spectrum of shallow-platform facies and these then are capped by shoreline deposits and horizons of exposure and erosion. Although almost exclusively secondary in nature, the type and distribution of pore fabrics appear to have been at least initially controlled by original depositional facies. In 1975, Union Oil Co. of Canada initiated a single well steam stimulation program to evaluate the potential for recovering bitumen from the upper Grosmont Formation. In 1977, the Alberta Oil Sands Technology and Research Authority (AOSTRA) and Canadian Superior Oil joined the project and further tests (steam stimulation, steam drive, combustion) were undertaken at a new site with Union continuing as operator. The 1980-81 huff and puff stimulation program consisted of one injection/production well and four closely spaced observation wells. Four production cycles have been completed to date, yielding attractively high production rates. An expanded evaluation program is currently under way and will utilize a five-spot scheme of injection/production wells augmented by four or more obse vation wells. End_of_Article - Last_Page 578------------