Cretaceous McMurray Formation Research Articles

Revised stratigraphic position of a volcanic-ash-derived maximum depositional age in the Lower Cretaceous McMurray Formation

Abstract The stratigraphic framework developed for the McMurray Formation and Wabiskaw Member of the Clearwater Formation (Aptian, Lower Cretaceous) provides a consistent nomenclature and allows for correlation across the Athabasca Oil Sands Region. Event horizons (e.g. volcanic ash layers) within the stratigraphic framework provide crucial geochronologic ages that constrain the timing of deposition and improve upon biostratigraphic estimations. Here, we provide recognition criteria for genetic depositional units in the Firebag sub-basin with the intent of revising the stratigraphic position of a previously published volanic ash-derived maximum depositional age of 115.07 +/– 0.16 Ma. Results demonstrate that the previous placement of this event horizon at the top of the B1 parasequence set was not consistent with the accepted definition of the McMurray Formation–Wabiskaw Member boundary or application of the stratigraphic framework. Here, we establish that the ash-bearing coal horizon is at the top of the A2 parasequence set/channel belt of the McMurray Formation. The implications of this change include constraining the age of the A2 channel belt system to approximately 115 Ma which, under the previous study, would have been younger and unconstrained. We also discuss the implications for our understanding of the Aptian-Albian boundary in the context of allogenic drivers and global environmental change at that time.

A typical point bar with atypical strata in the McMurray Formation, Alberta, Canada: Floods, tides and high suspended sediment concentrations

AbstractStratigraphic positions and the nature of some breccias and mudstones observed in the Lower Cretaceous McMurray Formation type section (Alberta, Canada) challenge the existing fluvial point bar facies models. The currently applied large‐scale, tidally influenced, sandy bedload facies model suggests that point bars fine upward, where breccias and cross‐strata occur at the base of the succession and mud content increases upward. Instead, this study documents that: (i) mudstone clast breccias are not exclusively associated with basal channel‐lag deposits and can occur throughout the point bar; and (ii) stratified mudstones are not limited to the top of the fining‐upward succession and can occur at or near the channel base. This outcrop data, including lithology, sedimentary structures, trace fossils, bed thickness and boundaries, and the nature of stratal packages, further suggests dynamic interplay between fluvial and tidal processes. This article discusses the potential role of high‐magnitude river floods in temporarily elevated suspended sediment concentrations and highlights that fluid mud processes played a crucial role in mud deposition in both tide‐influenced and fluvial parts of the system. Fluid mud processes allowed mud deposition along the channel base and across the point bar accretion surfaces. The breccias predominantly consist of broken pieces of inclined heterolithic strata and mostly occur on accretion sets of point bars, suggesting erosion and collapse of the point bar rather than of the cutbank. This work introduces a novel perspective to the existing models of point bar sedimentation, offering a new end member and concepts for the interpretation of subsurface data. The authors hope that this research encourages further investigations into similar phenomena elsewhere in the world.

Sedimentological and ichnological variations in fluvio‐tidal translating point bars, McMurray Formation, Alberta, Canada

ABSTRACTSedimentological and ichnological descriptions of fluvio‐tidal translating point bars are rare, and complex physico‐chemical processes make highly detailed but concise facies descriptions challenging. Herein, mesofacies are defined to describe and interpret three ancient translating point bars from the Lower Cretaceous McMurray Formation, Alberta, Canada. Twenty‐three mesofacies are defined, based on their recurring sedimentological and ichnological characteristics. These mesofacies form the building blocks of beds and bedsets that make up three depositional facies. Facies 1 reflects sand dune migration at the channel base, which grades into inclined heterolithic stratification of Facies 2 and 3. Facies 2 occurs in the centre and seaward portions of the translating point bars and records tide‐dominated deposition of sand and muddy sand during periods of reduced river discharge. Ichnological suites and bioturbation intensities in these beds reflect persistent but variable brackish‐water conditions, fluctuating deposition rates and the deposition of mud. Mud beds are derived from flows with high suspended‐sediment concentrations. Tidally derived mud beds are typically bioturbated with trace fossil suites indicative of slow deposition rates and brackish‐water conditions. Mud deposited during elevated river discharge is burrowed after the dewatering of the bed. Facies 3 occurs at the landward apex of the translating point bar and is marked by sand‐rich and mud‐rich dune deposits with abundant soft‐sediment deformation, indicative of elevated flow velocities and deposition rates. Bioturbation is rare and sporadically distributed owing to unstable substrates. The distribution of the facies reflect the hydrodynamic variations that occurred vertically and laterally across the bar in response to temporal variations in fluvial and tidal flow interaction, as recorded by their mesofacies. The detailed facies analysis strongly suggests that deposition of the three McMurray Formation translating point bars occurred in proximity to the turbidity maximum zone of a fluvio‐tidal channel system.

Fluvial character and architecture of an outcrop using sedimentology combined with UAV-based modeling, Cretaceous McMurray Formation, NE Alberta, Canada

ABSTRACT It is widely accepted that most occurrences of inclined heterolithic stratification (IHS) in the rock record form by laterally accreting point bars in freshwater fluvial, tidally influenced fluvial, or tidally dominated estuary channels. Despite the widespread distribution of IHS in the subsurface and outcropping strata of the lower Cretaceous McMurray Formation, the large-scale depositional architecture and lateral facies variability of these deposits remains controversial. The relatively limited lateral extent of many of the outcrops is a challenge, particularly when point-bar deposits on the scale of hundreds of meters to kilometers are interpreted in outcrops spanning anywhere from 100 to 300 meters laterally. This has in turn led researchers to leverage other datasets such as 3-D seismic to analyze the large-scale depositional architecture of the IHS, leading to two main interpretations for the IHS in the McMurray Formation: 1) a fluvially dominated environment owing to geomorphological features comparable to those in large modern fluvial systems, or 2) an estuarine environment owing to the presence of trace fossils characteristic of marine-derived faunal colonization in brackish-water settings and strong evidence of tidal modulation. The purpose of this study is to investigate the sedimentology and depositional architecture of IHS in a unique, kilometer-wide outcrop exposure of McMurray Formation strata and compare it to IHS observed at other McMurray Formation outcrops previously interpreted as estuarine channels. This is achieved by combining traditional field-based methods with Unmanned Aerial Vehicle-based outcrop modeling to create a 3-D outcrop model to visualize and analyze large point-bar geobodies deposited in a channel upwards of 25 meters deep and 750 meters wide exposed in outcrop at Crooked Rapids of the Athabasca River, west of the City of Fort McMurray. Importantly, this methodology uses bed orientation trends, paleocurrent data, and sedimentological observations to identify and map architectural elements, which constitute an eastward-accreting point bar crosscut by a southwestward-accreting counter point bar in the outcrop. The results strongly suggest that the IHS at Crooked Rapids was deposited in a freshwater fluvial environment. When compared to IHS deposited in estuarine depositional environments, fluvial IHS is driven by seasonal river discharge as opposed to the interplay between river discharge and the extent of the tidal prism. Therefore, fluvial IHS is: 1) dominantly sandstone with very minor waning-flow siltstone interbeds resulting from erosion by the succeeding freshet phase, and 2) completely devoid of bioturbation until flat-lying bar top or overbank strata is encountered. Using 3-D outcrop modeling to supplement sedimentological and ichnological observations strengthens the interpretation of complex fluvial geobodies and increases the overall understanding of the large-scale depositional architecture of point bars across the tidal–fluvial transition zone.

Early Cretaceous continental-scale sediment routing, the McMurray Formation, Western Canada Sedimentary Basin, Alberta, Canada

Alternative depositional models for the Early Cretaceous McMurray Formation between a dominantly continental and marginal marine settings remain a controversial topic. The source-to-sink model can be inverted to reconstruct ancient sediment-routing systems by utilizing methods to estimate ancient drainage basins, which can contribute to understanding of sediment routing and testing of alternative depositional models. New detrital zircon U-Pb ages from 31 samples were analyzed to identify source terranes of the McMurray Formation and the overlying Wabiskaw Member of the Clearwater Formation to test first-generation alternative sediment-routing models and estimate maximum depositional ages. In total, 9729 new concordant U-Pb ages identified multiple source terranes for the McMurray Formation within an interpreted continental-scale paleodrainage basin that extended from the SW to SE United States and eastern Canada. The paleodrainage basin then expanded to include the Western Cordillera arc system within the overlying Upper Mannville Group. Multidimensional scaling and mixing models independently support a paleo-upstream mixing of primary and recycled sources in the McMurray Formation axial system in the United States and a paleo-downstream confluence between this axial system and east-derived sediment-routing systems in Canada, which display an evolution from bedrock-confined to alluvial morphology from the lower to middle-upper McMurray Formation. The current dataset constrains the maximum depositional age of the McMurray Formation to the latest Barremian to Aptian, with an age range from ca. 122 to 115 Ma, which is significantly younger than previously reported. Age estimates in the overlying Upper Mannville units from ca. 115 to 110 Ma statistically overlap with the McMurray Formation age, suggesting continuous Mannville deposition that lasted 10−12 m.y. or less.

Reservoir Architecture in Tide-Dominated Estuary: A Case Study of McMurray Formation Oil Sand Reservoir in the Athabasca Block, Canada

This paper presents a detailed description of the sedimentary characteristics and architecture model of the tidal-dominated estuary reservoir in the Lower Cretaceous McMurray formation. Based on a thorough study of the core data, 13 lithofacies were recognized. The lithofacies associations were divided into nine types of architecture elements, that is channel, salt marsh, fluvial point bar, tidal point bar, mud flat, mixed flat, sand flat, tidal bar and offshore. Through the combination of high resolution seismic data and well data, the architecture model was established. The boundary surfaces of each architecture elements are depicted and the logging characteristics is concluded. The cross-section is constructed to reveal the superimposition model of different architectural elements with the seismic profiles. The plane and vertical distribution of architectural elements are revealed and the sedimentary model is constructed. In the longitudinal section of estuary, the architecture elements is fluvial deposits (channel, fluvial point bar, and salt marsh), tidal flat (mud flat, mixed flat, and sand flat), tidal bar, tidal flat and offshore from land to sea. In the cross section, tidal bar, sand flat, mixed flat, mud flat, tidal point bar and salt marsh are distributed successively from the center to the sides. Tidal bars and sand flats are the best reservoirs in terms of physical properties. We focus on the analysis of their architecture and summarize their stacking patterns. We propose tidal bars as compounds are lateral accretion. Tidal bar has high-angle continued lateral accretion, with a dip angle of 6–12°. Sand flat also has lateral-migrated characteristics, but their dip angle is 2–6° which is smaller than tidal bars. As a result, the dimension and stack of a single tidal bar and sand flat are quantified. The tidal bar was lateral stacking or vertical stacking with lateral-migrated or forward-migrated sand flat and existed vertical and lateral erosion-filling phenomenon. Tidal bars have width of 100–550 m and length of 800–2,400 m, and sand flats are 300–2,500 m long. Tidal bars and sand flats made up the best reservoirs. The study provides some insights for future research of estuarine sedimentary patterns and reservoir architecture.

Migrations of Devonian brine up-section and the origin of calcite-cemented sand in the Lower Cretaceous McMurray Formation, Athabasca Oil Sands

Calcite-cemented sand intervals are widely distributed in the lower interval of the McMurray Formation of the Lower Cretaceous Athabasca Oil Sands deposit, northeastern Alberta. Flows of Devonian brine were driven up-section upon mixture with meteoric-charged groundwater that came in contact with the Prairie Evaporite Formation only 200 m below. Voluminous migrations of carbonate-saturated Devonian formation water during lower McMurray Formation deposition account for the distribution of decimetre to multimetre thick calcite-cemented sand intervals and giant concretions within the syndepositional fill of the Bitumount Trough, a 100 km long salt dissolution–collapse structure that floored the northern Athabasca Oil Sands deposit. Precipitations of calcite cement concurrent with deposition of the lower McMurray Formation provide insight into the eventual disposition of the voluminous brine that would have resulted from the dissolution of as much as 130 m of Middle Devonian salt section below the Bitumount Trough in contrast to deep basin storage elsewhere. Breccia pipes developed in Middle–Upper Devonian carbonate beds flooring the Bitumount Trough provided migration pathways up-section for the carbonate-saturated Devonian brines. Giant concretions of calcite-cemented sand, 1–2 m across, developed in homogeneous sands proximal to collapse-breccia pipes that vent along the Trough floor, whereas brine flows directed along heterogeneous sands resulted in tabular calcite-cemented beds. Similar but volumetrically less significant Quaternary brine migrations diluted with glacial meltwater were discharged at the surface as saline springs along the modern river valleys, resulting in geochemical trends characterized by concentration of total dissolved solids but without associated precipitations of calcite cement.

Salt dissolution tectonism and origin of lacustrine carbonate beds: Mn‐Fe‐calcite and Mn‐siderite micro‐spherulite fabrics of the Lower Cretaceous McMurray Formation, Athabasca Oil Sands deposit, western Canada

AbstractSeveral lacustrine carbonate beds, each a metre‐thick interval of densely packed Mn‐rich sideritic micro‐spherulites or Mn‐rich ferroan calcite micro‐spherulites, are recorded for the first time within strata of the Lower Cretaceous McMurray Formation of the northern Athabasca Oil Sands deposit, western Canada. A lower McMurray lacustrine carbonate deposit is characterised by a metre‐thick bed fabric of Mn‐rich siderite micro‐spherulites. The middle and upper interval McMurray beds developed fabrics of Mn2+‐rich ferroan calcite micro‐spherulites. These carbonate beds represent saline lacustrine depositional environments that resulted from the lake bottom sediments ingressed from below by Mn2+‐Fe2+‐rich carbonate‐saturated brines. These up‐section migrations of Devonian formation water were sourced from dissolution trends developed in limestone and halite‐anhydrite beds of the underlying Devonian Prairie Evaporite during Cordilleran deformation of the Alberta Basin foreland. These brines ascended to the overlying McMurray Formation sediments along dissolution‐collapse structures such as breccia pipes, sinkholes and margins of differentially subsided Upper Devonian fault blocks. The up‐section migration of a sulphate‐saturated Fe2+ and Mn2+‐rich brine resulted in the ingress of a lower McMurray lacustrine bottom sediment at a site associated with the development of a peat mire terrain. Microbial redox of the lake bottom sediment resulted in a carbonate bed of micro‐spherulitic fabrics of Mn‐rich siderite interwoven with pyrite laminae. Subsequent salt dissolution events and up‐section migrations of Devonian brine during deposition of the middle and upper McMurray intervals resulted in similar carbonate‐saturated but sulphate‐poor chemistry. These saline flows also ingressed lacustrine bottom sediments below, and resulted in limestone beds of densely packed spherulitic fabrics of Mn‐ferroan calcite, not siderite. These deposits provide insight into largely unknown dispositions of voluminous brine resulting from salt dissolution trends below the Athabasca Oil Sands and further our understanding of controversial McMurray depositional processes.

Integrating facies analysis with dipmeter data to characterize point bars of the Lower Cretaceous McMurray Formation, Christina River, AB, Canada

The Lower Cretaceous McMurray Formation contains crude bitumen reserves of the Athabasca Oilsands that formed by channel amalgamation and lateral migration of fluvio-tidal point bars. These point bars are widely accepted as incised valley fills during episodic base-level rises in tidally influenced estuary (Mathison, 2004; Hein et al., 2013). Historically, well logs such as gamma-ray and spontaneous potential were used in combination with core data to interpret facies and to conduct stratigraphic correlations of the McMurray Formation. Microresistivity image logs, dipmeter logs, and seismic data have greatly improved the identification of facies and recognition of point-bar geometry. In comparison to microresistivity image logs and seismic data, dipmeter data is more regionally accessible; the tool indicates the orientations of fractures, bedding planes and contacts and it is commonly used to delineate boundaries between different point bars in the McMurray Formation. This work provides examples of dipmeter tadpole analyses and discusses how dipmeter-log interpretations can be supported with core-based facies analyses.This study integrates dipmeter tadpole patterns, sedimentological and ichnological characteristics from 47 sedimentary cores. Vintage dipmeter tadpole patterns were observed and digitized from 52 wells to establish a detailed facies characterization with a particular emphasis on McMurray point-bar complexes and sheltered shoreface/delta deposits. In particular, dipmeter data are useful in indicating paleocurrent directions and lateral accretion orientation of point bars, which helps to reconstruct paleodepositional history and stratigraphic relationships. Point bars in the study area are characterized by these sedimentological and dipmeter characteristics: 1) a lower point-bar section characterized by trough cross-bedded sandstone (locally inclined stratified master bedding) with local mud clasts and disorganized, low-to high-angle dipmeter readings; 2) a middle section represented by inclined heterolithic stratified lateral accretionary deposits that exhibit consistent low-angle dips (5°–18°), and display subtle shallow-to-steep-to-shallow dipmeter pattern; and 3) an upper section of the point bar or in some cases abandoned channel strata comprising laminated and lenticular mudstone or bioturbated heterolithic intertidal strata, each showing low-angle (<4°) dipmeter measurements.The dipmeter characteristics of estuarine point bars are integrated with core observations to delineate distinct point-bar stacking patterns and stratigraphic units. In particular, dipmeter tadpole patterns help to refine facies interpretations, such as cross-stratified sandstone facies. Similar point bar scales and stacking patterns are observed in four channel sets. Point bars show either NW-SW or NE-SE lateral accretion surfaces, suggesting northward paleocurrent flow direction.The analyzed datasets support a transgressive fill incised valley fill model that is consistent with earlier McMurray studies in different areas. The four channel sets descend from different stratigraphic levels, but share similarities in scale, orientation and sedimentology/ichnology. The similarity attests to persistent transgression over McMurray time punctuated with regular sea-level fall and rise until the McMurray Formation strata were replaced by Clearwater Formation sedimentation.

Using high-resolution microresistivity image logs to reconstruct paleoenvironments and stratal architectures: An example from the McMurray Formation, Leismer area, northeastern Alberta, Canada

Point-bar deposits, although readily recognizable in most successions, are notoriously challenging to map and correlate accurately, particularly in the subsurface. Sedimentologic interpretations from microresistivity image logs, by contrast, offer an unparalleled method of accurately reconstructing point-bar architectural elements and mapping their distribution. Using an example from the Lower Cretaceous McMurray Formation, we demonstrate that the bedding dip profiles of stacked downstream migration, lateral accretion, and channel abandonment elements record the semicontinuous point-bar accretion of two erosionally juxtaposed large-scale channels. Microresistivity image-log analysis clearly demonstrates that bedding and architectural elements of these heterolithic point bars display discrete and predictable changes in dip angle and direction that are independent of autogenic variations in lithology. We advocate, therefore, that this approach be used to map the facies of point bars and predict their reservoir trends. The laterally accreted mud-sand couplets display a cyclic pattern that is consistent with a nonrandom recurring process. Based on the distribution of bioturbation in the mud beds and variations in sand-bed thicknesses, we argue that this recurring process was probably a monthly occurrence. By measuring the lateral-accretion bed thicknesses for what appears to be a 1-yr interval, a sedimentation rate of 86 cm/yr is estimated for these lateral-accretion beds. This deposition rate provides an upper limit required to deposit these point bars and improves the framing of the depositional model. We estimate that the two point bars in this study were each deposited over a period of a few hundred years.

Acceleration in the rate of the Boreal Sea transgression recorded in the Lower Cretaceous McMurray Formation, Canada

Depositional environments are typified as low-to high-accommodation space settings based on the sediment accumulation rate. In low-accommodation settings, persistent top truncation of regressive units renders it challenging to accurately determine the rate of accommodation space creation. Herein, we analyze a nearly continuous succession of the Lower Cretaceous McMurray Formation, which was deposited during the foreland development of the Western Canada Sedimentary Basin in northeast Alberta, Canada (the McMurray Depocenter). Across the study area, the McMurray Formation consists of a vertical succession of 6 discrete regressive depositional units (DU) bounded by flooding surfaces and/or transgressive surfaces of erosion. The sedimentology, architecture and stratigraphy of these DUs are investigated and statistical analysis is used to quantitatively assess variations in DU thickness. Additional statistical analysis is used to quantitatively assess the vertical distribution of transgressive mudstone subfacies in the McMurray Formation.The mean thickness of each DU is 10.4 m, and each DU represents approximately 1.7 million years of deposition. This indicates that the McMurray Formation preserves deposition in a low-accommodation space setting, and that creation of accommodation space was primarily controlled by transgression of the Boreal Sea. An abrupt decrease in DU thicknesses is interpreted to result from concomitant increased erosional truncation of DUs via wave action during transgression and changes in basin morphology. DU thickness variations correlate to a change in the marine expression of the transgressive mudstone underlying each DU. Together, these data suggest an acceleration in the rate of the Boreal Sea transgression, and we hypothesize that the duration of still-stands during which DUs accumulated diminished in the latter stages of McMurray Fm deposition. This study demonstrates that detailed sedimentologic and stratigraphic research tied to statistical analysis can be used to quantify changes in the rate of transgression, especially in low-accommodation space settings.

Alignment of saline springs with evaporite karst structures in northeast Alberta, western Canada: analogue for cretaceous hypogene brine seeps to the surface

Meteoric and glacial meltwater charged groundwater, mixed with dissolved salts from Devonian sources at depth, discharged as saline springs along topographic lows of the Athabasca River Valley, which downcuts into the Cretaceous Athabasca oil sands deposit in northeast Alberta, western Canada. These Quaternary saline seeps have TDS measurements, isotope signatures and other chemical characteristics indicative of the groundwater flows coming in contact with Prairie Evaporite (M. Devonian) salt beds, 200 m below the surface. Migrations up-section of groundwater with dissolved chloride and sulphate salts occurred along salt dissolution collapse breccia zones that cross-cut Upper Devonian limestone strata. Seeps discharged along the karstic Devonian limestone paleotopography, the unconformity surface flooring the Lower Cretaceous McMurray Formation. Saline to brine springs along the Athabasca River Valley have TDS measurements that can exceed 100,000 mg/L. Quaternary salt removal was insignificant compared to the voluminous removal of the 80-130 m thick salt section for 1000s km2 during the Early Cretaceous configuration of the Devonian paleotopography, which partially controlled depositional patterns of the overlying McMurray Formation, principal host rock of the Athabasca oil sands. Little is known of the storage or disposition of voluminous brines that would have resulted from this regional-scale removal of the salt beds below the Athabasca deposit during the Cordilleran configuration of the foreland Alberta Basin. Holocene dissolution trends and discharges at the surface as saline springs are proposed as a modern analogue for voluminous Early Cretaceous brine seeps to the surface along salt dissolution collapse breccia zones, concurrent with deposition of the McMurray Formation. This model links several characteristics of the McMurray Formation as responses to Aptian brine seeps to the surface. These include: (1) the emplacement of a drainage-line silcrete along the margins of the Assiniboia PaleoValley, now partially exhumed by the Athabasca River Valley, (2) distribution of brackish-water burrowing organisms, and (3) diagenesis of calcite-cemented sand intervals.

Tidally-influenced deposition and microfacies sequences of fluid muds: Early Cretaceous McMurray Formation, Alberta, Canada

Fluid muds are common in estuarine environments, but their ancient examples have rarely been documented due to poor comprehension of their depositional processes and characteristics. Mudstone layers in the tidally-influenced channel sequences of the middle McMurray Formation are examined in detail through microscopic observations and interpreted on the basis of recent advances in the understanding of flow dynamics of high mud-concentration flows. The mudstone layers, < 1 to 25 mm thick, are classified into three microfacies. Structureless mudstone (Microfacies 1) consists mainly of clay particles with randomly dispersed coarse grains (coarse silt to fine sand). It represents cohesive mud flows with sufficient cohesive forces to support coarse grains (quasi-laminar plug flow). Silt-streaked mudstone (Microfacies 2) is similar to Microfacies 1 in texture, but contains discontinuous streaks of coarse-silt to very-fine-sand grains. It is interpreted as being also deposited by cohesive fluid muds. The silt streaks are, however, suggestive of the presence of weak turbulence under the cohesive plug (upper transitional plug flow). Heterolithic laminated mudstone (Microfacies 3) is characterized by alternations of very thin, silt and clay laminae, which are either parallel or low-angle cross-laminated. It is interpreted as the deposits of low-amplitude bed-waves formed in lower transitional plug flows. These microfacies reflect a range of flow phases of fluid muds, which changed as flow velocities and suspended sediment concentrations fluctuated with tidal cycles. Repeated vertical changes of microfacies are suggestive of an ideal sequence of fluid muds formed during tidal acceleration and deceleration. The sequence comprising microfacies 3, 2 and 1 in ascending order represents deposition from lower transitional plug flows through upper transitional plug flows to quasi-laminar plug flows as suspended sediment concentrations increase with flow deceleration. That of the reversed order of microfacies reflects the reversed change in flow types during acceleration. These results provide the basis for recognizing fluid-mud deposits and tidal signatures in ancient estuarine sequences.

A method of reconstructing 3D model from 2D geological cross-section based on self-adaptive spatial sampling: A case study of Cretaceous McMurray reservoirs in a block of Canada

An orthogonal 2D training image is constructed from the geological analysis results of well logs and sedimentary facies; the 2D probabilities in three directions are obtained through linear pooling method and then aggregated by the logarithmic linear pooling to determine the 3D multi-point pattern probabilities at the unknown points, to realize the reconstruction of a 3D model from 2D cross-section. To solve the problems of reducing pattern variability in the 2D training image and increasing sampling uncertainty, an adaptive spatial sampling method is introduced, and an iterative simulation strategy is adopted, in which sample points from the region with higher reliability of the previous simulation results are extracted to be additional condition points in the following simulation to improve the pattern probability sampling stability. The comparison of lateral accretion layer conceptual models shows that the reconstructing algorithm using self-adaptive spatial sampling can improve the accuracy of pattern sampling and rationality of spatial structure characteristics, and accurately reflect the morphology and distribution pattern of the lateral accretion layer. Application of the method in reconstructing the meandering river reservoir of the Cretaceous McMurray Formation in Canada shows that the new method can accurately reproduce the shape, spatial distribution pattern and development features of complex lateral accretion layers in the meandering river reservoir under tide effect. The test by sparse wells shows that the simulation accuracy is above 85%, and the coincidence rate of interpretation and prediction results of newly drilled horizontal wells is up to 80%.

Bioturbation, heavy mineral concentration, and high gamma-ray activity in the Lower Cretaceous McMurray Formation, Canada

In the Lower Cretaceous McMurray Formation (Alberta, Canada), many intervals of intensely bioturbated (Bioturbation Index = 5–6) fine-grained sediments are characterized by high gamma-ray (GR) readings. Several methods, including sedimentary facies analysis, thin-section petrography, handheld spectral gamma-ray, portable X-ray fluorescence, X-ray diffraction, inductively coupled plasma-mass spectrometry, microprobe of K-feldspar, energy dispersive spectroscopy, and detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry, were used to investigate the interval of interest in core samples. The mineralogical analysis shows that these intervals are enriched in heavy mineral grains, and particularly in zircons. The content of radioactive elements is variable. Thorium is commonly elevated up to three times, uranium nil to two times, and potassium content usually remains normal. The studied intervals consist of interbedded, bitumen-saturated cross-bedded and/or ripple cross-laminated sandstone (high-energy deposits) and light-gray bioturbated mudstone (low-energy deposits), commonly addressed as inclined heterolithic strata (IHS). IHS represent tidally influenced, brackish-water, upper point-bar deposits. The zircon grains become concentrated while hydraulic processes interact with bioturbation: the burrowing animals cause significant sediment mixing that allows the lightest sediment particles to go back into the suspension. Additionally, bioturbation increases the surface roughness along the sediment-water interface and, causes more turbulent flow, allowing for quartz and other light grains to be removed by traction and/or saltation, while dispersed heavier zircon grains become trapped and concentrated in open burrows.So far, this study is the first to demonstrate the importance of bioturbation in the enrichment of zircon grains in IHS. The interaction of bioturbation and hydraulic processes explains the apparently counter-intuitive enrichment of heavy minerals in a low-energy depositional setting. This scenario likely applies to numerous intervals characterized by similar GR and/or zirconium spikes across the McMurray Formation. Furthermore, it can be expected that in other sedimentary basins and stratigraphic units, similar studies will demonstrate that the proposed mechanism is universal.

Natural gamma-ray spectroscopy (NGS) as a proxy for the distribution of clay minerals and bitumen in the Cretaceous McMurray Formation, Alberta, Canada

Detailed examination of the mineralogy of the Cretaceous McMurray Formation within a facies framework is used to assess the use of natural gamma-ray spectroscopy (NGS) and a pulsed neutron generator (PNG) tool in delineating variation in clay mineral and bitumen contents. Characterization of the mixed-layer (interstratified) clay phases in the McMurray Formation provides an improved understanding of clay interaction in bitumen processing and tailings settling behavior, important for mine planning and tailings remediation schemes. Mineral diversity in the McMurray Formation was determined on facies attributed samples using whole rock X-ray diffraction (XRD), cation exchange capacity (CEC) measurements, elemental analysis (XRF), clay size fraction (<2 µm) XRD analysis, reflected light microscopy, and cryogenic-scanning electron microscopy (cryo-SEM). Kaolinite was ubiquitous in the entire McMurray Formation with lower and middle McMurray Formation sediments also containing mixed-layered illite–smectite (I-S) with a low expandability≈ 20–30%. Upper McMurray Formation sediments by contrast had higher expandability (≈ 60–70%). In floodplain sediments of the lower McMurray Formation an additional clay mineral was quantified as a kaolinite-expandable mixed-layer (clay) mineral. The associated CEC values of this mineral are 10 times the baseline for the McMurray Formation. NGS spectra from cores showed that yields of potassium (K), uranium (U), and thorium (Th) had distinct facies associations, correlated with a clay mineral signature. The resultant indicator is capable of highlighting zones within an oil sands ore body that are empirically known, by industry, to process poorly through extraction plants. A bitumen indicator from the carbon yield derived from a PNG logging tool assesses bitumen content. NGS and PNG allow a full assessment of clay mineral (fines) and bitumen profiles, with the future prospect that these techniques could be used to assess ore and tailings behavior in near-real time.

Refinement of the stratigraphic framework for the Regional C depositional unit of the McMurray Formation and implications for the early transgression of the Alberta Foreland Basin, Canada

ABSTRACTThe Lower Cretaceous McMurray Formation in Alberta and Saskatchewan, Canada, comprises a series of depositional units (DUs) consisting of stacked parasequences bounded by flooding surfaces and incised by fluvio-estuarine channel belts. The fluvio-estuarine channel belts of the McMurray Fm have been the focus of numerous studies whereas the regional DUs have received substantially less attention. Of the regional DUs, Regional C (equivalent to the middle McMurray) is the most understudied, yet this interval records the history of the McMurray Formation between deposition of fluvial strata in the lower McMurray and marine facies in the upper McMurray and overlying Clearwater Formation. Determining the history of the Regional C DU is fundamental for accurately reconstructing the stratigraphic evolution of the McMurray Fm and, by extension, the early evolution of the Alberta Foreland Basin.The Regional C is divided into two DUs separated by a regionally mappable flooding surface. This surface occurs 11 to 15 m below the top of the Regional C and is traceable over a 2,550 km2 area. This flooding surface divides the thick interval of undifferentiated Regional C into a lower C2 DU and an upper C1 DU, each with a maximum thickness of &amp;lt; 15 m. The thickness of the C2 and C1 DUs indicates that deposition at this time also occurred in a setting of low to moderate accommodation creation, which is consistent with the rest of the McMurray Formation. The limited available accommodation space was easily surpassed by sediment supplied by the paleo-distributive channel system, leading to a basinward progradation of the shoreline.The C2 and C1 DUs are retrogradationally stacked, with the maximum regressive paleo-shoreline of C1 lying landward of that of C2. This stacking arrangement indicates that the shoreline backstepped during the early stages of transgression of the Boreal Sea. The backstepping of the paleo-shoreline from C2 to C1 time is consistent with previous studies that show continued and stepwise retrogradation and/or transgression of the paleo-shoreline from the onset of deposition in the lower McMurray Formation through to maximum transgression in the Clearwater Formation. Together, these studies demonstrate that the early drowning of the Alberta Foreland Basin was persistent and slow.

Automated Approach Optimizes Flow-Control Device Placement in SAGD Completions

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 193364, “Optimization of Placement of Flow-Control Devices Under Geological Uncertainty in Steam-Assisted Gravity Drainage,” by Siavash Nejadi, Stephen M. Hubbard, Roman J. Shor, SPE, Ian D. Gates, SPE, and Jingyi Wang, SPE, University of Calgary, prepared for the 2018 SPE Thermal Well Integrity and Design Symposium, Banff, Alberta, Canada, 27–29 November. The paper has not been peer reviewed. Steam-chamber conformance in steam-assisted gravity drainage (SAGD) influences the efficiency and economic performance of bitumen recovery. Conventional SAGD well-completion designs provide limited control points in long horizontal well pairs, leading to development of nonideal steam chambers. The complete paper presents an automated approach to optimizing placement of flow-control devices (FCDs) in SAGD well-pair completions. The methodology uses a coupled wellbore/reservoir model to simulate both reservoir fluid-flow behavior and detailed wellbore hydraulics. The qualities of the well-completion-design parameters and their effect on production are assessed by calculating the net present value (NPV), which is considered the basis for•optimization. Introduction The Lower Cretaceous McMurray formation hosts the majority of bitumen in the Athabasca oils sands—the largest known resource of bitumen. The formation is composed of large-scale fluvial-estuarine point bars and other laterally accreting channel systems that are highly heterogeneous. The formation has been interpreted as having three stratigraphic subdivisions: a lower continental (fluvial), a middle fluvial-estuarine unit (point-bar dominated), and an upper marginal marine deposit. The repeated erosional cut and fill events within the McMurray have led to nested and multiple stacked structures. Similarly, laterally accreting channel systems, such as point-bar deposits consisting of inclined heterolithic strata of sandwiched sand-siltstone sequences and abandoned mud channels, lead to very complex sedimentary facies relationships in which rock types change both laterally and vertically over very short distances. SAGD completions offer the most promise of producing bitumen resources from Athabasca oil-sands deposits. The SAGD well configuration typically consists of two parallel horizontal wells within a short distance, one above the other. Steam is injected into the upper well and fluids are produced from the lower well. These design schemes provide limited control of steam injection and liquid production along horizontal sections and adversely affect steam-chamber conformance. Operational difficulties, drilling, well completion, and reservoir parameters moderate overall SAGD performance. More specifically, these factors include hydraulic gradients and pressure drop in the tubulars, injectivity and productivity variations along the wellbore from plugging and formation damage, heat exchange, energy loss to bottom water, nonparallel well-pair placement, well undulation, and—most importantly—reservoir heterogeneity and structure. The conventional SAGD well completion needs to be modified to deliver steam efficiently throughout the reservoir interval, improve liquid-production performance, and minimize steam breakthrough.

Using ichnological relationships to interpret heterolithic fabrics in fluvio‐tidal settings

AbstractIn fluvio‐tidal settings, the sediment is dominantly derived from the river systems. However, the importance of landward tidal transport of sediment in tidally influenced sedimentary environments is difficult to assess, particularly in the rock record. This problem is addressed using two intervals within the Lower Cretaceous McMurray Formation, each representing a distinct inclined heterolithic stratification motif. The ichnological variation between the heterolithic intervals is analyzed to determine which lithosomes are associated with brackish‐water (tidally influenced) colonization windows. From this, the relative fluvial influence responsible for the deposition of the fine and coarse members can be determined. Both of the inclined heterolithic stratification fabrics studied record the deposition of fluvio‐tidal point bars wherein the heterolithic bedding represents variations in river discharge. The first fabric comprises inclined heterolithic stratification in which bioturbation only occurs in mudstone beds. This fabric indicates that deposition occurred in more proximal positions within a fluvio‐tidal system (i.e. the outermost inner to middle estuary or distributary channels). In this example sand deposition is interpreted to represent high‐energy, freshwater dune migration within a fluvial‐dominated setting, whereas mud beds reflect brackish‐water suspension deposition during times of low river discharge. The second fabric, which is interpreted to have developed in more distal depositional positions (i.e. the middle estuary or seaward of the turbidity maximum in deltas), consists of inclined heterolithic stratification with laminated mudstone and bioturbated sandstone. In these inclined heterolithic stratification successions the mudstone beds were deposited under the influence of freshwater and heightened sedimentation rates, whereas bioturbated sandstone was colonized under brackish‐water conditions and in the presence of tidally facilitated sediment transport. In both examples, the bioturbated lithosomes are related to colonization windows that indicate the predominance at that time of marine or tidally influenced processes over fluvial processes.

Salt dissolution tectonism and microbial origin of drainage-line silcrete in the Lower Cretaceous Athabasca Oil Sands, foreland Alberta Basin

Abstract The Beaver River Sandstone is a 10 s cm to 1.7 m thick silicified interval emplaced enigmatically within unconsolidated fluvial sand beds of the Lower Cretaceous McMurray Formation, the principle host rock of the Athabasca Oil Sands in northeastern Alberta. The silicified interval was partially eroded into during the Late Pleistocene as glacial meltwaters enlarged and deepened the Athabasca River Valley. The silcrete consists of a 10 s km long linear trend of discontinuous caprocks that sourced the innumerable quartzite boulders along the floor of the Athabasca River Valley. Siliceous cementation was concurrent with Aptian deposition of the McMurray Formation. The silicified bed extends along the disconformity between the lower and middle intervals of the McMurray Formation. The silicification at the end of lower McMurray deposition was spatiotemporally linked to regional dissolution of halite-anhydrite beds in Devonian Prairie Evaporite substrate, 200 m below. A sulphate-rich brine migrated up-section into the uppermost sand beds of the lower interval accumulated along topographic lows of the ancestral Athabasca River Valley, a segment of the regional Assiniboia Paleovalley. Near surface redox of the seep occurred by sulphate reducing microorganisms sourced from groundwater emanating from widespread peat mires. Strongly acidic groundwater resulted in corrosion of the quartz sand grains and silica saturated groundwater in the shallow subsurface. A pH shift occurred at onset of the middle interval, triggering silica cementation as the acidic seep mixed with meteoric-charged oxygenated groundwater along topographic lows of the river valley. Microbial filaments of unknown taxonomic affinity, now silicified, are the first recognized in the McMurray Formation oil sands. These microfossils consist of silicified hollow branching sheaths covered with nanoscale bulbous protuberances that preserve stepped transition textures of hydrated silica into quartz crystallites. They are evidence of partial microbial controls on the sulphate redox process resulting in this unusual type of silcrete.