Integrating depositional models, ichnology and sequence stratigraphy in reservoir characterization: the middle member of the Upper Devonian-Lower Mississippian Bakken Formation of subsurface southeastern Saskatchewan revisited (original) (raw)
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The Upper Devonian-Lower Mississippian Bakken Formation hosts one of the most important oil reservoirs in Saskatchewan. Integration of ichnological and sedimentological data indicates that deposition of Bakken strata occurred in two different paleoenvironmental settings: open-marine and brackish-water marginal marine. This paper focuses on the open-marine deposits in the Lower Member, the basal and the upper part of the Middle Member, and the Upper Member. The open-marine deposits embrace shelf, lower and upper offshore, offshore transition, and a transgressive lag facies. The large extension and continuity of the open-marine sedimentary facies point towards deposition in a low-gradient system in a shallow epeiric sea. With the exception of the black shale from the Lower and the Upper Member, all these deposits are characterized by a high bioturbation index and a "distal" Cruziana ichnofacies with Phycosiphon incertum and Nereites missouriensis as dominant elements, and Asterosoma isp., Planolites montanus, and Teichichnus rectus as subordinate elements.
AAPG Bulletin, 2012
The Upper Devonian-Lower Carboniferous Bakken Formation is a widespread siliciclastic unit in the subsurface of the Williston Basin that is subdivided into three members: lower and upper organic-rich shale members and a dolomitic, silty, and sandy middle member. Although the unit has become one of the most active oil plays in North America and numerous sedimentologic studies have been made, no consensus about the depositional environments of the middle member has been achieved. Previous studies suggested several depositional and sequencestratigraphic scenarios, including lowstand offshore-shoreface, normal-regressive offshore-shoreface, incised estuary, and fallingstage shoreface complexes for the middle member. We propose a new depositional and sequence-stratigraphic model and compare it with some previous interpretations. This new model includes a basal transgressive systems tract (TST) embracing shelf deposits, a highstand systems tract comprising shelf to lower shoreface environments, and an upper TST encompassing a brackish-water embayment complex and offshore to shelf
In spite of the high interest on the Late Devonian-Early Carboniferous Bakken Formation as one of the most important oil plays in North America and the numerous studies carried out in this unit, no ichnologic analysis has been presented yet. Based on conventional sedimentologic data, previous studies of the Bakken interpreted the unit as formed entirely under open-marine conditions. However, integration of ichnology has been key to the identification of not only open-marine but also brackish-water marginal-marine conditions. Based on the geometry of the sedimentary bodies and the sequence-stratigraphic framework, the brackishwater interval is interpreted as an embayment with limited or intermittent connection to the open sea. Salinity, oxygen content, and storms are regarded as the most important environmental parameters that controlled the distribution and nature of the trace fossils in the Bakken Formation. Sparse bioturbation, relatively low ichnodiversity, and the "impoverished" Cruziana ichnofacies characterize the marginalmarine deposits, suggesting stressful brackish-water conditions. Marine deposits from the middle member, in contrast, are highly bioturbated, with a moderate ichnodiversity and the "distal" Cruziana ichnofacies that flourished during fully marine well-oxygenated conditions. There is only a slight difference between the total ichnodiversity of the open-marine interval (ten ichnogenera) and that of the brackish-water interval (seven ichnogenera). Ichnodiversity should not be considered at face value and isolated from other aspects when evaluating stress levels in order to detect a brackish-water signal. Lack of bioturbation in the black shale of the lower and upper members resulted from anoxic conditions. The passage from anoxic (lower member) to well-oxygenated conditions (middle member) was gradational, as is revealed by the occurrence of oxygen-deficient trace-fossils at the top of the lower member. Finally, the contrasting styles in tempestite preservation in upper-offshore deposits from the lower and upper open-marine intervals is attributed to variations in the intensity and frequency of storm during deposition of the highstand and transgressive deposits.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2012
In spite of the high interest on the Late Devonian-Early Carboniferous Bakken Formation as one of the most important oil plays in North America and the numerous studies carried out in this unit, no ichnologic analysis has been presented yet. Based on conventional sedimentologic data, previous studies of the Bakken interpreted the unit as formed entirely under open-marine conditions. However, integration of ichnology has been key to the identification of not only open-marine but also brackish-water marginal-marine conditions. Based on the geometry of the sedimentary bodies and the sequence-stratigraphic framework, the brackishwater interval is interpreted as an embayment with limited or intermittent connection to the open sea. Salinity, oxygen content, and storms are regarded as the most important environmental parameters that controlled the distribution and nature of the trace fossils in the Bakken Formation. Sparse bioturbation, relatively low ichnodiversity, and the "impoverished" Cruziana ichnofacies characterize the marginalmarine deposits, suggesting stressful brackish-water conditions. Marine deposits from the middle member, in contrast, are highly bioturbated, with a moderate ichnodiversity and the "distal" Cruziana ichnofacies that flourished during fully marine well-oxygenated conditions. There is only a slight difference between the total ichnodiversity of the open-marine interval (ten ichnogenera) and that of the brackish-water interval (seven ichnogenera). Ichnodiversity should not be considered at face value and isolated from other aspects when evaluating stress levels in order to detect a brackish-water signal. Lack of bioturbation in the black shale of the lower and upper members resulted from anoxic conditions. The passage from anoxic (lower member) to well-oxygenated conditions (middle member) was gradational, as is revealed by the occurrence of oxygen-deficient trace-fossils at the top of the lower member. Finally, the contrasting styles in tempestite preservation in upper-offshore deposits from the lower and upper open-marine intervals is attributed to variations in the intensity and frequency of storm during deposition of the highstand and transgressive deposits.
Bulletin of Canadian Petroleum Geology
In presenting this thesis in partial fulfillment of the requirements for a Postgraduate degree from the University of Saskatchewan, I agree that the Libraries of this University may make it freely available for inspection. I further agree that permission for copying of this thesis in any manner, in whole or in part, for scholarly purposes may be granted by the professor or professors who supervised my thesis work or, in their absence, by the Head of the Department or the Dean of the College in which my thesis work was done. It is understood that any copying or publication or use of this thesis or parts thereof for financial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of Saskatchewan in any scholarly use which may be made of any material in my thesis.
Sedimentary Geology, 2010
Lower Cretaceous deltaic sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. The influence of several factors on diagenesis has been investigated: depositional lithofacies, sea level changes, chemistry of basinal sediments and basinal fluid flux during burial. The distribution and chemistry of diagenetic minerals was determined from nine wells located along a dip section of the Sable Subbasin. Mineral type and paragenesis were characterized using a combination of optical petrography, back-scattered electron images, and electron microprobe analyses. Siderite is unusually abundant in marine sediments of the Scotian Basin and has thus been studied in detail. Siderite occurs in several generations. Early and late siderites are similar in chemical composition, suggesting buffering by pre-existing siderite, but rare low-Mg siderite is related to a greater contribution of meteoric water. Siderite has locally dissolved to create microporosity and has suppressed quartz overgrowths. Siderite is most common in those muddy prodeltaic lithofacies where there is the highest availability of detrital ilmenite. Reactive Fe released by breakdown of this ilmenite is responsible for the unusual presence of early siderite in marine sediments, with the Ca and Mg content of the siderite indicating fully marine waters. Lithofacies have a strong influence on early diagenetic mineral assemblages. Lithofacies deposited in the transgressive system tract have abundant early Fe-calcite and siderite. Early kaolinite occurs principally in proximal (fluvial and river mouth) lithofacies, where meteoric water was most likely available during the deposition. Contrary to other studies, we find little impact of sequence stratigraphy on diagenetic minerals except in the transgressive system tract. Mesogenetic minerals are related to flux of formation water and maturing hydrocarbon products, resulting first in pyrite and siderite and later in ankerite and ferroan calcite. The principal controls are interpreted to be high Fe 2+ and low Ca 2+ of formation waters and vertical movement of the waters along faults. Overall, the bulk chemical composition of terrigenous sediments and the depositional lithofacies are the most important factors controlling diagenetic minerals in the Lower Cretaceous of the Scotian Basin.
Marine and Petroleum Geology, 2017
The Late Devonian to Early Mississippian Bakken Formation in the Williston basin of North Dakota, USA, shows a tri-partite subdivision: a middle mixed carbonate-siliciclastic member is sandwiched inbetween two black siliciclastic mudstones, the lower and upper Bakken member shales. However, the transition from the lower shale member to the middle member does not represent a gradual coarsening but contains in places several millimeter-to centimeter-thick siliciclastic mudstones and carbonates that consist of three facies: (1) a glauconitic carbonate-rich siliciclastic mudstone, (2) a carbonate mud-to wackestone, and (3) an echinoderm wacke-to packstone with shell fragments. These three facies are present in many (all?) of the cores close and directly in the basin center in Mountrail County, North Dakota. At least one of these three facies is present in all 23 cores included in this study. This thin carbonate unit at the transition between the lower and the middle Bakken members is interpreted as representing the remnants of the transgressive systems tract. It is assumed that relative sea-level fell before deposition of the middle Bakken member establishing a proximal coarse-grained to distal fine-grained depositional transect that successively migrated into the basin. During the subsequent transgression, the siliciclastic input was low to absent, and the entire sedimentary system switched to depositing carbonates. The proximal to distal transect during this time showed coarse-grained packstones (and grainstones?) close to the shoreline, and a fining outwards towards the distal parts of the basin. This transgression also eroded what remained of the regressive and most of the subsequent transgressive sediments, leaving only the thin carbonate layer behind. Evidence for the regression, even though no sediment is directly preserved along the lower to middle Bakken member contact, comes from the fill of clastic dykes that cut through the lower Bakken member shale. The fill of the clastic dykes is partly siliciclastic and partly carbonate and not similar to any of the surrounding sediment. This indicates that these dykes must have originated before the middle Bakken member was deposited, yet the overlying sediment must have been carbonate at some point and siliciclastic another time. As it is not present anymore, this sediment must have been entirely removed by erosion. The here presented model suggests that the Bakken Formation reflects two entire sea-level oscillations. The first encompasses the lower Bakken member shale and the siliciclastic regressive portion of the lowstand only preserved as infill of the clastic dykes. The subsequent transgression deposited the carbonates now blanketing the lower to middle Bakken member transition, and the highstand and subsequent regression plus lowstand are represented by the middle Bakken member. The transgressive surface and therewith the onset of the topmost Bakken transgression is marked by the transition from the middle to the upper Bakken shale member.