Relationships between depositional environments, burial history and rock properties. Some principal aspects of diagenetic process in sedimentary basins

Reef development at the Frasnian/Famennian mass extinction boundary

Significant heterogeneity in petrophysical properties, including variations in porosity and permeability, are well documented from carbonate systems. These variations in physical properties are typically influenced by original facies heterogeneity, the early diagenetic environment, and later stage diagenetic overprint. The heterogeneities in the Mississippian Madison Formation in the Wind River basin of Wyoming are a complex interplay between these three factors whereby differences from the facies arrangement are first reduced by pervasive dolomitization, but late-stage hydrothermal diagenesis introduces additional heterogeneity.

The dolomitized portions of the Madison Formation form highly productive gas reservoirs at Madden Deep field with typical initial production rates in excess of 50 MMCFGD. In the study area, the Madison Formation is composed of four third-order depositional sequences that contain several small-scale, higher frequency cycles. The cycles and sequences display a facies partitioning with mudstone to wackestone units in the transgressive portion and skeletal and oolitic packstone and grainstone in the regressive portions. The grainstone packages are amalgamated tidally influenced skeletal and oolitic shoals that cover the entire study area. The basal three sequences are completely dolomitized, whereas the fourth sequence is limestone. The distribution of petrophysical properties in the system is influenced only in a limited manner by the smaller scale stratigraphic architecture. Porosity and permeability are controlled by the sequence-scale stratigraphic units, where uniform facies belts and pervasive dolomitization result in flow units that are basically tied to third-order depositional sequences with a thickness of 65–100 ft (20–30 m).

The best reservoir rocks are found in regressive, coarse-grained dolomites of the lower two sequences. Although the amalgamated shoal facies is heterogeneous, dolomitization decompartmentalized these cycles. Fine-grained sediments in the basal transgressive parts of these sequences, along with caliche and chert layers on top of the underlying sequences, are responsible for a decrease of porosity toward the sequence boundaries and potential flow separation. Good reservoir quality is also found in the third sequence, which is composed of dolomitized carbonate mud. However, reservoir-quality predictions in these dolomudstones are complicated by several phases of brecciation. The most influential of these brecciations is hydrothermal in origin and partly shattered the entire unit. The breccia is healed by calcite that isolates individual dolomite clasts. As a result, the permeability decreases in zones of brecciation. The late-stage calcite cementation related to the hydrothermal activity is the most important factor to create reservoir heterogeneity in the uniform third sequence, but it is also influential in the grainstone units of the first two sequences. In these sequences, the calcifying fluids invade the dolomite and partly occlude the interparticle porosity and decrease permeability to create heterogeneity in a rock in which the pervasive dolomitization previously reduced much of the influence of facies heterogeneity.

Hildegard Westphal studied geology in Tubingen, Brisbane, and Kiel, where she received her Ph.D. in 1997. After a postdoctoral position at Rosenstiel School for Marine and Atmospheric Sciences, University of Miami, she became an assistant professor at Hannover University. Currently, she is a member of the Paleontology Department at Erlangen University. Her work focuses on early diagenesis of carbonates; the genesis, diagenesis, and paleoenvironmental record of limestone-marl alternations; and paleoecological interpretation of carbonate platforms.Gregor Eberli received his Ph.D.s from the Swiss Institute of Technology (ETH), Zurich, Switzerland, in 1985 and the University of Miami in 1991. With his colleagues at the Comparative Sedimentology Laboratory, he conducts research in sedimentology, stratigraphy, geochemistry, and petrophysics of modern and ancient carbonates. In several projects, he investigated the influence of sea level changes on sedimentary architecture. He was an AAPG Distinguished Lecturer in 1996–1997 and a Joint Oceanographic Institutions/U.S. Science Advisory Committee Distinguished Lecturer in 1998–1999. Langhorne Smith currently heads the Reservoir Characterization Group at the New York State Museum. He holds a B.S. degree from Temple University, a Ph.D. from Virginia Tech, and did postdoctoral work at the University of Miami. He also worked for Chevron as a development geologist for two years. His current research interests are in carbonate reservoir characterization and hydrothermal alteration of carbonate reservoirs. G. Michael Grammer is an associate professor at Western Michigan University. His research includes high-resolution carbonate sequence stratigraphy and early diagenesis and their application to reservoir characterization. He was an AAPG Distinguished Lecturer for 2002–2003 and is a coleader of AAPG's modern carbonate field course. Previously, he was a senior research associate for Texaco and has consulted for domestic and international oil companies. He received his Ph.D. from the University of Miami in 1991. Jason Kislak received his bachelor's degree from Franklin & Marshall College in Lancaster, Pennsylvania. He is currently working toward his master's degree at the Rosenstiel School of Marine and Atmospheric Science at the University of Miami.

Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming

Carbonate platforms can commonly keep up with relative sea-level rise because of high rates of sediment accumulation and platform aggradation. Surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and episodically deposited carbonate sediment. If accumulation rates in basinal settings lag behind those of the platform, a bypass or erosional margin can develop. Under these circumstances platform and basin depositional sequences become physically detached and direct correlation of basinal and platform sequences is hindered. We report here the results of high-resolution stratigraphic analyses of two Upper Devonian isolated carbonate platforms in western Alberta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences. The slope and basin sequences surrounding the Miette and Ancient Wall platforms consist of basin-restricted, onlapping wedges of fine-grained background sediment deposited dominantly from suspension and coarse-grained platform-derived sediment redeposited by a variety of gravity-flow mechanisms. Sequence boundaries are identified within the redeposited carbonate intervals. Identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry, mineralogy, and clast content of redeposited carbonate units. Highstand carbonates contain sheet-like debris flows and turbidites with abundant slope-derived clasts and background facies with high total carbonate content. Lowstand carbonates contain sheet-like and channelized debris flows and turbidites with abundant platform-derived clasts and background facies with low carbonate content and locally high amounts of organic carbon. Transgressive facies are dominated by initially carbonate-poor and organic-rich background sediments that display a progressive increase in carbonate content and decrease in organic carbon content. These patterns are interpreted to record abundant background carbonate sedimentation during late transgression and highstand when the carbonate factory was robust. Highstand redeposited carbonates record slope erosion due to oversteepening and slope readjustment processes. Lowstand redeposited carbonates indicate platform and platform-margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions. High siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimentation but alternatively may represent continuous siliciclastic supply during times with little dilution by fine-grained carbonate sediment. Successive stages of platform development at Miette and Ancient Wall were controlled by accommodation changes driven by relative sea-level fluctuations. Backstripping analyses of strata from both platforms confirm that significant differential subsidence was a major control on variations in platform thickness and patterns of slope development. Greater subsidence at Ancient Wall fostered the development of a steeper bypass margin and different slope evolution compared to Miette. Slope oversteepening also initiated a process of slope readjustment that eventually reduced the platform-to-basin gradient and facilitated regressive platform progradation. In conventional siliciclastic sequence stratigraphy, basin-restricted wedges are interpreted as lowstand deposits on the basis of their geometry and position relative to an updip margin. Wedge-shaped basinal units along the Miette and Ancient Wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries. The results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through mineralogic and compositional analyses providing more accurate correlation of detached platform and basin sequences. Interpretation of carbonate basin-restricted wedges as purely highstand or lowstand deposits may lead to erroneous conclusions regarding sequence stratigraphy, platform-to-basin correlation, and the volumetric partitioning of sediments deposited in different systems tracts.

Bypass Margins, Basin-Restricted Wedges, and Platform-to-Basin Correlation, Upper Devonian, Canadian Rocky Mountains: Implications for Sequence Stratigraphy of Carbonate Platform Systems

Reservoir characterization of burrow-associated dolomites in the Upper Devonian Wabamun Group, Pine Creek gas field, central Alberta, Canada

Madison group sediments accumulated in two depositional realms: a basinal, predominantly limestone province in Montana and adjoining Dakotas; and the Wyoming shelf, characterized by prominent dolomitization on the south. The belt of transition in southern Montana and adjoining limestone and shale deposition took place in the Idaho geosynclinal trend. Southeastward to the Cambridge arch, Madison group sediments were affected by increasing amounts of pre-Pennsylvanian erosion in Wyoming and western South Dakota. Mississippian sediments are absent from a part of southeastern Wyoming. Early, medial, and late stages of sedimentation may be differentiated on the basis of lithologic cycles or sequences developed in the Madison group. Early sedimentation was characterized by deposition of the approximate Lodgepole formation and equivalents, Guernsey, Englewood, and lower Pahasapa formations. Deposition commenced earliest in the Williston basin and central Montana trough with the development of black shales, followed by dense limestone and shales; minor terrestrial to near-shore marine clastic sedimentation in the Black Hills area commenced somewhat later, preceding and accompanying the initial marine transgression on the Wyoming shelf. Widespread deposition of normal marine and fragmental oolitic limestones (mainly dolomitized equivalents in the shelf areas) occurred. hoaling was prominent, and as a result local minor restricted deposition took place. Marine limestone and dolomite sedimentation, modified by two episodes of evaporite precipitation, occurred during medial Madison group time. Mission Canyon carbonates containing an intermediate evaporite zone and overlain by the prominent basal Charles evaporites constitute the resulting middle unit. Evaporitic deposition was characterized by the precipitation of anhydrite and halite in the basinal areas at the north. Southward, progressively less restriction is demonstrated by the disappearance of halite, thinning of anhydrite, and the development of primary dense carbonate zones. Late sedimentation was characterized by restoration of normal marine conditions and development of a complex evaporite cycle (Charles formation, excluding basal evaporites). Big Snowy clastics were deposited in Chester time in the northern basinal areas. In other areas, pre-Amsden or pre-Pennsylvanian to pre-Middle Jurassic erosion had varying effects on the Madison group surface. Limestone reservoirs in the basinal areas of Montana and North Dakota, and dolomites in northwestern Wyoming yield group oil. Optimum facies changes and post-depositional tectonic effects make the madison group an excellent prospective unit in many areas. End_Page 2170------------------------------

Mississippian Madison group stratigraphy and sedimentation in Wyoming and southern Montana

In central Alberta, the Cooking Lake limestone or foundation unit for reef growth is divisible into four widespread lithologic units, in ascending order: basal dolomitic unit, lower calcarenite, middle argillaceous unit, and upper calcarenite. Carbonate buildup in the Cooking Lake in reef areas is marked generally by coalescence of the lower and upper calcarenites at the expense of the middle argillaceous unit, and by thickening of the upper calcarenite at the expense of the overlying Duvernay (brown to black shales and limestones). Limestones ranging from calcarenites to calcilutites and exhibiting negligible to complete dolomitization are present in the Cooking Lake and Leduc formations. The calcarenites comprise bioclastic and pseudo-oolitic (precipitated) types, and likewise the calcilutites include both bioclastic material and precipitates. Shallow- and agitated-water calcarenites and local patch reefs commenced to form in latest Beaverhill Lake time along a restricted north-south axis passing through the North Woodbend and South Acheson fields in the Leduc area. Reef sedimentation, initially confined to a narrow 2-3-mile belt along the western part of the shoal, onlapped eastward from a few to 10 miles by late Cooking Lake time. The ancient shoal is now marked by completely dolomitized limestones which extend End_Page 1------------------------------ farthest eastward in the area directly southwest of Edmonton ("Whitemud lobe"). On this relatively broad foundation, complete reef buildup was largely localized in the western part of the foundation. The final reef configuration, conforming closely to the productive Acheson, Leduc, Glen Park, and Wizard Lake fields, is marked by intervening channels or saddles. Formation of the northeast and southeast reef extensions at the Leduc field is believed to have been controlled in part by ocean currents in later Leduc time. In the Stettler area, late Cooking Lake sedimentation produced shallow-water limestones, now dolomitized, along three main trends termed Fenn-Big Valley, Stettler-Erskine, and Gadsby shoals. The Gadsby shoal was characterized by widespread biostromal carbonate deposition in the northern part of the Southern Alberta shelf. The south-southwest-trending Fenn-Big Valley shoal, apparently extending several miles southwest of Big Valley field, marked the site of earliest reefoid deposition (latest Cooking Lake time), followed somewhat later (early Duvernay time) by incipient reef foundation development in the Stettler and Erskine field areas. Later, mainly vertical carbonate buildup occurred along the central parts of the Fenn-Big Valley and Stettler-Erskine shoals. However, lateral growth nd onlap occurred west of the Fenn field and between the Stettler and Fenn-Big Valley fields, and caused the final Leduc configuration to extend south continuously from the Stettler to the Big Valley field, as well as west of these fields. Although a common carbonate foundation underlay the Stettler and Erskine fields, localization into separate reefs occurred. In the Redwater area, a prominent shoal exhibited deposition of relatively uniform limestone types and organic content throughout, except at the eastern margin. In the lower and upper Cooking Lake relatively narrow algal-stromatoporoid biofacies belts are developed at 6 to 10 and 4 to 5 miles, respectively, east of, and subparallel with, the Redwater oil field. Essential coalescence of the lower and upper calcarenites and carbonate buildup in the Cooking Lake at the expense of the overlying Duvernay characterizes the biostromal limestone foundation as far as its eastern margin at the algal concentration. It is interpreted that the biostromal margins in the Cooking Lake east of Redwater represent incipient reef margins or fronts, which migrated westward between Cooking Lake and late Le uc time. In the uppermost Leduc reef algal structures are concentrated in a narrow easternmost belt parallel with the reef front in the eastern part of the Redwater field. Stromatoporoids and associated organisms formed part of the "organic lattice" in a belt adjoining at the west. In turn, a non-stromatoporoidal biofacies, including precipitated and poorly fossiliferous limestones, was deposited west of the organic lattice. Calcarenite and biofacies patterns indicate that the organic lattice at Redwater represented an actively growing reef front subjected to ocean currents and possibly prevailing winds from the northeast. West of the organic lattice some relatively quiet, locally restricted lagoon-type sedimentation occurred. The upper deposits in the central part of the Redwater reef mass are believed to have been subject to solution-compaction effects, resulting in a slightly thinner Leduc section as compared with the margins. The highest level of occurrence of significant stromatoporoids, a biostratigraphic boundary, rises in the Leduc limestone section at least 500 feet from the interior to marginal parts of the Redwater reef mass. Lithofacies and biofacies studies of the Cooking Lake foundation may be applied in delineation of favorable areas for reef exploration. Patterns on facies maps showing early shoals or incipient algal-stromatoporoid belts may be used in determining possible reef extensions as well as outlining reef prospects in wildcat areas.

Stratigraphy and Facies Analysis of Upper Devonian Reefs in Leduc, Stettler, and Redwater Areas, Alberta

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

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

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

Late Devonian Geologic History in Stettler Area, Alberta, Canada

Devonian strata are traced throughout the Northern Rocky Mountain and Great Plains areas which include Wyoming, Idaho, Montana, southern Saskatchewan, and Alberta. The diverse Devonian nomenclatures between states and provinces are unified, and the areal limits of applicability of established formational names as cartographic units are demonstrated. Isopach and lithofacies studies are employed as a basis for interpreting the sedimentation processes and patterns responsible for the Devonian rocks now observed. In order to work out the dynamic picture of Devonian sedimentation, it is necessary to subdivide the total section into four operational rock units which are recognizable throughout the area on the basis of suitable lithologic criteria; the individual isopach and lithofacies maps of these units are interpreted in terms of the tectonic and environmental conditions responsible for the patterns exhibited. Finally, an integration of these four arbitrary stages of Devonian sedimentation presents the regional Devonian geologic history. The rock units include a basal Devonian unit, a lower limestone unit, a dolomite-evaporite unit, and a post-evaporite unit. Clastics (shale and sand), carbonates (limestone and dolomite), and evaporites (anhydrite and halite) constitute the three significant petrographic end members used in the statistical lithologic triangle. The two indices defining the lithologic variations are the clastic ratio and evaporite ratio. The important tectonic elements which were active during Devonian sedimentation include: Elk Point basin (Middle Devonian age) extending from east-central Alberta into southern Saskatchewan; Central Alberta basin; Southern Alberta arch; Central Montana positive axis; Southern Montana trough; Wyoming shelf; and Cambridge arch of Eardley in southeastern Montana and Wyoming (northwestern extension of Siouxia landmass). Evaporitic sediments are prominently developed in the basal and dolomite-evaporite units; and normal marine deposits, including reef-carbonates, are characteristic of the lower limestone unit.

Regional Stratigraphic Analysis of Devonian System in Wyoming, Montana, Southern Saskatchewan, and Alberta

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