Timing Of Diagenesis Research Articles

Uji Laboratorium Sampel Core Plug untuk Menentukan Porositas, Permeabilitas dan Saturasi Minyak pada Reservoir Batugamping

Porosity, permeability, and fluid saturation (gas, oil, water) are three important physical properties of rocks, especially in oil and gas reservoirs (oil and gas) which are carried out in core rock analysis. The purpose of this study is to determine the relationship between porosity, permeability, and oil saturation in limestone (limestone) from a horizontal plug core sample to a vertical core plug sample, to be associated with its distribution vertically and horizontally (laterally). The research method used is a Lab Test conducted at the Routine Core Analysis (RCAL) Laboratory at PPTMBG "Lemigas" Jakarta, and graphical analysis of the measurement results. The core plug samples used were 21 (twenty one) which were taken horizontally and 10 were taken vertically, which has been described, from conventional core samples from limestone reservoirs. The results of his research are that the porosity values ​​of the vertical and horizontal samples of limestone have an indistinguishable trend, while limestones without chalky have a deeper trend, the porosity is greater, while porosity with chalky has a scatter trend, this indicates that the porosity of limestones is higher. influenced by lithological conditions or depending on the facies and the time of diagenesis, not depending on the depth (as in the case of sandstone). The value of oil saturation in the vertical and horizontal samples has a distribution that can be said to not show a certain trend or scatter. The relationship between porosity and permeability is exponentially proportional, regardless of depth and vertical or horizontal core sampling position. The value of porosity and permeability of limestone with the presence of chalky is more dominant, significantly increasing the value of porosity and permeability compared to that without the presence of chalky.

Diagenesis and fluid evolution in the third member of the Eocene Shahejie Formation, Bonan Sag, Bohai Bay Basin, China

Diagenesis significantly influences sandstone reservoir quality, so that a detailed understanding of diagenesis is essential for the identification and prediction of favorable reservoirs. Several studies have been conducted on individual sandstone reservoirs in the Bonan Sag, a typical half-graben, oil-producing sag in eastern China, but few have explicitly considered the potential impact of the asymmetric shape of the sag, which is steep in the north and gentle in the south. This research takes full consideration of the tectonic setting and distribution of source rocks in the sag to compare the style, magnitude and timing of diagenesis in the northern Steep Slope and Sag Zone (SSSZ) and the southern Gentle Slope Zone (GSZ) by means of drill core observations, thin section petrography, high resolution scanning electron microscopy, fluid inclusion microthermometry, as well as cathodoluminescence and carbonate stable isotope analysis. Petrographic observations identify the wide-spread occurrence of authigenic quartz and clay minerals in association with dissolved feldspar, as well as abundant carbonate cements occurring as several distinct generations due to evolving pore fluid composition. The SSSZ has variable carbonate mineralogy while ankerite dominates in the GSZ. The homogenization temperature of fluid inclusions in authigenic quartz in the SSSZ has a wider range (93°C–157.9 °C) and lower starting temperature (93 °C) compared with the GSZ (106.2 °C) and is characterized by more lighter carbonate δ13C and δ18O values (δ18O: −14.8‰~-11.3‰, δ13C: 1.1‰–2.7‰ for the SSSZ; δ18O: −11.9‰~-5.8‰, δ13C: 2.9‰–5.5‰ for the GSZ). The results indicate that (1) Diagenetic conditions in the Bonan Sag evolved from alkaline to acidic to alkaline in general, but with distinct late stage carbonate cementation in SSSZ vs GSZ; (2) the diagenetic processes as well as the nature of pore waters varied across different zones and also temporally. Acidic conditions in the SSSZ started earlier and persisted for longer than in the GSZ. (3) The regional differentiation of the diagenetic processes and pore fluid evolution result from the source rock distribution and burial histories which differ across the Bonan Sag because of the asymmetric shape of the half-graben. We propose a model of fluid evolution over space and time which may provide a new framework for understanding the diagenetic processes and the fluid evolution of oil-producing sags and also contribute to the prediction of favorable reservoirs.

The Timing of Diagenesis and Thermal Maturation of the Cretaceous Marias River Shale, Disturbed Belt, Montana

AbstractThe hypothesis that chemical remanent magnetization (CRM) in argillaceous rocks may be due to release of Fe during smectite illitization has been tested by study of spatial and temporal relationships of CRM acquisition, smectite illitization, and organic-matter maturation to deformation in the Montana Disturbed Belt. New K-Ar ages and stacking order and percentages of illite layers in illite-smectite (I-S) are consistent with conclusions from previous studies that smectite illitization of bentonites in Subbelts I and II of the Disturbed Belt was produced by thrust-sheet burial resulting from the Laramide Orogeny. Internally concordant, early Paleogene, K-Ar age values (55–57 Ma) were obtained from clay subfractions of thick bentonites which were significantly different in terms of their ages (i.e. Jurassic Ellis Formation and late Cretaceous Marias River Shale), further supporting a model of smectite illitization as a result of the Laramide Orogeny. Internally concordant K-Ar ages were found also for clay sub-fractions from a thick bentonite at Pishkun Canal (54 Ma) and from an undeformed bentonite near Vaughn on the Sweetgrass Arch (48 Ma). In Subbelts I and II, a greater degree of smectite illitization corresponds to increased thermal maturation, increased natural remanent magnetization intensity, and increased deformation (dip of beds). A dissolution-precipitation model over a short duration is proposed for the formation of illite layers in Subbelts I and II. A characteristic remanent magnetization was developed before or just after folding began in the early Paleogene. More smectite-rich I-S, low thermal maturity, and the absence of a CRM were noted in one outcrop of an undeformed rock on the Sweetgrass Arch. Strontium isotope data allow for the possibility that internal or externally derived fluids may have influenced illitization, but the K-Ar age values suggest that illitization was probably in response to conductive heating after the overthrusting had occurred. The differences in K-Ar dates among the bentonites studied herein may be due to differences in the timing of peak temperature related to differences in distance below the overthrust slab, in rates of burial and exhumation, and in initial temperature.

Timing of diagenesis and very low‐grade metamorphism in the eastern sector of the Sierra de Cameros (Iberian Range, Spain): a U–Pb SHRIMP study on monazite

AbstractThe Sierra de Cameros is an intracontinental orogen and represents the north‐western part of the Iberian Range in Northern Spain. It comprises a thick sequence of syn‐rift continental sediments (mainly sandstones and carbonates) deposited during lower Cretaceous times. A unique characteristic of the Sierra de Cameros in relation to the rest of the Iberian Range is the presence of low‐grade metamorphism in certain parts of the basin, an event that predates basin inversion. This paper describes the presence, textural relationships and geochemical aspects of authigenic and/or metamorphic monazite within different lithologies from the deepest parts of the basin. Sensitive high resolution ion microprobe (SHRIMP) U–Pb dating of monazite records the presence of two age populations: the first with 206Pb/238U ages ranging from 122 to 116 Ma which is considered as diagenetic in origin, whilst the second is dated at 99 ± 2 Ma and postdates the metamorphic climax.

ABSTRACT: Source and Timing of Diagenesis in North Sea, Upper Jurassic Reservoirs

ABSTRACT: Source and Timing of Diagenesis in North Sea, Upper Jurassic Reservoirs

40Ar/39Ar illite dating of Late Caledonian (Acadian) metamorphism and cooling of K-bentonites and slates from the Welsh Basin, U.K.

The vacuum-encapsulation laser 40Ar39Ar technique allows extremely small (10−6 g) samples of fine-grained materials such as diagenetic clays to be dated. Here we show that the method can be extended to higher-grade clay minerals. The integration of transmission electron microscopic (TEM) characterization with 40Ar39Ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth/cooling from the time of diagenesis. We have applied this technique to well characterized Lower Paleozoic slates and K-bentonites from the Welsh Basin, which span the transition from anchizonal to epizonal grade, which had been previously studied using RbSr and SmNd dating.TEM observations of epizonal K-bentonites and slate showed that illite in these samples is of 2M1 polytype, of muscovite-like composition, and oriented parallel to cleavage, suggesting that they are of metamorphic origin. Total gas ages (equivalent to conventional KAr ages) for encapsulated epizonal K-bentonites and slate (340–408 Ma) are considerably variable. The Ar retention ages (calculated from 39Ar and 40Ar atoms retained in the sample after irradiation) are more consistent (383–411 Ma). The 39Ar recoil losses are minor for illites from whole rock samples of epizonal K-bentonites but very significant for clay separates of epizonal slate. Plateaus in age spectra were observed in epizonal K-bentonites and slate. The plateau ages (414–421 Ma) and retention ages (383–411 Ma) can be correlated with the onset of Acadian metamorphism and culmination of uplift and inversion of the Welsh Basin, respectively. These ages are significantly younger than the ∼ 450 Ma ages previously reported for diagenetic clays using the same method, suggesting that diagenetic history has been lost in these epizonal K-bentonites and slate.TEM observations of anchizonal slates showed that there are two modes of illite. The first mode is similar to that observed in epizonal samples, suggesting a metamorphic origin. The second mode consists of the 1Md polytype, has typical diagenetic illite composition, and is oriented parallel to bedding, suggesting a diagenetic origin. Total gas ages for encapsulated anchizonal slates vary considerably (361–422 Ma). The retention ages are more consistent (413–432 Ma) than the total gas ages. The 39Ar recoil losses are more significant than those for epizonal K-bentonites and slate. Plateaus in age spectra are generally not observed. However, the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples, and are inferred to represent the onset of Acadian metamorphism.These data, when combined with our previously published results for diagenetic shales, suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset Ar systems in shales and slates.

Rb-Sr Ages of Diagenesis of Mg-rich Clay in Permian Sediments, Palo Duro Basin, Texas Panhandle, U.S.A.

ABSTRACT Mid- to upper Permian sedimentary rocks in the shallow (< 3 km), intracratonic Palo Duro Basin, Texas Panhandle, occur in multicyclic carbonate-evaporite-redbed sequences, interpreted to be hypersaline shelf depositional assemblages. Certain marker horizons can be traced continuously over the entire basin. Clay minerals include distinctive varieties of Mg-rich smectite, chlorite, or mixed-layer species resulting from diagenesis in an evaporitic environment. Discrete particle-size fractions of clay were separated from core recovered from the Tubb (Leonardian), San Andres Unit #4 (lower Guadalupian), upper Seven Rivers (upper Guadalupian), and Salado-Tansill (Guadalupian-Ochoan) Formations to determine the times of mineral genesis. Rb-Sr isotopic data, for aliquots of a clay sample that has been leached by diverse ion exchange reagents, plot as a linear array in a Rb-Sr isochron diagram. For ultra-fine clay (< 0.05 µm), interpreted to be diagenetic, the slope of the data array corresponds to the time of diagenesis. Ages of clay diagenesis in the four sampled stratigraphic units agree generally with ages of deposition according to the geologic time scale, signifying that diagenesis was penecontemporaneous with deposition. Analyses of coarser clay fractions (0.2-0.5 µm and 2-4 µm) indicate the presence of a detrital component. In most instances, initial 87Sr/86Sr in diagenetic clay is higher than the Sr isotope ratio measured directly in associated anhydrite. Introduction of radiogenic Sr from occasional stream influx or from concurrent diagenetic dissolution of silicate detritus could have elevated 87Sr/86Sr in the ambient brine that mediated clay diagenesis.

K/Ar Dating of Authigenic Illite Constrains the Time of Diagenesis and Brine Migration in the Weber Sandstone of the Uinta-Piceance Basin, Colorado and Utah: ABSTRACT

K/Ar Dating of Authigenic Illite Constrains the Time of Diagenesis and Brine Migration in the Weber Sandstone of the Uinta-Piceance Basin, Colorado and Utah: ABSTRACT

Carbonate cement stratigraphy and timing of diagenesis in a Miocene mixed carbonate-clastic sequence, offshore Sabah, Malaysia: constraints from cathodoluminescence, geochemistry, and isotope studies

A mixed carbonate-clastic sequence is one of several Miocene hydrocarbon-bearing reservoirs in the northwestern Sabah Basin. This sequence consists of bioclast-rich sandstones interpreted as prograding storm shoal deposits. The sequence contains a complex variety of carbonate nodules.The sequence has undergone nine stages of cementation that completely occluded pores. Each stage represents a distinctive cement texture, precipitating at specific temperatures and burial conditions. The diagenetic evolution began with a syndepositional iron-free marine calcite (Ca1), followed by an early methane-derived dolomite (Do1), bladed calcite (Ca2), blocky, vein-filling Fe-calcites (Ca3Ca4, clay mineral-associated dolomite (Do2), Fe-dolomite and ankerite (Do3Do4, and late iron-rich calcite (Ca5). Each stage of cement has distinctive Sr2+, Mn2+, Mg2+, Fe2+, Ca2+, and Na+ contents, reflecting changes in the palaeo-porefluid systems with time. The cements are correlatable over the whole study area.Precipitation occurred from very early to late stages of diagenesis at near surface to 2.0 km depths. With progressive burial and temperature increase, oxygen isotope values become strongly negative (stage Ca4: δ13C = −0.5%. PDB, δ18O = −5.7%. PDB). The cements were precipitated at 20–75°C. The source of carbon varies from marine dissolved carbonates to fermentation of organic matter, methane, meteoric waters, and possibly hydrothermal fluids.Strontium isotope dating of Do1 (with δ13C = −32.5%. PDB, δ18O = +2.60%. PDB) and Do2 (δ13C = 3.0%. PDB; δ18O = −2.5%. PDB) dolomites indicates that these dolomitization events took place at 10.5 Ma and 8.9 Ma, respectively, assuming precipitation within a completely open seawater system. The Do1 dolomite was precipitated from a methane-derived brine at the boundary between the Serravallian and Tortonian stages after major uplift, and induced faulting, at a sea-level lowstand. The Do2 dolomite was precipitated during intermediate to deep burial diagenesis during smectite-illite transformation before hydrocarbon emplacement.

Timing of Diagenesis and Petroleum Entrapment in the Papuan Basin, Papua New Guinea: ABSTRACT

Timing of Diagenesis and Petroleum Entrapment in the Papuan Basin, Papua New Guinea: ABSTRACT

Mapping diagenetic fluid flow within a reservoir: KAr dating in the Alwyn area (UK North Sea)

The Alwyn area demonstrates that variation in the timing of diagenesis on the field scale depends on the structure and the proximity to the sources of diagenetic fluids. Variation in reservoir quality within the structure is the result of facies control on fluid migration. Illite in Alwyn South formed in the Middle Palaeocene to Middle Oligocene between 60 and 38 Ma. At this time the Brent Group was at depths of between 2400 and 3000 m and reservoir temperatures ranged from 110 to 120°C. Extensive KAr dating shows that illitization, pre-oil emplacement, proceeded from the deepest reservoir panels along and up-structure. Fluid migration may have occurred along the deep seated Alwyn South-Ninian Fault System. Where the reservoir is lithologically homogeneous illite precipitation proceeded systematically base-upwards at rates of 5.5 m Ma −1. The duration of illite formation at any one level was between 2 and 8 Ma. In contrast, in heterolithic reservoirs fluid migration occurred initially along high permeability fairways and consequently illite ages do not vary systematically with depth.

Determination of initial Sr isotopic compositions of dolostones from the Burlington-Keokuk Formation (Mississippian); constraints from cathodoluminescence, glauconite paragenesis and analytical methods

ABSTRACT Regionally extensive, fine-grained dolostones from the Mississippian Burlington-Keokuk Fm. comprise two major generations of dolomite that can be distinguished by their cathodoluminescence, chemical compositions, and times of formation. The petrography and geochemistry of glauconite, the predominant clay mineral in the dolostones, indicates that it is an authigenic phase, some of which formed episodically during various stages of postmarine diagenesis. This study evaluates techniques for the determination of the Sr isotopic compositions of the dolomite generations through the analysis of nearly pure whole-rock samples ( 95% soluble) and mineral separates, each composed predominantly of one cathodoluminescent type of dolomite. Sr isotopic compositions and Rb and Sr concentrations were measured on samples prepared by three separate methods: 1) leaching with 1.25N HCl, 2) leaching with 1.25N HCl and concentrated HF, and 3) complete dissolution with concentrated HF and HNO3, and 6N HCl. Leaching with HCl removes 24-60 percent of the Rb in the samples. Compared to acid leaching, complete sample dissolution yields higher 87Sr/86Sr ratios and Rb concentrations and more reproducible results. This indicates that most of the Rb and radiogenic 87Sr is associated with minor clay fractions in the samples and that HCl leaches Rb and Sr from clays to a variable extent. The relatively high Rb/Sr ratios of the dolostones necessitate an age correction of each sample's 87Sr/86Sr ratio. The determination of such initial 87Sr/86Sr ratios requires contemporaneous formation of carbonate and clay, closure of the Rb-Sr system after diagenesis, and constraints on the timing of diagenesis. Calculated initial 87Sr/86Sr ratios for the dolostones by method 3 differ considerably from the uncorrected measured 87Sr/86Sr ratios by any of the dissolution methods. These results demonstrate that even minor clay components can make significant contributions to the analyses, and that analysis for Rb and Sr, and constraints on the timing and diagenetic history of both the clay and carbon te phases can be critical for the determination of the Sr isotopic compositions of even nearly pure dolostones. An evaluation of the uncertainties associated with the timing and type of diagenesis indicates that the initial 87Sr/86Sr ratios of the dolostones are representative of the isotopic composition of the dolomite generations at the time of their formation. Two major cathodoluminescent types of Burlington-Keokuk dolomites preserve a record of two isotopically distinct diagenetic environments. Samples composed of the earliest dolomite (dolomite I) have initial 87Sr/86Sr ratios ranging from 0.70757 to 0.70808, while samples composed of dolomites II and II range from 0.70885 to 0.70942.

THE INFORMATIVENESS OF THE PRISTANE/PHYTANE RATIO AS A GENETIC INDEX

(1987). THE INFORMATIVENESS OF THE PRISTANE/PHYTANE RATIO AS A GENETIC INDEX. International Geology Review: Vol. 29, No. 9, pp. 1117-1120.

Diagenesis of US Gulf Coast shales.

The burial diagenesis of mudstones is of major interest in petroleum geology1-3. Key questions which remain incompletely answered include the relative importance of temperature, time, changes in pore-water chemistry and organic maturation as driving mechanisms, and the relationship between diagenesis in mudstones and neighbouring sandstones4-7. We reconsider here the nature and timing of diagenesis in Gulf Coast Tertiary mudstones using data obtained by backscattered electron microscopy (BSEM) and energy-dispersive X-ray analysis (EDS) of cuttings from two Texas wells. Our observations confirm the mineralogical trends with depth reported by Hower et al.1 and suggest new evidence concerning the timing and causes of the changes. The most important mineralogical changes occurred in the zone of organic matter decarboxylation (zone IV8). We show that the sequence of burial diagenetic events can be established quickly and reliably by BSEM examination of shale cuttings, and demonstrate the use of foram tests and their authigenic mineral infillings as indicators of diagenesis in soft, fine-grained shales.

Determination of Initial Sr-Isotope Compositions of Dolostones from the Burlington-Keokuk Formations (Mississippian) of Iowa, Illinois, and Missouri: ABSTRACT

Sr-isotope compositions, and Rb and Sr concentrations were measured on fine-grained, nearly pure dolostones, each consisting predominantly of one cathodoluminescent generation of dolomite. The results differ significantly, depending on the dissolution method used. For example, one sample gives replicate /sup 87/Sr//sup 86/Sr analyses by leaching with 1.25N HCl of 0.71032 and 0.71062. Adding HF to the leaching process gave a value of 0.71216, whereas complete dissolution with concentrated HF, HNO/sub 3/, and HCl yields a value of 0.71219. Using the time of dolomitization with Rb (12.4 ppm) and Sr (59.7 ppm) concentrations for the completely dissolved sample gives an initial /sup 87/Sr//sup 86/Sr ratio of 0.70943. Similar results for other samples indicate that most Rb and radiogenic Sr are associated with minor clay fractions (predominantly glauconite) in the samples, and that HCl leaches Rb and Sr from clays to a variable extent. Morphologic relationships between dolomite, calcite, and nonpelloidal glauconite suggest that some clay formed episodically during various stages of nonmarine diagenesis. Sr-isotopic exchange may have occurred between carbonate and clay during major influxes of diagenetic fluids. To obtain initial /sup 87/Sr//sup 86/Sr ratios on samples that are completely dissolved, the following corrections are needed: isotopic equilibration between carbonate andmore » clay, closure of the Rb-Sr system after diagenesis, and constraints on the timing of diagenesis. Such initial /sup 87/Sr//sup 86/Sr ratios give the isotopic composition of the carbonate phase at the time of its crystallization, whereas leaching with weak acid is not always carbonate selective and can produce ambiguous results.« less

Thermal History of Sandstones and Shales: Oxygen Isotope and K/Ar Evidence: ABSTRACT

The combined use of oxygen isotope and K/Ar dating can yield information about the diagenetic history of clay minerals in shales and sandstones. In the Tertiary shale sequences of the United States Gulf Coast, in which the dominant detrital clay mineral is mixed-layer illite/smectite, the progressive conversion of expandable clay layers to illite layers can be monitored by the isotope systematics. Oxygen isotopes of fine-grained clay and quartz approach equilibrium with one another, raising the (as yet unrealized) possibility of an O-isotope geothermometer. Fine-grained quartz becomes isotopically zoned as detrital grains are overgrown by diagenetic quartz that forms, accompanying the smectite-to-illite transformation. The K/Ar clock of the diagenetically formed illite la ers is set to zero age at the time of diagenesis (although that of existing illite layers within the crystals remains unaffected), and it is therefore possible to estimate the mean time of diagenesis in such shale sequences. Illite is a common cement in sandstones. The time of cementation by illite can be estimated from K/Ar systematics in cases, such as that of the Permian Rotliegendes of the North Sea, where the clay-sized fraction of the original detritus was relatively free of illite or other K-bearing phases. Conditions of cementation can be inferred from oxygen isotope measurements, augmented by knowledge of the geologic/tectonic history of the sandstone. In the case of the Rotliegendes, the timing and conditions of illite cementation were relatively uniform within fault blocks but varied from block to block. Oxygen isotope measurements indicate that meteoric water components were important in illite formation at times when nearby sections of the Rotliegendes were exposed to the surface by uplift an erosion. End_of_Article - Last_Page 304------------

A paleohydrologic model for early Proterozoic dolomitization and silicification

Primary depositional features observed in the intertidal and supratidal facies of the 2.25 Ga Malmani Dolomite (including tepees and finely laminated domal stromatolites) have equivalents in Holocene salinas on the southern coast of Australia. The hydrological regime, so important in the formation of these recent primary features, is still active and so it has been possible to interpret the palaeohydrological significance of analogous features in the Malmani Dolomite. The palaeoclimate of the Malmani Dolomite shoreline was probably humid to semi-arid and the supratidal zone is interpreted as a zone of groundwater resurgence. Tepees, indurated fenestral limestone sheets, algal mats, small domal stromatolites and intraclast breccias were forming on the supratidal flats in areas of periodically outcropping groundwaters. In the shallow subsurface, the mixing zone between these resurging continental groundwaters and the marine-derived phreatic waters was probably a zone of secondary coarse-grained dolomite precipitation and/or chert nodule formation. This groundwater-controlled mixing zone model explains why such fabrics are restricted to porous and immediately adjacent units which, at the time of diagenesis, were below the regional water table. In the upper supratidal zones, above the regional water table, no secondary dolomites or chert nodules could form, nor could diagenetic fabrics form in the subtidal zone for there the stromatolitic mounds contained no palaeoaquifer.

Comparison of Pore-Filling Material in Some Pennsylvanian and Cretaceous Reservoir and Nonreservoir Sandstones: ABSTRACT

SEM and thin-section study of core samples from producing sandstones and associated fine-grained facies from several stratigraphic intervals of Pennsylvanian and Cretaceous age in the Rocky Mountain region show different processes of pore-size reduction relating to: (1) original pore-fluid composition; (2) grain size and texture; (3) depositional environment; (4) timing of diagenesis; and (5) depth of burial. The sandstones studied and depositional environments are: Tyler Sandstone, southwestern North Dakota, shoreline (barrier island); J sandstone, northeastern Colorado, distributary channel and channel margin; Frontier Sandstone, northeastern Wyoming, offshore marine bar; Almond Sandstone, southwestern Wyoming, shoreline (barrier island or tidal channel); Terry and Hygiene sandstones, northeastern Colorado, offshore marine bar; and Raton Formation, southeastern Colorado, fluvial channel and channel margin. All the reservoir sandstones were deposited by relatively high-energy transport processes in fluvial, brackish, or marine depositional environments, and represent initially clean sandstones with relatively good porosity and permeability. Pore space in five of the six reservoir sandstones has been initially reduced by precipitated kaolinite or chlorite. The Tyler Sandstone, a nearshore sandstone which underwent early near-surface diagenesis, exhibits reduced porosity related to precipitation of caliche. Associated fine-grained, nonreservoir facies show a pore-filling composition that differs from the reservoir facies in both the marine and nonmarine sequences. End_of_Article - Last_Page 550------------

Allochthonous Carbonate Debris Flows at Devonian Bank (Reef") Margins, Alberta: ABSTRACT"

ABSTRACT Field work indicates that allochthonous carbonate debris flow deposits containing large blocks occur locally in upper Perdrix and Mount Hawk basin strata adjacent to three Devonian reef complexes -- Ancient Wall, Miette and Southesk-Cairn (Mount MacKenzie). The deposits are mostly pebble to boulder carbonate mudstone conglomerates and breccias with pervasive, dark, interstitial micrite. The largest of the deposits interpreted as debris flows occur southeast of Mount Haultain (Ancient Wall). Here, disoriented blocks (as large as 25 by 50m in cross-section) of shoal-water limestone occur in two sheet-like deposits of irregular thickness (up to 20 m); these deposits are exposed for more than a kilometre from the buildup margin. Similar, possibly correlative, deposits up to 12 m thick and containing disoriented blocks 10 m across occur three kilometers from the buildup margin. The allochthonous clasts are mostly limestone and vary from nonfossiliferous mudstones to grainstones rich in normal-marine fossils. Some clasts are coral growth frameworks several metres across. Finer debris commonly including abundant basin clasts occurs largely in sheets and in 6 The authors wish to acknowledge the substantial support provided by the Denver Research Center of the Marathon Oil Company; the National Research Council of Canada, Grant No. A2128 to Mountjoy; the Geological Survey of Canada, Grant No. 29-66 to Mountjoy; and McGill University. We are also pleased to acknowledge the very appreciable assistance received from our former co-workers and many colleagues in the gathering of data and formulating of ideas. Special thanks are extended to former and present Marathon Oil Company geologists who assisted in the field work: M. J. Brady, P. W. Chaquette, W. P. Gruman, W. A. Hogg, D. B. MacKenzie, W. J. Meyers, F. W. Rutledge, R. P. Steinen and J. L. Wray. J. L. Wray also identified the stromatoporoids, algae, and foraminifera. Discussions with J. C. Harms of the Marathon Oil Company, R. A. Bagnold and M. A. Hampton were helpful in clarifying ideas about the genesis of the debris flows. Mountjoy gratefully acknowledges the discussion and help received from C. W. Stearn and graduate students, J. Hopkins and P. Srivastava. Particularly helpful has been Hopkins' doctorate research concerning a detailed stratigraphic and petrographic investigation of the sediments adjacent to the Miette and Ancient Wall buildup margins. P. J. Coleman, P. E. Playford and F. Read have offered valuable perspective on parts of the manuscript. Manuscript assistance has been furnished by McGill University, the University of California (Riverside) and the University of Wisconsin (Madison). End_Page 439------------------------ some channels adjacent to buildup margins at all localities. A few debris deposits of both sheet and channel form have graded calcarenite-to-calcilutite tops a few cm thick. We believe these allochthonous materials were largely transported by submarine debris flows from upslope basin and buildup environments. The larger debris deposits may have formed when the relief and slope at the buildup margin was higher than normal, but relief in excess of 50 to 60 m over several kilometres or slopes as high as 10 degrees are unlikely; some deposits may be related to buildup margin unconformities. Allochthonous debris deposits containing large blocks may occur at other Devonian buildup margins in both surface and subsurface. Recognition of such deposits can assist in determining buildup proximity, in better interpretation of buildup and buildup margin genesis, in determining time of diagenesis -- particularly cementation and dolomitization, and in correlation. Presence of thick sheets of megabreccias interpreted as intraformational debris flows need not imply the high relief, active tectonism, or the steep slopes or scarps that are frequently envisioned for these deposits.

Geopetal Pyrite in Fine-Grained Limestones

ABSTRACT The Late Jurassic Oberalm beds of the Northern Austrian Alps consist largely of fine-grained limestone with abundant radiolaria and other microplanktonic fossils. Pyrite occurs mainly in microscopic spheroidal aggregates (framboids) within radiolarian tests. These framboids are normally lodged in the lower part of the fossil sphere, suggesting that the pyrite was precipitated within the fossil and sank to the bottom of the sphere during early diagenesis. Subsequent precipitation of mineral matter filled the remaining void space. Thus, the pyrite as seen in thin section provides evidence of the gravitational field at the time of diagenesis and is a potential tool for recognition of tectonic overturning. In penecontemporaneous slump folds the pyrite either formed subsequent to folding, r, more likely, adjusted its position to the gravitational field in the overturned limb. Details of pyrite framboids are illustrated in electron photomicrographs.