Thick Facies Research Articles

Facial composition of the deposits of the productive strata based on qualitative interpretation of the log data

As is known, most of the discovered oil and gas fields in Azerbaijan are associated with terrigenous deposits; thus, the successful discovery of new fields largely depends on determining the genesis of terrigenous formations.
 Determining the genesis of reservoir deposits allows apetroleum geologist to more accurately predict the reservoir’sshape and change its main parameters in both lateral and vertical directions.This information also allows us to choose a rational drilling operationsystem in the field. Comprehension of the genesis of the productive stratum deposits is also important in developing oil and gas fields.
 The research of the genesis of deposits of productive series is of great interest given the acceleration of exploration and evaluation of their oil and gas potential. In this regard, using the ««qualitative logging facies models» proposed by V.S. Muromtsev in the article, the sections of the wells of the Garbi Absheron, Shimali Absheron, Khazri, and Arzu structures of the North Absheron zone of the uplift PS suites «Fasila», «Upper Kirmaky Clayey» (UKC) and on the sedimentary rocks of the «Upper Kirmaky Sandy» (UKS) formation, a lithofacial analysis of sedimentary rocks was carried out, the genesis of sedimentary rocks was established, and an explanation of the results obtained was given.
 Geophysical logging data of the borehole potential curve of intrinsic polarisation were used to build spatial models and 3D models. The constructed models allow us to attribute various shapes of the well potential curve (WP) to deposits of a defined facies composition.
 In the research work, the total thickness of facies for the UKC and UKS suites was calculated, three-dimensional spatial models were built considering the variation in their thickness by structuresof the North Absheron zone of the uplift, and 3D models were built by the structures located on them.The facies composition of the sediments of the layers in the «Fasila», «UKC» and «UKS» formations was determined based on created models selected by their appearance and forms of display from WP diagrams of well sections in the Garbi Absheron, Shimali Absheron, Khazri, and Arzu structures.
 These models are major for accurately calculating oil and gas reserves in the area, for drilling and designing new wells, and refining the operation scheme.

Tectonic evolution of the Paranoá basin: New evidence from gravimetric and stratigraphic data

Field gravimetric and stratigraphic surveys were conducted with the aim to constraint the mechanisms responsible for the initiation of the Stenian-Tonian Paranoá basin, central Brazil, a subject not yet studied in detail. The Paranoá Group crops out in the external zone of the Brasília Belt, a Neoproterozoic orogen in the western margin of the São Francisco Craton. Detailed geological mapping confirmed the existence of a regional scale fault that controlled sedimentation of the Paranoá Group during the deposition of its basal formations, revealing important details about basin initiation and early evolution. Gravimetric modeling indicates the existence of paleorift structures beneath the Paranoá sequence in the study area. Results from both stratigraphic and gravimetric surveys show strong evidence of mechanical subsidence by faulting during basin initiation. Unsorted, angular, clasts cut by quartz veins and brecciated boulders present in the basal conglomerate, support this hypothesis. Basin initiation faults coincide with deeper paleorift faults and are thus interpreted as reactivations of the older Statherian Araí Rift. The reactivations favored an initial regime of mechanical subsidence, dominated by the development of epirogenic arches subsiding at different rates. Apart from faulting activity, the post-basal sequence presents no evidence of rift environment in the strict sense. Besides, the great lateral continuity and relatively constant thickness of facies, indicate that an initial mechanic subsidence rapidly gave way to flexural subsidence during subsequent stages of basin evolution. The Paranoá Group do not present reliable characteristics that would allow its strict classification as a passive margin. Its main stratigraphic characteristics, tectonic location and basement architecture, indicate that the Paranoá Group was deposited in a cratonic margin basin, and may have been either connected to a passive margin basin at times of sea level rise, or evolved to a passive margin later in time.

Does compaction-induced subsidence control accommodation space at the top of prograding carbonate platforms? Constraints from the numerical modelling of the Triassic Esino Limestone (Southern Alps, Italy)

The demise of the high-relief, steep-slope, prograding Ladinian-Early Carnian carbonate platforms of the Esino Limestone (Central Southern Alps of Italy) is marked by subaerial exposure of the platform top associated with different erosional (mainly karst-related), depositional and diagenetic processes (Calcare Rosso). The exposure-related deposits consist of three major facies associations: 1) residual soils with thin lenses of conglomerates with black pebbles, and, locally, weathered vulcanites; 2) chaotic breccia lenses irregularly distributed in the uppermost part of the Esino Limestone carbonate platform, interpreted as collapse breccias in karstic setting: 3) inter-supratidal carbonate cycles with dissolution and development of paleosols and tepee structures.Facies distribution follows the sub-environments of the underlying Esino Limestone. Facies 1 and 2 typically characterize the core of the platform, covering the underlying inner platform facies. Facies 3 instead develops toward the edge of the platform, above reef-upper slope facies of the prograding facies of the Esino Limestone. The thickness of facies 3 decreases toward the core of the platform. Facies distribution reflects differences in the accommodation space and sedimentary processes from the rim (highest accommodation, favouring the deposition of peritidal-supratidal carbonates) to the core (reduced accommodation, causing pedogenesis and karstification) of the carbonate system.The observed thickness changes may be controlled by different factors: 1) syndepositional tectonics, 2) subsidence induced by magmatic activity or 3) differential subsidence controlled by the stratigraphic architecture of the Esino Limestone platform and adjoining basins. As evidence of tectonics was not observed and the presence of volcanic bodies is only documented tens of km away from the study area, the scenario involving the creation of accommodation space by compaction of the basinal sediments (resedimented, fine-grained calciturbidites) during the progradation of the carbonate platform is here investigated. Numerical modelling was performed to verify the compatibility of compaction-induced subsidence with the observed depositional architecture. The models were built to simulate the architectural evolution of the platform by progressively adding layers from deepest to shallowest, while compacting the underlying sediments, in order to evaluate compaction-induced subsidence (and accommodation space for the Calcare Rosso) after the deposition of the youngest platform strata. Modelling results allow us to conclude that the wedge geometry of the Calcare Rosso, deposited on top of the extinct Esino carbonate platform, can be explained by subsidence controlled by compaction of the basinal sediments present below the early-cemented, fast prograding platform slope deposits.

Contribution of non-coal facies to the total gas-in-place in Mannville coal measures, Central Alberta

The contribution of non-coal, organic rich, fine grained lithofacies to total gas-in-place and producible gas in the Upper Mannville Group coal measures, has been investigated in the producing and prospective coalbed methane fairway in Alberta. Petrophysical well logs, core, fluid, and gas analyses of organic rich rocks have been evaluated in order to quantify the total gas-in-place resource that may be accessed by a horizontal well(s) completed in the thickest coal seam. Within a ±20m stratigraphic interval of the completed seam, significant additional gas storage exists in coal seams too thin to warrant separate completions and in organic rich, fine grained facies. Such gas, although not currently attributed to producible reserves, can reasonably be expected to be produced.Three non-coal, organic reservoir facies are recognised based on core desorption and laboratory analysis and are identified on log, through mainly a combination of the bulk density and gamma ray logs. The reservoir facies are carbonaceous mudstone (1.5–2.0g/cm3), organic rich mudstone (2.0–2.2g/cm3), and organic mudstone (2.2g/cm3–2.4g/cm3). The gas storage in the mudstone reservoir facies, similar to the coal, is mainly in the adsorbed state with minor amounts in solution within the water/brine, and in the free state within the pores and fractures. The average gas content from canister desorption of carbonaceous mudstone is 147scf/t,111cm3/g (SI unit) ≈ 34scf/t (oil field units, 15.6°C and 101kPs). organic rich mudstone is 83scf/t, and organic mudstone is 54scf/t. Gas in solution ranges from about 5scf/t for the carbonaceous mudstones to a low of 0.5scf/t for the organic mudstone reservoir facies. The measured as received porosity of the mudstones ranges from 2.5 to 14%.The organic matter in all lithologies is dominated by particulate carbonaceous material derived from vascular plants (Type III kerogen/vitrinite). The gas capacity of non-coal facies, as determined by adsorption isotherms, and gas content as determined by desorption analysis of core, increase with thermal maturity (rank), and percentage organic matter. Values for the carbonaceous mudstone approach those of coal.Regionally, the gas capacity and content of the coals and non-coal reservoir facies increases from northeast to southwest in parallel with the increase in current depth of burial and level of organic maturation (coal rank), although local variations exist. The gas composition of the coal and non-coal reservoir facies in the producing fairway is similar: the highest carbon dioxide contents occur in areas of highest and lowest maturity, whereas the percentage of heavier gases (C2–C5) increases with maturity/depth of burial. Anomalously high C2–C5 hydrocarbons (up to 10%) occur in the shallowest coal measures, which, in the absence of an alternate explanation, is attributed to migration. Isotopic composition of the methane has a thermogenic signature in the deepest strata, through mixed thermogenic–biogenic, to biogenic signatures at shallower depths.In order to quantify the stratigraphic and areal variation in gas content of the non-coal lithofacies in wells where no core exits, a protocol was developed that allows extrapolation of data from cored wells to non-cored wells of similar maturity and depth of burial using petrophysical logs. In the producing and prospective fairway within ±20m of the main coal seam, the net thickness of the carbonaceous mudstone facies varies from 0.3 to 9.3m and averages 2.7m. In this interval the organic rich mudstone ranges up to 7m thick and averages 2.6m and, the organic mudstone, ranges up to 7m thick and averages 2.7m thick.The total gas resource in the current area of production and in prospective areas was determined by applying the measured and calculated gas density (gas per unit volume) of the reservoir facies at estimated high median and low values to the net thickness of each facies in some 203 wells across the study area. The average well in the study area, using the estimate medium gas content, has a resource of 10.2BCF/mi2 (billions of cubic feet per square mile). Of the 10.2BCF/mi2, the contribution to the total gas-in-place of the thickest coal seam is 21%, the total cumulative coal is 57%, the carbonaceous mudstone facies is 21%, organic rich mudstone is 13%, and the organic mudstone is 9%.Most producing coalbed methane formations are similar to the Mannville coal measures in that they include non-coal organic rich lithologies that can be anticipated to have generated and retained hydrocarbons similar to coal. How much the non-coal lithologies will contribute to the producible ‘coalbed’ will vary with geology of the coal measures and the drilling and completion methods utilised. Not considering the contribution of non-coal facies to the gas resource and producible gas, will under value the resource and result in non-optimal field development.

Three-dimensional modeling of a shoreface-shelf parasequence reservoir analog: Part 2. Geologic controls on fluid flow and hydrocarbon recovery

Wave-dominated, shoreface-shelf parasequences are commonly modeled as simple layer-cake reservoirs, yet analysis of modern and ancient analogs demonstrates that these intervals contain a more complex physical stratigraphy. We investigate the impact of depositional and diagenetic heterogeneity associated with gently dipping clinoform surfaces on fluid flow and recovery during water flooding, using a three-dimensional model reconstructed from a well-exposed outcrop analog. We demonstrate that the volume of oil in place is affected by variations in facies thickness associated with interfingering along clinoforms, whereas waterflood sweep efficiency is affected by barriers to flow along clinoform surfaces, such as calcite-cemented layers, mudstones, and siltstones. Sweep efficiency is low when water flooding is down depositional dip because oil is bypassed at the toe of each clinothem as water flows preferentially through high-quality sandstone facies in the upper part of the parasequence. Sweep efficiency is higher when water flooding is up depositional dip because the gravity-driven, downward flow of water sweeps poorer-quality sandstone facies in the lower part of the parasequence. In both cases, injectors may offer limited pressure support to producers. Water flooding along depositional strike yields good pressure support but poor sweep because the gravity-driven flow of water into the lower part of the parasequence is significantly reduced. This yields highly variable fluid saturations but a uniform pressure gradient, which is consistent with pressure and fluid saturation data from the mature Rannoch Formation reservoir, Brent field, United Kingdom North Sea. Simple layer-cake models fail to capture the range of flow behaviors described above and overpredict recovery by up to 20% as a result.

Cambrian stratigraphy and depositional history of the northern Indian Himalaya, Spiti Valley, north-central India

Recent work on Himalayan tectonics indicates that prior to the Cenozoic collision of India and Asia, an enigmatic Cambrian– Ordovician event may have strongly infl uenced the regional geology of the Himalaya. Stratigraphic and sedimentological analyses of well-preserved Cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the Himalaya. The Parahio Formation, defi ned herein, of the Parahio Valley, Spiti region, in the Tethyan Himalaya of India, is the best biostratigraphically resolved section of Cambrian strata in the entire Himalaya. This formation consists of >1350 m of dominantly siliciclastic deltaic deposits. The formation ranges from uppermost Lower Cambrian (Lungwangmiaoan Stage) to middle Middle Cambrian (Hsuchuangian Stage), representing a time span of ~5–10 m.y. It contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross-bedded fl uvial facies. Many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits. The cycles are interpreted to have resulted from delta-lobe switching, based on a lack of systematic stratigraphic changes in cycle or facies thicknesses. This paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep-sea fl ysch to shallow-marine tidalites. In addition, our paleoenvironmental analysis and paleocurrent data suggest that the uppermost Lower to Middle Cambrian deposits of the Lesser and Tethyan Himalaya are parts of the same ancient northward-prograding, fl uvial-deltaic depositional system of the paleo-Tethys margin of India. An angular unconformity with overlying Ordovician conglomeratic rocks has considerable local relief, with meter-scale scours and a valley fi ll >100 m thick. The scours have northeast-southwest orientations, which parallel both the paleocurrents in the underlying Parahio Formation as well as published paleocurrent readings from the coarse red beds of the overlying Ordovician strata. The Cambrian–Ordovician unconformity is of regional extent, and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in Spiti is ~15 m.y. Our sedimentological analysis and associated paleocurrent data from the Parahio Formation, along with additional data from units both above and below the unconformity, indicate that published models portraying foreland basin development at this time with southward-directed thrusting are problematic. An alternate possibility— that uplift took place south of the Tethyan Himalaya—is also problematic, because no published stratigraphic or structural evidence exists for such an uplift to the south for either the Greater or the Lesser Himalaya lithotectonic zones.

Conodont biostratigraphy of the Lower to Middle Devonian Deserters Formation (new), Road River Group, northeastern British Columbia

A new Lower to Middle Devonian basinal unit of the Road River Group, herein formally named the Deserters Formation, contains argillaceous, crinoidal limestone and black shale deposited in a linear sub-basin of the Ospika Embayment, southern Kechika Trough. The abrupt lateral facies changes in the region, facies thickness, and occurrence of volcanics indicate a period of extensional tectonism. A total of 53 (4–5 kg each) samples from the Deserters Formation yielded 7766 conodont elements assigned to 14 genera representing 33 species. The formation ranges from the Lochkovian (eleanorae Zone, or lower part of the delta Zone of the Cordilleran Region) to Eifelian (australis Zone). The temporal constraints established by conodont biostratigraphy allow correlation to the Grizzly Bear Formation, a regionally restricted Lower to Middle Devonian unit in the Selwyn Basin to the north.

The Cuyama Strike-Slip Basin, California, U.S.A.: An Exemplar of Contrasting Syntectonic and Post-Tectonic Strata

ABSTRACT Miocene strata in the Cuyama basin of southern California exemplify syntectonic and post-tectonic stratigraphic development in a strike-slip basin. Integrated seismic, well-log, and outcrop data provide a detailed portrayal of the seismic stratigraphy and sedimentary facies in the previously unstudied middle and upper Miocene sequences. Results show sharp contrast among syntectonic and post-tectonic strata from composition and texture of sediments to lateral distribution and thickness of facies. Seven post-tectonic sequences were mapped across the basin. Sequences contain eolian to fluvial terrestrial facies, wave- and tide-dominated nearshore facies, biosiliceous, dolomitic, and phosphatic outer-shelf and slope mudstones, and shale-rich submarine fans in basinal environments. Authigenic dolomites and phosphates are pervasive in outer-shelf and slope facies and indicate the possibility of restricted sediment supply. Post-tectonic sequences are thin but laterally continuous, and they exhibit little depositional asymmetry. Minor erosion is distributed equally among regressive and transgressive relative sea-level changes. Asynchronous changes in tectonism, sediment supply, and eustatic sea-level change created dramatic differences among syntectonic and post-tectonic sequences. Interbasinal tectonic quiescence led to the abrupt end of stratigraphic mismatches and the accumulation of sediment during brief, episodic periods. Multiple points of progradation persisted through several million years of post-tectonic basin development and highlight the importance of extrabasinal sediment supply to the development of all sequences. The eventual loss of most accommodation space promoted the importance of eustatic sea-level change and reduced the prominence of asymmetric facies and abrupt lateral facies changes.

Estimation of subseismic nonreservoir layers within a turbidity oil-bearing sandstone, Campos Basin, using a geostatistical approach

This study has evaluated the relationships between several petrophysical characteristics and seismic attributes of a siliciclastic reservoir in the Campos Basin, Brazil. Very thin and nonreservoir sedimentary beds and cemented layers with random distribution are the main heterogeneities in this reservoir. Such heterogeneities are below seismic resolution but we show that some seismic attributes are useful to obtain a better estimate of the nonreservoir cumulative thickness in the productive stratigraphic unit. A better understanding of spatial distribution and thickness of nonreservoir facies is useful to build realistic net pay and heterogeneity maps and recoverable oil estimations. Using models with velocities based on the well logs and petrophysical analysis, we computed 10 seismic attributes from synthetic seismograms. Some of them showed a good correlation with nonreservoir facies thickness. The vertical distribution, thickness and amount of nonreservoir layers in the models were randomly generated. The analysis using seismic attributes computed from real seismic data and nonreservoir facies thickness derived from the drilled wells confirmed the relationships observed with synthetic data. The real seismic attributes were then used to constrain the estimates of nonreservoir facies spatial and cumulative thickness distributions through kriging with external drift (KED). Stochastic simulation was applied to quantify the estimation uncertainties. The generated risk maps allowed the understanding of the nonreservoir facies distribution.

Comparing statistical models of physical heterogeneity in buried‐valley aquifers

The hypothesis that physical heterogeneity has similarities in separate aquifers created by similar depositional environments is tested by comparing statistical characteristics of facies assemblages. The comparisons are made for a number of data‐rich sites in two buried‐valley aquifers in the North American midcontinent: the White River aquifer in Indiana and the Miami Valley aquifer in Ohio. These were proglacial valleys that directed drainage away from Quaternary ice margins and were filled with glaciofluvial sediments: predominantly sand and gravel (s) lithofacies, with interbedded mud and diamicton (m) lithofacies. At scales encompassing assemblages of both lithofacies m and s, permeability is strongly bimodal. We find that it is useful to compare statistics that characterize the proportions, geometry, and spatial distribution of each facies. The results give rise to a general model for heterogeneity in valley‐fill sediments along the proglacial sluiceway in both aquifers. The proportion of facies m is ∼15%. The mean thickness of facies m is 3.5 m and of the order of 10 m for facies s. The coefficient of variation in thickness for either facies is of the order of 1, with thickness ranging over orders of magnitude. Correspondingly, the vertical autotransition probabilities are exponential, and they are relatively symmetric with effective range of the order of 10 m. The lateral facies lengths are indicated to vary over orders of magnitude and to be multimodally distributed, with mean lengths of the order of 102 m, effective range in correlation structure of the order of 103 m, and lateral anisotropy ratio <2. There is some variation in how the facies m are vertically embedded within the facies s. The White River aquifer and areas in the Miami aquifer have facies proportions relatively stationary with elevation. In other areas of the Miami, there are near‐horizontal zones having relatively higher or lower proportions. However, such variations on the general model give rise to similar statistics for mass transport within the context of a relevant remediation problem and thus would lead to a similar conclusion or decision. Thus one general model is applicable to both aquifers in this context. In a broader sense, we have illustrated a method by which other examples developed from data‐rich sites can be compared.

Simulation models of salt-marsh morphodynamics: some implications for high-intertidal sediment couplets related to sea-level change

A salt marsh composed of a vegetated platform dissected by creek networks has at any time characters determined by the previous history of the system, because forcing factors constantly change in rate. Results from numerical simulation models of marsh growth are combined with engineering regime theory (channel stability) to form a comprehensive conceptual model which qualitatively describes the three-dimensional character of marshes and the stratigraphic sequences they generate. The morphostratigraphic evolution of marshes is examined for two main patterns of regional relative mean sea-level change. Where the level fluctuates smoothly about an underlying upward trend, there develops a stacked sequence of silt-peat couplets, each of which is symmetrical in facies states but asymmetrical in facies thicknesses. Palaeochannel forms and fills distributed through each couplet show that creek networks expand and densify as sea level rises and the hydraulic duty of the marsh platform increases to its maximum, but shrink and silt-up as sea level temporarily stabilises or falls and the duty declines. Great earthquakes, and also the abandonment of manmade flood defences on aseismic marshes, create catastrophic-episodic increases in relative sea level. Where there is also an underlying upward trend of sea level, stacked silt-peat couplets result that are strongly asymmetrical in both facies states and thickness. The creek networks enlarge rapidly under strong forcing within a short time of the sea-level increase but later, as the hydraulic duty declines, shrink and infill. Under each scenario, the tidal creek networks that functioned during the accumulation of one couplet are unlikely to be inherited by the next, except in the case of channels that also transmit fresh water from the hinterland.

Implications of Seismic Interpretation on Reservoir Architecture at Tengiz Field, Kazakhstan: ABSTRACT

Vintage 1982-89 2-D and 3-D seismic paper sections over the Tengiz Field were scanned and loaded to an interpretation workstation as a tight (500 meter) 2-D orthogonal grid. Tops from 80 wells were converted to time and tied to the seismic lines using recently recorded velocity surveys in workover wells. These velocity surveys showed little velocity variation across the carbonate platform, which allowed a simple velocity space to be developed for depth conversion. Previous mapping shows the field to be a large, unfaulted, dome-like carbonate platform/flank where platform facies grade smoothly into detrital flank facies, and facies thicknesses are thinned by erosion from platform to flank. The new mapping shows onlapping clinoform geometries for the flank sediments and at least four periods of faulting. Earliest faulting appears to be rifting of the Devonian Clastic section, which provided a subtly elevated horst block upon which later Devonian and Carboniferous carbonate platforms grew. Steep, syndepositional, small-offset faults that lose throw with depth occur within the carbonate platforms, and were probably caused by differential compaction of platform sediments. Later slump faults, probably of Bashkirian age, developed within the detrital material on the flanks and sole out with depth. The youngest faulting is due more » to movements of the Kungurian salt layer which immediately overlies the carbonate reservoir. These faults detach within the salt layer without penetrating the reservoir facies New features brought forth from the seismic interpretation include: possible reservoir compartmentalization due to flank faulting and facies onlap, possible enhanced permeability near fractures on the platform, and potential for drilling hazards very near to fault planes. Many of these feature. have since been corroborated by careful review of drilling and production histories. « less

Recognition of Subaerial Exposure and Flooding Surfaces in Carbonate-Siliciclastic Eolianites and Marine Carbonate Sequences in Southwestern Kansas

Recent work on the St. Louis Limestone in southwestern Kansas has demonstrated that these units contain a significant eolian facies component (up to 80-90% of total unit thickness). Reservoir intervals within the St. Louis are confined to relatively thin subtidal grainstones that, in turn, are capped by a muddy carbonate and shale facies. Critical to exploration and development of these grain-shoal reservoirs is an understanding of their spatial and stratigraphic distribution. Core through the St. Louis and St. Genevieve limestones has been examined and features have been recognized at the top of the eolianites. These surfaces are interpreted as long-term exposure surfaces. The contact between the subtidal grainstone shoals and the overlying muddy carbonate and shale facies is relatively sharp and is interpreted as representing a flooding surface separating shoal from muddy-open shelf facies. In the St. Louis Limestone, the subtidal carbonate grainstone reservoir intervals consist of primary interparticle porosity. Initial geologic models have viewed the reservoir zone as a single blanket-like layer with small structures and relatively random changes in facies thickness. However, a detailed integrated analysis of facies, well logs, and reservoir data within the framework of high-resolution sequence stratigraphy has resulted in a new model of St.more » Louis reservoir geometry. Development and exploration concentrates on recognizing and mapping the updip pinch outs of porous grainstone facies related to maximum landward extent of paleoshorelines caused by relatively major fluctuations in relative sea level. These shoal facies build-ups and pinch outs should form viable exploration targets as they cross structural elements formed or reactivated during the Pennsylvanian.« less

Ground penetrating radar of northern lacustrine deltas

Ground penetrating radar (GPR) was used in several selected deltaic sedimentary environments to better understand subsurface stratigraphy and reconstruct former depositional environments. The profiles provide high-resolution, continuous subsurface data on facies thickness and depths, orientation of major sedimentary structures, postdepositional failure planes, and depth of peat deposits.Field experiments were carried out on six river deltas. Records from four of the deltas exhibit sedimentary facies; a record from one delta shows a possible slump; and records from another delta reveal the thickness and stratigraphic relationships of peat deposits. The delta types are (i) sandy, wave influenced; (ii) sandy, immature wave influenced (steeper middle and lower shoreface); (iii) sandy braided; and (iv) gravelly, fan–foreset.In areas of limited subsurface control (stratigraphic logs from drill core, cutbank exposure, or geophysical logs), radar profiles can provide ''big picture'' perspectives of the subsurface, a view only available in laterally extensive exposures. High-resolution profiles of subsurface stratigraphy and sedimentary facies from GPR provide an opportunity for geomorphologists and sedimentologists to further advance field research. Although GPR has limited success in silt and clay, results from sand and gravel deposits often reveal detailed facies assemblages.

Use of Bed-Form Climb Models to Analyze Geometry and Preservation Potential of Clastic Facies and Erosional Surfaces

Based on a combination of Walther's Law of Facies and bed-form climb theory, we propose a model that explains how erosion surfaces and vertical sequences of clastic strata are preserved where deposition occurs in channelized or locally erosional environments including fluvial and submarine-channel deposits, barred beaches, and transgressive coastlines. The model considers both lateral and vertical migration of a scour surface and its associated depositional products. As in studies of bed-form climb, we recognize subcritical, critical, and supercritical climb of scour surfaces relative to adjacent depositional forms. For a given erosion surface, the amount of underlying material removed is dictated by the climb angle of the erosion surface. Preservation of sequences also d pends on whether conditions were relatively variable or invariable. One important conclusion of this study is the recognition of the bed-form climb angle's control on the thickness of a sedimentary facies. Estimates of paleochannel depth or paleowater depth based on facies thickness can be made only if the climb angle is zero. By combining depositional models of fluvial, nearshore, and offshore facies, it is theoretically possible to model depositional systems such as deltas and passive continental margins. Potential applications are limited by several factors, including influence of sea level, climatic fluctuations on sedimentation, and variable migration of facies (or nonideal behavior or sedimentary environments). In addition, negative climb angles or climb angles less than horizontal can result in the removal of large amounts of material. Such negative climb angles may be caused by drops in eustatic sea level and lowering of base levels of drainage systems. We believe that this heuristic model has considerable potential in helping sedimentologists and stratigraphers predict the consequences of facies mi ration.

Carbonate mud sedimentation on a temperate shelf: Bass Basin, southeastern Australia

Bass Basin (66,000 km 2) is unusual as a carbonate-dominated basin because calcitic carbonate muds are accumulating in relatively shallow (70–85 m) water depths in the central part of the basin (20,000 km 2). The carbonate muds are produced by the primary accumulation and disintegration of nannoplankton, as well as through the biodegradation of skeletal carbonate grains accumulating on the sea floor. The muds are transported to the south of the basin to the end of tidal current transport paths, where they accumulate in the lowest-energy environment available in enclosed Bass Basin, although they are still subject to periodic reworking by storm energy. The basin margins consist of coarser, partially palimpsest, carbonate sands, whereas the central muds overlie a Late Pleistocene disequilibrium surface. In cores these muds are up to 1 m thick and thin away from the centre towards the margins. Bioturbation is important in modifying the textural character of the muds by increasing the grain size as faecal pellets, and therefore creating a sea floor not in equilibrium with modern hydrodynamic conditions. Radiocarbon dates for an early post-glacial marine transgression embayment facies in the basin gave ages of 10,290 ± 250 to 11,660 ± 300 years B.P. An age of 8700 ± 710 years B.P. was obtained for the base of the modern strait facies. These ages and facies thicknesses were used to establish Holocene sedimentation rates of < 12 cm/1000 y for the basin centre, falling to < 6 cm/1000 y towards the margins.

Stratigraphy and Depositional Environment of Early Mississippian Joana Limestone of East-Central Nevada: ABSTRACT

The Early Mississippian Joana Limestone in the southern Schell Creek and Egan Ranges of east-central Nevada is divided into nine rock types: mudstone, fossiliferous mudstone, wackestone, peloidal wackestone, pelmatozoan wackestone, pelmatozoan packstone, pelmatozoan grainstone, and ooid packstone. From the combined rock type and larger scale outcrop information, three depositional facies were identified: (1) unbedded subtidal, (2) bedded subtidal, and (3) restricted subtidal, each containing a unique set of diagnostic microfacies. Facies thicknesses, lithologies, and contacts with adjacent stratigraphic units indicate a highly varied paleotopography of localized highs and basins during Joana deposition. It is suggested that Waulsortian-type buildups occur downslope of some paleohighs in the unbedded subtidal facies. An age of upper Kinderhookian to lowest Osagean within the Mississippian Period was determined for the Joana, based primarily on conodonts and foraminifera. In the middle beds of the Joana, the previously unreported upper Siphonodella crenulata conodont zone occurs and relates the timing of the Joana to regional geologic events. Color alteration indices of these conodonts are 1.5 to 2, and occur in the oil generation window. Additionally, oil staining was noted in numerous samples primarily from the lower half of the formation, represented by the unbedded subtidal facies. Porosities of themore » formation are varied, ranging from no visible porosity to over 20% interparticle porosity in some pelmatozoan grainstones.« less

Paleotopography and Substrate Lithology as Controls on Initiation of Waulsortian Reef Growth: Examples from Sacramento Mountains, New Mexico: ABSTRACT

Frameless mud mounds, usually called Waulsortian reefs, are common in Osagean and Kinderhookian rocks of North America and western Europe. Spectacular Waulsortian reefs crop out in the Sacramento Mountains of New Mexico along 10 mi of continuous exposure, where detailed petrographic studies and field measurements of the strata between the top of the Devonian and the base of the reefs reveal: (1) down-to-the-southwest paleoslope on an uneven, gently dipping ramp; (2) widespread deposition of skeletal packstones and siliciclastics to the northeast; (3) patchy, local thicks of skeletal packstones surrounded by shaly wackestones to the southwest; (4) relict highs on the Devonian surface beneath the skeletal packstone pods; (5) clusters of sheetlike reefs weakly associated with paleotopography in the northern outcrops; and (6) large, dome-like individual reefs strongly associated with depositional topography and skeletal packstone/grainstone substrates in the southern outcrops. The pre-reef strata do not exhibit abrupt changes in thickness or lithology to indicate a break in regional slope, and the reefs are not aligned with patterns in thickness of facies that would distinguish shelf-edge environments from lagoonal environments. Like their European counterparts, the Osagean reefs in the Sacramento Mountains grew on a ramp where the nonreef facies were grainy updipmore » and muddy downdip, and reef anatomy varied from sheetlike updip to dome-like downdip. The association between paleotopography, substrate lithology, and the initiation of Waulsortian reef growth provides new information about regional depositional patterns in the Early Mississippian.« less

Facies in Upper Part of Madison Group, Sawtooth Range, Northwestern Montana: ABSTRACT

Portions of the Mississippian Madison Group are gas reservoirs in the plains adjacent to the Sawtooth Range, northwestern Montana, and are equivalent to Mississippian carbonates that are major gas producers in the Canadian Foothills. In the Sawtooth Range, three facies are recognized in the upper 125 m (410 ft) of the Madison Group; they comprise a carbonate shelf sequence that is several hundred meters thick, shoals upward, and is unconformably overlain by Jurassic strata. Economically significant porosity may occur in the upper part of the Madison Group, controlled by the eogenetic secondary dolomitization of a conspicuous crinoidal grainstone unit within it. This dolomitized crinoidal grainstone unit (termed facies C) is the lowest of three facies in the upper part of the Madison Group, and it abruptly overlies lagoonal limestone that forms the major part of the group. Facies C is massively bedded and exhibits large-scale planar cross-stratification suggestive of its origin as a subaqueous dune field. Measured porosity in surface samples of the dolomitized grainstone of facies C is a maximum of 18% and consists of vuggy, intergranular, and intercrystalline pores. The upward transition from limestone to secondary dolomite commonly occurs in the lower part of facies C. The thickness of facies C ranges from 35 to 75 m (115 to 250 ft) and is inversely proportional to the thickness of the intertonguing and overlying facies B. The uppermost two facies, termed B and A, reflect the upward transition from and open platform to a restricted platform environment. Facies B ranges in thickness from 25 to 75 m (80 to 250 ft) and is a nonporous, dolomitized mudstone and wackestone sequence generally containing some 1 m (3 ft) interbeds of porous dolomitized grainstone. This sequence is capped by < 10 m (30 ft) of intertidal rocks of facies A, which are thin-bedded, partly algally laminated, dense dolomites. Locally, facies A and parts of B have been removed as a result of pre-Jurassic folding and erosion in the Sawtooth Range. However, all lateral thickness changes in facies C reflect its intertonguing with B. Although original facies patterns are greatly telescoped by thrusting, the porous grainstones of facies C are best developed in the vicinity of Blackleaf Canyon, Montana, the site of a recently developed commercial gas field in dolomite of the Madison Group. End_of_Article - Last_Page 1350------------

Organic Content of Devonian Shale in Western Appalachian Basin

In the organic-rich facies of the Devonian shale in the western part of the Appalachian basin, the distribution of organic matter determined from quantitative organic-content data derived from formation-density wire-line logs, provides an indirect measure of both gas in place and the capacity of the shale to supply gas to permeable pathways. Knowledge of organic-matter distribution is important for gas exploration and resource appraisal. The boundary between organic-rich (black) and organic-poor (gray) facies is defined here as 2% organic content by volume. The thickness of organic-rich facies ranges from 200 ft (61 m) in central Kentucky to 1,000 ft (305 m) along the Kentucky-West Virginia border. The average organic content of the organic-rich facies increases from 5% by volume on the eastern edge of the study area to 16% in central Kentucky. The net thickness of organic matter in the organic-rich facies (the product of volume-percent organic content and facies thickness) shows the amount of organic material in the shale, and is the most fundamental of the organic-content characterizations. Net thickness of organic matter ranges between 20 and 80 ft (6.1 and 24.4 m) within the mapped area, with local depositional maxima centered in Martin County, Kentucky, eastern Pike County, Ohio, and northern Ashland County, Ohio. The volume of organic material in the organic-rich facies within the mapped area is about 52 × 1012 ft3 (1.5 × 1012 m3).