Point-bar Deposits Research Articles

Vertical and Lateral Reservoir Inhomogeneities of a Big-River Bar Deposit, Mississippi River, Missouri: ABSTRACT

The Bryant bar, located on the Mississippi River in southeastern Missouri, is a potential sandstone reservoir that is 18,500 ft long, 3000 ft wide, and 55 ft thick (70,082 ac-ft). A river-meander belt reservoir model was developed from an extensive drilling, coring, trenching, and mapping program. Approximately 2100 horizontal and vertical gas permeability measurements were made of bedding units in trenches, vibracores, and Shelby and splitspoon cores from a variety of depositional environments within this major sand body. Permeability increases with an increase in mean grain size, although the presence of infiltrated mud is an important factor in reducing permeabilities. Avalanche sand and gravel foreset beds in large, laterally continuous, sand waves have permeabilities that range from 91 to 138 darcys. Lower bounding surfaces of foreset laminae are typically not as well sorted, and permeabilities are reduced by 10-50%. Tangential foresets of the laterally restricted, lunate sand dunes have permeabilities that range from 90 to 160 darcys. Permeabilities measured in the interbedded muddy sand, silt, and sandy silt beds from the levee, crevasse, and abandoned channel fill have widely varying permeabilities ranging from 0 to 50 darcys. Mud and silt beds and laminae are very common between and within beddingmore » units in the point-bar and surrounding environments, and are effective permeability barriers to vertical and horizontal fluid migration within point-bar deposits. The distribution patterns of these permeability barriers significantly control effective reservoir continuity. This river-meander belt reservoir model will enhance the understanding of vertical and lateral inhomogeneities in hydrocarbon and water reservoirs.« less

Vertical Grain-Size Progressions as an Aid in Depositional Environment Interpretation of Queen City Formation (Eocene), East Texas: ABSTRACT

Vertical grain-size progressions of sand deposits in the Queen City Formation (Eocene) of east Texas, were studied to find textural patterns capable of distinguishing depositional environments. Results from this study indicate that certain groups of environments are easily distinguished from other groups. Lower point bar, fluvial distributary channel, and distributary-mouth bar deposits are more readily differentiated from distal bar, lower shoreface, and upper point bar deposits by their coarser mean grain size and mode, higher sand content, and more prevalent saltation mode and smaller suspension component. The lower point bar may have the coarsest mean grain size and mode. Lower shoreface deposits may be distinguished from upper point bar and distal bar sands by having a higher sand content and a consistent 3.5 phi suspension mode. Flood-tidal deltaic sands are characterized by their consistent mean grain size, mode, sorting, and predominance of the saltation mode throughout a thick single sequence. Tidal-flat sands resemble certain fluvial and fluvial-deltaic sands. The overlap in grain size traits among many of the environments is attributed to the well-sorted and fine-grained nature of the source for the Queen City sand, similarities in transport mechanisms among environments and, to a lesser degree, diagenetic changes in themore » original grain size distributions.« less

Paleoenvironments of Vertebrate-Bearing Strata during the Cretaceous-Paleogene Transition, Eastern Montana and Western North Dakota

Exposures of the Hell Creek and Tullock formations in eastern Montana and the Ludlow Formation in western North Dakota allow detailed reconstruction of the paleoenvironments associated with the Cretaceous-Paleogene (K-P) faunal transition in the Western Interior of North America. Paleoenvironments were reconstructed from five facies: 1) a cross-stratified sandstone facies interpreted as afluvial channel environment; 2) a siltstone facies interpreted as flood-plain environments; 3) a facies of epsilon cross-stratification inferred to represent point-bar deposition; 4) a facies of organic accumulation, interpreted to reflect peat deposition; and 5) a variegated siltstonefacies, interpreted as the remnant of extensive ponded deposits. Facies associations demonstrate an ancient meandering fluvial environment in which gleization in unstable, poorly drainedfloodplains modified incipient soils. Sedimentologic and pedologic features indicate that concurrent with the faunal transition that occurred in the region, the amount of standing water increased dramatically, changing the earliest Paleogene soils and landscape. Depositional environment imposes taphonomic constraints on interpretations of K-P faunas and floras. Contrary to recent reports, fossil assemblages in Hell Creek channel deposits are reworked; bone and sediment clasts of the channel fills have been subject to traction transport. Age estimations based on supposedly unreworked fossils in channel deposits are thus unreliable. The chronostratigraphic resolution of the sediments under study is to date simply not comparable to the resolution required by researchers of the K-P boundary.

Trace Fossils Indicating Estuarine Deposystems for the Devonian-Mississippian Cloyd Conglomerate Member, Price Formation, Central Appalachians

The use of ichnologic data is crucial to the understanding of some paleoenvironmental regimes. The Cloyd Conglomerate Member of the Price Formation has been interpreted as entirely fluvial, partly fluvial and partly marine bar, and entirely marine bar. Trace fossils provide data to resolve these divergent opinions. The Cloyd preserves a thin sedimentary wedge composed of a lower, regressive nonmarine phase and an upper, transgressive marine phase bearing trace fossils and a restricted marine fauna. Massive, quartz-pebble conglomerates of the lower Cloyd were deposited in fluvio-distributaty systems during a regression near the Devonian-Mississippian boundaty. Regressive channel systems became estuaries during a subsequent transgression. Trace fossils in the upper Cloyd constitute a mixed Skolithos-Cruziana Ichnofacies of suspension-feeding sand dwellers and depositfeeding mud dwellers. Though not abundant, trace fossils occur most commonly in the uppermost sandstone beds of fining-upward, estuary pointbar deposits. A point-bar assemblage includes large, deep Skolithos and Cylindrichnus suspension feeders that inhabited bar slip faces. Deep burrowing behavior suggests that point-bar trace makers sought to moderate environmental extremes, e.g., salinity and temperature fluctuation and possible subaerial exposure of bar surfaces. The Teichichnus trace maker inhabited lower-energy, submerged sand flats on point-bar surfaces, possibly following channel abandonment. Tidal-flat facies composed of organic-rich silty shales with interbedded siltstones and sandstones overlie point-bar sandstones. Well-circulated tidal-flat facies are dominated by shallow-burrowing suspension feeders in sandstone interbeds. The trace assemblage includes Arenicolites, Diplocraterion, Monocraterion, Planolites, Rusophycus?, and shallow Skolithos. A backwater assemblage includes Chondrites and Planolites deposit feeders that inhabited organic-rich, black

Flow and Bed Profile in Meandering Sand‐Silt Rivers

Mathematical models for defining three-dimensional flow and bed topography in sinuous channels with suspendable bed material are presented. It is shown theoretically that the secondary flow in sinuous channels shows a reduction in its magnitude below that of a uniformly-curved channel, and also displays a phase-lag relative to the channel plan-form. The model for bed topography is derived by considering the sediment balance for bed load and suspended load, the transport rate and direction, all of which are governed by the three-dimensional flow. A laboratory test supports the model. The present study has made it possible to predict the large-scale bed topography of meandering sand-silt rivers, including such features as the magnitude and the location of local scour and point-bar deposits.

THREE-DIMENSIONAL FLOW AND BED TOPOGRAPHY IN SAND-SILT MEANDERING RIVERS

Mathematical models for defining three-dimensional turbulent flow and bed topography in meandering sand-silt rivers are presented, and interaction between them is analyzed. It is revealed theoretically that the secondary flow shows reduction in its magnitude from that of uniformly-curved open channels and shows phase-lag against the channel plan-form. A model for defining bed topography is derived by considering sediment balance for both bed load and suspended load, the transport rates and directions of which are obtained from the flow field thus determined. Laboratory tests have supported the present models. The study presented herein has made it possible to predict the large-scale bed topography of meandering sand-silt rivers such as the magnitude and the location of local scour and point-bar deposit.

Depositional Systems and Petroleum Potential, Mesaverde Formation Southeastern Wind River Basin, Wyoming: ABSTRACT

Depositional environments and systems of the Wind River basin Mesaverde Formation were interpreted from an analysis of outcrops along the Casper arch and Rattlesnake Hills anticline and cores and wireline logs from the adjacent subsurface. The Fales Sandstone and Parkman Sandstone/unnamed middle member are deposits of eastward progradational, wave-dominated strand-plain and deltaic complexes. Basal portions of the Fales Sandstone and the Parkman Sandstone are composed of a thickening- and coarsening-upward sandstone sequence whose facies represent storm-dominated inner-shelf and wave-dominated shore-zone environments. Facies sequences in the upper Fales Sandstone interval and the unnamed middle member are interpreted as deposits of lower coastal plain (marshes, bay fills, distributary channels, and crevasse splays) and upper coastal plain (alluvial channels, crevasse splays and fine-grained flood basin) sequences. The Teapot Sandstone is interpreted as an alluvial deposit. Analysis of facies sequences in the Teapot suggests a change in fluvial style, from braided-belt deposits along the southwest flank to meander-belt deposits along the northeast flank of the basin. These fluvial systems fed the Teapot deltas to the east. Stratigraphic plays for oil and gas include alluvial valley fills and point-bar deposits in the Teapot Sandstone, storm-dominated shelf sands in the upper Cody Shale and the Falesmore » and Parkman Sandstones, and a transgressive barrier-bar sequence in the upper Fales Sandstone. Laterally continuous shore-zone sandstones may form combination traps where pinch-outs occur on structure.« less

Wave Ripples in the Pleistocene Paleo-Tokyo Bay

Abundant wave ripples are found in the Pleistocene Shimosa Group distributed in Chiba and Ibaraki Prefectures, central Japan. Wave ripples in the Shimosa Group, which were deposited in the Paleo-Tokyo Bay, are intercalated in the following facies : the upper part of the point bar deposit of fluvial channel facies, distributary bay or lagoonal facies, tidal flat facies, tidal inlet facies, beach facies, shoreface to offshore facies, and deltaic facies.In this paper, it is introduced wave ripples in the distributary bay facies, tidal inlet facies, beach facies, and shoreface facies, which especially contain many typical wave ripples; and their paleo-environmental settings.Most of wave ripples were constructed under several meters deep in the Pleistocene Paleo-Tokyo Bay. Wave ripples in the distributary bay facies were constructed under the 'post-storm condition '. These in the shoreface facies were under both of the fair weather ' and post-storm ' conditions. These in the tidal inlet facies were mainly under the fair weather condition ' and in part under the post-storm condition '. Wave ripples are generally preserved by deposition of the overlying clay. Clay was supplied by floodwater of rivers running into the Paleo-Tokyo Bay.

古東京湾堆積物中のウエーブリップル

Abundant wave ripples are found in the Pleistocene Shimosa Group distributed in Chiba and Ibaraki Prefectures, central Japan. Wave ripples in the Shimosa Group, which were deposited in the Paleo-Tokyo Bay, are intercalated in the following facies : the upper part of the point bar deposit of fluvial channel facies, distributary bay or lagoonal facies, tidal flat facies, tidal inlet facies, beach facies, shoreface to offshore facies, and deltaic facies.In this paper, it is introduced wave ripples in the distributary bay facies, tidal inlet facies, beach facies, and shoreface facies, which especially contain many typical wave ripples; and their paleo-environmental settings.Most of wave ripples were constructed under several meters deep in the Pleistocene Paleo-Tokyo Bay. Wave ripples in the distributary bay facies were constructed under the 'post-storm condition '. These in the shoreface facies were under both of the fair weather ' and post-storm ' conditions. These in the tidal inlet facies were mainly under the fair weather condition ' and in part under the post-storm condition '. Wave ripples are generally preserved by deposition of the overlying clay. Clay was supplied by floodwater of rivers running into the Paleo-Tokyo Bay.

Point-Bar Deposits and Analysis of Subsurface Reservoir Dimensions, Toolachee Formation, Southern Cooper Basin, South Australia: ABSTRACT

Point-bar deposits occur ubiquitously within the lowest facies association of the Upper Permian Toolachee Formation (+500 ft thick). The association represents deposition in a moderate to high-sinuosity, mixed-load, fluvial channel system in which lateral bar deposits constitute major hydrocarbon sandstone reservoirs. Bars may be simple or punctuated in their upward-fining, vertical facies transitions, but heterolithic lateral accretion surfaces of upper point bars may occasionally be preserved in cores. Detailed facies studies of Association 1 in 17 cored wells reveal that pebbly thalweg lags locally include chaotic bank-collapse conglomerates. These basal point-bar deposits fine upward into cross-bedded sandstones. Ripple bedding is common in upper bar sequences. The bars are associated with a variety of abandonment fills, levees, and crevasse-splays, backswamp deposits, and autochthonous coals. Paleohydraulic studies of point bars in the Moomba field demonstrate an average bankfull depth of the Toolachee channels of 14 ft and bankfull width of 320 ft. Empirical relationships indicate a maximum meander loop diameter of 1,650 ft with meander belt width at 3,650 ft. These parameters allow an evaluation of reservoir geometry, storage potential, and permeability barriers. The latter, generated by clay-plug abandonment fills, can restrict fluid migration within the field and produce over-estimation of hydrocarbon reserves. Conversely, avulsive channel events may locally increase point-bar reservoir dimensions by partially, or totally, removing preexisting abandonment fills. End_of_Article - Last_Page 317------------

Stratigraphy and Depositional Environments--Krebs Formation in Southeastern Kansas: ABSTRACT

End_Page 1317------------------------------The Krebs Formation (Middle Pennsylvanian-Desmoinesian) forms the lower portion of the Cherokee Group in the Cherokee basin of southeastern Kansas. The Krebs Formation near its outcrop in Cherokee and Crawford Counties consists of 78% shale and mudstone, 18% sandstone and siltstone, 3% coal, and 1% limestone, comprising a total thickness of 120 to 220 ft (37 to 67 m). Integration of data from continuous cores, outcrops, and geophysical logs provides a detailed stratigraphic framework and facilitates interpretation of depositional environments. Coal beds and associated seat-rock units, some having an areal extent of several thousand square miles, provide excellent stratigraphic marker beds for correlation of discontinuous reservoir sandstones. Radioactive dark-gray shale units and argi laceous limestone units often overlie coal beds and may be equally widespread. Net-sandstone isolith maps reveal the presence of a lobate deltaic complex in southwestern Missouri, characterized by both stacking and offset of major sandstone bodies. Coal beds commonly cap upward-coarsening, mud-dominated sequences consisting of dark-gray shale with occasional argillaceous limestones overlain by lenticular-bedded shale or wavy-bedded siltstones. This vertical transition of lithofacies is interpreted to result from the progradational infilling of large interdistributary bays. Coarsening-upward sandstone sequences--consisting of lenticular-bedded shale grading upward into wavy-bedded siltstone, flaser-bedded sandstone, and rippled or cross-bedded sandstone--represent distributary mouth-bar or crevasse-splay deposits. Fining-upward sequences--composed of a basal scou surface overlain by mud-clast conglomerates, large-scale cross-bedded sandstone, and rippled or flaser-bedded sandstone--are interpreted to be channel-fill or point-bar deposits. End_of_Article - Last_Page 1318------------

Alluvial Architecture of Channel Belt Margins of Mississippi River, False River Region, Louisiana: ABSTRACT

The term alluvial architecture refers to the spatial arrangement and stratigraphic relationships between fixed channel belts (shoestring sands) and finer grained intervening flood-basin deposits. Overbank subenvironments associated with an active channel belt of the Mississippi River, including the levee, crevasses, and splay deposits, the upper point bar, and the abandoned channels, were cored using a standard vibracorer and Giddings Rig Soil probe. This study proposes that two end-member types of transition zones exist between channel belts and flood basins at an instant in time for a simple, sine-wave-shaped stream meandering in a fixed channel belt: (1) a levee to backswamp transition zone (type A), and (2) an upper point bar to backswamp transition zone (type B). T pe A transition zones, which consist of interstratified crevasse channel-fills, crevasse splay sheet sands, and fine-grained material of the natural levee, have greater potential for preservation than do type B transitions because crevasse splay deposits may extend for kilometers out into flood basins (type A) whereas preexisting point-bar deposits are destroyed during channel migration within the channel belt (type B). In type B transition zones, a sharp, erosional contact between the backswamp and upper point-bar subenvironments exists because of lateral migration of the point bar through preexisting facies within the fixed channel belt. End_of_Article - Last_Page 1420------------

Depositional Environment and Diagenesis of the Teapot Sandstone, Southern Powder River Basin, Wyoming

The Teapot Sandstone is interpreted as a regressive, wave/fluvial dominated deltaic sequence, which prograded eastward into the Cretaceous Seaway. Marine lithofacies coarsen upward from bioturbated offshore siltstone to nearshore sandstone with large, pellet-lined Ophiomorpha. The overlying well-sorted, horizontally-laminated, foreshore sandstone exhibits ridge and runnel topography. Marine foreshore sandstone is overlain by complexly interbedded sandstone and carbonaceous shale in stacked, lining-upward sequences of the delta plain. Rootlets and contorted beds are common. Fining-upward units are interpreted as abandoned channels. Coarsening-upward sequences are interpreted as interdistributary bay or lagoonal deposits. Capping the sequence is a thick, crossbedded, fluvial section consisting of levee, point bar, and channel sand deposits. Slumped beds, intraformational basal conglomerates, and minor eolian ripple laminations are present in fluvial sandstone. The Teapot Sandstone has a complex diagenetic history. Siderite and framboidal pyrite formed early in the diagenetic sequence at shallow depths of burial under anaerobic conditions. Pore-filling kaolinite, chlorite, and quartz overgrowths formed coevally following dissolution of relatively unstable framework grains. Poikilotopic calcite is locally abundant and extensively replaces framework grains. Depositional facies exert strong control of diagenetic patterns. Kaolinite occurs predominantly in fluvial sandstone. Chlorite is restricted to marine facies and calcite is further restricted to well-sorted foreshore marine sandstone. Quartz overgrowths occur only in relatively well-sorted sandstone, whereas pyrite and siderite are common in shaly sandstone and siltstone. Nearshore marine and fluvial sandstones are potential hydrocarbon reservoirs, although authigenic clays have significantly reduced permeability. Reservoir potential of we/I-sorted foreshore marine sandstone was destroyed by pore-filling calcite cement. Delta plain deposits are too thin, discontinuous, and poorly-sorted to form significant reservoirs.

Internal Geometry and Reservoir Potential of Some Modern Crevasse Splay Sands: ABSTRACT

Cores taken along strike and dip transects through the Baptiste Collette crevasse splay, modern Mississippi River Delta, have been analyzed to determine the sedimentologic nature and potential reservoir quality of modern crevasse splay sands. Internal geometry, lateral and vertical continuity, and sedimentary characteristics were determined to construct a model of crevasse splay depositional systems applicable to hydrocarbon exploration. The stratigraphic framework is more complex than previously recognized. This is demonstrated by the presence of several fine-grained (61-125µ) sand bodies (1-2 m or 3-6 ft thick) reflecting deposition in three distinct environments. Subaerial levee sands, which thicken toward the proximal end of the splay, contain 50-80% fine-grained (88µ) sand, 10% interlaminated muds, and 5-25% rooting. Distributary-mouth bar and point bar deposits (2 m or 6 ft below mean sea level) are 50-60% fine-grained sand (88µ) and 40-50% interlaminated mud, with 5% cross-bedding and a gradational base. The deeper (below 6 m or 18 ft) channel sands are 80-95% fine-grained (99-125µ) sand, with 20-75% cross-bedding, 10% interlaminated muds, and an erosional base. These correlatable sands are ncased in thick, organic-rich, bioturbated, bay and abandoned channel muds forming an impermeable seal. Channel sands have the greatest reservoir potential, being more laterally continuous along dip, clean ( This study should complement the limited knowledge of modern crevasse splay systems as well as provide insight into the exploration or enhanced recovery of hydrocarbons in ancient equivalents, such as the Admire 650-ft sandstone of Kansas. End_of_Article - Last_Page 1210------------

Lateral Accretion Channel Deposits in Athabasca Delta: Potential Modern Analog to McMurray Formation: ABSTRACT

Epsilon cross-stratification refers to large-scale, alternating sand and mud couplets that dip at right angles to the paleocurrent direction. This feature is of major concern when producing oil from ancient meandering river point-bar deposits. Mud epsilon cross-strata act as impermeable barriers to the movement of hydrocarbons within the reservoir. This and other related problems have been prevalent in steam enhanced recovery from the bitumen-rich middle member of the McMurray formation. Research on epsilon cross-stratified lateral accretion deposits has been focused for the most part on the study of ancient deposits. Investigation into the modern occurrence of these features has been limited in scale (0.5 m or 1.6 ft deep channels) and numbers of observations. Many unanswered questions remain as to the location of depositional environment (meanders in fluvial, tidal, or deltaic systems) and detailed sedimentologic characteristics. A recent investigation has found mud epsilon cross-strata in lateral accretion point-bar deposits in the Athabasca upper delta plain in northeastern Alberta. The overall sedimentologic trends of these particular lateral accretion deposits, including the variability, continuity, thickness, and geometry of the mud epsilon interbeds, has given new insight into the complex nature of these deposits and occurrence of depositional setting. Many aspects of the lateral accretion in the Athabasca upper delta plain appear very similar to the ancient lateral accretion deposits of the middle McMurray. Comparative sedimentology of the modern and ancient deposits may lead to a better understanding of these deposits. This may in turn allow for optimum site selection for in-situ steam injection and recovery wells as well as prediction of potential fluid movement patterns. End_of_Article - Last_Page 934------------

Sedimentologic and Stratigraphic Framework of Some Modern Crevasse Splay Sands: ABSTRACT

A series of cores taken along strike and dip transects through the Baptiste Collette crevasse splay, modern Mississippi River delta, have been analyzed to determine the sedimentologic nature and potential reservoir quality of modern crevasse splay sands. Internal geometry, lateral and vertical continuity, and sedimentary characteristics were determined to construct a model of crevasse splay depositional systems applicable to hydrocarbon exploration. The stratigraphic framework is more complex than previously recognized. This complexity is demonstrated by the presence of several fine-grained (61-125µ) sand bodies (1-2 m, 3-6 ft thick), reflecting deposition in 3 distinct environments. Subaerial levee sands, which thicken toward the proximal end of the splay, contain 50-80% fine-grained (88µ) sand, 10% interlaminated muds, and 5-25% rooting. Distributary mouth-bar and point-bar deposits (-2 m, -6 ft, MSL) are 50-60% fine-grained sand (88µ), 40-50% interlaminated mud, with Channel sands have the greatest reservoir potential, being more laterally continuous along dip, clean (5% silt and clay), well sorted, fine-grained and more homogenous, with few permeability barriers (i.e., mud layers and laminae). Conversely, the shallower bar and levee deposits have poorer reservoir quality, being less clean, less continuous laterally along dip, and with more permeability barriers. It is felt that this study will complement the limited knowledge of modern crevasse splay systems as well as provide insights into the exploration or enhanced recovery of hydrocarbons in ancient equivalents, such as the Admire 650^prime Sandstone of Kansas. End_of_Article - Last_Page 456------------

Ben South (Tannehill) Oil Field, Stonewall County, Texas: ABSTRACT

Ben South is one of 157 Tannehill oil fields on the northeastern shelf of the Permian basin. Texas Railroad Commission District 7B has 66 Tannehill oil fields while District 8A has 21. The discovery well for the Ben South field was the Ryder Scott Management (Sauder) 1 McMeans, completed in 1973. Ben South field production has totaled 749,340 bbl of oil through March 1983 from 13 wells. Oil production is from the lower Tannehill (lower Wolfcamp) sands underlying the Stockwether Limestone. These Tannehill sands were deposited in a fluvial environment. Channel-fill and point-bar deposits make up the pay sands. The trapping mechanism is both stratigraphic and structural. End_of_Article - Last_Page 116------------

Influence of Strike-Slip Movement on Terrestrial Sedimentation in Upper Carboniferous of Nova Scotia and New Brunswick: ABSTRACT

Lower Pennsylvanian (Namurian and lower Westphalian) sediments of maritime Canada were deposited in alluvial and lacustrine environments. Thick (> 3.5 km, 2.1 mi) sequences of these sediments accumulated within structural basins formed between strike-slip faults. Movement of the faults during the Namurian caused uplift in compressional areas. These positive areas, consisting of crystalline rocks and older Carboniferous sediments, provided a local supply of coarse sediment to the basin. In some cases, the positive areas excluded the introduction of sediment from large rivers sourced well outside the depositional basin. Large lakes were common in the Namurian and were probably a result of the relatively small amount of sediment that entered the basins. This sediment-starvation is also indicated by sequences of stacked paleosols that provide evidence of slow rates of sedimentation. A detailed study of the sediments indicates a progressive climatic change from arid conditions, with evaporites, in the lower Namurian to humid conditions, with coal deposits, in the Westphalian. This climatic change is reflected by an increase in the size of the extra-basinal river systems in younger formations. By the mid-Westphalian, the influx of sediment to the area was so great that the topographic basins were essentially infilled. Westphalian lakes were shallow and of limited lateral extent. Anomalously thick sequences of overbank sediments and stacked point-bar deposits are present, suggesting that tectonic movements were still sufficiently strong to influence the style of fluvial architecture. End_of_Article - Last_Page 505------------

Depositional Systems of Clinton Sandstone and Petroleum Exploration, Guernsey County, Ohio: ABSTRACT

The Lower Silurian Clinton Sandstone is the most commonly drilled formation in eastern Ohio. Successful exploration for subtle stratigraphic traps requires detailed knowledge of Clinton depositional systems. Two highly constructive cratonic delta systems (Claysville and Salt Fork deltas) are present in Guernsey County, Ohio. These deltas are typical of the small deltaic complexes present along the eastern margin of the Clinton-Medina production trend. Production from these deltaic deposits occurs in multistory and laterally discontinuous sandstone bodies deposited as distributary mouth bars, distributary channel fill, and delta-plain point bars. Criteria used to define depositional environments and patterns include: (1) sandstone isopach maps, (2) gamma-ray log cross sect ons, (3) log signature, and (4) slice isopach maps. Environmental interpretations are augmented by examination of two cores and thin sections. The three types of sandstone deposits are interrelated in a predictable manner and each has a unique isopach pattern, log signature, and production characteristics. Distributary mouth bar deposits are the most common reservoirs, and are characterized by coarsening-upward log signatures and elongate isopach patterns. Distributary channel-fill deposits are the most prolific reservoirs, and have eroded into underlying mouth-bar deposits. They are characterized by blocky log signatures and linear, narrow isopach patterns. Meander point-bar deposits have fining-upward log signatures and an ovoid to kidney-shaped isopach pattern. These methods and resul s provide a visualization of paleogeography and sedimentologic processes that should be used as a guide for development of and exploration for the Clinton Sandstone. End_of_Article - Last_Page 1922------------

Quantitative interpretation of an evolving ancient river system

ABSTRACTMultistorey sandstone bodies described from the Upper Devonian‐Lower Carboniferous of Kerry Head (Ireland) are interpreted as deposits of aggrading, perennial, river channels migrating laterally across alluvial plains. Point bars displayed surface features such as scroll bars, chute channels and chute bars. Relatively uncommon channel fills are both coarse‐ and fine‐grained.Quantitative interpretation of the sandstone bodies was accomplished by comparison with a physical model that predicts the sedimentology of single point bar deposits developed in channels of prescribed geometry and hydraulics. This analysis reveals that the separate storeys (point bars) in each sandstone body were deposited in a single channel belt in which channel geometry and hydraulics varied little with time (order of 103 yr) and space (order of 103 m). Two southerly flowing rivers of markedly different size were responsible for all sandstone bodies: bankfull widths, depths and mean velocities of both rivers varied little with time (order of 105 yr), implying a stable climatic setting. Channel sinuosities were usually 1.15–1.2 throughout the succession. Both rivers decreased in mean channel slope as time progressed, in association with a rising base‐level and a shoreline encroaching from the south.Using Bridge & Leeder's (1979) alluvial stratigraphy model, the nature and distribution of channel sandstone bodies relative to overbank deposits in the succession can be explained by an average (compacted) floodplain deposition rate of about 0.005 m yr−1, if avulsion occurred with a frequency of about once every 103 yr. Local variation in the relative amount of channel sandstone in the succession is probably due to local tectonic control of deposition.