Bar Complex Research Articles

Overbank and channelfill deposits of the modern Yellow River delta

The Huanghe is noted for its high transport rate of silt and clay, which may reach depth-averaged values of 200 kg m −3 during peak discharge. The sediment load transported through the river on entering the delta plain, amounts to 10 12 kg per year. In contrast to most other large deltas only one distributary channel is active at any one time. The high sediment load causes the rivermouth to prograde at a yearly rate of 1–4 km into the shallow (less than 20 m deep) Bohai gulf. The vertical aggradation of the channel belt and mouth bar complex is also rapid (decimetres per year on average), so that after a normal average of twelve years increasing channel instability and avulsion create the start of a new delta lobe. A series of satellite images covering the last fifteen years has provided insight in the evolution of the river pattern as well as the progradation of the delta front. A newly developed distributary passes from a multichannel to a single, straight channel system, and ends with the formation of meanders. The protruding mature delta lobe shows a radiating pattern of crevasse channels. Overbank/ crevasse deposits are made of vertically stacked dm-scale waning flow sequences, structurally characterized by (from bottom to top) small scour-and-fills, even (parallel) lamination, and climbing-ripple crosslamination. Accumulation rates on crevasse splays can be predicted on the basis of estimated river sediment discharge. It can be concluded that each sequence has been deposited within a few hours, and that tidal waterlevel fluctuations may have played a role in the generation of a single sequence.

Reconstructing Fluvial Macroform Architecture from Two-Dimensional Outcrops: Examples from the Castlegate Sandstone, Book Cliffs, Utah

ABSTRACT The Castlegate Sandstone is part of the Upper Cretaceous (Campanian-Maastrichtian) Mesaverde Group, a clastic wedge that prograded eastward from the Sevier Orogen of central Utah across the Western Interior Basin. The unit comprises the lowstand part of a regional fourth-order stratigraphic sequence. Earlier studies showed that the Castlegate Sandstone was deposited by braided rivers that deepened downstream from Price to east of Green River. Outcrop mapping employing photomosaics to document facies architecture, bounding surfaces, and paleocurrent information has been used to reconstruct the geometry of bars (macroforms) and bar complexes in the Castlegate rivers. The most common bar types are those that accreted laterally and downstream relative to local channel orientation. Gradations between these end-member accretion geometries may be present within the same depositional element. In one case, curved accretion surfaces are concave in map view, with the concavity oriented in a downdip direction, suggesting an origin as a counterpoint bar. Such bars are rare in braided systems. Accretion directions are determined relative to local paleoflow and are the basis for classification of bars as downstream- or laterally-accreted types. However, local paleoflow may be oriented at a high angle to regional or total-outcrop mean flow direction, indicating locally high channel sinuosity. The results of this study indicate that megascopic heterogeneity in the Castlegate Sandstone has a lateral dimension in the order of 100 m. It is suggested that this might be a suitable length dimension for scaling of grid blocks in simulation models of petroleum production of other, similar sandy braided fluvial systems.

Evolution of the Llanos Basin and the Deformation of the Eastern Cordillera, Colombia: ABSTRACT

The Llanos Basin is located on the flank of the Eastern Cordillera in northeast Colombia. Basin development commenced with the deposition of a synrift Triassic and Jurassic megasequence related to the separation of North and South America in the Caribbean. Basin development continued with the Cretaceous Back Arc Megasequence deposited in a back arc basin behind the Andean subduction zone. Three major sequences can be recognized corresponding to extensional pulses in the Tithonian, Albian, and the Santonian which control thickness and facies distributions. The primary reservoir in the basin is the Late Eocene Mirandor Formation which was deposited in a fluvial system which prograded from the Guyana Shield to the west-northwest. This was deposited as part of the Pre-Andean Foreland Basin Megasequence (Bartonian to Serravallian) which developed as a result of uplift onset and deformation in the Central Cordillera. This megasequence covered the Magdalena Valley the Eastern Cordillera ad the Llanos Basin. In the foothills of the Eastern Cordillera, the Mirador Formation begins to show evidence of marine influence and was probably deposited in a series of shoreface sands and offshore bar complexes in the Cordillera. The Pre-Andean Foreland Basin Megasequence includes the Eocene-Oligocene Carbonera Formation which was deposited inmore » a low every fluvial system that was mud dominated. Within the Carbonera Formation, a series of major, grossly coarsening upward cycles can be seen which are separated by maximum flooding surfaces that approximate to time lines. These cycles correspond to the early phases of development of the Central Cordillera with each pulse being seen as an influx of coarser clastics to the basin. The deformation style in the Eastern Cordillera is a mixture of thin-skinned thrust structures and the inversion of the thick-skinned basement involved extension faults. The inversion structures include the Cuisana field, a giant oil and gas-condensate discovery.« less

Anabranch confluence kinetics and sedimentation processes in gravel-braided streams

Abstract Based on observations of channel changes and bar sedimentation in small-scale hydraulic models, it is argued that confluence zones play the key role in sedimentation patterns and processes in gravel-bed braided streams. The control is exerted because confluences are nodes in the channel network and therefore sedimentation and channel pattern downstream of a confluence are mediated by the response of the confluence to changes in discharge, bedload, and planform of the upstream confluent channels. Examples of confluence kinetics and sedimentation occurring over time-scales equivalent to days or weeks of competent flows in the prototype are described for the first time. Renewed braiding is directly associated with flow expansion downstream of confluences and triggered by migration of unit bars or bedload sheets through the confluence. Lateral translation of confluences may lead to the accretion of point bar complexes, while downstream translation often causes erosion or complete removal of existing bars. The effects of a change in position or orientation of one of the upstream channels are similar to those of confluence translation. In addition to the changes caused by progressive shifting of the confluence zone, a second group of adjustments involving changes in the discharge or bedload transport rate in the confluent channels is also common. These include: rotation of the scour axis redirecting flow and sediment movement downstream of the confluence and leading to downstream channel avulsion in some cases; a change in the total discharge of the confluence causing expansion or contraction of the scour zone; and the virtual abandonment of one of the confluent channels resulting in obliteration of the scour hole. Observing a confluence for several hours in the model leaves the visual impression of sediment being distributed downstream of the confluence rather like water spurting from a hose as it plays about. This descriptive synthesis of confluence kinetics provides the basis for analysis and quantitative modelling of confluence dynamics, which requires the acquisition of field data, as well as further laboratory experiments.

The Use of Routine and Special Core Analysis in Characterizing Brent Group Reservoirs, U.K. North Sea

This paper describes the use of core analysis data, both routine and special, in characterizing the Brent Group reservoirs in the U.K. North Sea. The results of various special core analysis tests conducted over the years indicate that coring fluid, core preservation, and laboratory procedures are important in defining relative permeability and capillary pressure. Examples are given of the effect of oil-based mud filtrate on rock wettability; the effect of extraction, drying, and test procedures on laboratory waterflood performance; and variation of relative permeability among facies. Results also suggest how petrography may be used in assigning relative permeabilities by facies. Analysis of routine core data shows complexity within the Brent Group reservoirs even within relatively uniform sands.

Differential forms on loop spaces and the cyclic bar complex

In this article, we present a model for the differential graded algebra (dga) of differential forms on the free loop space LX of a smooth manifold X and show how to construct certain important differential forms in terms of this model. Our motivation is an observation of Witten (described by Atiyah in [2]) that the index theorem for the Dirac operator can be thought of as an application of the localization (or fixed point) theorem in T-equivariant homology, suitably generalised to the infinite dimensional case of the free loop space; here, T is the circle group. We can summarise our main results as follows. We are really concerned with equivariant differential forms and equivariant currents and we show how to reformulate these geometric objects as cyclic chains and cochains over the differential graded algebra Ω(X) of differential forms on X. In fact, the cyclic chain complex of Ω(X), if it is normalized correctly, is a sub-complex of the complex of equivariant differential forms on LX, and is a good enough approximation that it allows us to compute the ordinary and equivariant cohomology of LX, and to write down explicitly certain important differential forms and currents. We begin by explaining the motivation more carefully. Let S be a Clifford module on X with Dirac operator D. Witten observed that it should be possible to associate to D an equivariantly closed (inhomogeneous) current μD on the free loop space of X. The basic property of this current is that the index of D is given by pairing μD with the differential form 1 ∈ Ω(LX):

A Comparison of Reservoir Heterogeneity and Recovery Efficiency of Two Offshore Bar Complexes in an Intracratonic Basin

The US Department of Energy is developing a nation-wide reservoir classification system to update and upgrade its TORIS database. This new information will allow targeting of key reservoir types for focused research on improved an/or enhanced recovery. A comparison of two shallow marine siliciclastic reservoirs from the intracratonic Illinois basin offers an example of the level to which reservoir types need to be differentiated in order to predict optimal recovery methods. The lateral homogeneity of offshore bars within the Mississippian Cypress Formation at Bartelso field is a major factor contributing to a relatively high primary and secondary recovery efficiency. On the other hand, the lateral heterogeneity of a partially dissected offshore bar complex within the Mississippian Aux Vases Formation at King field is the major factor adversely affecting the efficiency of primary and secondary recovery techniques there. Although both these fields produce from sandstones that were deposited in shallow marine environments, a classification system must further differentiate between the two in order to stress the different exploitation procedures required to optimize recovery. Significant quantities of remaining mobile reserves may be recovered in King field-type Aux Vases reservoirs by programs involving infill drilling and selective waterflooding. Infill drilling would not bemore » effective in recovering additional reserves from the offshore bar sands of the Cypress Formation where compartments have been effectively drained by typical well spacing and waterflooding. Instead, tertiary methods will need to be applied. Reservoir studies from a variety of basins will be needed before it can be determined to what levels reservoir classification needs to be subdivided to enable prediction of optimal recovery methods.« less

Depositional Facies, Textural Characteristics, and Reservoir Properties of Dolomites in Frobisher-Alida Interval in Southwest North Dakota: Discussion (1)

The Mississippian Frobisher-Alida interval is an upward-shoaling cycle that began with open-marine sedimentation and culminated with the deposition of a widespread sabkha-salina evaporite. This cycle is the most prolific oil-producing interval in the North Dakota portion of the Williston basin. Most Frobisher-Alida production in the southern Williston basin is from dolomite reservoirs. The six major facies defined in this paper are lithologic suites that represent sediments and precipitates deposited in similar environments. Dolomite formed as a replacement of muddy sediments deposited in three facies: II, IV, and V. Facies II sediments consisted primarily of muds deposited in offshore lagoons. Facies IV sediments were composed of burrowed muds deposited in a restricted-marine environment in front of a barrier bar and island complex. Fossiliferous muds, which formed in an open-marine environment, were the dominant sediments deposited in facies V. The original sediment textures strongly influenced the textures and distribution of replacement dolomite. Some dolomite zones display relatively uniform textural and petrophysical properties over large portions of southwest North Dakota. Porous dolomite zones with larger average crystal sizes display better producing characteristics than zones with smaller average crystal sizes. The average crystal size in Frobisher-Alida dolomites ranges from about 8 to 44 µm. Most large Frobisher-Alida dolomite reservoirs in southwest North Dakota occur in combination structural-stratigraphic-hydrodynamic traps. Stratigraphic traps in dolomites are found where facies changes occur or where an updip decrease in the dolomitization intensity formed lower permeability rocks. A regional hydrodynamic gradient aids stratigraphic trapping in many dolomite zones that would not trap oil under hydrostatic conditions.

Specos Sa, a Computer Program for Managing, Graphing and Manipulating Laboratory Data

ABSTRACT A stand-alone version of the instrument automation and control program SPECOS (SPECtroscopy Operating System) has been developed for general laboratory data management, for data manipulation and processing, and for generation of presentation graphics and high quality plots. For data management, this program (SPECOS SA) provides features for data file creation, storage management, record keeping, report generation and automatic multiple file processing. For data manipulation it provides standard arithmetic functions, statistical procedures, smoothing, fitting, derivative and integral calculations, auto-and cross-correlation, and a wide variety of data transformations. The graphing and plotting functions support a variety of plotters and laser printers allowing variation of pen (color), line widths and types, plot symbols, bar graph characteristics, multipanel plots and complex graticule outlines. The program includes on-line help features and is provided with a set of instructional tutorials and test files.

Details of South Lake Arthur Field--Isopachous Maps and Log Correlations Utilizing Depositional Ideas: ABSTRACT

Continued development of South Lake Arthur field, Vermillion Parish, Louisiana, has given geologists some apparent answers as to the true nature of the productive Miogyp feature-that it is a large, structurally simple, pressure connected, east-west oriented anticlinal structure developed on the downthrown side of a major east-west, down to the coast, growth fault system. South Lake Arthur field is a structurally simple feature, the stratigraphic sequence of which ideally fits the unstable-shelf delta model. Deposition of a thick sequence of plastic prodelta shales immediately preceded a major regressive Miogyp delta complex. Component sand bodies within this complex conform paleogeographically to the model, with a central depopod of distributary mouth bars, flanked seaward by beach sands, and capped by a classic reworked sand. The Miogyp delta system built for a long duration of time owing to rapid subsidence associated with the dumping of a thick sediment load on the downthrown side of a large down to the coast growth fault, and flowage of the underlying shale. Although South Lake Arthur field is structurally simple, the Miogyp through Camerina section is stratigraphically very complex. Paleostructural analysis, using interval isopach maps, is necessary to properly interpret this section. Interval isopach maps reveal correctmore » depositional-strike, or paleo-strike, correlations. These isopach maps also depict a stratigraphically complex growth history of the South Lake Arthur field structure. Shifting depositional patterns and unconformities all relate to the movement of the plastic core of the prodelta shale underlying the Miogyp distributary mouth bar complex.« less

Concentrated and lean oolites: examples from the Nubia Formation at Aswan, Egypt, and significance of the oolite types in ironstone genesis

Summary Two textural types of oolitic ironstones occur in the Late Cretaceous Nubia Formation at Aswan, Egypt. These are: (1) relatively thin bedded (5–20 cm) muddy or ‘lean’ oolites containing up to 300oids, proto-ooids and peloids scattered in a matrix of ferruginous mud, and (2) thicker beds (as much as 2.5 m thick) of ‘rich’ or ‘concentrated’ oolites comprising 60–800oids. Sedimentologic and textural attributes of the ‘concentrated’ oolites suggest that these represent ooids transported and redeposited away from their site of formation. In fact, in ironstone-bearing successions elsewhere the concentrated oolites are either associated with winnowed sandstones or, in sand-poor deposits, they substitute for sandstone. Together the ooids and the detrital sand form bar and sand wave complexes in upward shoaling marginal marine sediments. In unbioturbated or less bioturbated occurrences these sandbodies of ferruginous ooids are replete with primary sedimentary structures indicative of strong current transport. Thin beds of similar ferruginous ooids in sequences of off-shore carbonate or argillaceous basinal mudstone, such as those in the Silurian Clinton Group of eastern United States, are storm deposits characterized by basal scours and hummocky cross-stratification. The muddy or ‘lean’ oolites, on the other hand, are commonly associated with prograding subtidal mudflat deposits. Textural characteristics of these ironstones suggest that they probably represent the primary sites of ooid formation. Different stages of ooid formation preserved in these ‘lean’ ironstones and their relationship with the concentrated oolites provide information which is significant for a more realistic and comprehensive genetic modelling for the Aswan ironstones. In the model proposed here the deposition of muddy, ferruginous sediments on prograding, near-shore, subtidal mudflats begins during the waning stages of detrital sediment supply. The ooids are formed on these shallow-water mudflats by mechanical accretion of detrital kaolinitic clay and precipitated iron hydroxide minerals, mostly around faecal pellets or flocculated peloids, under the influence of gentle wave or current agitation. Periodic high-energy events, such as storm-generated or tidal currents, transport the ooids to off-shore areas to be reworked there into oolitic sand bar and shoal complexes. Following deposition, the kaolinitic ooids, peloids and matrix are transformed into berthierine under a chemically reducing diagenetic environment.

Depositional Environments, Diagenesis, Hydrocarbons: Codell and Juana Lopez Members of Carlile Shale (Upper Cretaceous), Canon City-Raton Basins, South-Central Colorado: ABSTRACT

Codell and Juana Lopez strata in the Canon City and northern Raton basins comprise a nearshore marine system which was deposited in a series of barrier islands, lagoon fills, tidal deltas, and offshore bars. Codell thicknesses vary but average 6 m (20 ft). Three areally significant Codell paleoenvironments occur: barrier island, lagoonal, and offshore bar. Juana Lopez rocks are thinner, usually less than 1.8 m (6 ft). Five distinctive lithofacies/paleoenvironments occur in the Juana Lopez: (1) a calcarenite or limy sandstone (tidal flat); (2) a sandstone with limonitized borings (offshore bar complex); (3) a shaly to massive sandstone sequence (subaerial beach/dune ); (4) a sandy limestone or biosparite (lagoonal/bay molluscan biostromes); and (5) a sandy shale (offshore bar sequence). These deposits accumulated along a northeastward-trending coast that prograded southeastward in response to a gradual drop in sea level. Petrographic and scanning electron microscopy study reveals the following diagenetic sequence in the Codell Sandstone: (1) modification by authigenic, syntaxial quartz overgrowths; (2) chert cementation; (3) dissolution episodes causing corrosion of quartz, chert, and feldspar; (4) calcite cementation; (5) late-stage limonitization; and, in rare instances, (6) dehydration of limonite to hematite. Diagenetic changes in the Juana Lopez Member involve minor dolomitization, precipitationmore » of calcite rim cement, and limonitic staining. Stratigraphically trapped hydrocarbons occur in bioturbated, relict shelf Codell sandstones in the west-central portion of the Denver basin. Valley-fill( ) Codell sandstones of the northern Denver basin are generally tight but do produce. Juana Lopez calcarenites and fetid biosparities may lack commercial hydrocarbons.« less

Facies Architecture in a Fluvial-Deltaic Sequence, Upper Crooked Fork Group (Pennsylvanian), Tennessee and Kentucky: ABSTRACT

The upper Crooked Fork Group on the Cumberland plateau in northeastern Tennessee and southeastern Kentucky includes the Wartburg Sandstone (Corbin of Kentucky) and surrounding fine-grained strata of the Breathitt Formation below the Poplar Creek Coal. Detailed study of outcrops employs lateral profiles and architectural element analysis. Examination of bounding surfaces within and between elements indicates a sixfold hierarchy based on areal extent and geometry. The Wartburg Sandstone consists largely of three-dimensional macroform bar complexes. Foreset macroform elements predominate, with minor lateral accretion elements present and no apparent internal cyclicity. The channelized base of the Wartburg, predominance of large-scale planar and trough cross-stratification, low paleocurrent variance, and presence of soft-sediment deformation elements and massive sandstone elements with large mudstone intraclasts suggest a low-sinuosity fluvial setting. Strata overlying the Wartburg display fine-grained lateral accretion elements and associated overbank-fines. This suggests an upward change in fluvial style in the upper Crooked Fork Group from a predominantly coarse-grained low-sinuosity setting to a fine-grained high-sinuosity setting. The abundance of downstream and lateral accretion elements, plant debris, rooting, and thin discontinuous coals; paucity of burrows and bioturbation; and presence of thin marine horizons combined with the facies architecture suggest a channelized depositional setting with some marinemore » influence, indicative of fluvial or fluvial-dominated deltaic deposits.« less

Three-Dimensional Reconstruction of an Evolving Fluvial System: Chinle Formation, Northeastern Arizona: ABSTRACT

Exceptional three-dimensional (3-D) exposures of the Triassic Chinle Formation in northeastern Arizona allow detailed architectural analyses of a complex fluvial system. Lateral profiling, element analysis , and hierarchy of bounding surfaces demonstrate deposition in both low- and high-sinuosity channel systems. Two conglomerate bodies, the Shinarump and the Sonsela, were deposited in low-sinuosity, valley-confined systems as indicated by internal sequences of sedimentary structures and geometry of the macroforms. Stacked channel sequences consist of in-channel sandy bed forms separated from large, in-channel, downstream-accreting sandy macroforms (up to 5 m thick by 50 m long) bounded by fourth-order surfaces. Within localized channels, conglomerate bar complexes (up to 10 m thick by 150 m long) record downstream and oblique accreting macroforms bound both laterally and vertically by in-channel elements. Architectural features in the Chinle reflect intrabasinal as well as extrabasinal controls on sedimentation. Major changes in stream sinuosity are largely controlled by gradient changes associated with increased subsidence rates with time.

Linear Sand Body Trends in Mississippian Aux Vases Formation Outcrop Belt: Model for Subsurface Exploration: ABSTRACT

A regional analysis of depositional environments and facies relationships in the Mississippian Aux Vases Formation has identified two distinct linear sand-body trends. Field investigations from the type section in southeastern Missouri across southern Illinois to outcrops in Indiana and Kentucky show a major northeast-southwest trend and a secondary north-northwest-south-southeast trend. The major trend is interpreted to be a series of tidal-channel complexes, while the secondary trend is interpreted as multilateral and multistored offshore or tidal bar complexes separated by thin sheet sands or shallow marine shales. All sand bodies are composed of fine to medium-grained, subangular to rounded, sand-size quartz grains. Subsurface investigations by many previous workers have also identified linear sand bodies variously interpreted as channels or bars. A depositional model based on outcrop studies suggests that the tidal channel facies is volumetrically larger than the offshore bar facies and should therefore be potentially larger hydrocarbon reservoirs. Empirical models, based on detailed outcrop analyses, may greatly enhance hydrocarbon exploration.

Integrated Seismic-Petrologic Mapping of Ooid Bars and Associated Facies, St. Louis Formation (Upper Mississippian), Gray County, Kansas: ABSTRACT

Localized dim-spot anomalies on a large-amplitude, continuous seismic reflector correspond to porosity zones in the St. Louis Limestone. Anomalies are recognized as a decrease in wavelet amplitude on 12-fold CDP data and smaller amplitudes represent higher porosity. Mapped low-amplitude anomalies trend in a northwest-southeast direction. Continuously cored intervals from wells tied to the seismic data were studied to determine depositional environment, porosity types, and their relationships to the seismic anomalies. Porous ooid grainstones, up to 12 ft thick, encased in mud-rich skeletal wackestones and packstones document the presence of ooid shoals developed on a low-moderate energy shelf. The shoals, as seen on wireline logs and in cores, define a northwest-southeast-trending bar complex paralleling depositional strike. Primary interparticle and leach porosity totals around 10% in the ooid grainstone. Skeletal wackestones-packstones are locally cross-bedded and contain contorted shale laminations. These facies are tightly cemented and were deposited on a low-moderate energy shelf. Northwest-southeast-trending porosity zones in ooid grainstone shoals correlate with low-amplitude anomalies having the same orientation.

Depositional, Diagenetic, Production, and Seismic Characteristics of Mid-Dip Tuscaloosa Point Bar Complex, Little Creek Field, Mississippi: ABSTRACT

Little Creek field is oil productive from a structurally and stratigraphically trapped sandstone within the Upper Cretaceous lower Tuscaloosa Formation (Mid-Dip trend of southwestern Mississippi). The field was discovered in 1958 based on the seismic interpretation of simple closure at the lower Tuscaloosa horizon at a depth of 10,750 ft. Interpretation of core and log data indicates that the objective is a series of point-bar deposits and channel abandonment facies formed by the lateral migration of a north-to-south-flowing meandering river system. Development drilling, based on geologic concepts of point-bar architectures, occurred on 40-ac spacing with a total of 208 wells drilled; 162 were successful. New seismic data across the field have shown that the sandstone is a mappable soft seismic event; similar techniques have been successfully used for discovery and delineation elsewhere within the Mid-Dip Tuscaloosa trend. Little Creek has undergone primary recovery, waterflood, a carbon dioxide pilot, and recent initiation of a field-scale carbon dioxide flood. Approximately 70% of the original oil in place (102 million bbl) is estimated to be recovered by all processes. Reservoir quality and continuity are generally high and are controlled by both depositional and diagenetic processes. Average porosity and permeability are 24% and 100more » md, respectively. Depositional controls include slightly reduced vertical to horizontal permeability, local shale drapes, and channel abandonment facies. Diagenetic controls include compaction, chlorite grain coatings, and quartz and ankerite cements. The chlorite is most important in controlling the observed petrophysical properties of the sandstone, including the water saturation, porosity, and bimodal pore system.« less

Depositional Facies, Textural Characteristics, and Reservoir Properties of Dolomites in Frobisher-Alida Interval in Southwest North Dakota

The Mississippian Frobisher-Alida interval is an upward-shoaling cycle that began with open-marine sedimentation and culminated with the deposition of a widespread sabkha-salina evaporite. This cycle is the most prolific oil-producing interval in the North Dakota portion of the Williston basin. Most Frobisher-Alida production in the southern Williston basin is from dolomite reservoirs. The six major facies defined in this paper are lithologic suites that represent sediments and precipitates deposited in similar environments. Dolomite formed as a replacement of muddy sediments deposited in three facies: II, IV, and V. Facies II sediments consisted primarily of muds deposited in offshore lagoons. Facies IV sediments were composed of burrowed muds deposited in a restricted-marine environment in front of a barrier bar and island complex. Fossiliferous muds, which formed in an open-marine environment, were the dominant sediments deposited in facies V. The original sediment textures strongly influenced the textures and distribution of replacement dolomite. Some dolomite zones display relatively uniform textural and petrophysical properties over large portions of southwest North Dakota. Porous dolomite zones with larger average crystal sizes display better producing characteristics than zones with smaller average crystal sizes. The average crystal size in Frobisher-Alida dolomites ranges from about 8 to 44 µm. Most large Frobisher-Alida dolomite reservoirs in southwest North Dakota occur in combination structural-stratigraphic-hydrodynamic traps. Stratigraphic traps in dolomites are found where facies changes occur or where an updip decrease in the dolomitization intensity formed lower permeability rocks. A regional hydrodynamic gradient aids stratigraphic trapping in many dolomite zones that would not trap oil under hydrostatic conditions.

Diagenesis of Terry Sandstone (Upper Cretaceous), Spindle Field, Colorado

ABSTRACT This study discusses the influence that sedimentology, diagenesis, and basin subsidence history have on porosity and permeability of the Upper Cretaceous Terry sandstone of Spindle field in the Denver basin. The Terry sandstone consists of lithic subarkoses and feldspathic litharenites deposited in a marine bar complex. The reservoir facies, which was deposited in a marine bar crestal position, is a cross-bedded sandstone with a mixture of primary and secondary macropores. Approximately half of the pore volume is microporosity associated with clay minerals and lithic fragments. The nonreservoir facies was deposited on the bar-flank margin as lower-energy, alternating layers of mud and sand, which were extensively bioturbated to produce muddy sands. Ductile deformation of lithic fragments is a major factor in porosity loss for the reservoir facies. Chlorite and calcite are the most important mineral cements. Authigenic quartz, dolomite, ferroan calcite, kaolinite, feldspar, pyrite, and tourmaline are present in minor to trace amounts. Precipitation of these minerals is believed to be related to the evolution of marine water. Secondary porosity formed by dissolution of pore-filling and replacement calcite and framework feldspars. Sandstones with pervasive calcite cement have no secondary porosity developed, which suggests some permeability may be needed to develop secondary porosity. Plagioclase feldspars become more albitized with increasing depth. A temperature-depth subsidence profile predicts a maximum temperature of only 85°C, at least 15° lower than in Tertiary rocks, which suggests that kinetic considerations may also be important to albitization. Vitrinite reflectance (0.65%) and interbedded shales with ordered mixed-layered illite/smectite with up to 50% expandability also support the lower temperature regime. Calculations suggest that seawater can supply sufficient sodium to account for albitization. Albitization proceeded more rapidly in permeable sandstone, where the plagioclase was in direct contact with the pore fluids. Plagioclase in relatively impervious sandstones eventually became albitized, probably through diffusion.

Transgressive facies in the South Munster Basin, Ireland

ABSTRACTThis paper describes the sedimentation style associated with the basal Carboniferous transgression in southern Ireland and the influence which this event had on the palaeogeography of the region. The transgression as marked by the base of the Carboniferous succession is shown to represent one of several genetically related transgressive pulses which commenced during the Late Devonian. At this time an east‐west trending graben, the South Munster Basin, developed in southern Ireland. This was initially a non‐marine depositional site in which sediment was derived from the north and west. Subsidence and eustatic sea‐level rise resulted in a marine transgression which proceeded in a rhythmic style resulting in a number of transgressive‐stillstand pulses. The first transgressive pulse (T1) advanced in a westerly direction along the basin axis resulting in the development of an epicontinental‐like sea. The shoreline remained essentially static along the northern basin margin initially until a second transgressive pulse (T2) resulted in expansion of the marine area. The latter proceeded by gradual northward erosive advance of a barred coastal area as far as the northern basin margin where the stability of the bounding platform halted its progress. Erosion of the barrier shoreface was insufficient to destroy all the backbarrier lagoonal deposits which are preserved as a thin transgressive diachronous unit which grades northwards to a coastal alluvial plain.Immediately preceding the basal Carboniferous transgression (T3), a shallow, wave‐dominated, storm‐influenced shelf sea occupied the basinal area. Two sublittoral sand bar complexes developed on the shelf under the influence of shore‐parallel current regimes, apparently derived from source areas located on either side of the epicontinental seaThe basal Carboniferous transgression took place in two pulses. The first (T3a) resulted in a rapid reduction in sand supply to the shelf and deposition of clay. The barrier shoreline responded by erosively retreating across the lagoon, leaving a transgressive lag in its wake. Its northwards advance was, however, limited due to the relative stability of the northern platform. Sand supply to the shelf was completely terminated in the second pulse (T3b) and the barrier rapidly migrated erosively across the northern platform for a considerable distance such that the coastal plain is overlain by a thin transgressive lag. This transgressive phase was immediately followed by shoreline stillstand and progressive shallowing of the shelf. An open sandy shelf developed on which offshore sand bars accumulated under a storm and wave dominated regime. Clay deposition continued in the deeper part of the basinal area but was eventually terminated as the shelf sands prograded centripetally into the basin.The main factor that controlled the style of the overall transgression was an interplay between eustatic sea level rise and basin subsidence. The rate of relative sea‐level rise together with the effect of differential subsidence and fluvial input from the north appear to have diminished with time. The rate at which successive transgressive pulses advanced northwards shows an overall progressive increase.