Aeolian Facies Research Articles

Influence of Relative Sea-Level Change and Tidal-Inlet Development on Barrier-Spit Stratigraphy, Sandy Neck, Massachusetts

ABSTRACT Sandy Neck is a 10-km-long sheltered mesotidal barrier spit along the southeastern New England coast, bordering Cape Cod Bay. Its stratigraphy contains evidence of relative sea-level (RSL) rise and tidal-inlet development, and enables reconstruction of accretion and progradation of this barrier through time. Core, trench, and ground-penetrating-radar (GPR) data show that the lower boundary of the uppermost, eolian, facies climbs about 9 m from the proximal to the distal part of the spit, away from the mainland anchor point. This change is thought to be related primarily to a comparable rise in RSL. The same boundary also shows a shallowing trend in a seaward direction, indicating net spit progradation through time. The absence of inlet-channel facies in the stratigraphy of the proxima part of the barrier spit, in contrast to the presence of inlet facies in more distal parts of the spit, indicates that some time elapsed after barrier initiation before an inlet thalweg developed. In its earliest stages, this thalweg was poorly defined, wide, and shallow, but its characteristics changed as the barrier spit lengthened. In the central part of Sandy Neck, the assemblage of inlet-channel facies becomes markedly thicker, reflecting thalweg deepening in response to paleotopographic and substrate-lithologic factors. The stratigraphy of the distal part of the spit shows the effects of continued, but more gradual, inlet deepening that may be tied directly to increasing barrier length. The process-controlled stratigraphy presented here differs from current facies models because i accounts for temporal changes in coastal environmental processes and parameters. It suggests a high preservation potential for proximal eolian facies in the spit sequence, underneath a cover of fine-grained and organic back-barrier sediment. Once reliably dated, the new stratigraphic data will help to improve the regional RSL curve for the southern Cape Cod Bay area.

Modeling Heterogeneity in a Low-Permeability Gas Reservoir Using Geostatistical Techniques, Hyde Field, Southern North Sea

Hyde field is a small (133 Gcf (FOOTNOTE *) reserves) gas field in the United Kingdom southern North Sea. The reservoir, the Permian Rotliegende Group, contains eolian, fluvial, and sabkha facies. The eolian facies constitute the dominant flow units. Illite cementation results in low reservoir quality. In the eolian facies permeability averages 2.5 md and rarely exceeds 10 md. To maximize recovery from this field, with its thin gas column and low reservoir quality, Hyde has been developed with three long-reach horizontal wells. A reservoir model that could accurately predict future production had to include a full range of heterogeneities from the effects of thin muddy laminations at the core-plug scale to the spatial distribution of eolian, fluvial, and sabkha facies over the entire field at the largest scale. By carefully defining the model's layering scheme, we incorporated strongly deterministic elements where facies trends were influenced by laterally extensive stratigraphic surfaces. The distribution of facies within stratigraphic units was modeled using geostatistical techniques. Average permeability values were assigned to the poorest quality fluvial and muddy sabkha facies. Permeability values for the eolian and sandy sabkha facies were determined by upscaling models of their internal permeability structure. The resulting distribution of permeabilities across the field, present in a 2.2-million-cell model, was upscaled into a coarser grid suitable for simulating the full field. This new model produced a significantly better match to dynamic data than did an earlier simple layer model that was used to take the field to sanction. The results of our work suggest that stochastic modeling allowed us to represent a level of heterogeneity that was not captured by the earlier simple layer model. By capturing this level of heterogeneity, we were able to achieve a significantly better match of model predictions to production data.

Hydrocarbons Related to Early Cretaceous Source Rocks, Reservoirs and Seals, Trapped In Northeastern Neuqun Basin, Argentina: ABSTRACT

The Jurassic-Cretaceous backarc Neuqun Basin, located in the west central part of Argentina, is currently the most prolific oil basin of the country. The primary objective of this study is to evaluate an Early Cretaceous to Tertiary petroleum system in the northeastern portion of the basin, where oil and gas occurrences (e.g., Puesto Hernandez, Chihuido de la Sierra Negra, El Trapial and Filo Morado oil fields, among others) provide 82 MMBO/yr comprising 67% of the basin oil production and 31% of Argentina. The source rocks are represented by two thick sections of basinal kerogen type I and II organic-rich shales, deposited during transgressive peaks (Agrio Formation), with TOC content up to 5.1%. Lowstand sandstones bodies, 10 to 100 m thick, are composed of eolian and fluvial facies with good reservoir conditions (Avil and Troncoso Sandstones). The seals are provided by the organic-rich shales resting sharply upon the Avil Sandstone and a widespread Aptian-Albian evaporitic event (Huitrin Formation) on top of the Troncoso reservoir. Tertiary structural traps (duplex anticlines) are developed in the outer foothills, whereas structural, combined and stratigraphic traps are present in the adjacent stable structural platform. Oil-to-source rock and oil-to-oil correlation by chromatographic and biomarker fingerprints, carbon isotopicmore » composition and the geological evidences support the proposed oil system.« less

Loma La Lata Giant Gas Field from a Petroleum Paleosystem, Neuquen Basin, Argentina: ABSTRACT

The elements of a fully developed petroleum system may undergo later modifications, transforming it to a new system. We define the original system as a Petroleum Paleosystem. In the evolution of the Loma La Lata gas field, diagenetic processes have played a major role in the entrapment since reservoir, lateral and top seals consist of eolian sandstones. The gas origin can be explained as an early oil accumulation trapped between the Late Jurassic to Early Tertiary (petroleum paleosystem) that since Late Tertiary underwent a weak but sufficient thermal degradation to provoke the oil cracking resulting in the co-generation of wet gas+condensate+pyrobitumen-like residue. The elements of the paleosystem are: the Kimmeridgian eolian sandstones of the Sierras Blancas Formation+Catriel Formation as reservoir; the source rock and vertical seat is the rich organic matter marine shales associated with the 139 Ma MFS of the Vaca Muerta Formation; the lateral seal is the eolian facies of the Sierras Blancas Formation that experienced very early diagenesis under the strong influence of the phreatic oscillations in areas of maximum subsidence (Sauzal Bonito); the structural trap is related to the Cenomanian tectonic inversion phase; the expulsion peak was reached in the Late Eocene (40 Ma) and themore » migration from Vaca Muerta Formation. In the last 5 Ma the paleosystem was transformed by processes related to temperature increase. Subsequent calcite precipitation is extensive and resulted in the transformation of eolianites of the Catriel Formation in the vertical seal and coeval down dip remigration from the top of the structure. The intra-reservoir oil cracking has occurred as a consequence of an increment in the subsidence rate accompanied by an abnormally high geothermal gradient in the area.« less

Facies and Fracture Architecture of the Tensleep Sandstone, Bighorn Basin, Wyoming: Preiiminary Result of an Outcrop and Subsurface Study: ABSTRACT

The Middle Pennsylvanian to Lower Permian Tensleep Sandstone has been the most prolific producer in the state of Wyoming. Now in its advanced stages of production. it is critical to accurately characterize the Tensleep facies and fracture architecture. Two outcrop locations have been selected: Alkali Creek has cross-strata dip parallel exposures at close spacings for eolian facies architecture Ziesman Dome is an NW-SE trending asymmetric anticlinal closure with the SE axis having a marked dextral dislocation, and is selected for analysis of fracture architecture. Byron Field is also an anticlinal closure with NW-SE trend which is selected for location of subsurface study. Methods of study for outcrop includes high- precision electronic surveying, photomosaics, geologic mapping, and fracture spacing and orientation measurements. The subsurface study includes conventional wireline logs, cores, and porosity and permeability data. In addition, a microresistivity log was evaluated from the Lindsay 3H horizontal well. Tensleep facies architecture is defined by a hierarchy of elements. Parasequences are made up of marine to marginal marine dolomitic sandstones and sandy dolostones (<10[sup -2] to 10[sup 2] md) in the lower 20-50%, and eolian dune, interdune and large-scale deformed sandstones in the upper part. Eolian dune sandstones, the primary reservoir (2more » to 3x10[sup 2] md), am characterized by very large-scale (up to 40 ft thick) tabular-planar cross strata with lower permeability wind ripple strata (1 to 2x10[sup 2] md, strongly anisotropic) concentrated above 1st- and 2nd-order bounding surfaces. Medium-scale trough cross stratified intrasets are common; however, their spatial dimensions and distribution pattern are uncertain at this time.« less

Aeolian Facies Belts in the Taklimakan Desert1

Abstract:At least at the beginning of the last glacial epoch, the facies belts of dune sand, sandy loam and loess formed by winds had existed in the Taklimakan desert and areas south of it. There were no appreciable changes in the NE and NW wind systems and their wind forces that deposited dune sand, sandy loam and loess in the global cold stage since the last glacial epoch; accordingly no marked shifts of the boundaries of these aeolian facies belts took place. In the global warm stage since then, the climate in the Taklimakan desert and areas south of it became warm and dry, resulting in ablation of substantial volumes of ice and snow in their surrounding mountains and thus forming alluvial and diluvial deposits in the region. The alluvial‐diluvial actions, however, failed to change the general framework of aeolian facies belts.

Extraordinary preservation in a new vertebrate assemblage from the Late Cretaceous of Mongolia

WE report here a new locality, Ukhaa Tolgod ('brown hills'), from the Upper Cretaceous of the Gobi Desert of Mongolia, which shows an unmatched abundance of well preserved vertebrate fossils, including the highest concentration of mammalian skulls and skeletons from any Mesozoic site. In the main collecting area (about 4 km2), recovered and uncollected articulated skeletons of thero-pod, ankylosaurian and protoceratopsian dinosaurs represent over 100 individuals. Specimens collected also include skulls (many with associated skeletons) of over 400 mammals and lizards, skeletons (including the first known skull) of the bird Mononykus, and nest sites that preserve the first known theropod dinosaur embryos1. In contrast to other Mesozoic localities, the diversity and abundance of theropods, mammals and lizards are unusually high. The exceptional preservation of vertebrates from the red-bed facies of the Gobi Upper Cretaceous has been attributed to arid conditions2–4, possibly involving catastrophic death and burial during major sandstorms5. Although fossils are found in fluvial facies at Ukhaa Tolgod, high concentrations of excellent specimens in aeolian facies support the argument for rapid entombment in sand. This contrasts with conditions for the terrestrial Upper Cretaceous in North and South America, where accretionary preservation of fossils in fluvial deposits predominates.

Experimental and Natural Abrasion of Conodonts in Marine and Eolian Environments

Small skeletal sediment particles, by virtue of their size, experience physical sedimentological conditions different from those affecting large (i.e., > 4 mm) particles. In shallow, high-energy marine carbonate environments with an overall fine-grained (< 4 mm) sediment composition, grain-to-grain impacts are probably of insufficient magnitude to overcome the cushioning effect of water and cause abrasive wear. Very large skeletal grains within such a setting will promote physical degradation, but biological activity (microboring, bioerosion) is more effective in modifying particle surfaces and diminishing grain size. Only in eolian environments are collisions between small grains effective in causing abrasion. Quartz sand, a particularly important abrasive agent in the destruction of large skeletal parts under even moderate energy regimes, also appears to be appreciably important for the degradation of small skeletons only under eolian conditions. Conodonts are small (0.2–2.0 mm), phosphatic teeth, which may have belonged to an extinct (Cambrian-Triassic) group of jawless fish. They occur principally in marine and marginal marine sedimentary rocks; most specimens probably were deposited as discrete particles or as components of fecal masses. Because the extent and nature of degradation of a skeleton reflects its taphonomic history, evaluation of the susceptibility of conodonts to physical abrasion is important. Bioerosion, probably the principal cause of degradation of calcareous skeletons, was probably insignificant in alteration of conodonts. Their mineralogical composition made conodonts unlikely substrates for endoliths, and they were insufficiently abundant to serve as a recyclable source of phosphate for larger metazoans. Chemical breakdown of skeletal apatite would not have occurred, although atmospheric exposure may have promoted degradation of organic laminae, causing exfoliation. Experimental studies of abrasion (elements of Palmatolepis sp., Polygnathus sp. - U. Devonian, Iowa) plus petrographic examination of conodont bearing marine, marginal marine, and eolian facies of the Morgan Formation (M. Pennsylvanian, Utah and Colorado) suggests that abrasion of conodonts is insignificant in aqueous environments, and likely to occur to a major degree only under eolian conditions. High-energy (25.6 cm/sec) conditions were simulated in a tumbler containing quartz sand (4Φ − 2Φ), artificial sea water and moderate conodont abundances (50 specimens/kg). Abrasion produced under these artificially extreme conditions (e.g., continuous motion for up to 20 km travel distance) was at a scale detectable only with SEM. “Dry” tumbling experiments were ineffective, as noted by other authors simulating conditions for abrasion of mineral grains. Eolian conditions were simulated in a glass “wind tube” (see Kuenen, 1960, Journal of Geology) using a similar range of quartz sand sizes. Air velocity of approximately 100 cm/sec produced subtle visually detectable abrasion following 5 hours of exposure/transport in 2Φ quartz sand (1667 specimens/kg), but exhibited no visual evidence of abrasion under similar conditions with 3Φ sand. The Morgan Formation is a mixed carbonate/siliciclastic sequence that reflects repeated episodes of submergence and exposure of a carbonate shelf and laterally adjacent siliciclastic eolian “sand sea.” It represents a virtually ideal ancient example for the interpretation of the effects of natural sedimentary processes because of its well-documented environmental heterogeneity and contains both abraded and unabraded conodonts. Conodonts sampled from high-energy subtidal carbonate facies of the Morgan are unabraded, irrespective of the presence of quartz sand; platform and delicate ramiform elements are all represented. Conodonts sampled from mixed siliciclastic/carbonate eolian facies are extremely abraded; only the most durable Pa elements of Adetognathus sp. and Idiognathodus sp. are preserved, although fine surface details and denticles are worn and anterior blades are missing. Laboratory simulations confirm the interpretation that marine processes are generally insufficient to physically abrade small phosphatic skeletal grains and that highly abraded Morgan conodonts experienced extremely long residence times in the eolian environment where they were transported, repeatedly buried, and exposed to bombardment and abrasion by quartz sand. Supported by NSF EAR9004300

Triassic sandy braidplain and aeolian sedimentation in the Sherwood Sandstone Group of the Sellafield area, west Cumbria

SUMMARY During the Permo-Triassic the Sellafield area formed part of the eastern margin to the East Irish Sea Basin. Basin-margin alluvial fan, shallow marine and coastal mudflat sedimentation created a locally thick Permian succession. Triassic sediments are represented by the Sherwood Sandstone Group, which comprises the St Bees, Calder and Ormskirk sandstone formations. The basal few metres of the St Bees Sandstone are characterized by sheetflood sandstones, interbedded with playa-mudflat deposits. These pass upwards into thick, multistorey fluvial sandbodies, formed on sandy braidplain. Rare aeolian units are interbedded with the channel facies. The top of the St Bees Sandstone is sharply defined and represents the regional abandonment of the fluvial system. The overlying Calder Sandstone Formation is coarser grained and dominated by aeolian sediments, with dune and interdune facies recognized. These sands were blown into the area from the east. Minor episodes of fluvial reworking punctuated aeolian deposition and the top of the Calder Sandstone is represented by a fluvially deposited unit. The lower part of the overlying Ormskirk Sandstone is of aeolian facies and can be distinguished from the Calder Sandstone by its finer grain size. The two formations can also be differentiated using geophysical logs, as aeolian and fluvial sandstones have diagnostic geophysical log characteristics allowing recognition of different sedimentary facies associations.

A GLACIAÇÃO NEOPALEOZÓICA NA PORÇÃO MERIDIONAL DA BACIA SANFRANCISCANA

This text represents a stratigraphic and environmental study of two areas in the northwest of Minas Gerais State, Brazil. The work allowed the identification of the Permo-Carboniferous glaciation in the Sanfranciscana Basin, not yet described. A stratigraphic subdivision is proposed for the glaciogenic strata - The Santa Fe Group - which is composed of the Floresta and Tabuleiro Formations. The Floresta Formation is the basal unit and is composed of three members: Brocoto Member (tillites and tilloides), Brejo do Arroz Member (red mudstones and siltstones with a variety of dropstones) and Lavado Member (red sandstones). The Tabuleiro Formation (homogeneous sandstones) overlies all members of Floresta Formation. The arkoses of the Tres Marias Formation (Upper Proterozoic) are the regional basement for the glaciogenic sequence. The Santa Fe Group is a proglacial system with tillite and tilloide facies (Brocoto Member), fluvioglacial facies (Lavado Member), glacio-lacustrine and turbiditic facies (Brejo do Arroz Member) and periglacial eolian facies (Tabuleiro Formation). The glacial sedimentation was controlled by the advance and retreat of the ice sheet. Diagenetic reactions are not well developed because of the incipient burial history. The major diagenetic feature was a calcitic pervasive cementation. The provenance areas for the glaciogenic deposits are interpreted from heavy mineral analysis, striated pavement orientation and dropstone composition. The northern Espinhaco region is proposed as the source area.

Core Handling, Sedimentology, and Petrophysical Log Response of a Horizontal Core, Barque Field, United Kingdom Sector, Southern North Sea

With the growing number of horizontal holes being drilled, petroleum geologists must handle increasing amounts of material having greater lateral than vertical extent. This requires novel techniques in description, interpretation, and data manipulation. This paper describes the core handling and descriptive techniques used for a heterogeneous 28.6 m horizontal core from the eolian Rotliegende reservoir of the Barque gas field in the United Kingdom southern North Sea. The core is subparallel to structural dip, slowly cutting down stratigraphically but sampling only a limited extent (laterally or vertically) of the reservoir. One facies boundary can be traced for about 12 m along the core, and for a significant proportion of the core, two facies with markedly different rese voir properties occur across the same 10-cm core diameter. The core is unrepresentative of the reservoir, as described from vertical wells, in two ways: the overall lithofacies composition in the core is comparatively biased toward poor-quality (water-laid) facies, and the presence of a thin cemented layer in the normally better quality (eolian) facies markedly decreases their average porosity/permeability. Care must be taken in extrapolating plug data from the core to field models. Apparently, contradictory wireline responses are explained (through calibration to the core and by consideration of the physical basis of petrophysical log responses), allowing the construction of facies models covering the formation around the borehole.

Last interglacial sea levels on the south coast of New South Wales

SUMMARY Remnants of the Last Interglacial shoreline occur at Middle Lagoon on the far south coast of New South Wales. Relict beach sediments can be traced to a height of at least +4.8 m and are indicative of a former mean sea level of about +3 m. Thermoluminescence (TL) ages of 126 ± 13 ka and 114 ± 15 ka were determined for beach and aeolian facies respectively. Sands in the lower part of an exposure on the adjacent Gillards Beach gave TL ages of 108 ± 13 ka, but sands in the upper part of that exposure gave an age of 19.9 ± 3.5 ka. This chronological evidence of a stratigraphic unconformity in what was initially taken as pedogenic differentiation at Gillards Beach is supported by contrasting electron traps and colour centres in crystal lattices of quartz grains in these two samples. No tectonic displacement is apparent. This site provides the first evidence of the Last Interglacial sea level for 1000 km along the coast between Gippsland and Newcastle.

Thin eolianites interbedded within a fluvial and marine succession: early proterozoic whitworth formation, mount isa inlier, australia

The early Proterozoic Whitworth Formation in the Mount Isa Inlier, Australia contains the oldest known, thinly bedded eolian dune and amalgamated interdune deposits, and records the complex interplay of eolian, fluvial and marginal-marine depositional processes. The Whitworth Formation accumulated during the waning phases of rifting in a north-south trending trough that began between 1808 and 1790 Ma and ended before 1740 Ma. Three eolian facies are recognized: (1) cosets of cross strata, (2) sets of horizontal to low-angle strata, and (3) isolated sets of cross strata. Facies El records preservation of dunes with low angles of climb without intervening interdune deposits. Sets of facies E2 are interpreted as either sand sheet, preserved dune plinths or amalgamated interdune deposits. Facies E3 is encased in facies E2 and consists of lenses of cross strata with irregular tops. This association of facies is comparable to recent gypsum dune and interdune deposits at White Sands, New Mexico. Facies E3 reflects limited dune preservation during migration and accumulation at or near the groundwater table. Vertical change from amalgamated interdune deposits to dune without interdune deposits records dunefield initiation or lateral migration from a peripheral to a more centralized position within the dunefield. Increased sedimentation rates promoted preservation of dune deposits represented by facies E1. Decreased sedimentation rate and possible limited sediment bypassing resulted in amalgamation of interdune deposits on the dunefield periphery. Eolian facies E1 and E2 are interbedded with ephemeral-river deposits. Three ephemeral-river facies are recognized: (1) cosets of horizontal strata; 2) cosets of horizontal strata and overlying trough cross beds; and (3) cosets of medium-scale trough cross beds. Ephemeral-river facies F3 often are preserved in channels incised into eolian facies E3. Associated marginal-marine facies consist of: (1) thin-bedded arenites of intertidal to supratidal origin, and (2) trough and tabular planar cross-bed sets and cosets that resulted from migration of subaqueous dunes within a tide-dominated subtidal setting. Interbedded fluvial and eolian deposits reflect either autogenic processes, such as reworking of fluvial sands by eolian processes, or incursion of a fluvial system into a dunefield related to allogenic processes such as climatic change. Dune deposits in the Whitworth Formation probably developed under arid to semi-arid conditions as indicated by extensive ephemeral-river deposits, presence of evaporite pseudomorphs, and absence of wet interdune deposits. Interbedded eolian, fluvial, and marginal-marine deposits resulted from relative sea-level fluctuations related to a combination of eustasy and intrabasinal tectonics.

Controls on fluvial deposition in mixed fluvial and aeolian facies within the Tumblagooda Sandstone (Late Silurian) of Western Australia

The Tumblagooda Sandstone (? Late Silurian) of Western Australia was deposited dominantly by fluvial processes on a palaeoslope to the northwest. Interbedded fluvial and aeolian sandsheet deposits occur within the formation, in which winds blowing obliquely up the palaeoslope to the southeast reworked the sandy, unvegetated, and unconsolidated surface into aeolian sandsheets and small parallel crested aeolian dunes with NE-SW crest lines. The water table was generally high but flashy river discharge resulted in deposition of thin (up to 3 m) laterally extensive (hundreds to thousands of metres) sheets of medium-coarse sandstone with dominant trough cross-bedding and subordinate planar cross-bedding and parallel laminated beds. In the absence of vegetation and levees, floods were virtually unconfined and current directions are locally unimodal with narrow variation. Flood waters were sometimes diverted by aeolian surface morphology. Similar mixed fluvial and aeolian environments are considered to have been frequent in a wide range of climatic regimes prior to the advent of abundant land plants.

Late Quaternary evolution of coral reefs on a cool-water carbonate margin: the Abrolhos Carbonate Platforms, southwest Australia

Late Quaternary coral reefs have developed on the southwestern Australian margin, which has otherwise been characterised by cool-water carbonates since the Eocene. The Houtman Abrolhos coral reefs are at the limits of existence, extending, with the assistance of the Leeuwin Current, a poleward-flowing, warm water stream, into a region dominated by more temperate communities. Coring in the Easter Group reefs, supported by high precision dating, by both U Th TIMS and 14C methods, has shown vigorous coral growth, with reefs over 26 m thick in the Holocene and over 15 m thick in the Last Interglacial. Each of the three Abrolhos platforms consists of a central platform composed of Last Interglacial reefs, about which windward and leeward Holocene reefs developed asymmetrically. Reef, peritidal and eolian facies comprise the emergent Last Interglacial limestones which are extensively calcretized, with reef facies up to 5 m above MSL. The Last Interglacial high stand lasted for at least 10 ka from 130 to 120 ka, and possibly 15 ka, from 132 to 117 ka. Holocene reef facies are also emergent by 0.5 m, and are overlain by peritidal and storm ridge facies in an upward-shallowing sequence. Windward (10 m thick) and leeward (26 m thick) Holocene reefs in the Easter Group show contrasting lithofacies. The wave-exposed windward reefs consist of a slow-growing association of coralline algal bindstones and coral framestones, whereas fast-growing coral framestones dominate the more protected leeward reefs. The leeward reefs commenced growth 10,000 years ago and grew to the present sea level by 6500 years ago, generating Holocene constructional topography consisting of ‘blue-hole’ terrain in the leeward parts of the platforms.

The Rough Gas Field: distribution of Permian aeolian and non-aeolian reservoir facies and their impact on field development

Abstract The Rough Gas Field lies in United Kingdom Continental Shelf Blocks 47/3d and 48/8b in the Southern North Sea and is operated in a gas storage mode by BGE&amp;P involving the annual injection and production of large volumes of gas. To achieve full efficiency in this designated role it was necessary to carry out a detailed evaluation of the main producing reservoir, the Permian Rotliegendes Sandstone, after early development drilling demonstrated a need to re-evaluate the initial geological model. The present geological model is based on the detailed analysis of nine fully cored new development wells and well productivity surveys following an original six well conventional depletion. This resulted in the successful targeting of later development wells into specific geographic areas of the field for optimal field development. Reservoir facies development can be described in terms of complex fluvial processes associated with the basin margin and aeolian influences associated with the basin as a whole. Within the Rough Field the Rotliegendes can be subdivided vertically into three reservoir units or layers (Units A, B, and C) and laterally into regions characterized by differing degrees of aeolian or non-aeolian influence. It is observed that the highest reservoir permeabilities are associated with aeolian or reworked aeolian sands while flow profile studies indicate close correspondence between injectivity/productivity and sedimentary facies. Aeolian facies or reworked aeolian sands of the lowermost ‘A’ unit facilitate the bulk of gas injection and production while fluviatile sands in all three units show reduced or even zero flow.

Controls on reservoir quality in the Triassic Sherwood Sandstone of the Irish Sea

The Lower Triassic Sherwood Sandstone Group constitutes the principal reservoir target in the East Irish Sea Basin. Interpretation of sedimentary facies allows recognition of major low sinuosity braided fluvial channel, minor ephemeral fluvial channel, aeolian dune and sandsheet, unconfined fluvial sheetflood and playa lake deposits that are discussed in terms of facies associations. Reservoir quality is highly variable, with porosity and permeability data revealing significant differences between fluvial, sheetflood and aeolian facies associations. The latter, in particular, preserve very high reservoir quality reflecting an open grain fabric and paucity of blocky cements. Variations in porosity between the facies associations relate primarily to the distribution of quartz cement which is significantly more abundant in fluvial channel sandstones than other facies. Authigenic illite, while severely limiting permeabilities in some reservoir zones, is not facies specific and does not contribute to the observed variations. It is suggested that the disparity in the abundance of quartz cement between facies associations relates to two distinct grain types that correlate broadly with fluvial and aeolian associations, respectively, and probably derive from different provenances. The evolution of the basin, in controlling the distribution of the major facies, influenced the distribution of the two grain populations and, thus, also the reservoir quality of the sandstones.

Structural and Sedimentological Controls on Diagenesis in the Ravenspurn North Gas Reservoir United Kingdom Southern North Sea

The Ravenspurn area is divided into two main northwest-southeast-trending fault blocks which are markedly different in terms of their diagenetic evolution and reservoir performance. The northeasterly B structure contained gas earlier and was unaffected by Middle to Late Jurassic illitization. The southwesterly A structure was uplifted later and received accumulated gas after reservoir quality was reduced by pervasive illitization. The deposition of allogenic clay and the formation of early quartz, nonferroan dolomite, and anhydrite reduced the reservoir quality of fluvial sheetflood. Burial diagenesis resulted initially in ferroan dolomite, kaolinite, and later quartz precipitation in available primary and secondary porosity. Stable-isotope and fluid-inclusion studies indicate that Ferroan dolomite and later quartz precipitated at about 100[degrees]C in the Triassic-Early Jurassic from reduced fluids derived partly from the Carboniferous basement. Gas accumulation took place first in the northeasterly B structure, which had early closure. Elsewhere diagenetic fluids evolved to a more alkaline state, and widespread illitization took place which particularly affected more permeable eolian facies. The illitization reduced the reservoir quality of the lower Leman Sandstone and contributed to diagenetic sealing (to the northwest) of the field. K-Ar dating indicates that peak illitization took place between 150 and 170 Ma (Middle-Late Jurassic). Subsequentmore » periods of uplift in the late Cimmerian and particularly during the early Tertiary-Miocene produced the final structure of Ravenspurn North and the spillage of gas into this structure. The combination of structural and diagenetic events explains the differences in reservoir quality and well performance of the two structural blocks in the field.« less

Structural and sedimentological controls on diagenesis in the Ravenspurn North gas reservoir, UK Southern North Sea

The Rotliegend Lower Leman Sandstone Formation has been studied near the northern limit of its development in Ravenspurn North to determine the influence of structure, sedimentology and diagenesis on reservoir quality. The Lower Leman Sandstone in this area is about 300 ft thick and comprises six depositional cycles which reflect alternating periods of relatively dry and wet climate. Aeolian sands form the best reservoir and formed as small westward-migrating transverse dune fields and draa, at most 60 ft thick. These dunes were frequently reworked by southerly derived, strong fluvial sheetfloods which drained into the Silverpit playa lake. The Ravenspurn area is divided into main northwest-southeast-trending fault blocks which are markedly different in terms of their diagenetic evolution and reservoir performance. The northeasterly B structure contained gas earlier and was unaffected by mid- to late Jurassic illitization. The southwesterly A structure was uplifted later and received accumulated gas after reservoir quality was reduced by pervasive illitization. A hierarchy of factors influenced the diagenesis. These were the primary lithological variation, burial history and the structural development of the Ravenspurn North area. Depositional processes also influenced diagenesis; the deposition of allogenic clay and the formation of early quartz, non-ferroan dolomite and anhydrite reduced the reservoir quality of fluvial sheetfloods. Burial diagenesis resulted initially in ferroan dolomite, kaolinite and later quartz precipitation in available primary and secondary porosity. Stable isotope and fluid inclusion studies indicate that ferroan dolomite and later quartz precipitated at about 100°C in Triassic-early Jurassic times from reduced fluids derived partly from the Carboniferous basement. Mixing of these fluids with more alkaline connate water was the main triggering mechanism for authigenic mineral formation. Gas accumulation took place first in the northeasterly B structure which had early closure; the last authigenic phase to precipitate here was poikilotopic siderite at temperatures of about 120°C. Elsewhere diagenetic fluids evolved to a more alkaline state and widespread illitization took place which particularly affected more permeable aeolian facies. The illitization reduced the reservoir quality of the Lower Leman Sandstone and contributed to diagenetic sealing (to the northwest) of the field. K–Ar dating indicates that peak illitization took place between 150–170 Ma (mid–late Jurassic). Subsequent periods of uplift in the late Cimmerian and particularly during the early Tertiary–Miocene produced the final structure of Ravenspurn North and the spillage of gas into this structure. The combination of structural and diagenetic events explains the differences in reservoir quality and well performance of the two structural blocks in the field.

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