Facies Mosaic Research Articles

Depositional Environments and Reservoir Compartmentalization Within the Frio Zone 21-B Reservoir, Tijerina-Canales-Blucher Field, South Texas

Depositional Environments and Reservoir Compartmentalization Within the Frio Zone 21-B Reservoir, Tijerina-Canales-Blucher Field, South Texas

Depositional Facies and Hydrocarbon Reservoir Compartmentalization of the Mississippian Aux Vases Formation at King Field, Jefferson County, Illinois

King field has produced over 4 million bbl of oil from the Mississippian Aux Vases Formation along a north-south-trending anticlinal structure 3 mi long and 1.5 mi wide. The porous and permeable quartz sandstone, which comprises the principal hydrocarbon reservoir, can grade laterally into nonporous calcareous sandstone, siltstone, shale, or limestone within one well location (660 ft). The Aux Vases Formation at King field was deposited in an intertidal to subtidal, mixed siliciclastic-carbonate environment, and the resulting mosaic of depositional facies significantly affects reservoirs continuity. Impermeable to only slightly permeable siltstones, shales, and limestones of the various offshore facies, and shales and siltstones of the tidal-flat facies separate the producing sandstones of a tidal channel-offshore bar facies into distinct reservoir compartments. This reservoir heterogeneity results in an oil-water contact at various depths across the field. Lateral compartmentalization of the reservoir has allowed large sections of the Aux Vases reservoir to remain unswept by waterflooding. Volumetric analyses suggest that untapped reservoir compartments may contain an additional 1-2 million bbl of oil recoverable by primary and waterflood methods.

Dolomitization of Coeval Shelf and Fore-Shelf Carbonates, Lower Permian, Midland Basin, Texas

Coeval shelf and fore-shelf carbonates in Lower Permian (Leonardian) rocks in the Midland basin are pervasively dolomitized. Shelf strata consist of a regional mosaic of shelf margin reef, lagoon, and peritidal facies associated with contemporaneous evaporite deposition; fore-shelf facies consist of resedimented shelf margin deposits. Numerous relative sea-level fluctuations that resulted in the formation of Type 1 and 2 unconformities are recognized throughout this section. Mean stable oxygen and carbon isotopic compositions of the shelf strate suggest that dolomitization occurred essentially syndepositionally, likely by interaction with normal marine( ) and hypersaline fluids. In fore-shelf strata petrographic evidence, in combination with paleotemperature estimates from isotopic data and the burial depth-temperature history of these rocks, suggest relatively early (late Permian) dolomitization in rock-dominated systems. Three possible modes of dolomitization could have affected these fore-shelf beds: (1) dolomitization by reflux of normal-marine and/or hypersaline fluids during sea level highstands or lowstands; (2) dolomitization by mixed meteoric-marine fluids during lowstands; and (3) replacement by calcic dolomites via reaction with circulating marine fluids during deposition and early, shallow burial without organic matter influences. The latter mode of origin, however, is favored for most of the rocks examined on the basis of the slightly positive carbonmore » isotopic compositions, Sr versus MgCO{sub 3} compositions (mean Sr 61 ppm, mean MgCO{sub 3} 49.5), and low Mn contents (mean 61 ppm) of the dolomites, although later burial recrystallization is indicated by their relatively depleted isotopic compositions.« less

Shallow seismic refraction used to map the hydrostratigraphy of Nukuoro Atoll, Micronesia

Results from a shallow seismic-refraction survey on Nukuoro indicate that the distribution of fresh and brackish groundwater on that atoll island is controlled by a three-dimensional mosaic of carbonate facies. Each facies is characterized by a distinct seismic velocity which is dependent on the grain size, composition, and abundance of carbonate cement. The refraction survey further indicates that the upper saturated zone is composed of mostly unconsolidated sediments underlain by highly permeable, well-indurated limestone. The fresh-water lens and associated transition zone of fresh to saline groundwater occur within the upper unconsolidated sediments. Measurements of chloride-ion concentrations in water samples collected from sites across the island indicate an asymmetric fresh-water lens with the thickest part of the lens located near the lagoon shoreline. During the 1983 drought, saline water intruded into the island's central topographical depression where taro is cultivated. The intrusion was caused by tidal pumping (upward movement) of brackish water underlying a reef-flat plate, which forms a confining layer over a significant part of the island.

Sedimentology and Mineralogy of Dolomitic Coorong Lakes, South Australia

ABSTRACT Coorong dolomites are true primary dolomites; that is, Coorong dolomite is precipitating where there was nothing before and is not replacing an earlier carbonate mineral. Dolomites in the various lakes of the Coorong coastal plain in South Australia occur in three mineralogical associations; Dolomite ± Mg-calcite (widespread) Dolomite ± Magnesite (common in northwest) Dolomite ± aragonite ± hydromagnesite (rare and only in northwest). Most Holocene dolomite occurrences in the near coastal lakes of the Coorong region are associated with Mg-calcite. Only in the more arid settings found at the northwestern end of the Coorong region, near Salt Creek, is dolomite commonly found in association with magnesite and occasionally with hydromagnesite and aragonite. Dolomite in the near surface sediments of lakes near Salt Creek such as Pellet Lake and Milne Lake is often associated with magnesite in the lake center and Mg-calcite about the lake margin. The dolomites associated with magnesite tend to be more magnesian-rich than dolomites associated with Mg-calcite, a direct reflection of the chemistry of the mother waters. Waters which precipitate magnesite are typically more concentrated and more magnesium-rich than waters pre ipitating Mg-calcite, hence co-precipitated dolomites will also be more magnesium-rich. Textures found in vertical sequences from lakes in the Coorong coastal plain are largely independent of mineralogy; similar vertical transitions occur whether the lake is filling with carbonate, magnesian carbonates, or gypsum. Dolomite has been the main mineral of concern in previous geological studies of this area, yet it makes up no more than 10% of the carbonate minerals forming surficial deposits across the coastal plain. The hydrological model of the Coorong lakes should not be used to explain widespread dolomitization in ancient supratidal and shelf carbonates. Coorong counterparts in the rock record are a facies mosaic of lacustrine carbonate sitting in a much more extensive paleoaquifer.

Models of miocene marginal lacustrine sedimentation in response to varied depositional regimes and source areas in the Madrid Basin (Central Spain)

A succession of lake complexes formed through Miocene times in the Madrid Basin (Central Spain). The complex formed during the Aragonian stage provides an outstanding opportunity to evaluate the role of source areas and depositional regimes in the construction of coeval marginal lacustrine deposits. Sedimentary facies of the Aragonian lake indicate a shallow lake that was fringed to the NW and S by arkosic alluvium, to the N by litharenitic alluvial fans, and to the NE and E by alluvial fans distally dominated by large floodplains. Marginal lacustrine deposits in the arkosic realm mostly consist of Mg-smectites and dolomite sequences that laterally interfinger with sepiolite beds which were deposited in small discontinuous ponds in the distal alluvial fan areas. Occasionally, micaceous-rich sandy streams entered the lake forming sheets at their entry points. In contrast, the northern margin of the lake was formed by muddy flats which were influenced by fluctuation in the water table. In this setting, illitic mudstones are commonly interbedded with biotite-rich flood sands, showing a sheet-like geometry. Meandering channels occasionally cut into the mud flats. Facies assemblages on this side of the lake were mainly derived from low-grade metamorphic areas. To the northeast, the marginal lacustrine facies consist of silty clays, carbonate paleosols and paludal to lacustrine limestones. These facies reflect an interfingering between the lake and a wide stream-floodplain system derived from a carbonate source area. The mosaic of marginal lacustrine facies in the Madrid Basin during the Aragonian is discussed in the light of lithological and geochemical factors related to variations in source areas.

Deposition and palaeoecology of an Upper Silurian stromatoporoid reef on southernmost Gotland, Sweden

AbstractA stromatoporoid reef in the Ludlovian Sundre Beds at the sea stack field of Holmhällar I on southernmost Gotland was subjected to subaerial exposure during its period of deposition. The sea stack field shows a subcircular belt 500 m in diameter and 50 m wide which exposes part of a reef complex. One distinct erosional surface can be traced through all the localities. The surface cuts the stromatoporoid framework of a lower unit and undulates with a general dip 0–15° away from the centre of the subcircle. On the low‐lying parts of the undulating surface, limestone boulders and sediments containing a number of subordinate erosional surfaces occur. The geometry of the platform results in a complicated facies mosaic of the upper unit composed of stromatoporoid boundstone and its lateral facies, crinoidal debris. The boundstone generally rests on the topographic rises of the inner part of the platform. The crinoidal debris largely occurs on the outer part. Stromatoporoid skeletons occupy 50–65 per cent of volume of the boundstone. This value far exceeds the proportion of framebuilders in Quaternary reefs. The stromatoporoid assemblages are recognized. The assemblage of the boundstone is strongly dominated by Plectostroma scaniense which is laminar‐domical and mostly larger than 50 cm in basal diameter. In the other facies, the commonest species is Parallelostroma typicum which often exhibits a certain growth form as a result of growth on soft bottom and periodical sedimentation.

Sedimentology of coorong dolomite in the Salt Creek region, South Australia

Lakes in the Salt Creek area of the Coorong region, South Australia, contain a diverse suite of Holocene carbonates including aragonite, calcite, Mg-calcite, magnesite, hydromagnesite, and dolomite. Dolomite has been the main mineral of concern in previous geological studies of this area, yet it makes up no more than 5% of the carbonate minerals forming surficial deposits across the coastal plain. The hydrological setting of the Coorong lakes is such that the area should not be used to generate a model to explain the occurrence of widespread ancient supratidal and shelf dolomites. Coorong counterparts in the rock record are a facies mosaic of lacustrine carbonate sitting in a much more extensive paleoaquifer. Textures found in vertical sequences from lakes in the Salt Creek region are largely independent of mineralogy; similar vertical transitions occur whether the lake is filling with carbonate or gypsum. If the lake possessed an early Holocene connection to the marine waters of the lagoon then thebasal unit is a marine/estuarine skeletal grainstone/packstone. If there was no marine connection the basal unit is often a quartzose packstone to wackestone. Basal facies can contain a basal dolomite. Above the basal facies in some lakes is a massive to faintly laminatedorganic-rich unit. In lakes with an early marine connection the levels of T.O.C. in this unit can be as high as 12%, usually as an oil-prone proto-kerogen. Overlying this unit is amm-laminated unit of pelletal packstone to mudstone deposited once the estuarine connection to the open Coorong Lagoon was completely cut off by beach-ridge accretion. In a few lakes that were never connected to the estuary, such as Milne Lake and Lake 5, this laminated unit is composed of evaporative dolomite up to 4 meters thick. More typically it is composed of varying proportions of aragonite, hydromagnesite, magnesite, and/or magnesian calcite. In Halite Lake, it is composed of a laminated gypsum/aragonite. Capping the lake sediments is a“massive” unit of poorly layered packstone/mudstone usually less than 60 cm thick and showing a varying degree of induration. This unit usually holds the bulk of the evaporative dolomite found in the Coorong but can also be composed of aragonite, hydromagnesite, magnesian calcite and magnesite. The massive unit contains a characteristic suite of sedimentary structures including: bioturbation structures, stromatolites, mud cracks, extrusion structures, tepees, and breccia fragments; all these features indicate at least occasional desiccation. The presence of this capping unit in conjunction with laminated sediments provides the most reliable diagnostic for an ancient Coorong counterpart. Coorong dolomites are true “primary dolomites”; dolomite is precipitating as dolomite and not replacing an earlier carbonate mineral. Coorong dolomite occurs as two mineralogically distinct (type A and type B) forms, which are deposited as three stratigraphically distinct forms (evaporative, margin, and basal dolomite).Type A dolomites tend to be Mg-rich with a crystalline but heterogeneous microstructure, the unit cell is contacted in the ao dimension, and it has a heavier carbon isotope value and a wellclustered oxygen signature compared to type B.Type B dolomites tend to be calcian-rich to near-stoichiometric, with expanded unit cell dimensions (ao and co), they are more crystalline with a more homogenous microstructure compared to type A, and show isotopically lighter carbon values, as well as a larger variation in oxygen isotope values. Volumetrically the most important type of dolomite is anevaporative dolomite, a type A dolomite deposited as the last episode of sedimentation in those Coorong lakes which contain dolomite. In most lakes it is a capstone unit no more than tens of centimeters thick, although in Milne Lake it has infilled the lakes to form dolomitic units up to 4 – 5 meters thick. In some lakes there is a second type of surficial dolomite, a more calcian-richmargin dolomite (a type B dolomite) about the edges of the lake. Like the evaporative upper dolomite this dolomite is not intergrown with other carbonate phases and defines areas where resurging continental groundwaters first enter the lake margin. The third type of dolomite in some Coorong lakes is abasal dolomite (a type B dolomite) which is more crystalline than the other two forms of dolomite. It formed some 6000 years ago when the rising Pleistocene water table (driven by a transgressing sea) first caused continental groundwaters to emerge and evaporate at the surface. The Coorong Dolomites form by the evaporation of continental meteoric waters, and this controls the occurrence of dolomite versus other lacustrine carbonates.

Is cratonic sedimentation consistent with available models? An example from the Upper Proterozoic of the West African craton

The Atar Group, part of the Upper Proterozoic sequence covering the West African craton, stable since 2000 Ma, is characterized by an alternation of extensive carbonate beds and mixed siliciclastic and carbonate facies. The carbonate beds comprise essentially columnar stromatolite biostromes and bioherms which reflect sublittoral environments. The mixed facies contain a variety of laterally discontinuous facies which imply more variable environmental conditions. The settings of the mixed facies are not always clear but they do not contain thick sequences of high-energy facies. Few obvious facies sequences are discernable; those that are present are considered to be punctuated aggradational cycles (PACs) and they always start with biostromes of columnar stromatolites with very few sediments. Composite sequences are interpreted as due to shallowing upward or increasing energy environments that may be laterally contiguous, despite the fact that the contacts are not gradational. However, much of the stratigraphic sequence cannot be subdivided into cycles and seems to consist of unrelated individual facies, bound by sharp boundaries. The basin analysis reveals that biostromes of columnar stromatolites start after an instantaneous geological event corresponding to a sea-level rise. Consequently, their appearance can be considered as a time-line. We describe, in the Atar Group and its equivalents, three sedimentation trends, all of which are interpreted to be of shallowing upward character. The Atar Group appears to have been deposited in an epeiric sea (i.e. an extremely flat ramp). There are two contrasting styles of sedimentation: (1) after the submergence of the whole area, columnar stromatolites built extensive biostromes; (2) during the stable phase, sediments are deposited in a mosaic of laterally-discontinuous facies. Tidal influence cannot be recognized in the sequence, neither can a salinity increase toward the land; both common features in published epeiric sea models. A cratonic sedimentation area such as this is characterized by its size and flatness. Only during the stable phase of the cycle does small-scale topographic relief lead to deposition of a mosaic of facies. The sedimentation is storm- and wave-dominated.

Extent and bathymetry of North American Platform Seas in the Early Silurian

The North American craton is 25% covered by Lower Silurian (Llandovery) rocks based strictly upon the geographic area of surviving marine strata preserved in outcrop and the subsurface. This figure does not take into account extensive post‐Silurian tectonics and erosion. Isolated structural basins in which these strata are found today were not necessarily defined as corresponding topographic basins in the Silurian. Analysis of interbasin structural arches and outliers provides important clues as to the actual extent of Early Silurian seas. Information on the age of tectonic origin for 15 prominent arches is reviewed. The placement and significance of 11 key outliers is also discussed. Evidence of this kind indicates Early Silurian platform seas were among the most extensive of the Paleozoic. Probably more than 65% of the North American craton was then flooded. Nothing on so vast a scale serves as a recent guide to the past. The Pliocene‐Holocene Bahamian Banks do provide, however, worthy models for the role of bathymetry and water circulation as related to the changeover from laterally monotonous blanket deposits to a more complex mosaic of facies. The original bathymetry of Early Silurian environments may be determined, correlated, and mapped on the basis of widespread carbonate cycles linked to sea level fluctuations. The range of fully integrated facies patterns includes (1) coral‐stromatoporoid/stromatolite/paleosol cycles (Williston Basin), (2) pentamerid brachiopod/coral‐stromatoporoid/stromatolite cycles (Michigan Basin), and (3) stricklandiid brachiopod/pentamerid brachiopod/coral‐stromatoporoid cycles (East Iowa Basin). With shallow bathymetries of 0–30 m the Williston class tends to represent mosaic facies, while with bathymetries of 0–60 m and 10–90 m the deeper‐water Michigan and Iowa classes typically involve blanket deposits. Bathymetry of other basin deposits generally fits one of these three classes. Maximum and minimum stands in sea level during Late Llandovery time are represented by bathymetric maps.

Sedimentary models for extensional tilt-block/half-graben basins

Summary Extensional tectonism produces characteristic half-graben/tilt-block systems whose facies mosaics are influenced by tectonically induced slopes resulting from hanging wall downtilting and footwall uplift. The characteristic asymmetrical subsidence vectors that therefore develop across the graben also exert a fundamental control upon facies distributions. A number of predictive tectono-sedimentary facies models are presented in which these various influences are explored. Alluvial fans and cones react to tilting by becoming segmented, those in the hanging wall showing down-dip hanging wall off-lap and those sourced in the footwall showing progradation from the apex. Lake and coastal waters react instantly to tilting, causing transgression and seiche-induced erosion. Axial through-flowing river channels and delta lobes tend to migrate or avulse towards the axis of maximum subsidence but may be constrained by the toes of footwall-sourced fans. Peat accumulation or soil development are accentuated up the hanging wall dip slope away from the locus of maximum deposition. In coastal areas, fan deltas sourced in the footwall pass offshore into small submarine fans whilst axial fans issue from delta fronts where individual fan lobes may migrate under fault control. In carbonate provinces the footwall scarp may become a bypass margin whilst the hanging wall dip slope may undergo a ramp-to-rimmed shelf evolution with time.

Punctuated aggradational cycles: Implications for stratigraphic analysis

Application of the hypothesis of punctuated aggradational cycles (PACs), a model of episodic stratigraphic accumulation, has yielded results fundamentally in conflict with those predicted and generated by gradualistic models. The assumption that stratigraphic accumulation is controlled principally by rapid small‐scale base level fluctuations in the Milankovitch band leads to the adoption of the PAC, a thin time‐stratigraphic unit, as the fundamental unit of stratigraphic description and interpretation; the thicker generally diachronous formation is inadequate for stratigraphic analysis because it is an artifact of stratigraphic gradualism. Correlation of stratigraphic sections is accomplished by matching patterns of facies change, particularly the abrupt changes at the synchronous surfaces that mark PAC boundaries. This kind of physical event correlation is far more accurate than traditional biostratigraphic methods. Focus on synchronous PAC boundaries separating environmentally disjunct facies reveals discontinuities that preclude application of Walther's law across those surfaces. When viewed from this episodic perspective, the Manlius Formation of the Helderberg Group in New York state becomes a highly ordered set of time‐stratigraphic cycles rather than a disordered mosaic of facies representing short‐term autogenic processes in a single nearshore environment. Not only was Manlius deposition episodic with no gradual upward deepening as is predicted by stratigraphic gradualism, but also the Manlius‐Coeymans formational boundary is interpreted as a small‐scale unconformity rather than a paleoenvironmental boundary created by gradual transgression. Each of these interpretations is the direct result of assuming and applying a model of episodic stratigraphic accumulation.

Facies mosaic in the Ghuneri Member (Jurassic) of the Bhuj Formation, western Kutch, India

In marginal marine environments facies mosaics become complicated through time as and when the regular autocyclic changes are frequently intervened by allocyclic changes. The facies mosaic of the marginal marine Tithonian Ghuneri Member (476 m thick) exposed in western Kutch, India, is likewise fairly complex with frequent lateral as well as vertical alternations between six facies. The basic motif, built up of autocyclic facies transitions of the bar-lagoon spectrum in response to shift of the shoreline, is time and again interfered with by allocyclic changes caused by storm surges. These storm surges overtopped the bar and laid their sediments over the indigenous facies as washover. The autocyclic facies association is composed of three facies, namely: (1) bar; (2) lagoon-margin; and (3) lagoon-interior. The allocyclic facies association comprises: (4) proximal storm washover; (5) distal storm washover; and (6) proximal grain flow (of insignificant occurrence) facies. Statistical analysis indicates that the vertical facies transitions between these six facies are fairly unpredictable. Elimination of the products of instantaneous deposition (storm washover facies) from the same data base, however, renders the sequence of congruous autocyclic facies predictable.

Geology of Natural Gas Reservoir: Upper Travis Peak Formation, Western Flank of Sabine Uplift, East Texas: ABSTRACT

The Travis Peak Formation (Lower Cretaceous) in the eastern East Texas basin represents a sand-rich, fluvial-deltaic depositional system. This lobate, high-constructive deltaic system prograded radially to the southeast from an Upshur County locus. Regional studies of the Travis Peak established a threefold internal stratigraphic framework: a middle sand-rich fluvial and delta-plain sequence is gradationally overlain and underlain by a marine-influenced delta-fringe zone with a higher mud content. The entire Travis Peak succession thins over the Bethany dome on the western flank of the Sabine uplift. However, the delta-fringe sequences are relatively thicker over the structure because of a disproportionately greater thinning of the middle sandy fluvial-deltaic sequence. Lesser sand deposition over the Bethany dome reflects an active structural control over facies distribution. Gas production in the Bethany field and surrounding area is concentrated in thin zones (5-15 ft) of the upper delta-fringe sequence. This distribution probably reflects the increased abundance of mudstone beds in the upper delta-fringe interval, which may have served as source rocks or barriers to upward gas migration, or as both. The predominant trapping mechanism in this region is stratigraphic sand pinch-out in a structurally updip direction on the flanks of major structures. Studies of core andmore » closely spaced electric logs west of the Bethany dome help define the depositional systems in the upper delta-fringe producing interval. This sequence comprises a complex mosaic of continental and marine facies, and exhibits an overall upward trend of increasing marine influence that spans a gradual transition into transgressive carbonates of the Sligo Formation.« less

Depositional Facies Mosaics and Their Time Lines in Lower Ordovician Carbonates of Central Appalachians: ABSTRACT

A comparative sedimentology and facies stratigraphy study of the Lower Ordovician carbonate of the central Appalachians (Beekmantown Group and equivalents) has been carried out. Our approach used subfacies (rock record of subenvironments) as the basic units of section measurement. We differentiated related sets of subfacies into larger facies units (rock record of environments). Facies were then correlated from section to section using fossils and lithostratigraphy to make a 3-dimensional facies mosaic. Within this mosaic, time lines were constructed using onlap-offlap tongues and cyclic sequences. These time lines cut across facies boundaries. Using this approach, we have established that the lower 600 m of the Lower Ordovician carbonate sequence is made up of 4 main fac es: (1) cyclic laminite facies composed of a package of shoaling-upward shelf lagoon-peritidal cycles, (2) thin-bedded grainstone facies deposited in a shelf lagoon, (3) Renalcis bioherm facies recording a shelf lagoon patch-reef environment, and (4) Epiphyton bioherm facies recording a shelf-edge reef system. The distribution of these facies along time lines across the strike of the central Appalachian is markedly zoned. Epiphyton bioherm facies dominate the eastern margin while cyclic laminite facies dominate the western margin, with thin-bedded grainstone and Renalcis bioherm facies making up the central belt. This zonation of facies is a typical shallow carbonate shelf system with fringing reefs along the eastern, seaward margin and tidal flats along the western, landward margin. Ver ical distribution of these facies across strike records 3 major sea level changes during deposition of the lower 600 m of this extensive Lower Ordovician carbonate shelf. End_of_Article - Last_Page 292------------

Reservoir Facies Architecture in a Micro-Tidal Barrier System, Frio Formation, Texas Gulf Coast: ABSTRACT

Barrier-bar sand bodies are a complex mosaic of barrier-core, shoreface, inlet-fill, tidal-delta, and back-barrier facies. In addition, sandbody stratigraphy and internal depositional architecture are determined by the progradational, aggradational, or transgressive origin of the barrier complex. The Frio barrier/strandplain system of the middle Texas Gulf Coast has produced more than 3 billion bbl of oil. Examination of the Greta, Glasscock, and 41-A sands in West Ranch field illustrates the variability of barrier reservoirs. Each reservoir is a mosaic of variably interconnected compartments having sheet, tab, pod, or channel geometries. Conventional facies analysis (isolith and log-pattern mapping and limited core examination) combined with semiquantitative delineation of hydrocarbon-saturation distribution using resistivity logs defined the facies components of each reservoir. The 41-A sand consists of juxtaposed progradational barrier-core, inlet-fill, and flood tidal-delta units. The Glasscock sand is largely a transgressive barrier-flat and washoverfan deposit. The Greta sand is a complex of aggradational barrier-core and inlet-fill facies. Productive attributes of each reservoir are influenced by its facies architecture and attendant relative permeabilities. Natural water drive is ineffective in the volumetrically restricted transgressive Glasscock reservoir. Permeability distribution in the 41-A reservoir is facies defined. Erratic injection response, irregular oil-water contact advance, and variable water/oil ratios observed during the productive history of individual reservoirs document localized facies effects on fluid flow. Spatial variation of the gas/oil ratio may also reflect facies distribution. End_of_Article - Last_Page 478------------

Sedimentology and palaeoecology of a Miocene coralline algal biostrome from the Maltese Islands

A large coralline algal biostrome (20 km × 5 km × 16 m thick) is well exposed in western Malta and eastern Gozo. The biostrome is composed of limestones largely constructed by in situ coralline algae or transported algal debris. Some sediments are dominated by rhodoliths and others by crustose coralline frameworks. Six facies are distinguished: Crustose Algal Marl, Crustose Algal Wackestone, Rhodolith Pavement, Algal Branch Packstone, Crustose Pavement and Algal Crust Packstone. Each facies is characterised by different rhodolith morphologies and coralline crusts which are constructed mainly by Mesophyllum commune. The coralline morphogenesis is related to the palaeoenvironmental conditions. The biostrome is thought to have developed from a series of relic sand ridges to form a low relief platform of moderate depths containing a mosaic of sedimentary facies. The sequence is affected by growth faulting which divides the biostrome into eight units.

Comparison of Eogenetic and Mesogenetic Porosity-Diagenetic Trends in Deeply-Buried Limestone Reservoir in West Texas: ABSTRACT

Stratigraphically trapped gaseous hydrocarbons occur in porous limestones of Atokan (Middle Pennsylvanian) age in the Chapman Deep field, along the northern shelf edge of the Delaware basin in Reeves County, Texas. These rocks were deposited as a shallow-water mosaic of cyclic algal bioherms, grainstone shoals, and low-energy interbank facies. A relatively uninterrupted sequence of porosity evolution related to early and burial diagenesis is recognized in these rocks. Syndepositional subaerial exposure resulted in the formation of a secondary pore system, including biomolds, non-fabric-selective vugs and channels, and solution-enlarged fractures. However, most of this porosity was occluded rapidly in the vadose and phreatic environments by calcite cementation, dolomitizat on, and internal sedimentation. Progressive burial to minimum depths of 13,000 ft (3,962 m) was accompanied by bulk-volume reduction via physical and chemical compaction. However, simultaneous fabric-selective dissolution rejuvenated a pore system of relatively low permeability which, enhanced by natural fractures and the presence of open stylolites, comprises the principal reservoirs in the field. Pore types include ooid and cement-solution molds; although not recognized in these rocks, burial-solution channels and vugs are reported to be relatively abundant in Smackover carbonates of the Gulf Coast region. Such tertiary porosity is easily misinterpreted as of meteoric origin, and its occurrence attests to a possible parallel evolution of eogenetic and burial-diagenetic processes and pr ducts. End_of_Article - Last_Page 955------------

Jurassic Geology and Hydrocarbon Potential of Southwestern Alabama: ABSTRACT

The Jurassic Haynesville-Buckner-Smackover-Norphlet sequence in southwestern Alabama includes excellent deep reservoirs and laterally equivalent source rocks. The complex interaction of stratigraphy, syndepositional salt tectonics and faulting, and draping due to differential compaction over basement structures provides mechanisms for early entrapment of potentially significant hydrocarbon reserves in this area. Major Smackover-Norphlet production to the northwest and northeast, at Hatters Pond, Chunchula, and Jay fields, is from facies similar to those found in southwestern Baldwin County, Alabama. Detailed petrographic analyses of cores, chips, and cuttings samples from seven wells in southern Alabama and western Florida provide data for conceptualizing Upper Jurassic s ratigraphy (Norphlet-Lower Haynesville) and paleofacies relations in this area. The development of a regional Triassic Eagle Mills paleotopography in this area, including the interior salt basin, was influenced by existing Paleozoic basement structural trends and tensional tectonism related to incipient opening of the ancestral Gulf of Mexico. In the easternmost part of the salt basin, deposition of the Werner Anhydrite and Louann Salt was succeeded by the southward progradation of fluvial lower Norphlet facies. Upper Norphlet facies include littoral and possibly eolian sandstones, which are among the oldest Jurassic hydrocarbon reservoirs in the region. Rapid inundation of the Norphlet surface in lower Smackover time resulted in the deposition of argillaceous carbonates (brown dense facies) in the area. This event was followed by the establishment and rogressive southward progradation of a shoal-water limestone facies mosaic, including high-energy grainstones. Many of these subsequently dolomitized grainstones have porosities of as much as 24% and good permeabilities despite their burial below 19,000 ft (5,791 m) in southern Baldwin County. However, commercial hydrocarbon production from such reservoirs has not yet been extended south into this area. Evaporitic sabkha facies of the uppermost Smackover and Buckner-Lower Haynesville subsequently prograded over shelf carbonates throughout most of the study area. Whereas the area south of the Baldwin County graben generally appears to have remained basinal in character throughout all of Smackover time, Buckner-Lower Haynesville deposits in the Amoco 2 Amos well in Sec. 32, T7S, R4E, include thick grainstone shoal deposits which indicate the local existence of an emerging positive area south of the graben. This facies could provide for a significant stratigraphic-structural hydrocarbon accumulations in this heretofore unexplored region. End_of_Article - Last_Page 1561------------

The envjronment of deposition of the Caradoc Alternata Limestone and contiguous deposits of Salop

AbstractThe middle Caradoc rocks of Salop are a mosaic of sedimentary facies deposited in nearshore environments. The uppermost Horderley Sandstone in the Onny Valley is interpreted as formed by migrating marine bars and proximal and distal stonn deposits. The time equivalent Chatwall Sandstone of the Chatwall district is interpreted as a beach sequence. The overlying Alternata Limestone is a product of both proximal and distal storm deposits, associated with marine bars, in which different faunal assemblages are mized. Correlation of local sections is difficult due to facies and faunal changes and non‐deposition events. Based on a sedimentary model and palaeoecological information it is suggested that the Alternata Limestone in the Chatwall area is older than elsewhere, and that the base of the Cheney Longville Flags youngs southwards. Four faunal events are defined based on transect collecting, discriminant function and cluster analysis. These are the Howellites antiquior, Dalmanella lepta and Bancroftina robuste Associations and the Heterorthis alternata interregnum.