Algal Boundstone Research Articles

A steep‐fronted Carboniferous carbonate platform: clinoformal geometry and lithofacies (Picos de Europa, NW Spain)

A steep‐margined carbonate platform is developed in the Carboniferous synorogenic foreland basin of northern Spain. Dips of 60–90° produced during Late Carboniferous thrusting enable cross‐sections of a 4‐km‐wide portion of the marginal area of this platform (Las Llacerias outcrop) to be studied in aerial photographs at a seismic scale. Three stratal domains are observed: (1) a horizontal‐bedded platform; (2) a clinoformal‐bedded margin with a relief of up to 500 m; and (3) a low‐angle toe‐of‐slope, where slope beds interfinger with basin sediments. The slope shows well‐bedded sigmoidal clinoforms with depositional dips ranging from 15° to 32°. Based on lithology and stratal patterns, four facies groups have been recognized: (1) a flat‐topped platform, in which thick algal boundstone, skeletal packstone–grainstone and peloidal micrite wackestone with a poorly rhythmic character prevail; (2) the platform margin and upper slope, characterized by microbial boundstone spanning a bathymetric range of ≈150 m measured from the break of slope; (3) a slope, predominantly composed of margin‐derived rudstones and breccias; and (4) a toe‐of‐slope to basin zone, where a cyclic alternation of spiculitic siltstones, packstone to grainstone calciturbidites and rudstone/breccia is visible. Five successive stages of platform development are deduced: (1) Bashkirian: flooding of the pre‐existing Serpukhovian platform giving rise to the nucleation of a low‐angle ramp to the south‐east of the study area with microbial mud‐mound accumulations, and breccias and calciturbidites on the margins; (2) Early Moscovian: an influx of siliciclastic sediment buried part of the platform and reduced the area of carbonate sedimentation; (3) Moscovian: aggradation and progradation of the carbonate system produced an extensive steep‐margined and flat‐topped shallow‐water platform (shelf system); (4) Latest Moscovian–earliest Kasimovian: drowning of the platform; and (5) Kasimovian: covering of the platform by marly calcareous ramp sediments.

Abstract: Well-Preserved Geometries of Red Algal Boundstone Wedges: Transgressive or Highstand? 

Abstract: Well-Preserved Geometries of Red Algal Boundstone Wedges: Transgressive or Highstand? 

Evolution and Distribution of Porosity Associated with Subaerial Exposure in Upper Paleozoic Platform Limestones, West Texas

Middle Pennsylvanian-Lower Permian limestones in the subsurface of west Texas were studied to determine how subaerial exposure and freshwater diagenesis (karstification) affected porosity distribution in meter-scale cycles. Approximately 87 depositional cycles are present in the gross reservoir interval (depths of 2600-3000 m), and each cycle is interpreted to represent a glacio-eustatic sea level fluctuation. Using recent radiometric age dating, average cycle duration is estimated at 160,000 yr per cycle. Reservoir-grade porosity (>4%) occurs in 5-25% of the gross reservoir section. Porosity is stratified, occurring in 1-6-m-thick intervals in the upper part of cycles that were subaerially exposed; however, many cycles that were subaerially exposed now lack porosity. Diagenesis and porosity development have distinct patterns related to duration of subaerial exposure. Four stages of porosity development are identified. (1) Very brief or no subaerial exposure (estimated at less than 5000 yr) caused little or no diagenetic change. (2) Brief to moderate subaerial exposure (estimated at 5000-50,000 yr) resulted in most primary pores being filled with calcite cement, and dissolution creating fine matrix pores (molds and intercrystalline pores). (3) Moderately long subaerial exposure (estimated at 50,000-130,000 yr) resulted in cements filling primary pores and some fine secondary pores, and dissolution creating small conduit pores (vugs, fractures, fissures). (4) Prolonged subaerial exposure (estimated at greater than 130,000 yr) resulted in most primary and secondary matrix pores being filled with calcite cement, but dissolution enlarged conduit pores (vugs, fractures, fissures). Present subsurface porosity in this field preferentially occurs in thick grainstones, phylloid algal boundstones, and a few wackestone/packstones in cycles subjected to brief subaerial exposure (stages 2 and 3). Matrix porosity (molds, intercrystalline pores) is dominant because most conduit pores formed during prolonged subaerial exposure were filled with either shale during subsequent transgressions or burial cements derived from pressure solution associated with the shales. The distribution of porosity in the Southwest Andrews area indicates that duration of subaerial exposure and supply of clastics are major factors determining ultimate porosity in limestones subjected to subaerial exposure and karstification.

Continental, brackish and marine carbonates from the Lower Cretaceous of Kolone–Barbariga (Istria, Croatia): stratigraphy, sedimentology and geochemistry

During the Early Cretaceous, wide areas of the Dinaric–Adriatic Carbonate Platform emerged for long periods. The Hauterivian–Barremian carbonates from Kolone–Barbariga show a few typical examples of lacustrine facies with dinosaur bones and brackish/palustrine facies. The sequence of the platform is made for the most part by subtidal and intertidal limestones. The bone levels are located in a large depression few meters deep in the uppermost Hauterivian marine limestones. The filling facies of this depression are made by oncolitic rudstones and algal boundstones, which represent marginal lacustrine facies, and by laminated limestones, thin stromatolitic levels and distal fringes of rudstones which represent relatively open lacustrine facies. The fossil content is characterized by rare charophyte stems, ostracods, gastropods and plant remains, while typical marine fauna is absent. At the Hauterivian–Barremian boundary a major emersion event has been observed, then a slow transgressive phase occurred. The transgressive facies are primarily made by mudstones with ostracods, charophytes and Spirillina (brackish and probably freshwater facies), wackestones with Ophtalmidiidae and rare dasyclad algae, storm layers with gastropods and miliolids and breccia-like dinoturbated beds. Wackstones, packstones and very rich in dasyclad grainstones outcrop at the top of the section, representing the maximum of the transgression. Trace elements content, carbon and oxygen stable isotope analyses have been performed to aid the palaeoenvironmental interpretation. In this geological setting, Barium seems to discriminate between brackish and freshwater facies. The isotopic values of the marine carbonates appear to depend on early diagenetic processes, meanwhile lacustrine facies seem to show a weak signal of the depositional environment.

Late Viséan buildups at Kingscourt, Ireland: possible precursors for Upper Carboniferous bioherms

Abstract Two late Viséan (Asbian-early Brigantian) buildup complexes occur in the Kingscourt Outlier in Ireland, near the top of the Mullaghfin Formation, a shallow-water, grainstone unit. These massive buildups at Ardagh and Cregg accumulated on the margins of a carbonate platform bordering a deep-water basin. Both have a buildup facies of fine-grained, peloid-rich, algal lime mudstones and wackestones, interbedded with coarser grained intraclastic, skeletal packstones and grainstones (interbuildup facies). Microbial structures are well developed in the buildup facies, principally domal stromatolites, thrombolites and oncoidal fabrics of cyanophytes ( Ortonella and Girvanella ) with encrusting formainifers ( Aphralysia and Textrataxis ). Algal structures include rhodoliths ( Solenopora ) and fragments of Ungdarella and stacheiids, with less abundant chlorophytes ( Koninckopora ); in the interbuildup facies, Koninckopora is more abundant. Near the top of the Ardagh buildup is an unusual development of phylloid algal boundstone composed of the possible ancestral coralline red alga Archaeolithophyllum . This boundstone also contains encrusting bryozoans and foraminifers, and directly overlies abundant Brigantian in situ fasciculate rugose corals. This upper unit appears to be associated with a rapid shallowing event, which stimulated the development of a wave-resistant rigid framework. Laterally, bedded intraclastic skeletal packstones and grainstones, with thin interbedded clays and shales, pass into the lower part of the massive Ardagh buildup, but appear to onlap and drape the upper part of the buildup. Buildup clasts occur within the interbuildup facies and proximal flank facies, indicating synsedimentary cementation. Late Viséan buildups of the Ardagh type highlight an important period of diversification of colonial rugose corals and calcareous algae associated with the development of widespread shallow-water carbonate platforms worldwide. This allowed the emergence of new algal groups, particularly the red algae ( Archaeolithophyllum and Ungdarella ) and the palaeoberesellid green algae ( Kamaenella ), which subsequently dominated Upper Carboniferous wave-resistant bioherms.

Stratigraphic Sequences and Reservoir Facies of the Wapanucka and Spiro Formations: ABSTRACT

An understanding of the stratigraphic boundaries and lithofacies of the Spiro, middle shale (sub-Spiro shale in the subsurface), and the Wapanucka, will lead to a better prediction of the location of petroleum reservoirs in southeastern Oklahoma. Although the Spiro rests unconformably on the Wapanucka in some parts of the Arkoma basin; the base of the Spiro and the top of the Wapanucka are conformable surfaces throughout the frontal Ouachita outcrops. Small, discontinuous deposits of cross-bedded sandy calcarenites and bioclastic sandstones commonly occur within the middle shale sequence. These are commonly referred to as the `sub-Spiro sand` in the subsurface, and can make it difficult to distinguish the base of the Spiro and top of the Wapanucka sequences. At the Wapanucka shelf margin, subtidal subtidal cycles are capped by shoaling carbonate sand bodies (bioclastic and oolitic grainstones). In areas of paleotopographic highs, these shoal deposits can build up above sea level and form island beaches. Subaerial exposure is evidenced by the abundance of trees that grew on these islands. Sponge boundstones and phylloid algal bioherms grew adjacent to the shelf margin, while tubular algal (Donezella) boundstones occasionally formed small banks in slightly deeper water. The Spiro contains identical lithofacies to themore » Wapanucka; however, in certain areas, significant amounts of sand were transported to form shallow marine bars. In addition to the sandstones of the Spiro, several limestones facies of the Wapanucka and Spiro are potential hydrocarbon reservoir rocks. Fracture styles, related to certain lithofacies in outcrop, can aid in predicting fractured reservoirs in the subsurface.« less

Neogene Carbonate Exploration Play Concepts for Northern New Guinea: New Iteration from Field Work and Seismic Stratigraphy Along the Northern New Guinea Fault Zone: ABSTRACT

Recent field reconnaissance, petrography, nanno and foraminifera age determinations, and seismic stratigraphy of the Sepik and Piore subbasins of northern New Guinea reveal the existence of an extensive, tectonically unstable, Miocene-Pliocene carbonate shelf system. These findings represent the first recorded evidence of northern Papuan limestones coeval in age to those of the hydrocarbon productive Salawati Basin of Irian Jaya. Moreover, these observations also demonstrate the significance of episodic activities of the northern New Guinea fault zone upon the changes in carbonate sedimentation and diagenesis. During the Neogene, algal biosparites to foraminiferal biomicrites defined the clean portion of a mixed clastic-carbonate shelf system of the northern New Guinea basin, which began at the central New Guinea cordillera and deepened northward. This shelf was interrupted by coral-coralline algal boundstone fringing- to patch-reef buildups with associated skeletal grainstones. Clean carbonates were spatially and temporally restricted to basement blocks, which episodically underwent uplift while terrigenous dilutes carbonates were more common in adjacently subsiding basement block bathymetric lows. These tectonic expressions were caused by the spatially transient nature of constraining bends of the evolving north New Guinea faults. As shown by seismic stratigraphy, by the late Miocene to the early Pliocene the uplift of themore » Bewani-Torricelli Mountains sagittally divided the shelf of the northern New Guinea basin into the Ramu-Sepik and the Piore basins. Continued regional sinistral transpression between the Pacific and the New Guinea leading edge of the Indo-Australian plates led to the reverse tilting of the Piore basin, the shallowing of the former distal shelf with concomitant extensive biolithite development (e.g., on subsiding volcanic islands) eventual uplifting of the Oenake Range, and en echelon faulting of the Bewani-Torricelli Mountains.« less

Cycle Stratigraphy and Porosity in Pennsylvanian and Lower Permian Shelf Limestones, Eastern Central Basin Platform, Texas

Pennsylvanian and Lower Permian shelfal limestones were studied in core and wireline logs on the eastern side of the Central Basin platform in west Texas. Sixty-three (63) cycles were delineated in the study interval, which includes 200250 m of Canyon (Missourian), Cisco (Virgilian), and Wolfcamp strata. Four general lithofacies are present: fossiliferous wackestones and packstones, grainstones, phylloid algal boundstones, and shales. These lithologies typically occur in 118-m-thick cycles bounded by subaerial exposure surfaces. Grainstones in the upper part of some cycles indicate a shallowing of environments prior to subaerial exposure. Many cycles have subaerial exposure surfaces developed on subtidal fossiliferous wackestones or packstones suggesting rapid falls in se level. Long-term transgressive intervals (transgressive systems tracts or TST) are dominated by thick (>4 m) cycles, whereas long-term regressive intervals (highstand systems tracts or HST) are dominated by thinner cycles. Stable carbon isotope data suggest that thick cycles in TSTs were subjected to short periods of subaerial exposure, whereas thin cycles in the HSTs were subjected to much longer subaerial exposure. Porosity is quite variable beneath subaerial exposure surfaces. At any given well, reservoir-grade porosity (>4%) is present below only 1045% of the identified subaerial exposure surfaces. Where pres-ent, reservoir-grade porosity occurs in the upper part of cycles, 0.35 m below subaerial exposure surfaces. Porosity in thick cycles of the TST is relatively widespread and not facies-selective. Porosity is less abundant and facies-selective in the HST, with porosity occurring only in relatively thick (1.54 m) grainstones, which are concentrated near the shelf margin. Thin (

Middle Miocene carbonates from the Northern Plateau of the Western Desert (Egypt): Petrography and geochemistry

An extensive carbonate platform of predominately Middle Miocene age (Marmarica Formation) occupies the larger part of the northern plateau of the Western Desert of Egypt. The Marmarica Formation (up to 150m thick) exposed on the cliffs facing the Mediterranean coast consists mainly of alternating limestones and dolostones. Deposition took place in a shallow and normal marine environment. The limestones are dominated by algal boundstone, crossbedded packstone and bioturbated wackestone facies. The occurrence of the crossbedded packstone facies throughout the Marmarica Formation indicates that a shallow marine environment prevailed.

Petrographic studies of Permian carbonates in southern Thailand

Detailed petrographic studies were conducted on Permian carbonates from four areas: Ratburi, Prachuab Khirikhan, Ko Ang Thong, and Surat Thani-Phang Nga. Six carbonate facies were identified in these areas—mudstone, wackestone, packstone, grainstone, algal boundstone, and intramicrite. The allochems of these carbonates are similar and composed of bioclastic and non-bioclastic grains. The bioclastic grains are blue-green algae (stromatolite), green algae (dasycladacean), tabulate corals, solitary corals, crinoids, foraminiferas ( Shanita sp. and Hemigordius sp.), brachiopods, bryozoan, gastropods and unidentified skeletal fragments. The non-bioclastic grains are ooids, peloids, and intraclasts. Three phases of paragenesis were observed: micritization and neomorphism, dolomitization and non-ferroan/ferroan calcite fractures and veinlets. Micritization, neomorphism, and dolomitization are extensive. Rhombic and mosaic-crystalline dolomites had replaced carbonate mud, sparry calcite cement as well as the allochems. Abundant non-ferroan calcite veinlets cross cut allochems, mud matrix, and dolomite rhombs which suggests they postdate dolomitization. These carbonate facies were interpreted as interior-platform carbonate with a generally well developed shoaling-upward sequence starting from mudstone to wackestone, packstone, and grainstone. Local algal boundstone or algal mound and intramicrite could indicate low energy, regressive cycles.

Composition of Seismically Identified Satellite Mounds Surrounding Greater Aneth Field, Southeast Utah

Five different types of satellite mounds have been encountered during drilling and extensive coring of approximately 40 high-resolution CDP (common depth point) seismic anomalies in the Desert Creek interval of the Pennsylvanian (Desmoinesian) Paradox Formation around the periphery of Greater Aneth field. The composition and diagenetic modification of these mound types directly affect the quality of porosity and permeability within each satellite buildup. The mound types and their principal characteristics are (1) crinoid-sponge limestone with wackestone to packstone fabrics; (2) coralline algal boundstones that are slightly dolomitized; (3) bryozoan-dominated lime boundstones with no significant dissolution porosity; (4) phylloid algal bafflestones with extensive dissolution and some dolomitization occasionally overlain by dolomitized stromatolitic/thrombolitic boundstones; and (5) stacked bioclastic grainstones with extensive dissolution and complete dolomitization. Controls on the development of each mound type appear to be a function of water depth and prevailing water energy. Mound types 1 and 2 typically have low porosity, whereas type 3 preserved primary porosity. Types 4 and 5 commonly exhibit extensive porosity and permeability modification through freshwater dissolution and early dolomitization. Up to five cycles of buildup growth can occur within the Desert Creek satellite mounds. Mound composition types will recur or change to another growthmore » type depending upon local water depth and energy conditions. Calibration of seismic amplitude variations can be used in imaging reservoir size and porosity variation.« less

Petrology of modern carbonate hardgrounds from East Basin lake, a saline maar lake, southern Australia

East Basin Lake is a small maar lake located in the Western Volcanic Plains District of Victoria, Australia. The water in the basin is saline, alkaline, and strongly dominated by Na + and Cl - ions. The modern and Holocene shoreline and nearshore sediments of the lake contain well-indurated hardgrounds and crusts. These modern dolostones and limestones show a wide variety of textures, fabrics, and compositions. Morphologically, they range from flat, featureless mudstone and wackestone pavements with variable polygonal fragmentation to algal boundstones and microbialities having relief of up to a meter. The rocks are composed of variable amounts of siliciclastic and carbonate-clastic debris cemented by dolomite, monohydrocalcite, hydromagnesite, and magnesite. The rocks contain evidence of carbonate mineral formation by both inorganic precipitation and biologically induced cementation. Neoformation processes (precipitation directly from pore fluids) have been important in creating the carbonate minerals and lithifying the rocks. In contrast to many hardgrounds from peritidal-marine and other lacustrine settings, there is little direct evidence of diagenetic transformation processes (i.e., chemical and structural alteration of a pre-existing mineral). The unique carbonate mineral assemblage in the East Basin hardgrounds and crusts is a reflection of the high but fluctuating Mg/Ca ratios, the high alkalinity, and the high salinity of the brine.

Paleotopographic Control on Smackover Reservoir Evolution: Southwest Alabama

A number of Smackover fields in southwest Alabama occur over pre-Mesozoic paleotopographic highs that, in addition to providing a trapping mechanism, had considerable influence on Smackover deposition, diagenesis, and reservoir development. The overall influence of paleotopography is largely a function of relief on the pre-Smackover surface. Paleorelief ranges from less than 100 ft in association with low relief structures to over 500 feet with higher relief features. Lower relief structures (Appleton field) are characterized by thinning of both the Smackover and overlying Buckner Anhydrite sections. High energy depositional facies are localized across the crest of these structures and pass into lower energy facies off structure. Porosity enhancing diagenesis is most apparent across the crests of the structures, but had little effect on the Smackover in off structure wells. An appropriate exploration strategy would be to drill the crest of the structure. High relief structures (Vocation field) are characterized by thinning and, in some cases, pinchout of the Smackover section. Generally, nonporous deeper subtidal wackestones and packstones pass into porous, high energy grainstones and algal boundstones in a halo that surrounds the paleohigh. Porosity enhancing diagenesis is apparent across the entire structure, but extensive evaporite cementation plugs most porosity in crestalmore » areas. Because the updip Smackover is tight, successful exploration involves identification of the porosity halo that rims the structure.« less

Sequence Stratigraphy of the Smackover Formation in the Northern Half of the Manila Embayment, Southwestern Alabama

ABSTRACT The Smackover Formation in the northern half of the Manila Embayment, southwestern Alabama, ranges from 100 to 540 ft (30 to 165 m) thick and encompasses two genetic packages of facies. These genetic packages developed within a single cycle of relative sea-level change during the Late Jurassic (specifically late early Oxfordian to early Kimmeridgian) and comprise parts of a single depositional sequence. The base of the lower genetic package is the sharp contact between terrestrial clastic facies of the underlying Norphlet Formation and shelf carbonate facies in the Smackover. The lower Smackover genetic package (=transgressive systems tract) includes high-energy grainstones, shelf packstones and wackestones, and shelf-margin and patch-reef algal boundstones. The contact between the upper and lower genetic packages is a maximum-flooding surface delineated by a sharp facies change to a set of ramp carbonate facies. The upper genetic package (=prograding highstand systems tract) includes, in stratigraphic order, ramp packstones and wackestones, lagoon al wackestones and mudstones, and high-energy grainstone and clastic tempestite facies. The top of the upper genetic package is the sharp transition to anhydritic sabkha facies in the overlying Buckner Anhydrite Member of the Haynesville Formation.

Upper Jurassic Smackover Oil Plays in Alabama, Mississippi, and the Florida Panhandle

ABSTRACT Five Smackover (Upper Jurassic, Oxfordian) oil plays can be delineated in the eastern Gulf Coastal Plain. These include the basement ridge play, the regional peripheral fault trend play, the Mississippi Interior Salt basin play, the Mobile graben fault system play, and the Wiggins arch complex play. Plays are recognized by basinal position, relationships to regional structural features, and characteristic petroleum traps. Within two plays, subplays can be distinguished based on oil gravities and reservoir characteristics. Reservoirs are distinguished primarily by depositional setting and diagenetic overprint. The basement ridge play is updip of the regional peripheral fault trend where the Jurassic Louann Salt is thin or absent; structures in this trend formed on pre-Jurassic basement rocks. The basement ridge play is characterized by structural and combination traps. Reservoirs in the Choctaw ridge complex subplay are peritidal, partially to completely dolomitized, oolitic, peloidal, and oncoidal grainstone. Reservoirs of the Conecuh and Pensacola-Decatur ridge complexes subplay are subtidal to supratidal oolitic, oncoidal, intraclastic, and peloidal dolograinstone and dolopackstone, fenestral dolostone, quartz sandstone, and algal doloboundstone. The regional peripheral fault trend play is basinward of the updip limit of the Louann Salt and is typified by salt related structural features. These structural features occur in association with the Pickens, Gilbertown, West Bend, Pollard, and Foshee fault systems and are generally parallel to the basin margin. The regional peripheral fault trend play is exemplified by salt-related structural and combination traps. Reservoirs of the Pickens, Gilbertown, and West Bend fault systems subplay are peritidal, nondolomitic to completely dolomitized, oolitic, oncoidal, and peloidal grainstone. Reservoirs of the Pollard and Foshee fault systems subplay are subtidal to supratidal, partially to completely dolomitized, peloidal grainstone to wackestone, and dolomitized algal boundstone. The Mississippi interior salt basin play is downdip from the Pickens and Gilbertown fault systems and is characterized by structural and combination traps associated with salt tectonism in this basin. Reservoirs are peritidal, nondolomitic to completely dolomitized, oolitic and peloidal grainstone and packstone. The Mobile graben fault system play is located along the eastern limit of the Mississippi interior salt basin and is typified by salt-induced structural and combination traps and Smackover peritidal peloidal and oolitic dolograinstone to dolowackestone and dolostone reservoirs. The Wiggins arch complex play is in a downdip basinal position and is characterized by structural and combination-petroleum traps associated with stratigraphic thinning and salt flow. The traps occur along the flanks of pre-Mesozoic paleohighs associated with this complex. Reservoirs are subtidal to supratidal peloidal, oolitic and oncoidal dolograinstone and dolopackstone, thrombolitic dolostone, and crystalline dolostone.

Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia

ABSTRACTThe Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment‐water interface. The calcite is biologically precipitated as ostracod valves.In addition to the carbonates in the modern offshore (deep‐water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high‐Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units.Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.

Facies and Reservoir Characterization of an Upper Smackover Interval, East Barnett Field, Conecuh County, Alabama: ABSTRACT

Facies and Reservoir Characterization of an Upper Smackover Interval, East Barnett Field, Conecuh County, Alabama: ABSTRACT

Natural Gas Plays in Jurassic Reservoirs of Southwestern Alabama and the Florida Panhandle Area: ABSTRACT

ABSTRACT Three Jurassic natural gas trends can be delineated in southwestern Alabama and the Florida panhandle area. These include a deep natural gas trend, a natural gas and condensate trend, and an oil and associated natural gas trend. Trends are recognized by hydrocarbon types, basinal positions, and relationships to regional structural features. Within these natural gas trends, eight distinct natural gas plays can be identified. Plays are recognized by characteristic hydrocarbon traps and reservoirs. The deep natural gas trend includes the Mobile Bay area play that is characterized by structural hydrocarbon traps associated with salt tectonism and Norphlet sandstone reservoirs at depths exceeding 20,000 feet. The natural gas and condensate trend includes the Mississippi interior salt basin play, the Mobile graben play, the Wiggins arch complex play, and the Pollard fault system play. The Mississippi interior salt basin play is typified by salt-related structural and combination hydrocarbon traps and Smackover dolomitized oolitic, oncolitic, and peloidal grainstone and packstone reservoirs at depths of approximately 16,000 feet. The Mobile graben play is exemplified by salt-induced structural hydrocarbon traps and Smackover dolostone and Norphlet sandstone reservoirs at depths ranging from 12,400 to 18,400 feet. The Wiggins arch complex play is characterized by structural and combination hydrocarbon traps associated with stratigraphic pinch-outs and salt flow. These traps are salt-related and occur along the flanks of paleohighs associated with the Wiggins arch complex. Smackover dolostone reservoirs at depths ranging from 16,100 to 18,400 feet are typical of this play. The Pollard fault system play is typified by salt-induced structural hydrocarbon traps and reservoirs at depths of approximately 15,000 feet. These reservoirs are Smackover dolomitized oolitic and peloidal grainstones and packstones and Norphlet sandstones. The oil and associated natural gas trend includes the Gilbertown and West Bend fault systems play, the Foshee fault system play, and the basement ridge play. The Gilbertown and West Bend fault systems play is exemplified by salt-related structural or combination traps and Smackover dolomitized oolitic, oncolitic, and peloidal grainstone and packstone reservoirs and Norphlet sandstone reservoirs at depths ranging from 11,000 to 14,000 feet. The Foshee fault system play is characterized by structural and combination hydrocarbon traps related to salt movement and Smackover dolomitized peloidal grainstone and packstone and Norphlet sandstone reservoirs at depths of approximately 15,000 feet. The basement ridge play, which is typified by structural and combination traps associated with the Conecuh and Pensacola-Decatur ridge complexes and Smackover oolitic and peloidal grainstone and packstone and algal boundstone and Haynesville sandstone reservoirs at depths ranging from 11,800 to 15,500 feet, has potential for significant undiscovered natural gas.

Geometrie und Internbau der unterkambrischen Riffe im Untergrund der Sibirischen Plattform (Osinsky Horizont, Aldan; südliches Zentralsibirien)

Lower Cambrian buildups in the subsurface of the Nepsko-Botuobinsky anteclise in southern Central Siberia were investigated with respect to the geometry of the reefs and their biotic and lithological composition. The buildups of the Osinsky horizon are characterized by an abundance of non-skeletal and skeletal algae (Epiphyton, Renalcis, Markovella et. al.) which acted as binders, encrusters and probably also as framebuilders. Archaeocyathids are important elements in the climax communities of some successions. The buildup rocks are limestones and dolomites (algal boundstones, archaeocyathid/algal boundstones, algal wackestones and mudstones). Coarsegrained reef detritus is missing.

Occurrence of Pore-Filling Halite in Carbonate Rocks, Nesson Anticline, Williston Basin, North Dakota: ABSTRACT

Clear, colorless pore-filling halite of late diagenetic origin occurs locally in the Devonian Dawson Bay, Winnipegosis, and Ashern Formations, the Silurian Interlake Formation, and the Ordovician Red River Formation. The halite occludes a variety of pore types and individual pores are filled with single crystals or aggregates of only a few crystals. This halite is present in quantities ranging from a trace to approximately 12%. Cores from McGregor field, Williams County, show the Winnipegosis Formation consists of mixed-skeletal lime wackestones and mudstones. These contain vugs up to 4 in. (10 cm) in size, intraparticle pores, and shelter porosity within pelecypod shells, up to 4 in. (10 cm) in size, which are occluded with halite. Halite also fills common small discontinuous vertical fractures. The upper 200 ft (61 m) of the Interlake Formation locally exhibits the most striking occurrences of pore-filling halite. These dolostones consist predominantly of intraclast-peloid mudstones, wackestones, packstones, occasional grainstones, algal boundstones, and solution-collapse breccias containing vug, fenestral, interparticle, shelter, intercrystalline, moldic, channel, breccia, and fracture porosity types. All porosity types, except intercrystalline, can be halite filled. A rare occurrence of pore-filling halite exists in Red River cores from Blue Buttes field, McKenzie County, where a dolomitic, mixed-skeletal, more » lime mudstone and wackestone lithofacies contains vugs, discontinuous vertical fractures, and intraparticle porosity types occluded with halite. In most occurrences, the pore systems were noneffective prior to halite infilling and had no potential as hydrocarbon reservoirs. However, it has been demonstrated that halite plugging in the Interlake Formation has locally formed updip seals to hydrocarbon migration. « less