Buildup Facies Research Articles

Three-dimensional morphometric analysis and statistical distribution of the Early Kimmeridgian Hanifa Formation stromatoporoid/coral buildups, central Saudi Arabia

The high porosity and permeability stromatoporoid/coral facies partly characterize some of the most productive supergiant Late Jurassic carbonate reservoirs in the Middle East. Reservoir models often overlook detailed morphology and distributions of this facies due to the limited resolution of subsurface data. 3D morphological architecture and distribution of the Late Jurassic stromatoporoid/coral buildups are poorly understood as not many studies specifically investigate these aspects. This study performs a full three-dimensional outcrop investigation of the stromatoporoid/coral complex part of the Late Jurassic Hanifa reservoir analog in Wadi Birk, Saudi Arabia. We investigated spatial correlation, clustering tendencies, scaling relationships, and comprehensive statistical description over a 1.2 × 1 km area analog to a small-scale sectoral model of a typical Middle Eastern oilfield. This study utilizes integrated datasets including measured sections, drone-based digital outcrop model (DOM), core, ground penetrating radar (GPR), and shallow seismic to perform a full three-dimensional outcrop investigation. We identify and map three levels of the scaling hierarchy of the buildups: S1, S2, and S3. S1-scale buildups (a few meters in length) combine to form S2 clusters (∼35 m in length), which in turn may grow into larger S3 complexes (∼128 m in length). S1-scale buildups are near-circular and isotropic. However, as the S1 buildups cluster and grow into S2 and S3, their circular shape evolves into pseudo-ellipsoids with a strong preferential orientation towards northwest-southeast. The S2 and S3-scale amalgamated buildups are associated with asymmetric flank accumulation towards the northwest. We propose that the orientation of the S2 and S3-scale buildups and associated flanks are attributed to hydrodynamic currents driven by paleo-trade winds. This study also performs a brief static connectivity assessment to demonstrate the subsurface implications of these facies to field development planning. Results show that a vertical 5-spot pattern with a 1 km spacing can, in the best case, only access 14% of the Gross Rock Volume of the build-up facies due to the extreme lateral heterogeneity. Some 84% of the build-ups would be connected via flank and other facies with significantly different reservoir quality. This result underlines the extreme heterogeneity of the late Jurassic shallow-water platform carbonates and the potential for substantial quantities of bypassed hydrocarbons remaining in subsurface analog reservoirs.

Actinocrinitidae from the Lower Mississippian Fort Payne Formation of Kentucky, Tennessee, and Alabama (Crinoidea, Viséan)

AbstractThe Actinocrinitidae were among the most abundant crinoids worldwide during the Lower Mississippian. Recent systematic revisions of the family allow a revised genus- and species-level understanding of these crinoids globally and a more precise means by which to understand the temporal and facies distribution of genera and species in this important Mississippian family. Two genera with a total of five species of Actinocrinitidae (and five additional forms left in open nomenclature) are recognized from the Fort Payne Formation, including Actinocrinites jugosus (Hall, 1859), Actinocrinites spp. indeterminate, Thinocrinus gibsoni (Miller and Gurley, 1893), Thinocrinus lowei (Hall, 1858), Thinocrinus probolos (Ausich and Kammer, 1991), Thinocrinus akanthos new species, Thinocrinus sp. aff. T. gibsoni, Thinocrinus spp. indeterminate, and two taxa recognized as only Actinocrinitidae genus and species indeterminate. Actinocrinites tripus Ehlers and Kesling, 1963 is recognized as a junior synonym of Thinocrinus gibsoni. Thinocrinus, rather than Actinocrinites as previously thought, is the dominant Fort Payne Formation actinocrinitid. Fort Payne Formation carbonate buildup facies (wackestone buildups and crinoidal packstone buildups) each have characteristic species of Thinocrinus. Actinocrinites is relatively rare in the Fort Payne Formation, but occurs preferentially in crinoidal packstone buildups.

Crinoid assemblages from the Fort Payne Formation (late Osagean, early Viséan, Mississippian) from Kentucky, Tennessee, and Alabama

The Mississippian Fort Payne Formation of Kentucky, Tennessee, and Alabama is well known for its abundant crinoids and a diverse array of autochthonous and allochthonous carbonate and siliciclastic facies. Using Principal Coordinate Analysis and Non-Metric Multidimensional Scaling, it is demonstrated that distinct, contemporaneous, and geographically adjacent autochthonous facies in south-central Kentucky supported distinct crinoid assemblages. The two carbonate buildup facies had different assemblages dominated by camerate crinoids, carbonate channel-fill deposits were dominated by advanced cladid crinoids and the camerate Elegantocrinus hemisphaericus, and green shale facies supported a fauna dominated by disparids and primitive cladid crinoids. Allochthonous facies contain neither distinctive nor exotic taxa. Thus, these transported assemblages are considered a mixture of elements from the recognized, autochthonous facies. Faunal assemblages from Dale Hollow Reservoir are allochthonous; and faunas in north-central Alabama and south-central Tennessee are different from others, which may reflect slight biogeographic distinctions.

Corals from the Upper Viséan of the southern Azrou-Khenifra Basin (Carboniferous, Central Moroccan Meseta)

This is the first taxonomic treatment of coral faunas from upper Viséan shallow-water limestones of the southern Azrou-Khenifra Basin (eastern central Morocco). Corals recovered during reconnaissance work represent 8 genera and 11 species of rugose corals, 1 heterocoral species, and 3 tabulate coral taxa. The fauna is tentatively attributed to the late Asbian. Siphonodendron scaleberense is described from Northern Africa for the first time. Facies differentiations clearly control the coral distribution in the stratotype of the Bou-Rifi Formation near Sidi-Lamine. In the Tabaïnout Ridge, the massive build-up facies is almost devoid of corals except for specialised forms such as ? Sutherlandia. Bioclastic and oncolitic limestone facies below and above the build-up facies contain coral faunas, which differ in their compositions. The fauna of the Azrou-Khenifra Basin is part of the West European-North African faunal province.

Rugose corals from the upper Viséan (Carboniferous) of the Jerada Massif (NE Morocco): taxonomy, biostratigraphy, facies and palaeobiogeography

The Visean succession of the Jerada Massif contains a relatively diverse rugose coral fauna, which comprises 12 genera. Only two of these (Siphonodendron and Lithostrotion) are reported to include more than one species. Most taxa show distinctive facies dependencies. Small solitary corals are found in shaly environments of the Oued Es-Sassi Formation. On the northern flank a coral patch reef formed on an oolitic shoal in the Koudiat Es-Senn Formation. Its core mainly consists of Lithostrotion vorticale colonies, whereas in its surroundings literally hundreds of specimens of Siphonophyllia samsonensis occur. Associated with these dominant taxa occur colonial and further large solitary taxa (Palaeosmilia, Dibunophyllum). Tizraia and Pareynia are restricted to microbial-dominated buildup facies. Their presence might be strongly controlled by the development of this buildup type, because further occurrences in Algeria, Morocco, and Belgium are all in the same facies. The coral fauna at Jerada is a typical Late Visean assemblage for the Western European Province. The Eastern Moroccan Meseta may be an important pathway within the province for the connection between the Central Saharan basins and NW Europe. The biostratigraphic ages of the coral fauna partly contradict ages based on carbonate microfossils; the coral ages are slightly older and typical Brigantian coral taxa are absent.

Relationships between tectonics and sedimentation on the northeastern margin of the Subpyrenean trough during the late Santonian

Relationships between tectonics and sedimentation on the northeastern margin of the Subpyrenean trough during the late Santonian

Automated mapping of carbonate build-ups and palaeokarst from the Norwegian Barents Sea using 3D seismic texture attributes

Geological heterogeneity in hydrocarbon reservoirs is normally related to variability in depositional facies, diagenesis and structure. On the Loppa High, Norwegian Barents Sea, reservoir quality in prospective Upper Palaeozoic carbonates is considered to be controlled mainly by primary depositional facies variability linked to active faulting, with a secondary overprint of palaeokarst. The best quality reservoirs are anticipated in build-up facies. Although high-quality 3Dseismic data exist over the prospect, it is a challenge to map the buildups and palaeokarst in three-dimensional space using conventional 3Dseismic interpretation tools. This is mainly due to the internal heterogeneity of the build-ups and palaeokarst, which are characterized by a range of seismic reflection/attribute patterns. A procedure for multi-attribute mapping of seismic facies is described and utilized to provide a truly threedimensional interpretation of build-up and palaeokarst geobodies. A neural network classifier is used to analyse a set of 3D attributes that capture the seismic stratigraphical and structural patterns inherent in the data. These attributes are referred to as 3D texture attributes since they describe the reflector-geometry in a small 3D neighbourhood. Pattern recognition is interpreter-guided, with the desired seismic facies to be mapped input as ‘training data’ and analysed prior to their presentation to the 3D-classification system. Once the classification is made, the interpreter is presented with a 3D classified volume and an uncertainty analysis of each of the mapped facies/classes. This is used to evaluate the results prior to their visualization in 3D space. The method allows the interpreter to identify quickly a volume where the features of interest occur and to focus the interpretation work directly on them. In addition, the 3D visualization of the mapped geobodies brings important new information that might be overlooked when inspecting data in vertical or horizontal data windows. In the Norwegian Barents Sea case study, 3Dseismic facies mapping provides the first truly three-dimensional interpretation of the build-ups and palaeokarst, where their external form, juxtaposition, cross-cutting relationships and structural information are preserved. These data give new insights as to the stratigraphic distribution and internal variability of the build-ups and palaeokarst system; this information is important to estimation of reservoir volume, connectivity and variability.

Dynamic stratigraphy as a tool in economic mineral exploration: ultra-pure limestones (Upper Jurassic, SW Germany)

Ultra-pure limestones form valuable economic minerals used in the glass, paper, pharmaceutical, chemical and building industries. Both exploration and exploitation of economic materials require predictions of the subsurface stratigraphy. Such predictions are most reliable when the depositional processes creating these deposits are well understood. We present a simple, process-based methology (‘Dynamic Stratigraphy’) to break down stratigraphic packages into a hierarchy of easily recognisable genetically defined units of various scales. Each scale of analysis at the same time provides critical data on the composition, quality, volumes and preferred occurrence of economic minerals. In this study, Upper Jurassic carbonates of SW-Germany, deposited on a deeper epicontinental ramp, have been re-evaluated, based on newly available borehole data of an exploration programme for industrial minerals, especially for ultra-pure limestones, carried out by the Geological Survey of Baden-Württemberg. About 2.5 km of core were slabbed and continuously studied for micro- and macrofacies, geochemistry, colourimetry (‘whiteness index’) and calibrated to the gamma-ray signatures. In a number of key outcrops stratal and facies patterns as well as sedimentary geometries were mapped. The carbonate buildups form the most promising rock bodies hosting ultra-pure limestones. Gamma-ray signatures correlate both with major facies types and their bulk chemical composition and are thus useful for an initial evaluation of raw materials properties. In both major facies associations, (1) the bedded and (2) the massive microbial/sponge buildup facies, distinct sedimentary cycles were recognised, building systematic stacking patterns and a cycle hierarchy. Within the buildup facies, this cyclicity mirrors the ‘whiteness’ and ‘pureness’ of limestones. The cycles are used to carry out regional correlations between and across extensive buildup complexes, which are generally regarded as completely massive and homogenous. Based on this, the occurrence and distribution of economically valuable ultra-pure limestone bodies can be understood, and used to develop predictive genetic models. Dynamic or genetic stratigraphy integrated with petrophysical measurements (colourimetry, gamma-ray logs) leads to more cost-effective techniques of exploration and exploitation. While similar sedimentological concepts are commonly used in the petroleum industry, this study documents their usefulness also in the economic minerals industry.

Devonian carbonate buildup facies in an intra-oceanic island arc (Tamworth Belt, New South-Wales, Australia)

Biohermal and biostromal buildups were investigated in late Early and Middle Devonian carbonate complexes of the Tamworth Belt. The buildup types and subtypes were studied in three regions (Yarramanbully, Sulcor and, Wyaralong') to clarify their paleo-environmental position.

Organic buildups and reefs on the Palaeozoic carbonate platform margin, Pechora Urals, Russia

In the Pechora Urals, Palaeozoic organic structures are widespread and are represented by various types, from small bioherms to large barrier reefs which can be observed throughout Upper Ordovician–Lower Permian strata. In the pre-orogenic history of Palaeozoic carbonate sedimentation and reef growth three stages are distinguished. Each stage is characterized by a distinct reef geometry, growth rate and depositional setting. (1) The reefs of the lower Palaeozoic (Ashgillian–lower Emsian) stage were restricted to the outer shelf margin, and they were wave-resistant stromatolite-rich frameworks, except for upper Llandoverian knoll and patch reefs. The total thickness of the lower Palaeozoic reefs is 60–1200 m. (2) The middle Palaeozoic (middle Frasnian–lower Tournaisian) organic buildups are quiet-water carbonate bank-type structures generally between 300 and 700 m thick. They were formed on both the outer and the inner shelf margin; however, they are better developed and more characteristic of the latter setting. Stromatoporoid–microbial and microbial associations form the main buildup facies separated by bedded interbuildup sediments. The upper Palaeozoic (upper Visean–Sakmarian) organic structures are represented by both isolated wave-resistant shelf margin reefs, up to 200 m thick, and quiet-water carbonate banks, up to 330 m thick. These buildups were located in the outer and inner shelf margin. Framework-builders were mainly branching and platy stromatoporoids associated with stromatolites in the reefs, and calcified microbes and phylloid algae in the carbonate banks. The distribution and geometry of the buildups was mainly determined by regional tectonic events caused by the passive-to-active margin transition of the northeastern European Platform. The size of the reefs depended on the magnitude and duration of sea-level fluctuations.

Late Dinantian (Lower Carboniferous) platform carbonate stratigraphy of the Buttevant area North Co. Cork, Ireland

A thick sequence of late Dinantian (Asbian–Brigantian) carbonates crop out in the Buttevant area, North Co. Cork, Ireland. A mud-mound unit of early Asbian age (the Hazelwood Formation) is the oldest unit described in this work. This formation is partly laterally equivalent to, and is overlain by, over 500 m of bedded platform carbonates which belong to the Ballyclogh and Liscarroll Limestone Formations. Four new lithostratigraphic units are described within the platform carbonates: (i) the early Asbian Cecilstown Member and (ii) the late Asbian Dromdowney Member in the Ballyclogh Limestone Formation; (iii) the Brigantian Templemary Member and (iv) the Coolbane Member in the Liscarroll Limestone Formation. The Cecilstown Member consists of cherty packstones and wackestones that are inferred to have been deposited below fair-weather wavebase. This unit overlies and is laterally equivalent to the mud-mound build-up facies of the Hazelwood Formation. The Dromdowney Member is typified by cyclic-bedded kamaenid-rich limestones possessing shell bands, capped by palaeokarst surfaces, with alveolar textures below and shales above these surfaces. The carbonates of this unit were deposited at or just below fair-weather wavebase, the top of each cycle culminated in subaerial emergence. The Templemary Member consists of cyclic alternations of subtidal crinoidal limestones capped by subtidal lagoonal crinoid-poor, peloidal limestones possessing coral thickets. Intraclastic cherty packstones and wackestones characterize the Coolbane Member, which is inferred to have been deposited below fair-weather wavebase but above storm wavebase. The early Asbian Cecilstown Member has a relatively sparse micro- and macrofauna, typified by scattered Siphonodendron thickets, archaediscids at angulatus stage and common Vissariotaxis. Conversely, macro- and microfauna is abundant in the late Asbian Dromdowney Member. Typical late Asbian macrofossils include the coral Dibunophyllum bipartitum and the brachiopod Davidsonina septosa. The base of the late Asbian (Cf6γ Subzone) is recognized by the first appearance of the foraminifers Cribrostomum lecompteii, Koskinobigenerina and the alga Ungdarella. The Cf6γ Subzone can be subdivided into two biostratigraphic divisions, Cf6γ1 and Cf6γ2, that can be correlated throughout Ireland. Relatively common gigantoproductid brachiopods and the coral Lonsdaleia duplicata occur in the Brigantian units. The base of the Brigantian stage (Cf6δ Subzone) is marked by an increase in the abundance of stellate archaediscids, the presence of Saccamminopsis-rich horizons, Loeblichia paraammonoides, Howchinia bradyana and the rarity of Koninckopora species. Changes in facies at the Cecilstown/Dromdowney Member and the Ballyclogh/Liscarroll Formation boundaries coincide closely with the changes in fossil assemblages that correspond to the early/late Asbian and the Asbian/Brigantian boundaries. These facies changes are believed to reflect major changes in relative sea-level on the Irish platforms. The sea-level variations that are inferred to have caused the facies changes at lithostratigraphic boundaries also brought in the new taxa that define biostratigraphic boundaries. Moreover, many of the Dinantian stage boundaries that are defined biostratigraphically in Great Britain, Belgium and the Russian Platform also coincide with major facies boundaries caused by regressive and transgressive episodes. The integration of detailed biostratigraphic analyses with facies studies will lead to better stratigraphic correlations of Dinantian rocks in northwest Europe. © 1997 John Wiley & Sons, Ltd.

Facies sequences of the middle-upper jurassic carbonate platform (Amran Group) in the Sana'a region, Republic of Yemen

Rocks of the Amran Group in the Sana'a region of Yemen disconformably overlie the Kohlan Formation and unconformably underlie the Tawilah Group. Sea-level rises have produced landward migration of facies belts and the development of deeper water facies over shallow ones. The geometry of the depositional environments identified has enabled the passage of relatively short-lived transgressive-regressive cycles of sedimentation to be recognized. Three facies associations are introduced: (1) Carbonate platform facies, (2) Carbonate-marl alternation facies, and (3) Shallow water coral and stromatoporoid build-up facies. These facies are widely distributed and the whole sequence reflects deposition on a broad platform upon which shoals separated platform carbonates from basin sedimentation and an open marine environment. The repetition and interfingering of both fining- and shallowing-upward cycles within the study areas suggest that deposition occurred within the same basin with slightly different conditions in different places. The main factors controlling their deposition are sea-level changes and tectonics.

Waulsortian buildups of Waulsort, Belgium

The biostratigraphy and sedimentology of the late Tournaisian, Waulsortian buildups of the type area, at Waulsort (Belgium), are examined in detail. Four buildups are present. Overall, they span the stratigraphic interval from the base of the Ivorian stage to the lower part of the Moliniacian stage, ending just below the base of the Viséan. Petrographic study shows that all Waulsortian Phases are represented, the older buildups being of Phases A and B, whereas the youngest ranges through Phases A to D. The relatively crude Phase classification is refined by correspondence analysis, which shows that the grain types present in the Waulsortian and associated limestones conform closely to the relay (systematic shift of relative importance of grain types) described from other Waulsortian buildups in Belgium. A Relay Index, derived from the correspondence analysis and defining the position of each sample in the compositional relay, is plotted on stratigraphic logs of the sections studied to allow detailed comparison with other Waulsortian and peri-Waulsortian sections in the Dinant area for which similar curves already exist. The Waulsort buildups are found to fit closely into the depositional and palaeobathymetric patterns established or suspected in the area. Two shallowing upward sequences seem to be present, separated by an abrupt deepening. The earlier, relatively minor shallowing trend, in the Polygnathus communis carina Zone, is here recorded in buildup facies for the first time: previous evidence had come from the laterally equivalent facies. The other shallowing trend, which extends through the Scaliognathus anchoralis Zone and culminates near the base of the Moliniacian stage, is the regional, late Tournaisian regression which had a major influence on the biota of the Waulsortian banks in Belgium and was probably largely responsible for cessation of bank growth over most of the area.

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.

The Stratigraphic Hierarchy: Framework for Integrated Basin Analysis at the Exploration and Production Scales- Paradox Basin Case Study: ABSTRACT

Analysis of seismic, outcrop, core, and log data into a stratigraphic hierarchy for the Middle Pennsylvanian mixed carbonate, siliciclastic, and evaporate strata of the Paradox basin, southwest USA allows for an improved prediction of reservoir, source, and seal. Facies stacking patterns indicate that 5th-order depositional cycles ([ge] 100 k.y.), stack into 4th-order depositional sequences ([approximately]400 k.y.), each with lowstand, transgressive, and highstand systems tracts. This framework comprises the reservoir architecture of the giant Aneth field. Fourth-order sequences stack into seismically resolvable, biostratigraphically defined 3rd-order composite sequences ([approximately]1.5 m.y.). Secondary porosity is most pronounced in highstands beneath 3rd-order sequence boundaries. Five 3rd-order sequences make up part of an Atokan-Missourian 2nd-order transgressive-Tegressive cycle. This framework is used to predict the regional distribution of reservoir, source, and seal. A second-order relative rise in sea level during the lower three 3rd-order sequences (Atokan-E. Desmoinesian) resulted in a basin-rimming carbonate shelf that backsteps through time. During 3rd-order lowstands, basin restriction and drawdown resulted in accumulation of subaqueous evaporates. Third-order transgressions over vast evaporate deposits led to dissolution, elevated salinity, and high organic productivity. These source rocks are basinwide in extent. The ensuing turnaround and initiation of the 2nd-order fall in sea level resulted in rapidmore » progradation of the carbonate shelf. Drawdown was sufficient during 4th-order lowstands to allow shallow subaqueous evaporates to accumulate in each 3rd-order sequence. World-class reservoirs formed in algal buildup and grainstone facies of the 3rd-order highstand above the 2nd-order turn- around. Seal facies overlie reservoirs and are composed of lowstand evaporates and transgressive carbonates.« less

Geological and Reservoir Characterization of Shallow-Shelf Carbonate Fields, Southern Paradox Basin, Utah: ABSRACT

Abstract The Paradox Basin of Utah, Colorado, and Arizona contains more than 50 small oil fields producing from carbonate buildups within the Pennsylvanian (Desmoinesian) Paradox Formation. These fields typically have one to four wells with primary production ranging from 700,000 to 2,000,000 barrels (112,000-318,000 m3) of oil per field at a 15- to 20- percent recovery rate. Five of these fields (Heron North, Runway, Mule, Blue Hogan, and Anasazi) within the Navajo Nation of southeastern Utah, were chosen for case study to develop geologic reservoir models to be used for waterflood and carbon-dioxide- miscible flood simulations. Conventional cores, modern geophysical logs, reservoir data, closely spaced seismic lines, and outcrop analogs for the five case-study fields were used to determine the reservoir facies, geometric relationships, flow units, and areal extent of regional facies belts and localized hydrocarbon traps. Local scale, phylloid algal buildup facies are the dominant hydrocarbon traps. These buildups typically consist of basal platform, middle mound-core, and upper supra-mound intervals. Reservoir heterogeneity is caused by combinations of lithotypes, mound relief, flooding surfaces, and diagenesis. The basal platform contains combinations of low-permeability carbonate fabrics and is rarely productive. The mound cores are generally homogenous, with porous and permeable algal lime bafflestones. The supramounds are very heterogeneous, with lower overall permeability but higher average porosity than the underlying mound cores. This heterogeneity is due to the mixed mineralogy as well as the wide range of depositional fabrics. Primary recovery is estimated to be approximately 40 percent from the mound-core and less than 15 percent from the supra-mound intervals. Oil production rates are highest from permeable algal-lime bafflestones of the mound cores. However, most of the oil reserves reside in porous dolomites of the overlying supra-mound intervals. As oil is produced from algal-lime bafflestones, oil from the overlying dolomites continually replenishes the bafflestone pore system, resulting in a production capacity far greater than can be attributed to the mound cores alone. This study provides the basis for several areas of future work. First, waterflood and/or carbon-dioxide-miscible flood projects can be designed using models and reservoir simulations based on this study. The objective is to drive oil trapped in the supra-mound rocks into the mound-core reservoir where recovery is highest. Second, the results of this study can be used to increase recoverable reserves by identifying untapped compartments created by reservoir heterogeneity and prevent premature abandonment of numerous small fields throughout the Paradox Basin. Third, the results of this study may apply to small carbonate- buildup reservoirs in other areas in the Rocky Mountain Region, the Michigan and Illinois basins, and the Midcontinent.

Meteoric diagenesis below a submerged platform: implications for δ13C compositions prior to pre-vascular plant evolution, Middle Ordovician, Alabama, U.S.A.

The Middle Ordovician “Chickamauga” Formation at Red Mountain Expressway (RME) and Tidwell Hollow (TWH) sections in Alabama (U.S.A) has a complex early diagenetic history. Early diagenetic history of the buildup facies consisted of two phases of marine cementation separated by two generations of meteoric, equant calcite with moldic dissolution separating equant calcite generations. A third generation of meteoric, equant (drusy) calcite is present after secondary marine diagenesis. Equant (drusy) calcite is interpreted as meteoric because its stable isotopic composition ( δ 13 C = −0.1 to −1.6‰ , δ 18 O = −4.8 to −7.3‰ ) is too variable to be accounted for by marine or mixing zone precipitation. Equant (drusy) calcite from buildup facies is mostly non-luminescent and non-ferroan. This is unlike early, ferroan equant calcite (dull to bright luminescent) from below the unconformity that overlies the “Chickamauga” Formation at RME. Meteoric diagenesis below the unconformity is unrelated to early diagenesis in the underlying buildup facies. Equant (drusy) calcite from the buildup facies at RME has a covariation of δ 13C and δ 18O values that forms a truncated inverted “J” and has elevated Mg values. These geochemical signatures indicate precipitation at a site that was distal from the recharge zone. Meteoric diagenesis at RME occurred during rising relative sea level and flow of meteoric fluids under a submerged platform is the only mechanism able to reconcile available stratigraphic, petrologic, and geochemical data. Ordovician meteoric δ 13C values (from TWH and other formations) are slightly depleted (2–4‰) relative to coeval marine carbonate, especially when compared with post-Silurian meteoric carbonate. Minor depletion of Ordovician carbonate is the result of limited generation of light soil-gas CO 2 before the development of vascular plants.

Significance of Cyclic Pennsylvanian-Permian Coral/Algal Buildups, Snaky Canyon

Five cyclic algal, hydrozoan, and coral buildups occur within a thick sequence of Pennsylvanian-Permian (Virgilian through Wolfcampain) carbonates in south-central Idaho. The Juniper Gulch Member of the Snaky Canyon Formation, as described by Skipp and coworkers, is approximately 600 m thick and contains four depositional facies, including: (1) open circulation outer( ) platform, (2) hydrozoan and phylloid algal mound-dominated carbonate buildup, (3) backmound, restricted platform/lagoon, and (4) restricted inner platform facies. Several microlithofacies, including lime mud-rich bafflestone, diversely fossiliferous packstone and grainstone, bryozoan lime floatstone, and phylloid algal and hydrozoan (Palaeoaplysina) lime bindstone are described within the phylloid algal mounds. Successional faunal assemblage stages are recognized within the buildups. Colonial rugose corals comprise a stabilization stage. When the algal communities of the diversification stage reached wave base, because of their rapid upward growth, cross-bedded oolitic grainstone and occasional cross-bedded dolomite shoals developed. Supratidal to high intertidal platform sedimentation is represented by dolomitic Palaeoaplysina bindstone, algal mat bindstone, and vuggy dolomite. Five vertical sequences of buildup development, each terminate by intertidal, supratidal, or erosional events, are seen in the Juniper Gulch Member in the North Howe stratigraphic section of the southern Lost River Range. The carbonate platform was constructed within amore » depositional basin that includes an eroded highland to the west, and a mixed siliciclastic-carbonate inner platform with craton uplifts to the east.« less

Upper Permian Carbonate Buildups and Associated Lithofacies, Western Hubei-Eastern Sichuan Provinces, China (1)

In the Upper Permian Changxing Formation in western Hubei and eastern Sichuan, China, three types of carbonate buildups have been recognized, including platform-margin reefs, organic mud mounds, and patch reefs. Carbonate buildups in the Changxing Formation have yielded natural gas in the well J-7 area of the Jiannan gas field in western Hubei and in well B-1 on the Dashigan structure in eastern Sichuan. Distribution and nature of these buildups were controlled by movement of basement blocks along paleorift zones, such as the Anping-Xianfeng rift, that experienced resurgent Late Permian movement. For example, one of these buildups, the linear platform-margin Jiantianba reef, formed along the upthrown western side of this rift zone, and thin starved-basin deposits accumulated on the downdropped eastern side of the zone. Paleotopographically high blocks west of the rift became sites of carbonate platforms upon which patch reefs developed in the Late Permian. Carbonate mud mounds developed along gentle margins of the platforms. Calcareous sponges, tabulozoans, hydrozoans, and algae, the latter including Archaeolithoporella, are the dominant reef-forming or baffle-forming organisms in all three types of structures. The Changxing Formation in the area is composed of five lithofacies, including (1) black, thin-bedded, widespread, basinal limestones, (2) light-gray, massive, areally limited boundstones, grainstones, wackestones, and breccias of buildup and fossiliferous carbonate platform facies, (3) dark-gray, medium-bedded limestone, (4) light-gray, structureless, areally limited dolostone, and (5) thin, interbedded shales of probably volcanic ash origin. These facies are arranged in sections that show (1) completely basin facies through the formation in the east, (2) basin facies grading upward to shallow platforms and buildups and then to submerged shelf facies in intermediate areas, and (3) basin facies grading upward to submerged shelf facies successions in western areas. Platform-margin reefs have only a few meters of overlying Permian strata, whereas Permian rocks over the patch reefs on the platforms typically are 10 to 50 m thick. Permian strata over mud mounds that formed on gentle slopes may be as much as 100 m thick. Dolomitization is most extensive in the platform-margin reefs and these contain the best hydrocarbon reservoirs. Dolomitic reservoirs occur most commonly in the upper 50-80 m of these reefs. Dolomitized reef rocks also occur in the patch reefs where the altered zone may be up to 50 m thick. Mud mounds typically have little dolomite and poor reservoir potential.

Facies variation in Waulsortian buildups, Part 1; A model from Belgium

AbstractA composite cross‐sectional model of a Mid‐Dinantian Waulsortian buildup and its lateral equivalents is assembled from 10 measured sections. Five of these are in the Furfooz reef southeast of Dinant and the others, well correlated, are in non‐reef facies in the Dinant and Namur synclinoria (ranging up to an original 100 km or more from the area of the buildups).This model provides the framework for a petrographic study (over 700 samples) in which 25 components, mainly allochems, were recorded. The distributions of 15 of the most interesting of these are figured and discussed. Major patterns, extending well beyond the limits of the Waulsortian facies, can be discerned. Four component assemblages (A‐D) are distinguished. Broadly arranged in strati graphic order (A at the bottom; D at the top), their constituents are.Echinoderm (mainly crinoid) debris, fenestellid bryozoans, and ostracods.As A plus sponge spicules (notably the large, hyalosteliid ones) and moravamminids.As B plus plurilocular foraminifera, small gastropods, intraclasts and peloids.As C plus micritization, cryptalgal coatings on grains, and calcareous algae.The simple A assemblage typifies the classic Waulsortian facies–and many of its lateral equivalents. The others have increasing compositional diversity, with B and C occurring in the Waulsortian buildup facies and assemblage D mainly limited to flank deposits.Many components of these assemblages (calcareous algae, micritization, plurilocular foraminifera, etc.) have distribution patterns showing marked diachronismdownslopeon the flank of the buildup. This is interpreted as representing a lowering of critical depth thresholds for individual components due to a major drop in sea level late in Tournaisian times. Different components shift downwards ‘in step’ at slightly different stratigraphic levels and this allows construction of a relative depth scale. Taking the lower limits of the calcareous algae as a reference level (0 m), other lower limits are, approximately: micritization and cryptalgal coating (‐90 m), plurilocular foraminifera (‐120 m), hyalosteliid sponges (–150 m). This relative scale is tentatively converted into a water depth scale using as criteria the distribution of the calcareous algae, the limit of micritization and cryptalgal coating, and evidence indicating that the upper parts of the buildup were in the shallows at the end of Tournaisian times when the buildup had a sea floor relief of about 220 m.Changes in the depositional environments throughout the formation of the buildup are then traced, from initiation of growth in subphotic depths, probably exceeding 300 m, through shallowing phases possibly leading to emergence.Acknowledgements.We gratefully acknowledge: E. Groessens of the Service Géologique de Belgique for much unpublished information on conodonts, particularly for the sections at Pont‐à‐Lesse and Furfooz 2, 5, and 6; R. Conil and M. Laloux for help with various aspects of the micropalaeontology; V. Leclère for sampling and initial study of the Pont‐à‐Lesse section; L. Hance for information on biostratigraphic problems in the Lower Viséan and for useful comments on the manuscript; and M. Hennebert and J. Miller for critically reading the manuscript.