Ooid Grainstone Research Articles

Waulsortian-Type Bioherm Development and Response to Sea-Level Fluctuations: Upper Visean of Bechar Basin, Western Algeria

ABSTRACT An exceptionally well-developed and well-preserved upper Visean succession rich in sponge-bryozoan-crinoid mounds in the foreland Carboniferous Bechar Basin (northwestern Algerian Sahara) is described and discussed. The succession is composed of recurrent facies mosaics forming individual superposed members. An ideal member is made up of two distinct fades assemblages. The lower assemblage forms the actual mounds, and is composed of sponge bafflestone-wackestone at the base, overlain by massive sponge-fenestellid bafflestone-wackestone, and capped by massive crinoid wackestone with bedded flanks of lithoclastic wackestone. In contrast to massiveness of the mounds, the upper facies assemblage is composed of well-bedded crinoid packstone-grainstone and ooid grainstone, with local rugose coral and algal-foraminiferal banks. On the basis of benthic assemblages and nature of substrate, seven bathymetric zones are defined, and indicate that each member of the bioherm-rich formations is a shallowing-upward parasequence controlled by an asymmetrical transgressive-regressive cycle. The local curve of relative sea level for the late Visean shows 13 cycles interpreted as fourth-order eustatic sea-level changes, each cycle averaging half a million years in duration. Third-order cycles could not be identified. The platform model proposed is a distally steepened ramp, 15-20 km wide, that developed in a rapidly subsiding foreland basin. Lateral distribution of facies on the ramp depended strongly on local tectonic setting, whereas vertical development was controlled by sea-level fluctuation. The deep-water, mud-rich, sponge-bryozoan-crinoid mounds developed in 100-150 m water depths during phases of sea-level highstand, whereas deposition of shallow-water facies occurred on top of the mounds, in less than 70-80 m water depths during regressive phases. The Bechar mounds share similarities with the classical Lower Carboniferous Waulsortian mounds, but they differ in two aspects: the abundance of large sponges, which is unique, and their vertical zonation, wherein basal facies consist of sponge-dominated assemblages. The proposed model can serve as a tool in developing exploration strategies in the search for hydrocarbon reservoirs in the Bechar Basin, because the peculiar architecture of the platform may provide suitable plumbing systems for fluid migration, and the depositional signature of the mounds and associated facies is possibly recognizable on seismic profiles.

Lithostratigraphy and Depositional Environments of the Upper Jurassic Arab-C Carbonate and Associated Evaporites in the Abqaiq Field, Eastern Saudi Arabia

Lithostratigraphy and depositional environment of the Arab-C carbonate in eastern Saudi Arabia were studied using cores and well logs from the Abqaiq oil field. A 275-m Upper Jurassic evaporite sequence occurs between the Sulaiy-Yamama (Thamama Group) and the Tuwaiq-Hanifa-Arab-D (Tuwaiq Group) carbonate sequences. Dolomite and limestone interbeds of 1-30 m in thickness within the evaporite sequence are continuous over long distances and have uniform thicknesses and log characteristics indicating an aggradational type of deposition. The lithologically heterogeneous Arab-C carbonate is the thickest interbed, reaching 26-30 m in thickness. Lithology types recognized in the Arab-C carbonate are (1) ooid bioclast grainstone, (2) oolite grainstone, (3) pellet grainstone, (4) p llet packstone, (5) skeletal wackestone, (6) mudstone, (7) anhydritic limestone, and (8) dolomite. The Arab-C carbonate has been divided into seven layers based on lithology and porosity variations. Four layers (1, 3, 5, 7) are porous calcarenitic carbonates and the other three layers are nonporous. A basinal (subaqueous) rather than a sabkha-type deposition is interpreted for the evaporite sequence. Lithological and sedimentological successions in the Arab-C carbonate indicate abrupt storm-type deposition and an open-marine connection with the restricted evaporite basin. This sequence was followed by a subtidal carbonate deposition and then a gradual upward return to an evaporitic environment. As a consequence of depositional environments, a massive, very porous interval developed in the lower Arab-C carbonate sequence, and alternating porous and nonporous layers developed in the upper Arab-C.

Serpulid-bryozoan-foraminiferal biostromes controlled by temperate climate and reduced salinity: Middle miocene of the Styrian Basin, Austria

Reduced salintiy and a temperate climate prevailed during the Sarmatian in the Styrian Basin, the westernmost embayment of the Central Paratethys. At its northern margin (in the study area) tectonic processes initiated a transgression causing the incision of a cliff into the metamorphic basement and the formation of a carbonate buildup in the latest Sarmatian. The buildup consists of two serpulid-bryozoan-foraminiferal biostromes separated by a microbialite. Serpulids, bryozoa and the encrusting foraminifer Sinzowella caespitosa (Steinmann) as well as microbial mats formed a rigid framework, in which biogenic debris and siliciclastics were baffled. The different biota show complex growth relationships. Stromatolitic crusts and laminated micritic microbial crusts with birdseyes grew in a small sheltered lagoon. The upper biostrome is truncated by a ravinement surface of eustatic orgin and is overlain by ooid grainstone. The buildup drowned during maximum sea-level rise.

Borings in an Oomoldic Rockground, Pennsylvanian of Southeast Utah

The upper surface of an ooid grainstone within the Upper Pennsylvanian Hermosa Formation of southeastern Utah bears abundant borings and is encrusted by syringoporid coral colonies. Aragonite-dominated ooids were cemented on the sea floor by high magnesium calcite and were later leached by meteoric waters to form abundant oomolds. Two lines of evidence show that the bored, encrusted, marine-cemented surface is not a simple synsedimentary hardground: 1) The margins of the micrite-filled borings are mammillate: micrite fills truncated oomolds along the perimeter of the boring. Oomolds (and a leaching episode) therefore predate borings. 2) Rhizoliths within the ooid grainstone (previously interpreted as penetrating from a higher unconformity) are also bored along the encrusted surface, providing corroborative avidence of an ex- posure evnt that predated boring and postdated marine cementation

Paleoceanographic and Paleoclimatic Controls on Ooid Mineralogy of the Smackover Formation, Mississippi Salt Basin: Implications for Late Jurassic Seawater Composition

ABSTRACT The Late Jurassic Smackover Formation in the Mississippi salt basin consists of two 150 m thick shoaling-upward cycles, each capped by ooid grainstones. During deposition of the lower cycle, originally calcite ooids (preserved radial fabric, 18O = -3.8 PDB, 13C = 4.5 PDB, Sr2+ = 315 ppm) formed on the seaward side of the basin and former aragonite ooids (calcitized textures, 18O = -3.0 PDB, 13C = 5.5 PDB, Sr2+ = 1790 ppm) were precipitated on the landward side. In the upper cycle, originally calcite ooids (preserved radial fabric) were precipitated on both the seaward (18O = -2.5 PDB, 13C = 4.2 PDB, Sr2+ = 300 ppm) and the landward (18O = -2.5 PDB, 13C = 3.2 PDB, Sr2+ = 250 ppm) sides of the basin. Because kinetic variables (Mg2+/Ca2+, rate of CO32- supply, PO43-, SO42-) are incapable of totally preventing aragonite formation, we suggest that Smackover calcite ooids were precipitated from seawater with low carbonate saturation state (possibly undersaturated relative to aragonite). The shift from seaward calcite to landward aragonite ooids in the lower cycle was controlled by a shoreward increase in seawater salinity. The net effect of the salinity gradient was a landward increase in the carbonate saturation state in response to decreasing dissolved CO2 and increasing CO32-, Ca2+, and temperature. In seawater supersaturated with respect to both arago ite and calcite, kinetic variables favored dominance of aragonite over calcite. The landward increase in seawater salinity reflects extensive evaporation in an arid climate, resulting in antiestuarine circulation. The monomineralogic (calcite) nature of ooids of the upper cycle suggests that the salinity gradient across the basin was not sufficient to alter the seawater saturation state. This is attributed to a less arid climate and/or a less restricted circulation.

Diagenetic trends of the Pleistocene calcareous ridges, Mersa Matruh area, Egypt

Ooids are a major constituent of the Pleistocene carbonates along the northwestern coastal zone of Egypt. The Mersa Matruh area was selected for a detailed diagenetic study of these rocks. δ 18O, δ 13C and Sr values were determined on whole-rock samples on manually separated ooids and calcite cements. Plerographic examination shows that these rocks are partly consolidated ooid grainstones with low-Mg calcite cement in granular voids. Oxygen isotopic and Sr values for the whole rock (+ 0.5% vs. PDB and 9300 ppm, respectively) are consistent with data on limestones of the same age. In contrast, oxygen isotopic values for ooids (ave. +0.5% vs. PDB) are heavier relative to values for low-Mg calcite cements (ave. −1.4% vs. PDB), and Sr contents are higher for ooids (10,000 ppm). In addition, δ 13C-values are identical for the whole rock and ooids (ave. +4.5% vs. PDB), and heavier relative to δ 13C-values of the calcite cements (ranging from +1.4 to +3.8% vs. PDB). These data may suggest that calcite cements are formed in non-diluted pore water under relatively high rock/water ratios.

The sedimentological and diagenetic evolution of the Ratburi Limestone, Peninsular Thailand

The late Middle to early Late Permian Ratburi Limestone in Peninsular Thailand is a warm-water carbonate deposit formed in an extensive platform setting, suggesting that the Shan Thai craton occupied a subtropical-tropical position at this time. The distribution of facies, from peritidal, fenestral and algal laminated muds, to subtidal skeletal and ooid grainstones, bryozoa boundstones and carbonate slope deposits, is controlled by the platform topography, which is thought to have been controlled by extensional tectonics. Early diagenesis of the platform began with seawater-influenced dolomitisation, followed by uplift, producing karstification, in response to the Late Permian Indosinian orogeny. East dipping subduction of the Indian Ocean Plate resulted in granite magmatism in the peninsula during the Cretaceous. The effect on the Ratburi Limestone included neomorphism, dissolution, calcite cementation and dolomitisation. The petroleum potential of Ratburi Limestone comes from the extensive and long karstic history (Late Permian and Tertiary-present day) and the source potential of the widespread platform carbonate mudstone ( TOC <4.5%) .

Integrative Analysis of a Lacustrine Carbonate Reservoir, Kambala Field, Cabinda, Angola

The presalt lacustrine rift basins of west Africa are the continued focus of exploration and development in this prolific petroleum province. Kambala field produces from the Barremian-Aptian lacustrine carbonates of the TOCA Formation. Source rocks are the rich lacustrine shales of the Bucomazi Formation. TOCA carbonates were deposited along major shorelines and around islands in large lake systems. Reservoir studies provide the necessary models and analogs to guide exploration and evaluation efforts. The TOCA reservoirs in Kambala field have been subdivided into two units. The lower TOCA is characterized by limestones and cherty dolomites, which range texturally from high-energy ooid and oncoid grainstones and packstones to lower energy ostracod wackestones and marls. The upper and lower TOCA are separated by a unit of middle Bucomazi shale. The upper TOCA is composed of thin beds of low-energy facies and thick porous carbonate units, which are pervasively dolomitized. Kambala field sits over a northwest-southeast-trending basement high. The field is bounded by intense faulting on the west and dips gently to the northeast. The faults on the eastern side are more widely spaced and have less throw than the ones on the west. Seismic amplitude maps on the upper TOCA show markedly highermore » amplitudes on the western and southern edges of the field. The high seismic amplitudes and intense faulting generally correlate with the zone of porous dolomite.« less

Carbonate Bank Sedimentation in a Volcaniclastic Arc Setting: Lower Carboniferous Limestones of the Eastern Klamath Terrane, California

ABSTRACT Carboniferous volcaniclastic-arc deposits of the eastern Klamath terrane, California, include Late Visean/Namurian limestone lenses that formed as small carbonate banks. The limestone lenses, within the Bragdon and Baird formations, reach 17 m in thickness and 1.2 km in length. Slope deposits consist of argillaceous spiculitic wackestone, and bank-edge deposits include ooid grainstone, Striatifera packstone, argillaceous phylloid algal packstone, and argillaceous skeletal packstone. Bank-interior deposits include skeletal wackestone/packstone and argillaceous sandy mudstone. The limestone lenses overlie proximal deltaic deposits of thick-bedded volcaniclastic sandstone and conglomerate. Carbonate banks developed on delta lobes during intervals of minimal clastic sedimentation, ossibly related to sea-level rise and volcanic quiescence. The carbonate banks were short-lived depositional systems, and they were covered by prograding deposits of younger volcaniclastic sands.

The Reservoir Potential of the Zechstein-3 Carbonate Member (Platten), Offshore Netherlands

The Platten has been examined from 31 released and nonreleased wells, dominantly from northern P and Q quadrants, Dutch North Sea, and including a limited number of adjacent onshore wells. Wireline log interpretations have been allied with available core log, sidewall core, and ditch sample data. This has allowed a division into four broad facies associations: lower ramp, shoal shoreface, lagoonal, and the dominantly siliciclastic peritidal. Primary porosity is concentrated in ooid grainstones of the shoal/shoreface facies association. Diagenesis has also been examined. While the Platten has been broadly affected by early dolomitization, which generally occluded porosity, later diagenesis was more selective and had a significant control on porosity creation more selective and has a significant control on porosity creation as well as occlusion. Location of Platten reservoir potential is shown by facies association, diagenetic texture, and percentage porosity maps. The best reservoir conditions in the Platten exist where the shoal/shoreface facies association coincides with faults and with diagenesis associated with fluid movements through fractures. Low-temperature hydrothermal fluids are shown in this study to be the most important control on the hydrocarbon potential of the Platten in much of the P and Q quadrants.

Upward-Shoaling Cycles in Smackover Carbonates of Southwest Alabama

ABSTRACT Upper Smackover strata in Alabama commonly consist of one or more upward shoaling cycles ranging from 15 to 50 feet in thickness. These are fourth or fifth order cycles within the third order upper Norphlet to lower Haynesville depositional sequence. Multiple forcing functions (subsidence, salt halokinesis, and autogenic sediment aggradation) and position relative to sea level at the start and end of each cycle generated an array of sedimentary responses. The Brittain No. 1 well illustrates nucleation of an offshore bar. Bar deposits are capped by anhydritic sabkha deposits, gradationally overlain by subtidal lagoonal strata. Varying rates (and directions?) of halokinesis controlled this succession and created as many as five sabkha-capped cycles in the eastern Mississippi interior salt basin. The International Paper Company 20-5 No. 1 well contains three upward shoaling cycles capped by evaporites. Because of the limited aggradational potential of supratidal evaporitic settings subsidence caused immersion, which eventually permitted reactivation of the carbonate factory and formation of the next cycle. The Chatom Unit 20-14 No. 1-04 well contains four different cycles. The lower cycle consists of subtidal lime mudstone, capped by a 5 foot thick paleosol. The paleosol underlies an intraclastic storm deposit followed by a deepening-upward lagoonal succession. A thin ooid grainstone containing exposure surfaces caps the second cycle. The third cycle consists of a deepening upward peloidal carbonate succession capped by closely spaced exposure surfaces. In the upper cycle, peritidal carbonate strata underlie sabkha deposits. The first and third cycles were probably caused by halokinesis; the second and fourth could have been autogenic. Some fourth or fifth order upper Smackover cyclic depositional sequences in southwest Alabama may represent parasequences formed in response to eustatic sea-level change. However, many cycles were caused or influenced by other factors as outlined above.

Shongaloo Field: A Recent Smackover (Jurassic) Discovery in the Arkansas-Louisiana State-Line Graben

ABSTRACT The new North Shongaloo/Red Rock/Haynesville/East Haynesville field (herein called Shongaloo) is the largest discovery in the mature Gulf Coast Jurassic Smackover play in the past 20 years. Field reserves are 20-30 MMBOE, developed from over 50 producing wells. The field is located within the Arkansas-Louisiana state line graben. The previously under-explored graben bisects the mature state line Smackover play, where exploration dates back to the 1940's. Shongaloo field is an upthrown fault trap within the graben. Vertical separation on the trapping fault ranges from 500 ft. to less than 200 ft. The trapping fault can only be resolved on modern, post-1982 seismic data. Shongaloo field pay is from the Smackover C sand and B carbonate. The C sand is overlain by a key transgressive marker horizon, the marine shale. The marine shale is overlain by prograding carbonate cycles including porous ooid grainstones of the B carbonate. The C sand is a tight gas-condensate bearing sandstone which requires hydraulic fracture stimulation for economic completion. B carbonate grainstone reservoirs have average porosity of 10% and average permeability of 60 md, ranging up to 1,800 md. The majority of the field reserves are from the B carbonate. Two technical methods were advanced in the discovery of the field. First, sub-surface correlations and core descriptions of Smackover facies documented that reservoir rocks follow paleo-shorelines across the graben in the Shongaloo area. Regional stratigraphic study indicates that the vast majority of porous grainstone facies within a mappable cycle of the Smackover B are parallel to and immediately down-dip (< 1.5 mi) of paleo-shorelines. The Smackover B facies in the state line area are associated with paleo-shorelines and are not greatly affected by Jurassic fault movements. Second, the critical trapping fault was resolved by shooting and interpreting modern proprietary seismic data. The discovery of Shongaloo Field demonstrates that significant reserves can be found in mature domestic plays by the application of modern technology and stratigraphic concepts.

Sequence Stratigraphy of Middle and Upper Jurassic Strata of Southwestern Alabama

Middle and Upper Jurassic systems tracts of southwestern Alabama differ from those of the western Gulf rim, showing: (1) profound influence of antecedent topography; (2) low early subsidence rates; and (3) greater clastic influx from adjacent uplands. Werner Anhydrite and Louann Salt represent the earliest marine incursion onto the Gulf rim following initial rifting; they onlap upper Paleozoic basement and garben-filling Eagle Mills red beds. Because basin-wide evaporative drawdowns overprint even higher order eustatic sea level changes, transgressive systems tracts (TST) and highstand systems tracts (HST) are indistinguishable. Anhydrite and shale caps accumulated via interstratal halite dissolution. Oxfordian Norphlet siliciclastics form a continental lowstand systems tract as illustrated by abrupt contact with underlying marine evaporites without intervening progradational marginal marine facies. Marine-reworked uppermost Norphlet sandstone marks the base of a subsequent TST, which includes overstepping lower Smackover lithofacies (laminated mudstone, algal-laminated mudstone, and pellet wackestone). The upper Smackover HST is characterized by formation of rimmed shelves upon which algal mounds and aggrading ooid grainstone parasequences accumulated. Shallow lagoonal carbonate and evaporite saltern deposition occurred behind ooid shoals; fine-grained siliciclastics accumulated in updip areas. Equivalents of Smackover A, Smackover B, Bossier, and Gilmer sequences are largely masked by influx of Haynesvillemore » and Cotton Valley continental clastics. Lack of biostratigraphic data, a consequence of restricted fauna, precludes useful age assignments for these sequences in Alabama. Middle and Upper Jurassic systems tracts of southwestern Alabama are regionally atypical and cannot serve as a model for Gulf-wide sequences.« less

Petrophysics of Bimodal Porosity: Lower Cretaceous Rodessa Limestone, Running Duke Field, Houston County, Texas

ABSTRACT The Running Duke field in Houston County, Texas is developed on an east-west oriented domal structure with production from the Lower Cretaceous Rodessa Limestone. The reservoir rocks consist of ooid grainstone, ooid skeletal grainstone and skeletal grainstone. Micritization of the ooids by early fresh water diagenesis has created intragranular microporosity. The ooid grainstone reservoir, therefore has a bimodal pore system which consists of intragranular microporosity and intergranular megaporosity. The presence of the bimodal pore system in the ooid grainstone results in high water saturations (55 to 87 percent), because the intragranular microporosity is water-filled. In order to determine the true productive potential of these bimodal reservoirs the effective water saturation in the intergranular megaporosity and the amount of effective intergranular megaporosity was determined by combining mercury injection and petrographic analysis with log analysis. First the percentages of both the micro- and megaporosities were determined using mercury injection and petrographic analysis. Then total water saturation was corrected to effective water saturation. Uncorrected total water saturations for the ooid grainstones averaged 69.4 percent. The corrected effective water saturations averaged 34.9 percent. The much lower effective water saturations calculated by a combination of log analysis plus mercury injection and/or petrographic analysis indicates the true productive potential of these bimodal reservoirs.

Sea Level Fluctuations and Ooid Diagenesis in the Round Top Palo Pinto Field, Fisher County, TX

The principal reservoir facies in the Missourian Round Top Palo Pinto field are ooid grainstones and skeletal packstones/wackestones which cap shallowing-upward depositional cycles. Individual cycles form flat-bedded, tabular limestones 10-50 ft thick, and at least 15 cycles are identified in the 650-ft-thick interval. Individual cycles are laterally extensive and correlatable throughout most of the field. The cycles exhibit abrupt erosional truncation at the contact with an onlapping Virgilian-age clastic sequence, and intraformational erosional scarps with relief up to 150 ft are present. A complete cycle is comprised of basal black shale/mudstone/wackestone, overlain by crinoid/foram wackestone/packstone, capped by ooid/skeletal grainstone. The composition and morphology of the cycles are the result of the interaction between subsidence, carbonate production rates, and sea level fluctuations. The ooid grainstones are 5-20 ft thick and characterized by oomoldic porosity and intergranular spar cement (porosity inversion). The uppermost 2-5 ft of the grainstones are commonly tightly cemented. The ooid grainstones formed during sea level highstands as carbonate production exceeded subsidence, resulting in aggradation. During subsequent lowstands, freshwater lenses were established on exposed oolite islands. Calcite spar cement filled interparticle space and partially replace ooid cortices. Replacement and dissolution fabrics in the ooids are similar to those reportedmore » from freshwater phreatic lenses of modern oolite islands in the Bahamas.« less

Sequence Stratigraphic Framework of the Upper Jurassic Smackover and Related Units, Western Gulf of Mexico

Post-Norphlet Upper Jurassic subsurface rocks of the western Gulf may be divided into four sequences encompassing some 12 million years (Lower Zuni A-4). The transgressive systems tract of the lower sequence (Smackover) consists of laminated muddy carbonate rocks and is the source for upper Jurassic hydrocarbons. The highstand systems tract of the Smackover consists of blanket ooid grainstones deposited on a platform. The Buckner 'B' begins with a siliciclastic lowstand fan ('C' sand). Highstand deposits of the 'B' consist of subaqueous lagoonal evaporites and an ooid grainstone prograding barrier system. Reflux of Buckner brines into Smackover grainstones resulted in regional dolomitization. The Buckner 'A' is similar to the 'B' except that the lagoon is dominated by siliciclastics. The Gray Sands of northern Louisiana may represent a lowstand fan initiating the 'A' sequence. The last sequence (Gilmer) is dominated by siliciclastics in Arkansas and Louisiana and limestones in east Texas. Its highstand systems tract in Texas is ooid dominated and is similar to the Buckner 'B' and 'A.' The regionally extensive Bossier Shale represents the transgressive systems tract of the next major sequence (Cotton Valley). Jurassic hydrocarbon production is controlled by sequence architecture: porosity of Smackover highstand deposits are dominantly secondary,more » developed by exposure during sea level fall and by dolomitization during the subsequent Buckner 'B' highstand, while Buckner and Gilmer highstand porosity is primary; Smackover traps are structural because of the blanket nature of the highstand system, while Buckner traps are stratigraphic developed in progradational highstand wedges.« less

Diagenetic History, Porosity Development, and Permeability Pathways in Mississippian Greenbrier Oolites of West Virginia

In the Rhodell gas field of southern West Virginia, grainstones of the union and Pickaway oolite members have initial potentials that exceed 2.0 MMCF GPD and reserves of up to 1 bcf per well, Thirty-two wells in the field are productive from the Greenbrier. Porosity is fair with a mean value of 9.5%, and permeability is variable but generally poor, averaging only 0.15 md. The production rate for these wells is directly related to the oolites' diagenetic history. Pores are intragranular and 1-5 {mu}m in size. Ooids underwent recrystallization during meteoric diagenesis, and they now consist of aphanocrystalline calcite. Micropores with relatively large throats and numerous interconnections exist between the very fine calcite rhombs. The pores are arranged in alternating laminae of the ooids' cortex, hinting that the ooids were originally bimineralic. Intergranular pore space has been largely occluded by successive generations of cement, including isopachous calcite rims that precipitated in the marine environment. Following mineral stabilization, some grainstones were subjected to chemical compaction, which produced a fitted fabric. The compacted limestones possess large contact areas between neighboring ooids and, consequently, and enhanced permeability and exceptional production. In contrast, limestones with an early marine cement had the strength to resistmore » compaction, and the porous ooids have small point-to-point contacts. Because the pathways for gas migration are narrow, tortuous, and discontinuous, permeability in these rocks is low. The best reservoirs, therefore, are ooid grainstones that escaped early marine cementation but underwent meteoric diagenesis and chemical compaction.« less

Jurassic Sequence Stratigraphy of the Eastern Gulf Coastal Plain: Applications to Hydrocarbon Exploration

Based on regional stratigraphic and sedimentologic data, three unconformity-bounded depositional sequences associated with cycles of relative sea-level change and coastal onlap are recognized for Jurassic strata in the eastern Gulf Coastal Plain area. These sequences are designated, in ascending order, the LZAGC (Lower Zuni A Gulf Coast)-3.1, the LZAGC-4.1, and the LZAGC-4.2 sequences and include Callovian through Kimmeridgian Stage strata. An understanding of the relationship of Jurassic reservoirs to sequence stratigraphy can serve as an aid to hydrocarbon exploration in the eastern gulf area. The most extensive and productive Jurassic hydrocarbon reservoirs in the study area occur within the progradational, regressive highstand deposits of the LZAGC-3.1 and LZAGC-4.1 depositional sequences. For example, the majority of Norphlet sandstone reservoirs in the onshore and offshore Alabama area are interpreted to have accumulated in eolian dune, interdune, and wadi (fluvial) depositional environments, which occurred in association with the highstand regressive system of the LZAGC-3.1 sequence. The most important Smackover reservoirs generally consist of partially to completely dolomitized ooid and peloid packstones and grainstones in the upper portion of the unit. These reservoirs occur in subtidal to supratidal, shoaling-upward carbonate mudstone to grainstone cycles in the highstand regressive system of the LZAGC-4.1 sequence. Inmore » addition, minor reservoirs that are discontinuous and not well developed are associated with the shelf margin and transgressive systems of the LZAGC-4.1.« less

Cyclic Ramp-to-Basin Carbonate Deposits, Lower Mississippian, Wyoming and Montana: A Combined Field and Computer Modeling Study

ABSTRACT The Lower Mississippian Lodgepole/lower Madison Formations (20-225 m thick) developed along a broad (> 700 km), storm-dominated pericratonic ramp. Three types of fifth-order upward-shallowing cycles are recognized across the ramp-to-basin transition. Peritidal cycles consist of very shallow subtidal facies overlain by algal-laminated tidal flat deposits, which are rarely capped by paleosol/breccia layers. Shallow subtidal cycles consist of stacked ooid grainstone shoal deposits, or deeper subtidal facies overlain by ooid-skeletal grainstone caps. Deep subtidal cycles located along the outer ramp consist of basal sub-storm wave base limestone-argillite, overlain by storm-deposited limestone, which are capped by hummocky stratified to massive skeletal-ooid grainstone. Deep subtidal c cles pass downslope into ramp-slope facies composed to rhythmically interbedded limestone and argillite, with local deep-water mud mounds; no upward-shallowing cycles occur within the ramp-slope facies. Average cycle durations calculated along the outer ramp are between 30-110 ky. The fifth-order cycles are stacked to form three third- to fourth-order depositional sequences, which are recognized by regional transgressive-regressive facies trends and cycles stacking patterns. Ramp-margin wedges (RMW) developed during long-term sea-level fall and lowstand and consist of cyclic crinoidal bank and oolitic shoal facies, which pass downdip into deep subtidal cycles. Transgressive systems tracts (TST), which onlapped the ramp during long-term sea-level rise, include thick shallow and deep subtidal cycles; peritidal cycles are restricted to the inner ramp. Highstand systems tracts (HST) developed during long-term sea-level highstand and fall, and along the ramp are composed of early HST shallow subtidal cycles overlain by late HST peritidal cycles; shallow through deep subtidal cycles characterize the HST along the ramp-slope. Two-dimensional computer modeling of the cyclic sequences suggests that for the assumed water depths of facies, fifth-order sea-level oscillations of at least 20-25 m were required to generate deep subtidal cycles along the ramp-slope. Synthetic sequences run with fifth-order sea-level amplitudes End_Page 1194----------------------- and TST deposition, a feature not observed in the actual sequences. Other factors, in addition to fifth-order sea-level oscillations, likely played a role in generating synchronous peritidal and deep subtidal cycles during HST deposition. These factors may include short-term climatic changes, which influenced the depths to storm-wave reworking. The moderate-amplitude sea-level oscillations suggested by the cyclic sequences may reflect the initial effects of Carboniferous glaciation that was occurring in Gondwana.

Lower Cambrian shelf and shelf margin buildups, Flinders Ranges, South Australia1

ABSTRACTCarbonate buildups in the Flinders Ranges of mid‐Early Cambrian age grew during a period of high archaeocyath diversity and are of two types: (1) low‐energy, archaeocyath‐sponge‐spicule mud mounds, and (2) high‐energy, archaeocyath‐calcimicrobe (calcified microbial microfossil) bioherms. Mud mounds are composed of red carbonate mudstone and sparse to abundant archaeocyath floatstone, have a fenestral fabric, display distinct stromatactis, contain abundant sponge spicules and form structures up to 150m wide and 80 m thick. Bioherms are either red or dark grey limestone and occur as isolated small structures 2–20 m in size surrounded by cross‐bedded calcarenites and calcirudites or as complexes of mounds and carbonate sands several hundreds of metres across. Red bioherms comprise masses of white Epiphyton with scattered archaeocyaths and intervening areas of archaeocyath‐rich lime mudstone. Grey bioherms are complex intergrowths of archaeocyaths, encrusting dark grey Renalcis and thick rinds of fibrous calcite cement. The bioherms were prone to synsedimentary fracturing and exhibit large irregular cavities, up to 1.5 m across, lined with fibrous calcite.The buildups are isolated or in contiguous vertical succession. Mud mounds occur alone in low‐energy, frequently nodular, limestone facies. Individual bioherms and bioherm complexes occur in high‐energy on‐shelf and shelf‐margin facies. The two types also form large‐scale, shallowing‐upward sequences composed of basal (deep water) mud mounds grading upward into archaeocyath‐calcimicrobe bioherm complexes and bioherms in cross‐bedded carbonate sands. The uppermost sequence is capped by ooid grainstone and/ or fenestral to stromatolitic mudstone.The calcimicrobe and metazoan associations form the two major biotic elements which were to dominate reefs throughout much of subsequent Phanerozoic time.