Shoal Deposits Research Articles

Depositional and Diagenetic Controls on Reservoir Quality and Gas Composition in a Mixed Siliciclastic/Carbonate Setting, Thomasville Field, Mississippi: ABSTRACT

The Smackover Formation in central Mississippi is sour gas productive from siliciclastics interbedded with tight carbonates. The sandstones have low reservoir quality (porosity and permeability of 7.0% and 0.35 md, respectively), but wells are capable of producing at high rates and with large volumes because of the extremely large geopressures and locally thick continuous sandstone packages. Production is from a hostile subsurface environment which includes high temperatures (>365{degree}F), high pressures (>0.88 psi/ft), great depths (>20,000 ft), and variable and corrosive sour gas mixtures (CH{sub 4} = 55%, H{sub 2}S = 36%, CO{sub 2} = 9%). Five fields are productive in this trend; Thomasville, the largest field, is used to illustrate the depositional and diagenetic controls on productivity. Deposition occurred as an upward-shoaling sequence of outer ramp to nearshore and sabkha environments. The lower sandstones are bioturbated, low-quality reservoirs interbedded with carbonate mudstones and packstones; the middle interval consists of the thickest and most continuous sandstones that are interpreted to be amalgamated inner-ramp shoal and ridge deposits; the upper interval consists of the common but tight Smackover ooid grainstone shoals that are interbedded with continuous but thin shoal and shoreface sandstone. The middle interval has the best reservoir quality and contains more » 75% of the hydrocarbons. Seismic may be used to map the sandstone packages but not the individual sandstones. In addition to the larger net/gross and more continuous sandstones in the middle zone, two postdepositional controls also govern the reservoir quality. These include small-scale faulting but more importantly diagenetic events which strongly overprint but do not obscure the original depositional control. At least 15 different diagenetic events have occurred in the sour gas fields that have generally decreased reservoir quality. « less

Intraplatformal basin-fill deposits from the Infracambrian Huqf Group, east Central Oman

Abstract The Infracambrian Huqf Group, well exposed in the Huqf-Haushi area of Central Oman, is an alternating sequence of clastic (Abu Mahara and Shuram Formations) and carbonate (Khufai and Buah Formations) sequences overlain, in the subsurface, by the evaporitic Ara Formation. The Khufai and Buah Formations are dolomites whose lowest members have previously been interpreted as lagoonal deposits; they are here reinterpreted as sub-wave base deposits which were probably formed in an intraplatformal basin. The upper parts of both formations consist of shoal and back shoal (peritidal) deposits with evaporites. The intervening Shuram Formation is a mixed siliclastic-carbonate unit deposited in an extensive shallow wave and tidally-influenced setting.

Inhomogeneities, Paleomorphology, and Architectural Elements in Shoal Deposits of Salem Limestone (Mississippian) in Southern Indiana: ABSTRACT

Inhomogeneities, Paleomorphology, and Architectural Elements in Shoal Deposits of Salem Limestone (Mississippian) in Southern Indiana: ABSTRACT

Facies analysis and palaeogeography of the Rytteråker Formation (lower Silurian, Oslo region, Norway)

The Llandovery Rytteråker Formation is a carbonate-dominated lithologic unit which extends over the whole Oslo Region. Eight composite sections reveal five characteristic facies associations. These represent different, laterally coexisting depositional settings of an incipient, storm-dominated carbonate platform, which developed out from the transition zone between an open and a restricted marine basin. A mixed carbonate/siliciclastic shelf-lagoonal unit, a bioclastic shoal/barrier unit, a bioherm unit, a tidal delta/shallow shelf association and a mud-dominated deeper neritic unit are presented with their typical inventory of sedimentary structures. A possible tidal inlet fill sequence is reported from the Skien district where it is incised into bioclastic shoal deposits. Biostratigraphic correlation points to diachroneity of facies units and reveals their lateral and vertical association as being due to the migration of depositional belts. General palaeogeographic implications of the observed facies pattern are discussed and an attempt is made to place them into their global palaeoclimatic and geotectonic context.

Structure, age and origin of the bay-mouth shoal deposits, Chesapeake Bay, Virginia

The mouth of Chesapeake Bay contains a distinctive shoal complex and related deposits that result from the complex interaction of three different processes: (1) progradation of a barrier spit at the southern end of the Delmarva Peninsula, (2) strong, reversing tidal currents that transport and rework sediment brought to the bay mouth from the north, and (3) landward (bayward) net non-tidal circulation and sediment transport. Together, these processes play a major role in changing the configuration of the estuary and filling it with sediment. The deposits at the mouth of the bay hold keys both to the evolution of the bay during the Holocene transgression and to the history of previous generations of the bay. The deposit associated with the shoals at the mouth of the bay, the bay-mouth sand, is a distinct stratigraphic unit composed mostly of uniform, gray, fine sand. The position and internal structure of the unit shows that it is related to near-present sea level, and thus is less than a few thousand years old. The processes affecting the upper surface of the deposit and the patterns of erosion and deposition at this surface are complex, but the geometry and structure of the deposit indicate that it is a coherent unit that is prograding bayward and tending to fill the estuary. The source of the bay-mouth sand is primarily outside the bay in the nearshore zone of the Delmarva Peninsula and on the inner continental shelf. The internal structure of the deposit, its surface morphology, its heavy-mineral composition, bottom-current studies, comparative bathymetry, and sediment budgets all suggest that sand is brought to the bay mouth by southerly longshore drift along the Delmarva Peninsula and then swept into the bay. In addition to building the southward- and bayward-prograding bay-mouth sand, these processes result in sand deposition tens of kilometers into the bay.

Carbonate Facies and Stratigraphic Framework of Middle Magdalena (Middle Pennsylvanian), Hueco Mountains, West Texas: ABSTRACT

The middle Magdalena of the Hueco Mountains, west Texas, is best exposed in the vicinity of Pow Wow Canyon, particularly along the western scarp of the range. It can be divided into two major depositional sequences, the lowermost of which consists predominantly of alternating bank, interbank, and shoal deposits of Atokan age. These banks are low-relief accumulations of Donezella, with coeval slackwater skeletal wackestones and interbedded deposits of foraminiferal sands. A prominent zone of intertonguing Chaetetes biostromes punctuates the middle part of this sequence. The Desmoinesian sequence begins as a series of rubbly limestones in association with abundant silicified plant remains and is interpreted as a set of paleosols. These are overlain by argillaceous, low-diversity wackestones and packstones of lagoonal origin, followed by carbonates of more open-marine circulation with abundant corals and other stenotypic fossils. In turn, these deposits are succeeded by a sequence of phylloid algal banks that increase in resistance upward to where they represent strata of the upper Magdalena. At this locality, however, the upper Magdalena is very thin because it is truncated by the pre-Hueco unconformity which, so prominent at the head of the canyon, can now be traced to the western scarp. These deposits aremore » directly analogous to subsurface reservoir facies of the same age on the opposite side of the Diablo uplift in the Permian basin and thus provide an opportunity to generate reservoir models based on extensive outcrop exposure.« less

Facies relations, depositional environments and diagenesis in a major early Proterozoic stromatolitic carbonate platform to basinal sequence, Campbellrand Subgroup, Transvaal Supergroup, Southern Africa

The Campbellrand Subgroup and its correlative, the Malmani Subgroup of the Transvaal Supergroup, represent a major 2300–2600 Ma old carbonate buildup. It is on average between 1500 and 1700 m thick and covers an area of approximately 190,000 km 2 on the Kaapvaal craton. The original depository probably extended across the entire 600,000 km 2 surface area of the craton and beyond. The buildup consists essentially of two major lithofacies assemblages, namely a basinal, non-stromatolitic, laminated carbonate and shale sequence (with minor chert, iron-formation and mafic tuff interbeds) off the craton to the west, and a shallow-water stromatolitic carbonate sequence on the craton proper. Growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite, oolite and carbonate arenite shoal deposits. These are flanked basinward by “reef-like” giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites. Platform lagoonal deposits consist of dark grey, laminoid fenestrate, stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal, light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites, laminoid fenestrae, tepee structures and flat pebble conglomerates. The buildup consisted primarily of limestone which was replaced by early diagenetic dolomite. The chemical composition of the dolomite is related to depofacies. Platform margin and lagoonal dolomites are manganese-rich, basinal dolomites are iron-rich and intertidal to supratidal deposits are virtually free of iron and manganeses. Indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf. Eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the Kuruman iron-formation.

Deposition and Diagenesis of a Pettet Reservoir at Lisbon Field, Claiborne Parish, Louisiana: ABSTRACT

ABSTRACT Rocks of the Pettet Zone at Lisbon Field consist of three shoaling upwards oolite cycles that were deposited over a paleotopographic high as dip trending, lenticular sand bodies. The lower 5 feet of the ooid shoals consist of superficial ooid packstones and grainstones. These rocks are succeeded vertically by ooid packstones and grainstones. Adjacent to the shoals are marine rocks consisting of intraclast, ooid, bivalve packstones and oyster wackestones and packstones. The open marine shelf downdip from the shoals and the marine lagoon updip from the shoals were blanketed by skeletal mudstones and wackestones. Shoal deposition is interpreted to have started during periods of relative sea-level rise. The basal two oolites are capped with unconformities that marked sea-level retreats. During these retreats, the topographically high shoal areas were partially exposed as islands, and silty shales were deposited as laterally equivalent facies in a quiescent marine environment. Diagenetic alteration began in the marine environment and continued into the subsurface. Marine diagenesis includes mainly micritization of grains in the oolites. Later, vadose and freshwater diagenesis occurred during the periods of exposure. Aragonitic allochems were selectively dissolved, and magnesian calcites were altered to low magnesium calcite. Lime mud underwent aggradational neomorphism, and columnar to blocky calcite rim cements were precipitated in the lower portions of the leached, sand buildups. Selective dissolution of aragonite produced abundant moldic porosity in the superficial ooid packstones and grainstones where the majority of the ooid nuclei were composed of mollusk fragments. In contrast, many ooid corticies around the moldic voids were preserved, suggesting that the corticies were of a different mineralogy--probably Mg-calcite. Compaction and attendant brittle fracture are common in the moldic superficial ooid facies, but microstylolitic grain contacts mark the more resistive regular ooid facies. Stylolitization is common in clay rich facies adjacent to the shoals. Virtually all of the diagenetic and primary porosity was partially filled by coarse calcite cement at this time. Subsequently, minor replacement and cementation by saddle dolomite and anhydrite occurred along with late dissolution. These features are most common in the lower two oolitic zones, and they characterize the deeper subsurface environment. The late dissolution event enlarged much of the porosity that had been partially occluded by the pore-filling calcite mosaic spar. Late leaching has significantly enhanced reservoir porosity. The reservoir at Lisbon Field consists primarily of facies selective, diagenetic porosity. Exploration and development in similar situations will require careful petrographic study combined with conventional stratigraphic mapping. End_of_Record - Last_Page 109-------

Carbonate Platform Sedimentology, Upper Cambrian Richland Formation, Lebanon Valley, Pennsylvania

ABSTRACT The Richland Formation is exposed on both limbs of the Lebanon Valley nappe in southeastern Pennsylvania. The nappe probably contains the farthest transported carbonate platform rocks of the Cambro-Ordovician continental margin sequence. Intertidal to deep subtidal environments are represented. Intertidal deposits include cryptalgalaminates and tabular-laminated dolomites. Ooid grainstones, in part altered to coarsely crystalline dolomites, are shoal or beach deposits. Subtidal deposits include mudstones, peloid packstone/wackestones, thrombolites, and domal stromatolites. Ribbon carbonates were probably deposited in an open-platform setting below fair-weather wave base. Lithofacies occur in cyclic peritidal, subtidal, and thrombolitic facies associations. Facies sequences on each limb are reciprocally related. Upper-limb rocks occupied a more seaward platform position. The upper limb features a 180-m-thick cyclic peritidal sequence followed by 320 m of deep to shallow subtidal deposits. On the lower limb a 300-m-thick cyclic subtidal sequence is succeeded by 100 m of cyclic peritidal deposits. The contrasting facies successions on each limb imply two major depositional phases. Phase one featured a shelf system with a broad shoal complex (upper limb) situated in front of or beside lagoon or embayment deposits (lower limb). In the second phase, deep subtidal conditions spread across the area of the upper limb while cyclic peritidal facies were established in the lower-limb setting. The upper-limb shoal complex probably was incipiently drowned during a transgression that culminated in a late Cambrian-early Ordovician sea-level highstand.

Shelf to Basin Transition in Middle Ordovician Carbonates in Alabama Appalachians: ABSTRACT

In the Alabama valley and ridge, Middle Ordovician carbonates are exposed in two northeast-southwest-trending outcrop belts separated by the Helena fault. Northwest to southeast transits across these outcrop belts illustrate a well-defined shallow to deep water transition. West of the Helena fault, the Middle Ordovician is represented by a transgressive-regressive sequence of peritidal and shallow subtidal carbonates of the Chickamauga Limestone, deposited in tidal-flat, low-energy open-shelf, and high-energy shoal environments. Tidal-flat deposits consist of peloidal and intraclastic wackestones and packstones containing abundant exposure indicators. These grade into light-colored, skeletal wackestones and packstones containing a diverse faunal assemblage rich in algae, indicating deposition in a shallow, low-energy, open-shelf setting. High energy shoal deposition is represented by a 20-80 ft thick sequence of cross-bedded skeletal grainstone. Included within the grainstone are pods of bryozoan-sponge-algal bafflestone and bindstone that represent small mud-rich bioherms. East of the Helena fault, the Middle Ordovician series consists of deeper water carbonates of the Lenoir and Little Oak Limestones and graptolitic shales of the Athens Formation. The Lenoir and Little Oak are composed of dark-colored, even-bedded, skeletal wackestones which, with the exception of scattered Nuia, lack algae, indicating deposition in the deeper part of the photic zone. Thesemore » wackestones grade southeastward into very finely laminated, argillaceous mudstones and calcareous shales of the Athens Formation. The Athens is dark colored, organic-rich, lacking in bottom-dwelling fauna, and contains common synsedimentary slump structures suggesting deposition in an anoxic, lower slop environment.« less

Trinity Shoal: a Reworked Deltaic Barrier on Louisiana Continental Shelf: ABSTRACT

Abandonment and reworking of deltaic complexes of the Holocene Mississippi River have produced a series of sandy shoals on the muddy Louisiana continental shelf. Trinity shoal, one of these transgressive deposits, is located 30 km offshore of Atchafalaya Bay and the Point AuFer-March End_Page 309------------------------------ Island shell reefs. Approximately 1,000 km of high-resolution uniboom and 3.5 kHz subbottom-profile seismic data, taken in this area in 1983 and 1984, provide the data base for this study. Trinity shoal, associated with the abandoned Maringouin delta complex, is a lunate shore-parallel feature approximately 36 km long and 5-10 km wide. Relief on the shoal ranges from 2 to 3 m, and minimum water depths over the shoal vary from -5 to -2 m. The shoal sand body is from 5 to 7 m thick and is composed largely of parallel to low-angle clinoform reflectors. Several levels of buried fluvial channels, ranging in age from early Wisconsinian to Holocene, are associated with the shoal deposit. The occurrence of channel features within the shoal sand itself suggests the presence of tidal inlets, indicating a possible barrier-island origin for the shoal. The underlying deltaic sediments reach approximately 15 m in thickness and are made up of low-angle clinoform reflectors dipping to the southwest. Distributary, bay-fill, estuarine, and buried oyster-reef deposits can be recognized, making these similar to modern Atchafalaya delta deposits. Continued progradation of the Atchafalaya delta will probably result in burial of the Trinity shoal and Maringouin delta deposits by fine-grained sediments, giving these shoal deposits a high-preservation potential and creating an excellent stratigraphic trap. End_of_Article - Last_Page 310------------

Petroleum Potential and Stratigraphy of Holitna Basin, Alaska: ABSTRACT

The Holitna basin, an interior Alaskan basin, is flanked by Cambrian to Cretaceous sedimentary rocks that have been highly folded and faulted. Gravity mapping and modeling indicate up to 15,000 ft of sedimentary section is present within the basin. Cambrian rocks consist of trilobite-hash lime mudstone, red siltstone, and basinal chert. Ordovician through Devonian basinal facies rocks consist of platy limestone to graptolitic shale with minor interbeds of limestone turbidites and turbidite-channel debris flows. Ordovician through Middle Devonian shallow-water platform carbonate rocks prograde over the basinal rocks and are composed of algal boundstone buildups with associated inboard lagoonal, oolitic shoal, and tidal-channel deposits. Toward the interior of the platform, restricted subtidal and intertidal to supratidal carbonate rocks were deposited. Upper Devonian to Permian platform carbonate deposition occurred to the east of the older platform rocks and conformably(?) over basinal rocks. Minor mixed carbonate-clastic deposi ion occurred into Triassic time. The Cretaceous Kuskokwim Group is in fault contact with and/or unconformably overlies the Paleozoic carbonates. This unit varies from lithic-rich conglomerates to marine turbidite deposits. Maturation values for Cretaceous and Paleozoic rocks are within the oil window, with most of the shales showing a thermal alteration index (TAI) from 2 to 3. Organic carbon content exceeds 3% in some samples, however, deep surface weathering has resulted in low hydrocarbon values. The lithic-rich Cretaceous sandstones are well indurated, poorly sorted, and are considered to have low reservoir potential. Partly dolomitized, shallow-water Ordovician to Devonian carbonate rocks are the best potential reservoir rocks exhibiting vuggy porosities greater than 10% and good intergranular porosity. End_of_Article - Last_Page 308------------

Stratigraphy of Yorktown (Lower and Middle Pliocene) and Chowan River (Upper Pliocene) Formations in Southeastern Virginia: ABSTRACT

The Yorktown and Chowan River formations exhibit vertical and lateral variations in lithology and fauna. The Yorktown, which rests disconformably on the Eastover Formation (upper Miocene), is comprised of a fossiliferous shallow marine sand with a discontinuous basal pebbly sand (Sunken Meadow Member). The upper Yorktown consists of intertonguing and intergrading shelf, shoal, and restricted marine deposits (Rushmere, Mogarts Beach, Moore House, and a possible unnamed uppermost member). The shelf deposits are principally fossiliferous, marine silty fine sand, and the shoal sediments are planar and cross-bedded biofragmental sands. The restricted marine sediments range from silty fine sand to silty clay and contain a limited fauna. Near the Fall Zone, the Yorktown is compr sed of nonfossiliferous quartz-rich sand. Differential upward movement of the outer Atlantic coastal plain created an offshore shoal, west of which were embayed conditions. The Bacons Castle, Windsor, and higher formations rest with a regional angular unconformity on the Yorktown. The Chowan River formation is restricted to the outermost coastal plain in Virginia and is composed of planar and cross-bedded shelly fine sand. The basal Chowan River contains scattered allochthonous pebbles and boulders, sideritic nodules, and brackish to marine fossil assemblages. The Yorktown and Chowan River formations represent the last incursions of relatively warm seas into southeastern Virginia during the late Tertiary. End_of_Article - Last_Page 1438------------

Prolific Overton Field Gas Reservoirs Within Large Transverse Oolite Shoals, Upper Jurassic Haynesville, Eastern Margin East Texas Basin: ABSTRACT

Late Triassic rifting along a northeast-southwest spreading center in east Texas resulted in basement highs along the eastern margin of the East Texas basin that became sites of extensive ooid shoal deposition during Late Jurassic time. Reservoirs within oolite facies at Overton field contain over 1 tcf of natural gas. These large shoals, each approximately 15 mi (24 km) long and 3 mi (4.8 km) wide, trend north-south as a group and northeast-southwest individually. They are oblique to the basin margin but parallel with Jurassic diffracted tidal currents within the East Texas embayment. Modern Bahamian ooid shoals of similar size, trend, and depositional setting occur at the terminus of the deep Tongue-Of-The-Ocean platform reentrant. Overton field reservoirs are in ooid g ainstone shoal facies and in transitional shoal margins of skeletal-oolitic-peloidal grainstones and packstones. Adjacent nonreservoir facies are peloidal-skeletal-siliciclastic wackestones and mudstones. Early diagenesis of grainstone reservoir facies included meteoric dissolution and grain stabilization, resulting in abundant chalky intraparticle porosity and equant and bladed calcite cements filling interparticle porosity. Subsequent burial diagenesis resulted in intense solution compaction and coarse equant calcite and saddle crystal dolomite that occluded remaining interparticle porosity. Whole-rock trace element analysis indicates greatest diagenetic flushing (less magnesium, strontium) in porous zones. Stable isotopes for grains and cements show strong overprint of later burial diagenesis, with greater depletion of ^dgr18O in reservoir facies. However, hydrocarbons were emplaced prior to late cementation, and unlike other Jurassic Gulf Coast reservoirs, deep burial iagenesis provided no late-stage formation of porosity. End_of_Article - Last_Page 257------------

Carbonate Facies and Landsat Imagery of Shelf Off Belize, Central America: ABSTRACT

A reevaluation of Holocene sediments on the Belize shelf is based on (1) a newly constructed composite of 7 Landsat images, enhanced and registered to form a regional base map, and (2) a Holocene facies map based on a rigorous treatment of compositional and textural parameters for approximately 600 bottom samples. The sediments are mapped in terms usually applied to lithified carbonate rocks, allowing direct comparisons with carbonate facies in the subsurface. By combining Landsat imagery with this facies map, it is possible to point out the following geologic features: (1) major tectonic elements, such as the Maya Mountains, the Yucatan Plateau, several offshore ridges, and 3 large atolls, (2) major physiographic features such as the Belize barrier reef with its reef platform and crest, middle-shelf shoal deposits, middle-shelf patch reefs (including lagoon reefs or rhomboid reefs), (3) Holocene facies patterns with potential reservoir facies of foraminifera-grainstone bars, Halimeda grainstones, and branching-coral, encrusting red-algae boundstones, and (4) nearshore clastics and a sharp transition eastward to carbonate sediments. An understanding of Holocene facies patterns on the Belize shelf is important to the explorationist, because these facies patterns are living examples of exploration fairways and invite comparisons with several petroleum provinces: (1) Cretaceous reefs of Texas, (2) upper Paleozoic skeletal-grainstone bars in west Texas, and (3) Devonian reefs of the Alberta basin. End_of_Article - Last_Page 271------------

Tidal deposits of carbonate facies and their microfacies types in the Qiziqiao Formation of Middle Devonian, South China

Tidal-flat and shoal deposits of carbonate fades in the Qiziqiao Formation are widely distributed over the vast areas of Guangxi, Guangdong and Hunan provinces, constituting an important stratigraphic unit where strata-bound and stratiform ore deposits (galena-sphalerite-pyrite) are found. These types of ore deposit seem to have close relations to the tidal deposits. Recognition of tidal deposits is based upon rock fabric, texture, structure, fossil assemblage, and particular sedimentary cycle. The typical sequence of tidal-flat deposits consists mainly of three units:Amphipora limestone, laminated limestone and dolostone (from the bottom to the top). This sequence represents a complete process of sedimentation from low-tideflat through intertidal to high-tideflat or supratidal. The sequence of shoal deposits of carbonate facies consists chiefly of grainstone and algal oösparite.Two major types of tidal deposit (open and restricted marine facies) can be distinguished, with eleven microfacies as follows: 1) calcarenite (grainstone) with sparite (MF-1); 2) algal oösparite (MF-2); 3) oncolite (alga-, or stromatoporoid-encrusted grains) (MF-3); 4) bioclastic grainstone (biosparite) or rudstone with sparite (MF-4);5) Amphipora limestone (MF-5); 6) dark fossil-poor micrite (MF-6); 7) pelsparite or peloidal grainstone with sparite (MF-7); 8) laminated pellet mudstone-wackstone (MF-8); 9) micrite with onkoids (MF-9); 10) rudstone or floatstone (MF-10; and 11) bedded dolomite-gypsum-dolomite (MF-11).

Depositional History of Smackover Carbonates in Manila Embayment of South Alabama: ABSTRACT

Upper Jurassic Smackover carbonate deposition in south Alabama was significantly influenced by a number of paleotopographic highs which divided the region into several subbasins or embayments. The Manila embayment in Monroe, Conecuh, and Clarke Counties was bounded to the south by the Conecuh ridge and the Wiggins arch and to the west by diapiric salt ridges. These paleotopographic highs protected the interior of the embayment from wave and current energy and significantly influenced Smackover deposition. Initial carbonate deposition, represented by intertidal oncolitic and peloidal packstones and algally laminated mudstones, occurred in the deeper portions of the embayment, and was accompanied by the continuation of subaerial clastic deposition on the still-emergent highs. With sea level rise, these intertidal deposits graded upward into subtidal peloidal packstones containing sparse faunal assemblages indicative of the restricted circulation within the embayment. By middle Oxfordian time, sea level rise had inundated the paleohighs, eliminating restrictive conditions in the embayment. Deposition in the embayment is represented by peloidal, oolitic, and oncolitic packstones and grainstones containing open marine faunal assemblages, while the paleohighs became the site of oolitic shoal with local patch-reef development. A decrease in the rate of sea level rise during late Oxfordian time allowed the carbonate deposits to build toward sea level. The oolitic shoal deposits on the paleohighs pass upward into evaporitic sabkha deposits, whereas the deposits of the embayment grade into oolitic shoal and intertidal flat deposits and finally into sabkha and evaporitic lagoon deposits. End_of_Article - Last_Page 468------------

Shelf Morphodynamics of Drowned Barriers on Louisiana Shelf: ABSTRACT

Since the 1950s, Mississippi-delta researchers have hypothesized that Ship and Trinity Shoals, on the Louisiana continental shelf, were formed during the transgressive phase of the delta cycle following the abandonment of the Maringouin delta lobe during the Holocene. Historic, stratigraphic, and sedimentologic data strongly support this theory and suggest that the shoals originated through the drowning of delta-flank barrier islands. Maps published prior to 1840 show islands in the approximate location of the present shoals. Following submergence, the barrier/shoal system was subject to extensive modification by shelf currents. A comparison of U.S. Coast and Geodetic hydrographic surveys (1853, 1889-90, 1936) and a 1983 survey conducted by the authors illustrate that the shoals have migrated landward up to 2 km (1.2 mi) and been reduced in subaqueous elevation up to 3 m (10 ft). The overall trend is of erosion at the shoal crest and deposition seaward below 10-m (33-ft) water depth. Large scale ridge and swale features are also present in water depths below 6 m (20 ft). The shoals are composed of clean, well-sorted, fine-grained sands essentially devoid of bedforms detectable by side-scan sonar. Three days after Hurricane Alicia's passage within 160 km (100 mi) south of Ship Shoal, there were still no noticeable bedforms. This suggests that hurricanes may be of too short a duration to initiate shelf-wide flow of a magnitude great enough to significantly affect shelf-shoal sediment dispersal, at least in the shoals' present state. Ship and Trinity Shoals are still being modified today, and as long as they remain highs on the sea floor they will continue to be modified by shelf currents. It is believed that the original barrier morphology played an important part in the development of the present morphology. It is inferred that the presence of the original barrier chain, its composition, and its orientation relative to dominant wave and current conditions controlled the development of the present morphologic features. The only hope of preserving the present shoals would be if they were covered by prodelta deposits of another progradational cycle. Otherwise Ship and Trinity Shoals will continue to be modified until they reach a stable configuration for the Louisiana continental shelf. End_of_Article - Last_Page 468------------

Adell Field and Vicinity--Sheridan and Decatur Counties, Kansas: ABSTRACT

The Adell field in northeastern Sheridan County, Kansas, and other smaller fields in the area, produce oil from multiple zones of the Lansing-Kansas City. Porous zones of this age are shoal deposits consisting of rounded carbonate grains, many of which have superficial oolitic coating. Carbonate grain distribution was controlled by water turbulence and relief on the sea floor. The Jennings anticline existed during Lansing-Kansas City deposition and provided the necessary relief on the sea floor for shoals to occur. Differential rates of sea-floor subsidence and cyclic nature of sediment accumulation account for the multiple porous pay zones which are stacked on top of each other. End_of_Article - Last_Page 1323------------

THE PETROLEUM POTENTIAL OF THE EASTERN ARROWIE BASIN AND FROME EMBAYMENT

The eastern Arrowie Basin is a descriptive term for the present-day structurally defined Cambro- Ordovician basin approximately coinciding with the Curnamona Shelf depositional feature which formed on the eastern stable flank of the Adelaide Trough. Remnants of the Arrowie depositional basin are preserved in outcrops of the Flinders Ranges and in the subsurface to the east and west of the ranges. The eastern subsurface sub-basin occupies the area around Lake Frome in South Australia and across the border in New South Wales. Much of the eastern sub-basin is overlain by the Jurassic and Cretaceous rocks of the Frome Embayment of the Eromanga Basin.The area has been explored sporadically over at least the past 20 years but no deep wells have been drilled to test exploration targets. This assessment of the prospectivity of the basin is assisted by shallow stratigraphic and mineral drilling, by extrapolation from outcrop, and by regional geophysics.In the deepest part of the eastern sub-basin the total thickness of the Cambro-Ordovician sediments is expected to exceed 2500 m. Potential reservoirs can be expected within both carbonate and sandstone rocks. Good secondary porosity exists in sandstones of the Middle to Late Cambrian Lake Frome Group and good porosity development is predicted within possible ooid shoal deposits.Source material has been demonstrated in outcrop samples with Total Organic Carbon measured at up to 0.35 per cent. It is suggested that petroleum generation probably occurred in Cambrian sediments in the Adelaide Trough (now the area of the Flinders Ranges) and the petroleum migrated east and west to the stable shelf areas. Sufficient depth of burial may also have been attained on the Curnamona Shelf to allow local generation.In the past, the very poor quality of available seismic data has frustrated the mapping of the subsurface in sufficient detail to justify exploration drilling. Recent experimental seismic surveys, however, have produced data of a much better quality. Potential exists for anticlinal, fault and unconformity traps. There is also some potential for stratigraphic traps such as ooid shoal deposits. At this stage an exploration programme to evaluate the potential of this area is being carried out by the licence holders.