Laminated Lime Mudstone Research Articles

Microfacies, diagenesis and hydrocarbon potential of the Neoproterozoic cap carbonate of the southern Amazon Craton

The low-latitude Marinoan glaciation (635 Ma) record in the southern Amazon Craton has been one of the most important sites with strong evidence for the Neoproterozoic Snowball Earth hypothesis. In Central Brazil, the 50-m-thick carbonate succession of the post-Marinoan cap carbonate exposed in the Tangará da Serra region, Central Brazil, represents a complementary section of the classical occurrence of the Puga cap carbonate in Mirassol d'Oeste region, that overlies glaciogenic deposits marking the well-preserved Cryogenian-Ediacaran boundary in South America. The 8-m-thick cap dolostone that represents the Mirassol d'Oeste Formation consists of pinkish microcrystalline peloidal dolomudstone, exhibiting even parallel to low-angle lamination locally tubestone, and megaripple bedded dolopackstone. The cap dolostone is succeeded upsection by the cap cementstone limestone (basal Guia Formation), comprising laminated lime mudstone rich in silt-sized terrigenous grains (quartz), and subordinately bituminous shale, dolomitic marls, intraformational breccias, and abundant calcite crystal fans and crusts. Crystal fans occur in isolated and decimetre-scale arrays and are considered aragonite pseudomorphs. Primary precipitation of dolomite, crystal fans, and micrite and the formation of tubestone are related to depositional processes. The cap dolostone was affected by the neomorphism of the dolomicrite, development of vug and intercrystalline porosity, euhedral dolomite precipitation, synsedimentary fracturing, calcite and quartz precipitation, chemical compaction, Fe-oxide substitution, and precipitation. The diagenetic processes of the cap cementstone limestone were pyrite precipitation, calcite cementation, neomorphism of micrite and crystal fan, partial crystal fan dissolution, dolomitization, chemical compaction, fracturing, ferrous dolomite and calcite precipitation, pyrite (pseudomorph) and micrite replacement with Fe-oxide and replacement of rhombohedral dolomite by Mn-oxides. Hydrocarbons are rare and its migration represents the last diagenetic event being found: 1) in fractures and interpeloidal, vug and intercrystalline pores (~2%) with low permeability and connectivity of the cap dolostone; and 2) in intercrystalline pores, fractures, and rare pores in crystal fans of the cap cementstone limestone.During the post-Marinoan sea-level rise occurred the establishment of a shallow to moderately deep microbially induced dolomitic platform locally influenced by storm-action. The progressive transgression led the implantation of a deep anoxic and CaCO3-oversaturated aragonitic platform. The similarities of cap carbonate microfacies among the post-Marinoan succession worldwide reflects an extraordinary and non-recurring global precipitation event in geological history. In contrast, besides the majority of cap carbonate have the same textural and compositional framework, the diagenetic history seems to be regional and completely dependent on the tectonic, thermal, and burial history of the depositional basins where they were deposited.

Sequence Stratigraphic Context of Extensive Basin-Margin Evaporites: Middle Jurassic Gypsum Spring Formation, Wyoming, U.S.A.

Abstract Ancient marine evaporites are commonly far more extensive than any known modern evaporite system. This is true not only for basin-central saline giants, but also for evaporites that occur in basin-margin settings, and the origin of these basin-margin systems is unclear. Extensive basin-margin evaporites of the Middle Jurassic Gypsum Spring Formation are well exposed along the eastern flank of the Bighorn Basin in Wyoming, USA, and a facies and sequence stratigraphic analysis was undertaken to understand the origin of these evaporites and associated deposits. The Gypsum Spring Formation was deposited on a northwestward-dipping mixed evaporate–carbonate–siliciclastic ramp, with facies and depositional environments that include distal ooid shoal grainstone, open-marine shallow-subtidal peloidal–skeletal wackestone to packstone, restricted shallow subtidal laminated lime mudstone, peritidal dolostone and microbialite, as well as red mudstone and gypsum deposited on mud flats, sabkhas, and salinas. Three depositional sequences are present, with the J-1, J-1a, and J-1b sequence boundaries characterized locally by chert-pebble lags, silicified horizons, and iron-rich paleosols. Evaporites accumulated in a basin-margin position, with 1–6 m beds of gypsum forming in salinas and sabkhas, and nodular anhydrite and gypsum forming in red mudstones deposited on sabkhas and mud flats. The limited lateral extent of salina-type gypsum beds and the variable number of these beds among the exposures indicates substantial autocyclic control on the formation of salinas. Salinas are well developed in the TST of the J-1 sequence, and sabkhas and mud flats are well developed in the J-1 HST, the J-1a HST, and the J-1b TST. The great lateral extent of the Gypsum Spring evaporites reflects the original expanse of the depositional environment in which they formed, and is not the result of diachroneity of a narrow evaporite-precipitating belt, such as the modern-day coastal sabkhas of the Trucial coast. The great lateral extent of Gypsum Spring evaporites suggests that they were precipitated mainly from continental brines on an expansive desert mudflat.

MICRO‐ AND NANOPORES IN TIGHT ZECHSTEIN 2 CARBONATE FACIES FROM THE SOUTHERN PERMIAN BASIN, NW EUROPE

The study evaluates pore systems in samples of tight Upper Permian Zechstein 2 Carbonate (Z2C) facies from widely dispersed locations in the eastern and western parts of the Southern Permian Basin, NW Europe. Samples of Z2C drill cores comprising platform to shallow‐basin deposits were examined by petrographic techniques, porosity measurement and SEM analysis, in order to acquire a better understanding of porosity development and pore microstructure. Four carbonate lithofacies were identified by core‐scale and thin‐section analyses: subtidal / intertidal planar stromatolites and thrombolites; slope facies (laminated dolo‐ and lime‐mudstones and grain‐dominated turbidites); toe‐of‐slope / lower slope facies (laminated lime mudstones with turbidite beds); and shallow‐basin (embayment) facies (laminated lime mudstones and hemipelagic deposits). Porosity ranges from 0.08% to 9.6% and is lowest in the subtidal / intertidal planar and columnar stromatolites. The highest porosities, in the slope and shallow‐basin (embayment) lithofacies, probably resulted from catagenetic processes and microbial activity. High porosity in subtidal thrombolites is related to the original pore network. SEM analyses showed that pores are present within organic matter, pyrite framboids and microfossils, and between crystals. Subtidal / intertidal planar and columnar stromatolites and thrombolites have a high proportion of nanopores, probably resulting from microbial activity. Although porosity development is a combined function of the presence of organic matter and mineral components, together with sediment fabric and fractures, it is mostly a result of the early diagenetic transformation of mineral phases, microbial activity, and evaporite and carbonate dissolution.

Classification and oil system of continental shale: Es3L sub-member of Bonan sag, Jiyang depression, Eastern China

On the basis of shale textures, carbonate contents, and total organic carbon (TOC) contents, five types of continental shale are identified: organic-rich laminated argillaceous limestone, organic-rich laminated lime mudstone, organic-lean laminated argillaceous limestone, non-laminated massive argillaceous limestone, and non-laminated massive lime mudstone. Criteria for shale oil system is also proposed, characterized by Ro value higher than 0.5 %, TOC content higher than 0.5 wt%, or S1 (free hydrocarbon) value higher than 0.5 mg/g, with more than 2-m-thick shale top and bottom layers, and the whole system thickness is no less than 30 m. The Es3L (lower sub-member of the third member of Paleogene Shahejie formation) continental shale has total organic carbon (TOC) values ranging from 0.71 to 9.32 wt%, with an average of 3.1 wt%. The organic matter (OM) is predominantly type I kerogen, with minor amounts of type II kerogen. Vitrinite reflectance ranges from 0.7 to 0.9 %, indicating that most of the shales are in the oil generation window. The temperature of maximum yield of pyrolysate (Tmax) ranges from 424 to 447 °C, with an average of 440 °C. Free hydrocarbon (S1) values range from 0.03 to 13.12 mg HC/g rock, with an average of 2.14 mg HC/g rock, the hydrocarbons cracked from kerogen (S2) yield values in the range of 1.19–78.59 mg HC/g rock, with an average of 17.07 mg HC/g rock, and hydrogen index (HI) values range from 160.58 to 1041.82 mg/g TOC, and with an average of 495.87 mg/g TOC. The dominant minerals of the Es3L shale are carbonates (including calcite and dolomite), ranging from 38 to 92 wt%, with an average of 65 wt%, along with secondary clay and quartz, both at an average of 17.5 wt%. The continental shale oil resource potential of Es3L in the Bonan sag was evaluated through further research on TOC content, and free oil content (S1). The evaluation results show that the shale oil accumulation intensity ranges from 0.3 to 4 × 106 ton/km2. The oil content of the Es3L continental shale is similar to that of Monterey shale and Antelope shale of the USA.

Upper Cretaceous-Lower Eocene succession of the Monte Postale and its relationship with the “Pesciara di Bolca” (Lessini Mountains, northern Italy): deposition of a fossil-fish lagerstätte

The Lower Eocene (Cuisian) deposits of Pesciara di Bolca in northern Italy represent one of the most important Konservat-Lagerstatte in the world, well known since the 16th century. Over a period of around 500 years, numerous high-quality fossils have been found in the Pesciara and nearby olistoliths. In order to explain the exceptional nature of these finds, the relationship between the Pesciara and the nearby Monte Postale successions are considered in terms of their facies and depositional environments. The sites are approximately 300 m apart, but the presence of volcanics, the local tectonics, and the dense vegetation make it difficult to correlate between the two. The complete M. Postale succession has been studied from the Maastrichtian Scaglia Rossa basinal lime mudstones to the Cuisian bioclastic grainstones. Volcanic activity that began in the Late Paleocene provided topographic highs that allowed the development of Ilerdian (Spilecco Limestone) and Cuisian (Nummulite Limestone) platforms carbonates. The Ilerdian platforms were successively dismantled. The Cuisian platforms included the Pesciara olistolith and the main part of the M. Postale succession. Within the latter, three lithofacies were identified: massive bioclastic packstone (lithofacies 1), bedded bioclastic packstone (lithofacies 2), and laminated lime mudstone with fish and plants (lithofacies 3). Lithofacies 1 is absent in the nearby Pesciara olistolith, whereas lithofacies 2 and 3 are interdigitated in different arrangements in the Pesciara and M. Postale. The palaeoenvironmental model envisaged for the Pesciara-M. Postale system consists of a coastal lagoon in a subtropical zone, which is bordered by an emergent area (most likely represented by a volcanic archipelago surrounded by a rather deep basin), and a carbonate buildup, in which larger benthic foraminifera played a fundamental part. Brief anoxic episodes occurred within the more distal back-reef areas, close to the center of the lagoon, which enabled the preservation of the fossil fish.

Enrichment of diamondoids and 13C in condensate from Hudson Canyon, US Atlantic

Gas condensate migrated vertically to Upper Jurassic and Lower Cretaceous reservoir rocks in the Hudson Canyon (HC) area of the Baltimore Canyon Trough, US Atlantic. Condensate samples are enriched in diamondoids (4742–4774 ppm) and 13C ( δ 13C = −23.7‰ to −24.6‰ PDB). Several lines of evidence suggest that the main source rock is within the Middle to Lower Jurassic section of Hudson Canyon. Condensates from Hudson Canyon are similar to condensates generated by the Upper Jurassic Smackover Formation of the US Gulf Coast, a laminated lime mudstone with shale facies. The petrologic fabric of the lime mudstone favors early generation of diamondoids. Hydrogen rich kerogen and clay catalysts are concentrated within thin laminations of depositional origin separated by thicker barren carbonate layers.

Lithofacies, diagenesis and cyclicity of the ‘Lower Member’ of the Khuff Formation (Late Permian), Al Qasim Province, Saudi Arabia

The ‘Lower Member’ of the Khuff Formation, exposed along the western margin of the Arabian Basin (Al Qasim Province, Saudi Arabia) comprises seven carbonate and two siliciclastic lithofacies. The carbonate lithofacies include laminated lime mudstone, ostracode wackestone, foraminiferal wackestone, intraclastic wackestone, burrowed dolostone, lithoclastic grainstone and caliche. The clastic lithofacies are represented by claystone and quartzarenite. These lithofacies are arranged vertically into shallowing-upward or emergence cycles, the boundaries between which were delineated by field observations. Peritidal as well as subtidal cycles built-up the Lower Member of the Khuff Formation. The peritidal cycles consist of laminated lime mudstone or subtidal claystone, overlain by intertidal burrowed dolostone, which is capped in most cases by supratidal lithoclastic grainstone, and locally by subaerial caliche deposits. These cycles were probably formed by allocyclic mechanism due to the effect of sea level changes and intrabasinal tectonics. The subtidal cycles comprise laminated lime mudstone, claystone, foraminiferal wackestone, and ostracode wackestone, capped by intertidal burrowed dolostone or intraclastic wackestone. These cycles may be formed due to internal factors, e.g. carbonate production, changes in water circulation, salinity, depth, chemistry, energy, temperature and nutrient content. Diagenetic studies on the depositional cycles show that the basal part of cycle (subtidal zone) includes micrite cementation, micritization and silicification. The middle part (intertidal zone) was affected by early dolomitization, while the upper part (supratidal zone) was subjected to dedolomitization, aggrading neomorphism, sparry calcite cementation and evaporite-carbonate replacement.

Paleo-plateau/-seamount Limestone of the Cambrian Accretionary Complex in the Gorny Altai Mountains, Southern Siberia

Vendian-Cambrian Baratal limestone occurs as large allochthonous blocks in the Cambrian accretionary complex of the Gorny Altai Mountains, southern Russia. We analyzed the primary stratigraphy and depositional environments of Baratal limestone in the Kurai and Akkaya areas in the eastern part of the Gorny Altai Mountains.In the Kurai and the Akkaya areas, Baratal limestone conformably overlies basaltic greenstone. Geochemistry of this greenstone is similar to that found in modern oceanic plateau basalt or oceanic island basalt. The limestone lacks terrigeneous elastic influx. These suggest that the Baratal limestone was originally deposited on and around a plateau or seamount far from the continents in a mid-oceanic environment.Baratal limestone in the study area is lithologically divided into four types; 1) massive lime mudstone, 2) massive limestone conglomerate 3) bedded lime mudstone with slump structures, and 4) laminated lime mudstone. Massive lime mudstone contains stromatolites and ooids. This evidence suggests that the massive lime mudstone was formed in a shallowmarine environment. The massive limestone conglomerate contains angular clasts of lime mudstone, greenstone and chert. Its poorly graded and poorly sorted characteristics suggest that the limestone conglomerate was formed as debris flow deposits. Some parts of bedded lime mudstone have slump structures, and are interpreted as sliding deposits. In addition, laminated lime mudstone that shares an affinity with limestone turbidite, is associated with the massive limestone conglomerate.Sedimentary environments of these four types of limestone are inferred, respectively as follows ; massive lime mudstone may have been diposited on the top of a paleo-plateau/ -seamount, while massive limestone conglomerate, bedded lime mudstone with slump structures, and laminated lime mudstone on the slope of a paleo-plateau/-seamount.

Loch Macumber (early Carboniferous) of Atlantic Canada

Following the Acadian Orogeny, Atlantic Canada accommodated several, large, relatively deep lakes within a wrench-fault basin complex called the Maritime Basin. Late Devonian and Tournaisian lakes were hydrologically open, shallow to deep, mainly fresh water bodies. Middle Visean lakes, here collectively called Loch Macumber, were closed, deep, and meromictic. Their deposits comprise the first and thickest of five sequences in the Maritime Basin. Salinity in the loch increased with time from restricted marine or penesaline, to saline. Basin-centre facies consist of a thin, but extensive, sheet of black, peloidal laminated lime mudstones and an overlying thick evaporite complex. The carbonate sheet grades laterally into both laminated to thinly bedded marlstones, siliciclastic sandstones, and microbial, biocementstone mounds. Laminae consist of alternating carbonate and either silty carbonaceous shale or siliciclastic clay and silt. The mudstone and marlstone are locally interbedded with siliciclastic and carbonate turbidites, resedimented (?deep water) breccias, and olistostromes. Seasonal changes in anoxia and/or carbonate production produced rhythmic laminae of carbonate and carbonaceous shale. Carbonate grains consist of silt-sized microbial clots and rare arthropod carapaces and brachiopod shells. The mounds originated as tufa precipitated around subaqueous hydrothermal springs that supported chemosynthetic communities. Resedimentation processes including incipient brecciation, sliding, slumping, debris flows, and turbidity currents were common. The mounds trapped hydrocarbons from the surrounding laminite and sulphides from underlying hydrothermal vents. Increasing salinity with time resulted in sulphate and chloride precipitation that filled the basins and ended the life of Loch Macumber. After the deposition of thick evaporites the topography became less accentuated, the seas less saline, and the faunas more normal marine.

Mid‐Cretaceous basin margin carbonates, east‐central Mexico

ABSTRACTIsolated, high relief carbonate platforms developed in the intracratonic basin of east‐central Mexico during Albian‐Cenomanian time. Relief on the platforms was of the order of 1000 m and slopes were as steep as 20–43°. Basin‐margin debris aprons adjacent to the platforms comprise the Tamabra Formation. In the Sierra Madre Oriental, at the eastern margin of the Valles‐San Luis Potosi Platform, an exceptionally thick (1380m) progradational basin to platform sequence of the Tamabra Formation can be divided into six lithological units.Basinal carbonate deposition that preceded deposition of the Tamabra Formation was emphatically punctuated by an allochthonous reef block 1 km long by 0·5 km wide with a stratigraphic thickness of 95 m. It is encased in Tamabra Formation unit A, approximately 360 m of peloidal‐skeletal wackestone and lithoclastic‐skeletal packstone that includes some graded beds. Unit B is 73 m of massive dolomite with sparse skeletal fragments and intraclasts. Unit C, 114m thick, consists of structureless skeletal wackestone passing upward into graded skeletal packstone. Interlaminated lime mudstone and fine grained bioclastic packstone with prominent horizontal burrows are interspersed near the top. Unit D is 126 m of breccia with finely interbedded skeletal grainstone and burrowed or laminated mudstone. The breccias contain a spectrum of platform‐derived lithoclasts and basinal intraclasts, up to 10 m in size. The breccias are typically grain supported (rudstone) with a matrix of lightly to completely dolomitized mudstone or skeletal debris. Beds are up to several metres thick. Unit E is 206 m of massive, sucrosic dolomite that replaced breccias. Unit F is approximately 500 m of thick bedded to massive skeletal packstone with abundant rudists and a few mudstone intraclasts. Metre scale laminated lime mudstone beds are interspersed. The section is capped by El Abra Formation platform margin limestone, consisting of massive beds of caprinid packstone and grainstone with many whole valves.Depositional processes within this sequence shift from basinal pelagic or peri‐platform sedimentation to distal, platform‐derived, muddy turbidity currents with a large slump block (Unit A); through more proximal (coarser and cleaner) turbidity currents (Unit B?, C); to debris flows incorporating platform margin and slope debris (Units D, E). Finally, a talus of coarse, reef‐derived bioclasts (Unit F) accumulated as the platform margin prograded over the slope sequence. Interspersed basinal deposits evolved gradually from largely pelagic to include influxes of dilute turbidity currents.Units containing turbidites with platform‐derived bioclasts reflect flooding of the adjacent platform. Breccia blocks and lithoclasts were probably generated by erosion and collapse of the platform during lowstands. Laminated, black, pelagic carbonates, locally cherty, are interbedded with both breccias and turbidites. At least those interbedded with turbidites may have been deposited within an expanded mid‐water oxygen minimum zone during relative highstands of sea level. They are in part coeval with mid‐Cretaceous black shales of the Atlantic Ocean.

Island Arc Carbonates from the Silurian Heceta Formation of Southeastern Alaska (Alexander Terrane)

ABSTRACT The Heceta Formation of Early to Late Silurian age forms part of a well-preserved Paleozoic island arc sequence that is exposed within the Alexander terrane of southeastern Alaska. Limestones and limestone breccias predominate within the formation and are interbedded with polymictic conglomerates and volcaniclastic sandstones. These deposits conformably overlie the Lower Ordovician-Lower Silurian Descon Formation, which is characterized by volcaniclastic wackes and mudstones that accumulated as turbidites in a deep-water setting. Interbedded with the turbidite deposits are basalt-andesite pillow flows and volcanic breccias that formed during incipient stages of island arc construction. The limestones within the Heceta Formation record the earliest occurrence of widespread carbonate deposition within the arc. Although previously described as having only a shallow marine origin, the limestones can be differentiated into shallow-water platform, platform margin, and slope facies. Shallow-water deposits comprise a diversity of skeletal grains that are preserved as micritized fragments associated with cyanobacterial encrustations. They are also characterized by low-diversity brachiopod communities, Amphipora, and oncolites that have a predominance of dasycladacean alga nuclei. These units reflect restricted, quiet areas of deposition that were affected by intermittent turbulence on the platform. Stromatolitic buildups and stromatoporoid reefs constructed at the pla form margin generally are not preserved in situ. They occur predominantly as slumps that initiated carbonate debris flows and deformed the underlying slope deposits during periodic collapse of the platform margin. Graded skeletal grainstones containing shelly debris of shallow-water origin and finely laminated lime mudstones represent turbidites that were affected by synsedimentary deformation in a slope environment, as indicated by the convoluted bedding, slump folds, microfaults, and truncation surfaces. Rapid facies changes, shallowing- and deepening-upwards sequences, and thick polymictic conglomerates record phases of tectonism within the arc that periodically disrupted carbonate deposition. The predominance of cyanobacteria in the limestones suggests that these organisms were largely responsible for stabilizing the substratum by constructing stromatolitic buildups and encrusting skeletal debris during early stages in carbonate platform development and following periods of tectonic disturbance within the arc.

New Exploration Targets in Northern Midland Basin: Depositional and Diagenetic History of West Smyer Field: ABSTRACT

West Smyer field, Hockley County, Texas, produces from the upper part of the Wichita Group (Wolfcampian) in the northern part of the Midland basin. The field produces from a resedimented dolomitized grainstone at the base of the slope separating the Northwestern shelf from the deeper parts of the Midland basin. Since its discovery in 1984, the field has produced over 248,000 bbl of oil from just two wells. Shelf facies such as reefs or oolite shoals have been the primary exploration targets in the northern Midland basin. Areas of basinal facies remain relatively unexplored, but resedimented grainstones along the base of the slope are proven reservoirs in West Smyer and Levelland Northeast fields and potential reservoirs elsewhere. Laminated lime mudstones representing normal basinal sedimentation and fossiliferous grainstones composed of shallow-water sediments carried into the basin by turbidity currents alternate in core from West Smyer field. Fossils in the grainstones underwent micritization, silicification, and glauconitization while still residing on the shelf and were then resedimented into deeper water. During burial, the grainstones were cemented by calcite and then partially dolomitized. Next, the remaining calcite was dissolved, creating intraparticle, interparticle, and moldic pores. These secondary pores were then partially occluded by dolomite,more » followed, in turn, by pyrite and anhydrite, and later filled by hydrocarbons.« less

Framework of Hydrocarbon Generation and Destruction in Eastern Smackover Trend

Laminated lime mudstones of the lower member of the Jurassic Smackover Formation are significant source rocks for crude oil across Mississippi, Alabama, and Florida. The source facies was deposited in an anoxic and perhaps hypersaline environment that preserved algal-derived kerogen. The distribution of kerogen along laminations of depositional origin and along stylolites of diagenetic origin resulted in efficient expulsion of crude oil. With increasing thermal maturity, crude oil initially emplaced in reservoirs was cracked to yield gas condensate and then methane rich in nonhydrocarbon gases such as hydrogen sulfide, carbon dioxide, and nitrogen. Early destruction of methane was driven by thermochemical sulfate reduction. The thermal maturity framework of the Smackover rend explains the distribution of hydrocarbon discoveries and suggests areas previously overlooked by exploration.

Distribution of hydrocarbon source potential in the Jurassic Smackover formation

There are regionally consistent differences in source potential between the upper and lower members of the Smackover Formation. Measurements of total organic carbon and visual kerogen assessments indicate that the Lower Smackover has source potential for crude oil from Texas to Florida. Source potential is largely restricted to the laminated lime mudstone facies of the Lower Smackover where deposition occurred in a subtidal environment that was anoxic and probably hypersaline. Algal-derived amorphous kerogen is concentrated in laminations of depositional origin. Carbonate diagenesis during burial enhanced source potential by further concentrating kerogen along stylolites. We suggest that the efficiency of hydrocarbon expulsion has been enhanced by depositional and diagenetic factors that concentrate source rock kerogen along conduits for fluid movement. The Upper Smackover serves mainly as a reservoir rock, and other Jurassic formations lack source potential.

Upper Jurassic Depositional Systems and Hydrocarbon Potential of Southeast Mississippi: ABSTRACT

ABSTRACT Upper Jurassic sedimentation in southeast Mississippi was controlled by eustatic sea level fluctuations and locally modified by salt tectonism and basement structure. The present study using conventional core data and geophysical logs indicates that a stable carbonate platform developed along the updip margin of the Mississippi Interior Salt Basin. The basin was partially barred from the main Gulf of Mexico water mass by the Wiggins Uplift, and became evaporitic during the upper Jurassic. The Smackover and Haynesville formations are separate sedimentologic packages deposited during two distinct sea level cycles. High energy carbonates extended across the northern basin margin as sedimentation kept pace with subsidence and slowing sea level rise near the close of each transgression. An effective shelf margin barrier did not develop. Lime mud generally accumulated under subtidal conditions within the interior salt basin. However, the basin was sufficiently shallow for wavebase sedimentation to occur on crests of salt-cored bathymetric highs, resulting in localized reservoir facies. To the south, eolian conditions initially prevailed on the Wiggins Uplift Later, as the Wiggins subsided, subaqueous shoals fringed and capped individual basement islands. Eustatic sea level fluctuations and salt movement controlled diagenesis and porosity formation. Limestone porosity dominates in upper Smackover platform carbonates, with a transition from secondary moldic porosity updip to preserved primary porosity downdip. Moldic, intercrystalline, and vuggy dolomite porosity is developed on the crests of intermediate and high amplitude salt highs and on the Wiggins Uplift. Jurassic source rocks are lower Smackover laminated lime mudstones. Migration into adjacent reservoirs postdated formation of porosity and the growth of salt anticlines, the most common trap type. A large potential Norphlet-Smackover gas play extends along the southern flank of the Wiggins Uplift. Salt anticlines within the interior basin remain viable targets. Small oil discoveries should continue in stratigraphic traps, subtle salt structures, and basement blocks on the platform.

Geochemical Studies of Crude Oil Generation, Migration, and Destruction in Mississippi Salt Basin: ABSTRACT

ABSTRACT The main source for crude oil in the Mississippi Salt Basin is the laminated lime mudstone facies of the Lower Smackover. Crude oil generation and migration commenced at a level of thermal maturity equivalent to about 0.55% vitrinite reflectance. Short-range lateral migration of crude oil was focused to Upper Smackover and Norphlet reservoirs, but vertical migration also charged some overlying Cotton Valley, Rodessa, Lower Tuscaloosa, and Eutaw reservoirs. Following migration from the Lower Smackover, thermal maturity history of reservoir rocks controls the preservation of crude oil, gas-condensate, and methane. Slow thermal cracking of crude oil occurred in deep Upper Smackover reservoirs, resulting in formation of gas-condensate and precipitation of solid bitumen. The maximum thermal maturity for preservation of condensate is equivalent to about 1.3% vitrinite reflectance. Only methane, pyrobitumen, and nonhydrocarbon gases including hydrogen sulfide persist at higher levels of thermal maturity. Early destruction of methane in deep Upper Smackover reservoirs near the Wiggins Arch is driven by thermochemical sulfate reduction. Lesser availability of sulfate in Norphlet reservoirs in Lower Mobile Bay could account for methane preservation at higher levels of thermal maturity. One basic geochemical strategy for further exploration of the Mississippi Salt Basin is to focus exploration effort on traps with reservoirs in the thermal maturity window for hydrocarbon preservation. Another strategy is to avoid drilling traps with overmature reservoir rocks.

Carbonate-Evaporite Cycles in Lower Duperow Formation of Williston Basin: ABSTRACT

The Duperow (Frasnian) sediments of the Williston basin consist of approximately 12 regular cycles. Stratigraphic and petrographic studies were made of the lower 300 ft (91 m) of these strata using cores, cuttings, and radioactivity logs from selected locations in North Dakota, Montana, and Saskatachewan. Each cycle consists of three members. The lower member consists of either dark-brown, burrowed, lithoclastic-bioclastic brachiopod-crinoid limestone with a mud matrix, or a stromatoporoid boundstone. A middle member consists of brown lime mudstone with a restricted microfauna of ostracods and calcispheres interbedded with unfossiliferous pelletoid beds or laminated lime mudstone. Bedded anhydrite and gray-green, silty, very fine-grained dolomite displaying intertidal and supratidal sedimentary structures cap each cycle. End_Page 870------------------------------ The Duperow cycles are widespread and constituent beds only 10-15 ft (3-5 m) thick can be traced completely across the Williston basin. Deposition occurred within a vast back-reef lagoon lying south of the Woodbend reef platform of southern Alberta and stretching to a sandy shore in South Dakota and northern Wyoming. This lagoon was periodically and rapidly flooded with normal marine water, permitting organisms to flourish; the sea then gradually shallowed as sediments filled the basin. Desiccation produced extensive tidal flats and evaporitic sabkhas and was perhaps responsible for some dolomitization of the carbonates on shelves outside the basin. The cause of such cyclic sedimentation might have been slow, steady subsidence of the basin with a superimposed climatic rhythm that may have speeded up reef growth and periodically choked off seawater from the basin. Perhaps this process operated coincidentally with sporadic eustatic sea level fluctuation or with abrupt periodic subsidence of the whole basin. The low bathymetric relief that permitted rapid flooding certainly would have aided in development of such cycles. End_of_Article - Last_Page 871------------

Carbonate-Evaporite Cycles in Lower Duperow Formation of Williston Basin

ABSTRACT The Duperow (Frasnian) sediments of the Williston basin consist of about twelve regular cycles. Stratigraphic and petrographic study was made of the lower 300 ft. of these strata using cores and cuttings from 45 wells and about 200 radioactivity logs from selected locations in North Dakota, Montana, and Saskatchewan. Each cycle consists of a lower member, either dark brown, burrowed, lithoclastic-bioclastic brachiopod-crinoid limestone with a mud matrix, or a stromatoporoid biostrome and a middle part of brown lime mudstone with a restricted microfauna of ostracods and calcispheres interbedded with unfossiliferous pelletoid beds or laminated lime mudstone. Bedded anhydrite and greygreen, silty, very fine-grained dolomite displaying intertidal and supratidal sedimentary structures cap the cycle. The Duperow cycles are amazingly widespread and constituent beds only 10-15 ft. thick can be traced completely across the Williston basin, a distance of several hundred miles. Deposition occurred within a vast backreef lagoon lying south of the Woodbend reef platform of southern Alberta and stretching to a sandy shore in South Dakota and northern Wyoming. This lagoon was periodically and rapidly flooded with normal marine water, permitting benthonic organisms to flourish; the sea then gradually shallowed as sedimentation filled the basin. Desiccation resulted in extensive tidal flats and evaporitic sabkhas and perhaps in some dolomitization of the carbonates on shelves outside the basin. The cause of such cyclic sedimentation might have been slow, steady subsidence of the basin with a superimposed climatic rhythm which may have speeded up reef growth and periodically choked off sea water from the basin. Perhaps this process operated coincidentally with sporadic eustatic sea level fluctuation or with abrupt periodic subsidence of the whole basin. The low bathymetric relief which permitted rapid flooding certainly would have aided in development of such cycles. Comparison of patterns of replacement dolomite in lower Duperow sediments of the basin itself with lateral and vertical changes in interpreted depositional facies and with thickness variations of the cycles leads to the conclusion that most replacement dolomitization occurred in the formation before complete lithification, probably not during Duperow time, but later in Devonian time during shelfward evaporite deposition and tectonic adjustment preceding the Mississippian transgression.