Tuscarora fault, an Acadian(?) bedding-plane fault in central Appalachian Valley and Ridge Province
No abstract available.
Pennsylvania↗
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Stratigraphic and tectonic framework of Libya
Libya is situated on the Mediterranean foreland of the African shield. Marine strata of Paleozoic, Mesozoic, and Cenozoic ages abound in northern Libya, but continental rocks of Paleozoic and Mesozoic ages predominate in southern Libya. Marine incursions in Ordovician, Silurian, Devonian, Carboniferous, late Cretaceous, and early Tertiary times reached far into the country, some crossing the southern border. Compressional folds are almost wholly absent, but uplift, subsidence, block faulting, and tilting have occurred, and angular and parallel unconformities are common. The major diastrophic disturbances include the Caledonian and Hercynian, as well as disturbances during late Cretaceous and Oligocene through Miocene or Recent times. The chief regional structures are the Gefara basin, Hamada basin, Gargaf arch, Marzuq basin, Tibesti-Haruj uplift, Kufra basin, Cyrenaican uplift, and Sirte embayment. Several large basalt flows of Cenozoic age are present. Sand and gravel conceal the bedrock in about a third of the country. In the Sirte embayment marine sedimentation, differential compaction, reef development, subsidence, and block faulting, beginning in late Cretaceous time, favored the development of source and reservoir rocks. Recoverable oil has been found chiefly in limestone and sandstone of early and late Cretaceous and Tertiary ages, and in knobs of probable early Paleozoic sandstone and in fractured granite. In the Hamada basin oil accumulations have been found in sandstones of Triassic age and of several Paleozoic systems. Most of the oil and gas discovered to date in Libya are in anticlinal structures, but several unconformities within the section probably influenced these accumulations; oil and gas may well await discovery in other types of traps.
AAPG Bulletin↗
Geophysical observations on northern part of Georges Bank and adjacent basins of Gulf of Maine
Continuous-seismic-reflection and magnetic-intensity profiles provide data for inferences about the geology of the northern part of Georges Bank and the basins of the Gulf of Maine adjacent to the bank. Basement is inferred to be mostly sedimentary and volcanic rocks of Paleozoic age that were metamorphosed and intruded locally by felsic and mafic plutons near the end of the Paleozoic Era. During Late Triassic time, large fault basins formed within the Gulf of Maine and probably beneath Georges Bank. The fault basins and a possible major northeast-trending fault zone beneath the northern part of the bank probably formed as a result of the opening Atlantic during the Mesozoic. Nonmarine sediments, associated with mafic flows and intrusive rocks, were deposited in the fault basins as they formed. The upper surface of the Triassic and pre-Triassic rocks that comprise basement is an unconformity that makes up much of the bottom of the Gulf of Maine. Depth to the basement surface beneath the gulf differ greatly because of fluvial erosion in Tertiary time and glacial erosion in Pleistocene time. Beneath the northern part of Georges Bank the basement surface is smoother and slopes southward. Prominent valleys, cut before Late Cretaceous time, are present beneath this part of the bank. Cretaceous, Tertiary, and possibly Jurassic times were characterized by episodes of coastal-plain deposition and fluvial erosion. During this time a very thick wedge of sediment, mostly of Jurassic(?) and Cretaceous ages, was deposited on the shelf. Major periods of erosion took place at the close of the Cretaceous and during the Pliocene. Fluvial erosion during the Pliocene removed much of the coastal-plain sedimentary wedge and formed the Gulf of Maine. Pleistocene glaciers eroded all but a few remnants of the coastal-plain sediments within the gulf and deposited a thick section of drift against the north slope of Georges Bank and a thin veneer of outwash on the bank. Marine sediments were deposited in the basins of the Gulf of Maine during the retreat of the last ice and the postglacial rise in sea level.
Georges Bank, Gulf Of Maine↗
Geology of Bulgaria: A review
Bulgaria is in southeastern Europe between reasonably well-described areas of predominantly Alpine crustal deformation on the north and northwest (Carpathians in Romania, Hungary, and Czechoslovakia; Alps in Austria), on the east (Pontides and Taurides in Turkey), and on the west and south (Dinaric Alps in Yugoslavia; Pindos Mountains in Greece) but has not been well described in easily available literature. There are three major east-west trending morphotectonic units. The low-relief Moesian platform of northern Bulgaria and southern Romania owes its heritage to Hercynian deformation. It is mantled by flat-lying, shelf-type Mesozoic and Tertiary sedimentary (mostly carbonate) rocks. Along its southern boundary a foredeep developed during Jurassic-Cretaceous time in which thick flysch-like sediments accumulated. There is small oil and gas production. The Lorn basin, North Bulgarian swell, and Varna trough are other major structural elements within the platform. The Rhodope massif of southern Bulgaria and northern Greece is a rugged mountainous terrain of Precambrian and early Paleozoic crystalline rocks. It is a fragment of a once much larger crustal block that began to break up in the Paleozoic and which has experienced uplift of 2,000 m in the Pliocene-Pleistocene. Between the Rhodope and Moesian crustal blocks the narrow (10-20 km) Balkan Mountains (Stara Planina zone) consist of tightly folded and metamorphosed Paleozoic sedimentary rocks that have been deformed recurrently during the Hercynian and Alpine orogenies, culminating in 3 to 4 km of vertical uplift since the Miocene. The sub-Balkan fault bounds the south side of the Balkans and is at many places a spectacular scarp. The Balkanide zone has been compressively molded between the large, relatively stable Rhodope massif and Moesian platform. Locally, north-moving gravitational glide masses lie on the platform but there has been minimal crustal shortening. A fourth morphotectonic unit, the Kraishtide zone, trends northwest through western Bulgaria and into adjacent Yugoslavia. This rugged area, 15-60 km wide, is a megashear zone within which both right-lateral strike-slip and vertical movements have been common since the Paleozoic. It parallels the better known Vardar zone of Yugoslavia. In addition to the long-lived, deep-seated faults that parallel the trend—and which are the boundary for some—of these four crustal units, a northeast-southwest fault system (Tvarditsa and Etropole), further breaks the Bulgarian crust into a giant block mosaic. Predominantly vertical movement along the major faults alternately has elevated or depressed individual blocks of the mosaic and has influenced profoundly the location and the character of sedimentation, igneous activity, occurrence of mineral deposits, and erosion.
AAPG Bulletin↗
Structure and evolution of Bering Sea shelf south of St. Lawrence Island
The virtually featureless Beringian shelf south of St. Lawrence Island is underlain structurally by at least 14 basins. Encompassing a total area of more than 300,000 sq km, most of the basins are either elongate structural sags, grabens, or half (asymmetric) grabens beneath the outer shelf. The regional trend of these basins is northwest, parallel with that of the continental margin. Two of the basins, St. George and Navarin, contain 7 to 10 km of Upper Cretaceous(?) and Cenozoic sedimentary strata. A major divergence in dip of beds in the upper half of the sedimentary section may reflect an abrupt shelf-wide change in the rate of sedimentation and/or subsidence, probably during the Miocene. The outer sub-shelf basement grabens and adjacent ridges (horsts) are bounded by high-angle normal faults that exhibit growth-type structure. St. Matthew basin, an elongate, southwest-trending feature of the inner shelf, lies along the offshore expression of the Kaltag fault of western Alaska. The Kaltag fault, like the Denali fault in southwestern Alaska, does not extend to the outer Bering Sea shelf but ends or turns parallel with the margin within the inner shelf. The inner shelf is underlain by a broad basement high, Nunivak arch, the seaward half of which is characterized by an arcuate belt of high-frequency and high-amplitude magnetic anomalies. This zone of intense magnetic anomalies along the shelf is probably the signature of a Mesozoic magmatic arc that extends from southwestern Alaska to eastern Siberia and consists of Jurassic to Cretaceous plutonic and volcanic rocks. We speculate that this magmatic arc resulted from oblique convergence and subduction in the Mesozoic between the Kula(?) and North American plates along the eastern Beringian margin. Folding and uplift in the area of the present outer shelf occurred contemporaneously with magmatism along the inner shelf. Plate convergence apparently ceased by the end of the Mesozoic or t e beginning of the Cenozoic. Subsequently, the foldbelt underlying the outer shelf was eroded extensively and rifted extensionally to form large, deep basins. On the average, the shelf has subsided more than 1.5 km. Subsidence and sediment burial of the eroded orogen formed the modern Beringian shelf.
AAPG Bulletin↗
Upper Cretaceous Ferron Sandstone: Major coalbed methane play in central Utah
Recent drilling for coalbed gas in the Upper Cretaceous Ferron Sandstone Member of central Utah has resulted in one of the most successful plays of this kind. Exploration to date has resulted in three fields and a potential fairway 6-10 mi (10-16 km) wide and 20-60 mi (32-96 km) long, corresponding to shallow coal occurrence at depths of about 1800-3500 ft (545-1060 m) in the Ferron, a sequence of interbedded fluvial-deltaic sandstone, shale, and coal in the lower part of the Cretaceous Mancos Shale. Coalbed methane (CBM) reservoirs in this interval consist of thin to moderately thick (3-10 ft [1-3 m]) coal beds of relatively low rank (high-volatile B bituminous) and variable gas content, ranging from 100 scf/ton or less in the south to as high as 500-600 scf/ton in the north. Productive wells have averaged more than 500 mcf/day and, after several years, continue to typically show negative production declines. In the major productive area, Drunkards Wash unit, the first 33 producers averaged 974 mcf and 85 bbl of water per day after five years of continuous production. Estimated ultimate recoverable reserves for individual wells in this unit range from 1.5 to 4 bcf. Based on several criteria, including gas content, thermal maturity, and chronostratigraphy, the play is divided into northern and southern parts. The northern part is characterized by coals that have the following characteristics: (1) high gas contents; (2) moderate thermal maturity (e.g., vitrinite reflectance [R o ] values of 0.6-0.8%); (3) good permeabilities (5-20 md); (4) lack of exposure; and (5) overpressuring, due to artesian conditions. Southern coals have much lower average gas contents (<100 scf/ton) and lower thermal maturity (R o = 0.4-0.6%), and they are exposed along an extensive, 35 mi (56 km) outcrop belt that may have allowed a degree of flushing. These coals, however, are also thicker and more extensive than those to the north and thus may retain significant potential. Northern coals appear to contain a mixture of gas from three sources: in-situ thermogenic methane, migrated thermogenic methane from more mature sources, and late-stage biogenic gas. Current development is focused on the northern part of the stated fairway, where well control and an existing infrastructure are present. Indications are that CBM exploration in the Ferron will expand considerably in the near future.
Utah↗
Simulations of the origin of fluid pressure, fracture gen eration, and the movement of fluids in the Uinta Basin, Utah
The Altamont oil field in the deep Uinta basin is known to have reservoir fluid pressures that approach lithostatic. One explanation for this high pore-fluid pressure is the generation of oil from kerogen in the Green River oil shale at depth. A three-dimensional simulation of flow in the basin was done to test this hypothesis. In the flow simulation, oil generation is included as a fluid source. The kinetics of oil generation from oil shale is a function of temperature. The temperature is controlled by (1) the depth of sediment burial and (2) the geothermal gradient. Using this conceptual model, the pressure buildup results from the trade-off between the rate of oil generation and the flow away from the source volume. The pressure increase depends primarily on (1) the rate of the oil-generation reaction and (2) the permeability of the reservoir rocks. A sensitivity analysis was performed in which both of these parameters were systematically varied. The reservoir permeability must be lower than most of the observed data for the pressure to build up to near lithostatic. The results of the simulations indicated that once oil generation was initiated, the pore pressure built up rapidly to near lithostatic. We simulated hydrofractures in that part of the system in which the pressures approach lithostatic by increasing both the horizontal and the vertical permeability by an order of magnitude. Because the simulated hydrofractures were produced by the high pore pressure, they were restricted to the Altamont field. A new flow system was established in the vicinity of the reservoir; the maximum pore pressure was limited by the least principal stress. Fluids moved vertically up and down and laterally outward away from the source of oil generation. The analysis indicated that, assuming that one is willing to accept the low values of permeability, oil generati n can account for the observed high pressures at Altamont field.
AAPG Bulletin↗
Deep structure of northern Mississippi embayment
In September 1980, the U. S. Geological Survey conducted a seismic refraction investigation of the northern Mississippi Embayment. During the investigation, 34 shots from nine shotpoints were recorded along a series of profiles. The profiles were parallel to and across an inferred Precambrian rift zone which is outlined by a series of magnetic anomalies and covers an area at least 200 km (125 mi) long and 70 km (45 mi) wide. The paper presents an interpretation of the survey results. The tectonic model proposed to explain the origin of the embayment velocity structure includes a late Precambrian mantle plume that intruded the lower crust of the northern embayment, causing uplift (and/or crustal stretching) and subsequent rifting of the axial area. This was followed by erosion, subsidence, and subsequent deposition of sediments in the resulting trough.
Northern Mississippi embayment↗
Introduction to Special Issue: Gas Hydrates in Green Canyon Block 955, deep-water Gulf of Mexico: Part I
No abstract available.
AAPG Bulletin↗
Lisburne Group (Mississippian and Pennsylvanian), potential major hydrocarbon objective of Arctic Slope, Alaska
The Lisburne Group, a thick carbonate-rock unit of Mississippian and Pennsylvanian age, is one of the most widespread potential reservoir-rock units in northern Alaska. A comprehensive review of the Lisburne in the subsurface of the eastern Arctic Slope indicates attractive reservoir characteristics in a favorable source and migration setting where numerous trapping mechanisms appear to be available. Evaluation of this group as a potential exploration objective is particularly timely because of impending offshore sales in the Beaufort Sea and current exploration programs under way in the Prudhoe Bay area and the Naval Petroleum Reserve. Dolomite and sandstone have been identified as reservoir rocks. Oolitic grainstone is a common rock type, but observations indicate little reservoir potential owing to complete void filling by calcite cement. The most important reservoir rock as judged by thickness, areal extent, and predictability is microsucrosic (10 to 30μ) dolomite of intertidal to supratidal origin. It is present throughout the Lisburne and is most abundant near the middle of the sequence. Northward it decreases in thickness from 1,000 ft (300 m) to less than 100 ft (30 m). Porosity of the dolomite as determined in selected wells averages between 10 and 15% and attains a maximum of slightly more than 25%. Net thickness of reservoir rocks (i.e., rocks with greater than 5% porosity) ranges in these wells from 140 to 390 ft (40 to 120 m). Oil shows are common, and drill-stem tests have yielded as much as 1,600 bbl/day of oil and 22 MMcf/day of gas in the Lisburne pool of the Prudhoe Bay field and as much as 2,057 bbl/day of salt water outside the field area. The occurrence of dolomite over such a large area makes its presence in the offshore Beaufort Sea and adjacent Naval Petroleum Reserve 4 fairly certain. The presence of sandstone as thick as 140 ft (40 m) in the middle and upper part of the Lisburne in two coastal wells suggests that larger areas of sandstone may be found on the north in offshore areas. Shows of oil and gas and a saltwater flow of 1,470 bbl/day have been recorded from this sandstone facies. Shales of Permian and Cretaceous ages unconformably overlie the Lisburne, providing adequate sealing beds above potential reservoirs. Impermeable limestone (completely cemented grainstone) and thin beds of shale may serve as seals within the Lisburne, but the possibility of fractures in these units may negate their sealing capability. The most favorable source rock for Lisburne hydrocarbons appears to be Cretaceous shale that unconformably overlies the Lisburne east of Prudhoe Bay. This shale is reported to be a rich source rock and is the most likely source for the entire Prudhoe Bay field. A source within the Lisburne or within the underlying Kayak Shale is postulated for oil shows in the southernmost Lisburne wells. This postulated source may be in a more basinal facies of the Lisburne and may be similar to dark shale in the upper Lisburne in thrust slices to dark shale in the upper Lisburne in thrust slices in the Brooks Range. Coal in the underlying Endicott Group is a possible source for dry gas. At present, much of this coal probably is in a gas-generating regime downdip from the Prudhoe Bay field. Stratigraphic traps involving the Lisburne Group may have resulted from widespread Permian and Cretaceous unconformities. Structural traps related to normal faulting may be present along the trend of the Barrow arch, and faulted anticlines are numerous in the foothills of the Brooks Range. Combination traps are possible along the trend of the Barrow arch.
Alaska↗
VAMPs—Possible hydrocarbon-bearing structures in Bering Sea Basin: Geologic notes
Narrow (1 to 2 km) subsurface columns of concave reflection horizons are common time-base seismic profiles collected in the Bering Sea basin. The columns of recorded downflexures are thought to be velocity pulldowns and commonly are associated with one or more arched or gently domed high-amplitude reflection horizons about 100 m higher in the section. Inferred from this association is that subsurface deposits characterized by anomalously low acoustic velocity are present. We refer to the velocity-anomaly and reflection-amplitude association as a velocity-amplitude feature, or VAMP, and speculate that VAMPs are deep-seated “bright spots” underlain by a strong velocity pulldown possibly caused by gas-charged deposits.
AAPG Bulletin↗
Lithofacies and stratigraphy of the Lisburne and Etivluk groups in the Lisburne 1 well and adjacent outcrops
The Lisburne 1 well in the thrust belt of the central Brooks Range penetrated 17,000 ft of imbricated, chiefly Ellesmerian sequence strata in the Endicott Mountains allochthon. Five thrust repeats of the Lisburne Group (Carboniferous) and overlying Etivluk Group (Permian-Jurassic) were drilled. Lithofacies analyses of >350 thin sections of cores and cuttings, and biostratigraphy based on foraminifers and conodonts, allow detailed correlation with coeval units in adjacent outcrops and provide data on the depositional setting and reservoir and source rock potential of these strata. The late Early- Late Mississippian (Osagean-Chesterian) Lisburne Group consists mainly of skeletal wackestone to grainstone, locally completely dolomitized. An interval of abundant glauconite and detrital quartz in the lower Lisburne may mark a sequence-bounding unconformity. Dolostone in the upper part of the unit has maximum porosities of 10-13% and common residual hydrocarbons. The uppermost Lisburne is thinly interbedded mudstone, chert, and shale that are locally dolomitic, phosphatic, spiculitic, and organic-rich; conodonts from this interval in outcrop represent an outer shelf to slope biofacies. The Etivluk Group here encompasses the Siksikpuk and Otuk Formations. The Siksikpuk is mainly varicolored shale and radiolarian chert, with a basal interval of glauconitic, pyritic sandstone. Phosphatic and organic-rich shale, radiolarian chert, and pelecypod coquinas make up the Otuk. Outcrop and subsurface data indicate that the Lisburne Group in this area accumulated near the seaward margin of a shallow-water carbonate platform that drowned during the Late Mississippian; outer shelf or deeper conditions predominated throughout deposition of the upper Lisburne and the Etivluk Group.
AAPG Bulletin↗
Structural architecture of the central Brooks Range foothills, Alaska
Five structural levels underlie the Brooks Range foothills, from lowest to highest: (1) autochthon, at a depth of ~9 km; (2) Endicott Mountains allochthon (EMA), thickest under the northern Brooks Range (>15 km) and wedging out northward above the autochthon; (3) higher allochthons (HA), with a composite thickness of 1.5+ km, wedging out northward at or beyond the termination of EMA; (4) Aptian-Albian Fortress Mountain Formation (FM), deposited unconformably on deformed EMA and HA and thickening northward into a >7-km-thick succession of deformed turbidites (Torok Formation); (5) gently folded Albian-Cenomanian deltaic deposits (Nanushuk Group). The dominant faulting pattern in levels 2-3 is thin-skinned thrusting and thrust-related folds formed before deposition of Cretaceous strata. These structures are cut by younger steeply south-dipping reverse faults that truncate and juxtapose structural levels 1-4 and expose progressively deeper structural levels to the south. Structural levels 4-5 are juxtaposed along a north-dipping zone of south-vergent folds and thrusts. Stratigraphic and fission-track age data suggest a kinematic model wherein the foothills belt was formed first, by thrusting of HA and EMA as deformational wedges onto the regionally south-dipping authochon at 140-120Ma. After deposition of FM and Torok during mid-Cretaceous hinterland extension and uplift, a second episode of contractional deformation at 60 Ma shortened the older allochthonous deformational wedges (EMA, HA) and overlying strata on north-vergent reverse faults. To the north, where the allochthons wedge out, shortening caused duplexing in the Torok and development of a triangle zone south of the Tuktu escarpment.
AAPG Bulletin↗
Regional structural framework and petroleum assessment of the Brooks Range foothills and southern coastal plain, National Petroleum Reserve, Alaska
New interpretations of the frontal part of the Brooks Range orogen beneath the foothills and coastal plain in the National Petroleum Reserve-Alaska (NPRA) are based on reprocessed regional seismic reflection data, recent geologic field observations, and new apatite fission-track analyses. Three long north-south transects illustrate the configuration of thrust faulting above a basal detachment that, within the southern part of NPRA, steps up from the Triassic Shublik Formation, to the Jurassic Kingak Shale, and finally into Cretaceous Torok mudstones. This thrust system represents the youngest recognized pulse of major shortening, about 60 Ma. The transects, along with other seismic-reflection examples, illustrate four play concepts being used in the deformed area for the 2002 U.S. Geological Survey oil and gas assessment of the National Petroleum Reserve-Alaska (NPRA). The Brookian topset structural play includes broad west-northwest-trending anticlines in the Cretaceous Nanushuk Group, developed above structurally thickened Torok mudstones in the incipiently-deformed, most northerly part of the thrust system. The Torok structural play includes prominent anticlines affecting deep-basin sandstones, many of which are detached from folds exposed at the surface. The Ellesmerian structural play includes closures developed in the clastic part of the Ellesmerian sequence, mainly above a detachment in the Shublik Formation. The thrust belt play includes antiformal stacks of allochthonous Endicott Group clastic rocks and Lisburne Group carbonates; these stacks were assembled at about 120 Ma, and were transported to their present positions in the foothills at about 60 Ma.
Alaska↗
Book review, Uranium resource processing and secondary resources
No abstract available.
AAPG Bulletin↗