Offshore Portions Research Articles

Tectonics of Cauvery basin (India) in onshore and offshore portions

Morphometric studies coupled with geophysical investigations from petroliferous basins provide valuable information about tectonics of the regions. We deduce six morphologic parameters from the River Cauvery's designated (sub)watersheds and perform dendrogram clustering analysis. The main channels of the (sub)watersheds showcase steeper gradient and meandering nature with less concavity. On the other hand, the distributaries portray lesser gradient and straight nature with greater concavity. From the offshore region of the Cauvery basin, we perform gravity modeling. The results show variations in gravity values from low to high, -150 to 50 mGal probably because of the basement ridge-depression features. This anomaly orientation indicates the extension of the sub-basin in the region. A strong NE-SW gravity trend possibly suggests a high-density plug at mid-crust. The low gravity values near the Vedaranyam region indicate low-density rocks within the basement. Places of high gravity anomaly show a decrease in bathymetry depth and vice versa. This indicates isostatic compensation of the Moho geometry, possibly due to crustal thinning in the offshore areas. Isostatic regional gravity fields are manifested by Moho relief and can have implications on the lithospheric mechanical strengths. Bathymetric studies indicate multi-directional slopes. Relatively flat areas can indicate depocenters. E-W trending lineaments mapped on the Precambrian basement from a portion of the study area is presumably a manifestation of Cauvery Shear zone, which also persists in the offshore region.

A Database of Topo-Bathy Cross-Shore Profiles and Characteristics for U.S. Atlantic and Gulf of Mexico Sandy Coastlines

A database of seamless topographic and bathymetric cross-shore profiles along with metrics of the associated morphological characteristics based on the latest available lidar data ranging from 2011–2020 and bathymetry from the Continuously Updated Digital Elevation Model was developed for U.S. Atlantic and Gulf of Mexico open-ocean sandy coastlines. Cross-shore resolution ranges from 2.5 m for topographic and nearshore portions to 10 m for offshore portions. Topographic morphological characteristics include: foredune crest elevation, foredune toe elevation, foredune width, foredune volume, foredune relative height, beach width, beach volume, beach slope, and nearshore slope. This database was developed to serve as inputs for current and future morphological modeling studies aimed at providing real-time estimates of coastal change magnitudes resulting from imminent tropical storm and hurricane landfall. Beyond this need for model inputs, the database of cross-shore profiles and characteristic metrics could serve as a tool for coastal scientists to visualize and to analyze varying local, regional, and national variations in coastal morphology for varying types of studies and projects related to Atlantic and Gulf of Mexico sandy coastline environments.

Macrobenthic infaunal assemblage structure in nearshore and offshore seabeds of Ghana

Abstract. Commey NA, Armah AK, Lamptey E. 2021. Macrobenthic infaunal assemblage structure in nearshore and offshore seabeds of Ghana. Indo Pac J Ocean Life 5: 50-60. This research aimed to compare and contrast the sediment properties and benthic macrofaunal assemblage composition of nearshore and offshore habitats. Also, the existing state of benthic macrofaunal assemblages in the Jubilee Fields, Ghana in 2015 is determined. A baseline is established for these communities before constructing a coal-powered plant in the nearshore area of Ekumfi Aboano, Ghana (deep-sea). The research was conducted in the shallower nearshore Ekumfi Aboano (average depth: 15 m) and deeper offshore portions of the Jubilee Fields (average depth: 1,250 m). A Van Veen grab was used to collect sediment samples from the nearshore area, while a box corer was used to collect samples from the deep water. PRIMER was used to look into the distribution of benthic macrofauna. Using GRADISTAT, it was determined that the nearshore sediment (mean grain size of 99.80 m) is fairly sorted with very fine sand. In contrast, offshore sediment (mean grain size of 109.79 m) was poorly sorted and comprised of extremely coarse silty fine sand. It was determined from studies of macrobenthic infauna that there were 11,131 individuals in the nearshore environment, with a density of 38 individuals/m2, and 22,105 individuals in the deep-sea environment, with a density of 47 individuals/m2. Data for the nearshore and offshore environments showed a total of 194 and 983 polychaetes, 827 and 696 crustaceans, 35 and 229 mollusks, 14 and 37 echinoderms, and 43 and 260 "others" (foraminiferans, nematodes, nemerteans, and sipunculids). Crustaceans > polychaetes > molluscs > others > echinoderms dominated the shallow Ekumfi seabed, whereas polychaetes > crustaceans > molluscs > others > echinoderms dominated the offshore Jubilee seabed. There were 116 species discovered close to shore, while 188 were discovered in the deep water. Both locations displayed a high level of diversity (H' > 4; 1-D > 0.9) and were evenly distributed (J' > 0.8). The nearshore was dominated by arthropods (74.30%), while the offshore was dominated by polychaetes (44.58%). A greater variety of macrobenthic infauna was found in the deep sea compared to the nearshore, with a difference of 75% between the two environments.

Large-scale forcing of environmental conditions on subarctic copepods in the northern California Current system

In the ocean, dominant physical processes often change at various spatial and temporal scales. Here, we examined associations between large-scale physical forcing indexed by the Pacific Decadal Oscillation (PDO), regional ocean conditions including alongshore currents in relation to the abundance of two subarctic oceanic copepods, Neocalanus plumchrus, and N. cristatus in the offshore portions of the northern California Current (NCC) system in spring of 1998–2008. We found significant relationships between the abundance of copepods, water temperature, and alongshore currents with a lag of two or four months in response to the PDO in the NCC system. During the growth season in March/April both subarctic copepod species displayed consistent cross-shelf patterns with shoreward decreasing gradient in abundance, and were negatively correlated with the PDO, sea water temperature, and alongshore currents. Our studies highlight the responses of regional ocean conditions to large-scale physical forcing and illustrate the potential for Neocalanus copepods as unique vectors for a new understanding of the ecological response in the offshore oceanic waters of the NCC system to climate variability.

Continental breakup and the dynamics of rifting in back-arc basins: The Gulf of Lion margin

Deep seismic profiles and subsidence history of the Gulf of Lion margin reveal an intense stretching of the distal margin and strong post-rift subsidence, despite weak extension of the onshore and shallow offshore portions of the margin. We revisit this evolution from the geological interpretation of an unpublished multi-channel seismic profile and other published geophysical data. We show that an 80 km-wide domain of thin lower continental crust, the “Gulf of Lion metamorphic core complex”, is present in the ocean-continent transition zone and exhumed mantle makes the transition with oceanic crust. The exhumed lower continental crust is bounded upward and downward by shallow north-dipping detachments. The presence of exhumed lower crust in the deep margin explains the discrepancy between the amount of extension deduced from normal faults in the upper crust and total extension. We discuss the mechanism responsible for exhumation and present two scenarios, a first one involving a simple coupling between mantle extension due to slab retreat and crustal extension, and a second one involving extraction of the lower crust and mantle from below the margin by the south-eastward flow of hot asthenosphere in the back-arc region during slab rollback. In both scenarios, the combination of Eocene crustal thickening related to the Pyrenees, the nearby volcanic arc and a shallow lithosphere-asthenosphere boundary weakened the upper mantle and lower crust enough to make them flow south-eastward. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after breakup.

Validation of a Risk-Based Numerical Model for Predicting Deep-Draft Underkeel Clearance

AbstractThe channel analysis and design evaluation tool (CADET) is a numerical program to determine the optimum dredge depth for the offshore portions of entrance channels. A brief description of the theoretical basis and risk assessment in CADET is presented. Validation comparisons between CADET predictions and field measurements of wave-induced vertical motions at Ambrose Channel, New York, are presented. Prototype ship motions were measured with a differential global positioning system in April 1998 along three 488-m-long reaches. Comparisons were made with six ships, three tankers and three containerships, during inbound and outbound transits. In general, the CADET predictions matched the field measurements reasonably well, especially considering the wave heights were relatively small during the measurement period.

New perspectives on transitions between ecological-evolutionary subunits in the “type interval” for coordinated stasis

Northern Appalachian Basin deposits and associated fossils have served as exemplars for ecological-evolutionary investigations, and as the reference interval for the concept of coordinated stasis. Here, we examine faunal and environmental changes within the uppermost Hamilton and lowermost Genesee Groups of the late Middle Devonian succession of New York State. Dramatic diversity loss, faunal migrations, and ecological restructuring recognized in these strata have been used previously to define the end of the Hamilton ecological-evolutionary subunit, and, furthermore, these strata and corresponding faunal changes represent the type region for the global Taghanic Biocrisis. We present and analyze a new, high-resolution data set of post-Taghanic Genesee fossil assemblages, in which we recognize 11 biofacies corresponding to an onshore-offshore (depth) gradient. The Genesee Fauna shows an unexpectedly high taxonomic similarity to nearshore biofacies of the pre-Taghanic Hamilton Fauna, related to the persistence of siliciclastic-dominated nearshore settings through the Taghanic Biocrisis, whereas the onset of anoxic/dysoxic conditions typified offshore portions of the environmental gradient. The “Nearshore Refugium Model” of Erwin offers a possible explanation for the persistence of taxa through the biocrisis in nearshore settings. This constriction was followed by subsequent expansion of these residual taxa to offshore environments in relatively similar associations, as increased Acadian orogenic activity and resultant delta progradation increased habitable space offshore by decreasing the extent of deeper-water, oxygen-poor settings. Although taxonomic similarity was high between the Hamilton and Genesee Faunas, biofacies structure differed primarily because of tectonically driven physical transformations to the basin and associated biotic turnover. Nevertheless, the combination of high taxonomic persistence of Hamilton nearshore taxa and the introduction of relatively few new taxa in the Genesee Fauna resulted in a taxonomic holdover that was much higher than observed in the original formulation of coordinated stasis.

Hydrocarbons and petroleum geology of Tierra del Fuego, Argentina

The history of the hydrocarbon exploration and present production, as well as a compilation of their main tectosedimentary features in the Argentine territory of Tierra del Fuego Island are summarized in this paper. The exploration and production of hydrocarbons in the studied region is mostly restricted to both onshore and offshore portions of the Austral-Magellan Basin. Their infil is constituted by a Late Cretaceous to Tertiary sedimentary and volcanoclastic 8,000 m thick succession deposited on a folded and eroded basement cropping out along the northern foothill of the Fueguian Andes. The main productive levels are sandy layers of the Springhill Fm and Tobifera Fm in the eroded basement highs. Also, levels of the Tertiary sequences are currently under evaluation adding an exploratory potential ranging from conventional plays related to transpression and inversion structure to new scenarios based upon tectosedimentary concepts.

Infosys Technologies LTD.: Growing Share of a Customer's Business

Infosys Technologies Ltd. provides a full range of IT and consulting services. One of Infosys' major customers is Prairie Four Square Insurance (PFS). PFS maintains extensive and mission critical IT systems across the United States. Relentless pressure from Wall Street to cut costs dramatically prompted the PFS senior management to outsource offshore portions of its IT maintenance activities. A PFS white paper had indicated that the purchasing department was one area where IT could make significant contributions in lowering operating costs. PFS managers decided to forego their traditional best of breed approach to outsourcing in favor of a pilot test of single sourcing. Thus, PFS managers bundled all activities related to procurement process re-engineering, vendor selection, Ariba software customization, system installation and maintenance into a single project. PFS managers narrowed candidates for the project down to two consulting firms (i.e. Excalibur and Merrimac) plus Infosys. A win for Infosys would enable it to not only grow its share of PFS business but also expand the scope of its market offerings. Successful completion of the project, in turn, would increase the likelihood that PFS would turn to Infosys for other end-to-end IT solutions.

Infosys Technologies Ltd.: Growing Share of a Customer's Business

Infosys Technologies Ltd. provides a full range of IT and consulting services. One of Infosys' major customers is Prairie Four Square Insurance (PFS). PFS maintains extensive and mission critical IT systems across the United States. Relentless pressure from Wall Street to cut costs dramatically prompted the PFS senior management to outsource offshore portions of its IT maintenance activities. A PFS "white paper" had indicated that the purchasing department was one area where IT could make significant contributions in lowering operating costs. PFS managers decided to forego their traditional "best of breed" approach to outsourcing in favor of a pilot test of single sourcing. Thus, PFS managers bundled all activities related to procurement process re-engineering, vendor selection, Ariba software customization, system installation and maintenance into a single project. PFS managers narrowed candidates for the project down to two consulting firms (i.e. Excalibur and Merrimac) plus Infosys. A win for Infosys would enable it to not only grow its share of PFS business but also expand the scope of its market offerings. Successful completion of the project, in turn, would increase the likelihood that PFS would turn to Infosys for other end-to-end IT solutions.

Putative historical occurrence of North Atlantic right whales in mid-latitude offshore waters: ŒMaury¹s Smear¹ is likely apocryphal

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 282:295-305 (2004) - doi:10.3354/meps282295 Putative historical occurrence of North Atlantic right whales in mid-latitude offshore waters: ‘Maury’s Smear’ is likely apocryphal Randall R. Reeves1,*, Elizabeth Josephson2, Tim D. Smith2 1Okapi Wildlife Associates, 27 Chandler Lane, Hudson, Quebec J0P 1H0, Canada 2Northeast Fisheries Science Center, 166 Water Street, Woods Hole, Massachusetts 02543, USA *Email: rrreeves@total.net ABSTRACT: The seasonal distribution of North Atlantic right whales Eubalaena glacialis is generally well known in coastal areas but poorly known in offshore portions of the species’ range. Two main areas of offshore occurrence have been known or suspected, based largely on evidence from 19th century American whaling logbooks: the Cape Farewell Ground east and south-east of the southern tip of Greenland, and ‘Maury’s Smear’, a mid-latitude area bounded by 35 to 43°N and 25 to 48°W. While the modern occurrence of whales on the old Cape Farewell Ground has been demonstrated by recent sightings, this is not the case for Maury’s Smear. Moreover, by examination of original sources we provide direct evidence that the offshore distribution illustrated by Maury in 1852 and 1853 was at least partly and possibly entirely erroneous, based on faulty data extraction and transcription procedures. Our results cast doubt on Maury’s charts of 19th century whale distribution more generally, leading us specifically to question the reliability of his depictions of right whale occurrence in offshore portions of other ocean basins. KEY WORDS: North Atlantic right whale · Eubalaena glacialis · Distribution · Matthew Fontaine Maury · North Atlantic Ocean Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 282. Online publication date: November 16, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Distribution of Trialkylamines and Coprostanol in San Pedro Shelf Sediments Adjacent to a Sewage Outfall

The fate of sewage from the Orange County Sanitation District (OCSD) ocean outfall was assessed by measurements of trialkylamines (TAMs) and coprostanol (COP) in bottom sediments from the San Pedro Shelf (southern California). The wastewater discharge is an important source for steroidal sewage markers, as deduced from ratios of COP to other sterols (COP/epicoprostanol, COP/dinosterol and COP/total sterols) and from the very high concentrations of TAMs (35 μ g g −1) and COP (8.3 μ g g −1) in sediments in the immediate vicinity of the outfall. Decreasing concentrations of both molecular markers with increased distance from the outfall were observed, except in the north-west, which corresponds with the predominant bottom current direction in the region. The offshore portions of Newport and San Gabriel submarine canyons act as efficient traps for fine-grained sediments containing high concentrations of TAMs, which have a high clay affinity characteristic of their amphiphilic chemical properties. The TAM/COP ratio in sediments (0.3–11.2) was larger than in the effluent (0.07), indicating a relatively higher stability of TAMs than COP in sediments under prevailing oxic conditions. Because the two markers exhibit different distributions and fates, an integrated use of both markers provides an effective tool to assess the effects of wastewater discharges to the marine environment.

Drake F-76, In-Situ Abandonment of a High Arctic Offshore Completion and Facilities

Abstract The Drake F-76 well was drilled in 1978 from an ice platform, in 55 m of water, offshore Melville Island, NWT, Canada. The well was completed as a prototype gas producer using a subsea production tree and subsea pipeline bundle to shore. Well testing and project feasibility testing were completed by 1979 and the project suspended. In 1993 a decision was made to abandon the well. An extensive program of research, planning, equipment refit and design was undertaken and completed over two years. Technical and environmental approvals were solicited and received from regulatory bodies. In the winter of 1995 - 1996 the wellhead was relocated and an ice platform constructed. A selection of purpose built and contracted equipment was mobilized to site and assembled. A subsea reconnection was completed, the wellhead was successfully function and pressure tested and the well subsequently re-entered and permanently plugged. The subsea tree and all associated flowline equipment were decommissioned and abandoned in-situ. This paper describes the subsea completion, the technical evaluation of the abandonment operation, and the methods used to successfully abandon the installation. FIGURE 1: Location of Drake F-76 project (Available in full paper) Introduction The Drake gas field is located on the east coast of the Sabine Peninsula, Melville Island, NWT as shown in Figure 1. The field, with approximately 5.7 tcf of proved and probable reserves is about 30 km in length and straddles the coastline with the reserves distributed almost equally between land based and offshore portions of the reservoir. The land-based portion of the field was explored and developed in conventional fashion in the early 1970s. While exploratory wells drilled from sea ice platforms had proven up the offshore reserves, there was a desire to develop and test production methods for this offshore gas. It was intended to show this gas could be delivered to production facilities for subsequent sale into the gas marketing projects then proposed for the early 1980s. In 1978, Panarctic Oils Ltd. successfully drilled, completed, and flow tested the Panarctic Hmstd et al. Drake F-76 well from a strengthened ice platform located approximately 1 km offshore Melville Island. The well was drilled to a depth of 1,128 m (KB) in water 54.9 m deep. The well was completed with a subsea production tree, two bundled 152 mm subsea flowlines to shore and gas processing and testing facilities onshore as shown in Figure 2. FIGURE 2: Drake Point facilities (Available in full paper) Remote wellhead control was provided from a shore-based control unit, through subsea control lines in the flowline bundles to a control module located on the wellhead. The Drake F-76 project was designed to demonstrate that offshore gas could be delivered into a land-based gathering system, through remotely operated subsea wells, despite the problems of sea ice, permafrost and generally challenging conditions. Although the project in itself was a success, no market for Arctic gas has been developed and the facilities have been dormant since 1978.

Structural styles in the Perth Basin associated with the Mesozoic break-up of Greater India and Australia

The Perth Basin constitutes one of the principal tectonic features along the western continental margin of Australia and formed through Permo-Cretaceous rifting of Greater India and Australia. It is composed of northerly striking sub-basins, troughs and ridges bounded by faults which display evidence for normal dip–slip, and possibly dextral strike–slip movement. Transfer faults divide the basin into compartmentalised regions of similar structural style. Extension and transtension occurred during the Jurassic–Early Cretaceous associated with continental break-up. Stretching was accommodated mainly by the reactivation of pre-existing northerly oriented faults which display both planar and listric geometries. The Darling Fault system along the eastern margin of the basin has a planar profile. The hanging wall succession in the adjoining Perth Basin has undergone block rotation and tilting with syn-sedimentary depositional thickening of rock units towards the fault. Listric normal faults occur in both the onshore and offshore portions of the basin and influence geometry of the sub-basins. Dip on these faults decreases through the basin sequence and probably soles out in the upper level of basement. Rollover anticlines develop within the hangingwall of these structures. Extension oblique to the main structural trends of the basin in the Late Jurassic–the earliest Cretaceous resulted in dextral strike–slip movement along major north-striking faults and the development of en-echelon folds in the basin. Compressive deformation developed during this event at constraining bends of the major fault systems. Two orientations of transfer zones are recognised and significantly influence basin structures. The first are east–west trending transfer faults which formed in the Permian and were reactivated during Jurassic extension but can only be recognised in the northernmost Perth Basin. These transfer faults link basin segments of different characteristics and transfer dip displacement among the northerly striking faults. The transfer zones striking NW only influence Late Jurassic–Early Cretaceous age deformation features. They are characterised by either a swing in a left-lateral sense in the orientation of northerly striking fractures as they cross the transfer zones or a termination of the fracture at the transfer zone. The transfer zones are parallel to, and may be related to, transform belts in the Indian oceanic crust. Basin-wide uplift occurred during the break-up and resulted in basin inversion and erosion of up to thousands of metres of the sedimentary succession. Folding by dextral strike–slip motion along the Darling-Urella fault system also inverted the pre-break-up sequences in major onshore depocentres.

Resolution of fault slip along the 470‐km‐long rupture of the great 1906 San Francisco earthquake and its implications

Data from all available triangulation networks affected by the 1906 earthquake have been combined to assess the trade‐off between slip resolution and its uncertainty and to construct a conservative image of coseismic slip along the rupture. Because of varying network aperture and station density, slip resolution is very uneven. Although slip is determined within uncertainties of ±1.0 m along 60% of the fault, constraints are poor on the remaining, mostly offshore portions of the rupture. Slip decreases from maxima of 8.6 and 7.5 m at Shelter Cove and Tomales Bay to 4.5 m near Mount Tamalpais and 2.7 m at Loma Prieta. The geodetically derived slip distribution is in poor agreement with estimates based on analysis of S wave seismograms, probably because these waves register only 20–30% of the total seismic moment obtained from longer‐period surface waves. Consideration of a range of fault geometries for 1906 slip near Loma Prieta indicates right‐lateral motions lie between 2.3 and 3.1 m. These values are considerably greater than the 1.5 m of measured surface slip on which several assessments of high earthquake hazard for this fault segment were based. This factor, along with the absence of 1989 slippage where 1906 surface slip was used to make the forecasts, casts doubt on some claims of success in predicting the 1989 M=6.9 Loma Prieta earthquake.

Sonic imaging reveals new plate boundary structures offshore New Zealand

Recent bathymetry and sonar imagery studies of offshore portions of the plate boundary transecting New Zealand allow the first confident mapping of detailed tectonic and sedimentary patterns of the region. Working in late 1993 aboard the R/V L'Atalante of the Institut Francais de Recherche pour l' Exploitation de la Mer (IFREMER), we recorded soundings of a wide swath of seabed to elucidate major structural transitions along the plate boundary. Results of the study, part of the GEODYNZ‐SUD program developed jointly by institutions in France and New Zealand, will be complemented by New Zealand cruises to the Puysegur and Hikurangi margins. The total data set will be processed and interpreted during the next two years.

Early Devonia benthic communities of the Alexander Terrane, southeastern Alaska

Emsian (upper Lower Devonian) limestones exposed on Kasaan, Round, and Wadleigh Islands in southeastern Alaska are part of an allochthonous suitc of island are deposits preserved within the accrctionary Alexander Terrane. Ten benthic marine communities, including several new brachiopod associations, are defined on the basis of field data correlated with a cluster analysis. Biologic and sedimentologic evidence is integrated with community group assignments in order to substantiate the paleoecology and evolutionary history of each community. These faunas were adapted to a spectrum of quiet and rough water habitats, including restricted, shallow subtidal areas nearshore, offshore biostromal banks, and open and restricted portions of a lagoon or shelf. Comparisons with coeval assemblages from the Cordillera and elsewhere reveal that these communities are unique to southeastern Alaska. Their origin within an early Paleozoic island are appears to have led to biogeographic isolation from other parts of the Cordillera and the development of a distinct biota at a site offshorc from the ancient continental margin. ▭Emsian, paleocommunities, community groups, cluster analysis, Alexander Terrane.

Ordovician Petroleum Source Rocks and Aspects of Hydrocarbon Generation in Canadian Portion of Williston Basin: ABSTRACT

Accumulation of rich petroleum source rocks - starved bituminous mudrocks in both the Winnipeg Formation (Middle Ordovician) and Bighorn Group (Upper Ordovician) - is controlled by cyclical deepening events with a frequency of approximately 2 m.y. Tectonics control both this frequency and the location of starved subbasins of source rock accumulation. Deepening cycles initiated starvation of offshore portions of the inner detrital and medial carbonate facies belts. Persistence of starved offshore settings was aided by marginal onlap and strandline migration in the inner detrital facies belt, and by low carbonate productivity in the medial carbonate facies belt. Low carbonate productivity was accompanied by high rates of planktonic productivity. Periodic anoxia, as a consequence of high rates of planktonic organic productivity accompanying wind-driven equatorial upwellings, is the preferred mechanism for suppressing carbonate productivity within the epeiric sea. The planktonic, although problematic, form Gloecapsamorpha prisca Zalesskey 1917 is the main contributing organism to source rock alginites. A long-ranging alga (Cambrian to Silurian), it forms kukersites in Middle and Upper Ordovician rocks of the Williston basin as a consequence of environmental controls - starvation and periodic anoxia. Source rocks composed of this organic matter type generate oils of distinctive composition at relatively highmore » levels of thermal maturity (transformation ratio = 10% at 0.78% R/sub o/). In the Canadian portion of the Williston basin, such levels of thermal maturity occur at present depths greater than 2950 m within a region of geothermal gradient anomalies associated with the Nesson anticline. Approximately 193 million bbl (30.7 x 10/sup 6/ m/sup 3/) of oil has been expelled into secondary migration pathways from thermally mature source rocks in the Canadian portion of the basin.« less

Geology and Petroleum Habitat for Brazilian East Atlantic Margin Basins: ABSTRACT

Part I--Sergipe-Alagoas, Jequitinhonha, Espirito Santo, and Campos Basin Data. Part II--General Model for Petroleum Habitat along the Brazilian East Margin. Sergipe-Alagoas, Jequitinhonha, Espirito Santo, and Campos basins are located along the east Brazilian continental margin in South America. These basins present 3 major Mesozoic and Cenozoic sedimentary sequences ranging from continental to open-marine environments. Two structural styles produced 3 major exploratory plays, both structural and stratigraphic. Structures are related, first, to an early tectonic framework of the basins caused by extensional forces, when an irregular hinge line was developed, and later, to gravity sliding and sedimentary loading when an entire set of structures was developed. Structures include listric faults, folds, and collapse structures that depict complex structural and sedimentary patterns from the border of the basins toward the hinge line, and from the hinge line down to the basins. The tectonic-structural model defines trends for hydrocarbon exploration. More than 80 oil and gas fields have been discovered in both onshore and offshore portions of the east Brazilian margin basins. Oil-field data plus a large amount of subsurface and seismic reflection data support the proposed model used to establish the global exploratory strategy for the east Brazilian margin basins. Such a model can also be useful for homotaxial analogy in areas where similar geologic conditions occurred; however, it is necessary to keep in mind that each basin has unique features. End_of_Article - Last_Page 469------------

Construction of Process-Response Models for Ancient Epicontinental Seaway Depositional Systems Using Partial Analogs

Epicontinental seaways, such as those that existed during the Jurassic through the Cretaceous in North America, do not have modern counterparts. Thus, exact analogs are not available for use as sedimentologic models of ancient epicontinental seaway depositional systems. As a result, well-documented modern shoreline features traditionally have been used as physical analogs to describe most paleodepositional features. During the past decade, marine geologists have accumulated a sufficient amount of data concerning modern continental-shelf sedimentologic processes and resulting features to relate these to similar features recognized in epicontinental deposits. For example, studies along the Atlantic epicontinental margin of North America and the North Sea between Great Britain and Europe reveal the dynamics of sand movement in response to storm-generated and tidal currents. These processes interact with sediment-covered sea bottoms to form transverse and longitudinal bed forms. Transverse bed forms, which include sand waves and ripples, form in response to the opposing current couplets created by tidal circulation systems. Longitudinal bed forms, which include sand ribbons, longitudinal furrows, and large-scale sand ridges, are formed predominantly on storm-dominated sand-covered shelves. The mechanisms which create and maintain these features are poorly understood but sand ridges and related longitudinal features apparently may form in response to cellular flow structure in storm-generated currents. Similar processes probably operated within ancient epicontinental seaways. Using present-day continental-shelf environments as partial analogs, we can construct process-response models to explain the development of some epicontinental paleodepositional systems rather than relying solely upon shoreline physical models. This method of model construction consists of an empirical, or data-gathering phase, followed by an interpretive phase. The objective is to match sedimentary sequences to process elements, and then combine these elements with information concerning the regional geologic setting to make paleoenvironmental interpretations. In this way, interpretations are based upon possible processes within particular geologic settings rather than the physical comparison of stratigraphic sections and well-documented present-day depositional systems. The process- esponse approach to sedimentary models frees us from the constraints imposed by the lack of well-documented sedimentologic studies of offshore depositional systems. This method of model building has been used to recognize and delineate offshore depositional systems in the Jurassic and Cretaceous epicontinental seaways of North America. It is postulated that currents generated by tides, storms, and regional circulatory mechanisms played major roles in creating and distributing continental-shelflike depositional systems within the offshore portions of these and other ancient epicontinental seaways.