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At least 1,747 records · Page 97Linked to original sources

USGS2025

No evidence for an active margin-spanning megasplay fault at the Cascadia Subduction Zone

It has been previously proposed that a megasplay fault within the Cascadia accretionary wedge, spanning from offshore Vancouver Island to Oregon, has the potential to slip during a future Cascadia subduction zone earthquake. This hypothetical fault has major implications for tsunami size and arrival times and is included in disaster-planning scenarios currently in use in the region. This hypothesis is evaluated in this study using CASIE21 deep-penetrating and U.S. Geological Survey high-resolution seismic reflection profiles. We map changes in wedge structural style and seismic character to identify the inner-outer wedge transition zone where a megasplay fault has been previously hypothesized to exist and evaluate evidence for active faulting within this zone. Our results indicate that there is not an active, through-going megasplay fault in Cascadia, but instead, the structure and activity of faulting at the inner-outer wedge transition zone is highly variable and segmented along strike, consistent with the segmentation of other physical and mechanical properties in Cascadia. Wedge sedimentation, plate dip, and subducting topography are proposed to play a major role in controlling megasplay fault development and evolution. Incorporating updated megasplay fault location, geometry, and activity into modeling of Cascadia earthquakes and tsunamis could help better constrain associated hazards.

British Columbia, Oregon, Washington↗

USGS2026

Paleocene-Eocene dinoflagellate biostratigraphy in the central Delaware coastal plain

Dinoflagellate cyst palynology and lithostratigraphy from three continuously cored boreholes (Blackbird, Smyrna, and Woodland Beach), supplemented by calcareous nannofossil biostratigraphic data, were used to refine the age and correlation of three Paleocene-Eocene formations in northern Delaware: the Vincentown, Manasquan, and Shark River. Lithologic logs reveal significant facies changes in the upper Paleocene Vincentown Formation over a short distance (*9 km) southeastward, transitioning from very permeable glauconite-quartz-shell-rich sands that form the regionally important Rancocas/Aquia aquifer to much less permeable muddy sands and sandy muds. These lithologic changes represent a shift from nearshore, clear, warm-water, carbonate-rich sand shoals to mud-dominated deeper-water, offshore shelfal environments. Dinocyst assemblages including Areoligera gippingensis, Cribroperidinium giuseppei, and Stenodinium meckelfeldensis confirm the Paleogene ages and track these paleoceanographic changes from shallow coastal settings updip to neritic or outer marine settings downdip. The Vincentown Formation is unconformably overlain by the lower Eocene Manasquan Formation, which consists predominantly of silt and clay with relatively consistent lithology across the study area, representing general neritic or open marine conditions. Lithology, dinoflagellate cyst assemblages, and calcareous nannofossil data indicate that the Marlboro Clay and its record of the onset of the Paleocene-Eocene Thermal Maximum (PETM) global warming event is absent from the Paleocene-Eocene transition at these sites, with an unconformity marking the boundary between the Vincentown and Manasquan formations. The youngest unit examined, the Eocene Shark River Formation, unconformably overlies the Manasquan Formation and consists of interbedded clayey silts and sands, with dinoflagellate assemblages suggesting a return to coastal or inner neritic settings. Calcareous nannofossil zones recognized from the Woodland Beach core corroborate the depositional ages determined using dinoflagellates throughout the sequence, confirming the chronostratigraphic framework for these important coastal plain units.

Delaware↗

USGS1983

Pressure core barrel; application to the study of gas hydrates, Deep Sea Drilling Project Site 533, Leg 76

A pressure core barrel (PCB), developed by the Deep Sea Drilling Project, was used successfully to recover, at in situ pressure, sediments of the Blake Outer Ridge, offshore the southeastern United States. The PCB is a unique, wire-line tool, 10.4 m long, capable of recovering 5.8 m of core (5.8 cm in diameter), maintained at or below in situ pressures of 34.4 million Pascals (MPa), and 1.8 m of unpressurized core (5.8 cm in diameter). All excess internal pressure above the operating pressure of 34.4 MPa is automatically vented off as the barrel is retrieved. The PCB was deployed five times at DSDP Site 533 where geophysical evidence suggests the presence of gas hydrates in the upper 600 m of sediment. Three cores were obtained holding average in situ pressures of 30 MPa. Two other cores did not maintain in situ pressures. Three of the five cores were intermittently degassed at varying intervals of time, and portions of the vented gas were collected for analysis. Pressure decline followed paths indicative of gas hydrates and/or dissolved gas. The released gas was dominantly methane (usually greater than 90%), along with higher molecular-weight hydrocarbon gases and carbon dioxide. During degassing the ratio of methane to ethane did not vary significantly. On the other hand, concentrations of higher molecular-weight hydrocarbon gases increased, as did carbon dioxide concentrations. The results from the PCB experiments provide tentative but equivocal evidence for the presence of .gas hydrates at Site 533. The amount of gas hydrate indicated is small. Nevertheless, this work represents the first successful study of marine gas hydrates utilizing the PCB.

Initial Reports of the D.S.D.P.↗

USGS1987

Eocene siliceous and calcareous phytoplankton, Deep Sea Drilling Project Leg 95

Eocene siliceous and calcareous phytoplankton, with emphasis on silicoflagellates, were studied in 62 samples from DSDP Sites 612 and 613 on the continental slope and rise off New Jersey. The mid-latitude assemblages correlate well with assemblages from California, Peru, and offshore of southern Brazil, but are distinctly different from high-latitude cold-water assemblages of the Falkland Plateau off southern Argentina. Coccoliths and silicoflagellates provide evidence for the presence of a fairly complete middle and upper Eocene sequence, represented by a composite of Sites 612 and 613. A major unconformity occurs at the middle Eocene to upper Eocene contact at Site 612. The genus Bachmannocena Locker is emended and proposed as a replacement for genus Mesocena Ehrenberg for ring silicoflagellates. Six new silicoflagellates and one new diatom are described: Bachmannocena apiculata monolineata Bukry, n. subsp., Corbisema amicula Bukry, n. sp., C. bimucronata elegans Bukry, n. subsp., C. hastata incohata Bukry, n. subsp., C. jerseyensis Bukry, n. sp., Dictyocha acuta Bukry, n. sp., and Coscinodiscus eomonoculus Bukry, n. sp. Also, one new replacement name, B. paulschulzn Bukry, nom. nov., and 24 new combinations are proposed for genus Bachmannocena.

Initial Reports of the D.S.D.P.↗

USGS1988

Aliphatic hydrocarbons in sediments from Prydz Bay, Antarctica

High molecular weight aliphatic hydrocarbons were extracted from sediments at two sites (741 and 742) drilled during ODP Leg 119 in Prydz Bay. The distributions of n-alkanes and triterpenoid and steroid hydrocarbons suggest that the n- alkanes and steranes are mainly of terrestrial origin and that the hydrocarbons are immature to slightly mature in the Lower Cretaceous sediments and immature to mature in the Tertiary sediments. At Site 741, the Lower Cretaceous depositional sequence, which is generally characterized by immature hydrocarbons, is interrupted by sediment having more mature components, suggesting a change of source during part of Early Cretaceous time. At Site 742, the mature geochemical parameters of a Pliocene sample correlate with results reported elsewhere for Site 739. In all but one of the other Tertiary samples, the geochemical parameters indicate intermediate maturity. The Lower Cretaceous and Pliocene sediments average about 1.9% organic carbon, a value of interest from the point of view of potential sources of petroleum offshore of Antarctica.

Proceedings of the Ocean Drilling Program: Scienti↗

USGS2000

Sedimentary record of the California Current system, middle Miocene to Holocene: A synthesis of Leg 167 results

During Ocean Drilling Program Leg 167, the California continental margin was drilled from about 30°N to 42°N to sample high-resolution paleoceanographic and paleoclimatic records in the California Current system. Because of typically high sedimentation rates along the margin (80 to >200 m/m.y.), drilling has proved necessary to collect late Pleistocene sedimentary records that usually are sampled by standard piston cores in other oceanic regions. Triple piston coring on Leg 167 enabled us to construct continuous submillennial paleoceanographic records to about 2 Ma. In the offshore drill sites, continuous records were constructed into the Miocene. The oldest sediments recovered on Leg 167 have an age of ~14 Ma. The California margin has an active diagenetic system driven by the degradation of organic matter. Leg 167 drilling provided a means to quantify the diagenetic processes within the deep sediment column and to study the links between sediment diagenesis and primary productivity. This synthesis chapter also documents the oceanographic variability along the California margin at all time scales, from 10 3 to 10 6 yr. Millennial-scale variability is found in Santa Barbara Basin (drilled during Leg 146) and in nearby basins drilled during Leg 167. Leg 167 cores also captured millennial-scale variability in the northern and central California margin. Orbitally forced insolation changes invoke a strong response throughout the California margin. Sea-surface temperature (SST) measured by the alkenone U k' 37 paleothermometer is highly coherent with the oxygen isotope record, being cold in glacials and much warmer in interglacials. Faunal and floral plankton assemblages vary strongly on the glacial-interglacial scale. Coastal plant communities show a glacial-interglacial variability that is most pronounced in the north, near the Cordilleran Ice Sheet. Major changes in the sediments prior to the Pleistocene mark major late Neogene oceanographic events. Opaline silica burial in the middle and upper Miocene sections have step-like drops from high opal deposition in the middle Miocene. One major drop occurs at ~11 Ma and is roughly correlative with the eastern equatorial Pacific Miocene carbonate crash. A second major drop occurs at about 8 Ma, equivalent in age to the end of the Monterey Formation. A third drop occurs slightly younger than the end of the Miocene. A lower Pliocene interval, roughly from 5 to 4.2 Ma, is low in all biogenic components. It separates the Miocene high-opal sediments from upper Pliocene high-carbonate sediments. High CaCO 3 deposition occurred all along the entire California margin in the late Pliocene, but CaCO 3 burial dropped abruptly with the beginning of Northern Hemisphere glaciation (2.6 Ma).

Baja California, California↗

USGS2020

Dietary patterns in black abalone (Haliotis cracherodii Leach, 1814) as indicated by observation of drift algal and seagrass capture at San Nicolas Island, California USA, 1982‒2019

Black abalone Haliotis cracherodii Leach, 1814 are known to feed on drift plant macrodetritus moved about in the intertidal zone by waves and currents. Drift capture is a trait shared by at least several other abalone species. Drift materials are entrapped beneath the anterior foot and held for ingestion. The quantitative significance of feeding on entrapped drift macrodetritus for black abalone is unknown. Furthermore, there are no published data on the extent to which local and mesoscale spatial distributions of source plant populations influence the composition of drift plant material in black abalone diet as acquired by entrapment. From February 1982 through March 2019, occurrences of macrodetrital entrapment by black abalone were observed in nine rocky intertidal study plots, with a summed surface area of 2,054 m 2 , on the periphery of San Nicolas Island (SNI), California (Island centroid at ∼33.25°, –119.50°). A small preliminary survey and 27 complete surveys were performed during the study period (mean of ∼1.4 y between complete surveys). During the study, more than 1.5 × 10 5 black abalone were examined. The total likely included repeated observations of many individuals as a result of the known longevity and limited mobility of the species. Of those observed, ∼1.65 × 10 3 black abalone were recorded as apparently ingesting entrapped items. Frequency data were dominated (∼95% of all records) by three species of kelp Macrocystis pyrifera (Linnaeus) C. Agardh; commonly known as “giant kelp”, Egregia menziesii (Turner), and Eisenia arborea Areschoug. Of those, giant kelp was the most frequently observed entrapped category (∼76%). Living, attached giant kelp is rarely observed in intertidal habitats at SNI, and it follows that utilization of giant kelp by black abalone requires physical importation of the kelp from other locations. Frequencies of occurrence of giant kelp entrapment by individual study site were clearly associated with the relative surface canopy sizes and persistence patterns of offshore kelp forests adjacent (≤2 km) to the respective study sites. The pattern suggests that subsidies of drift giant kelp to black abalone diet involve mesoscale physical processes largely proximate to SNI but probably not subsidies from more distant locations such as other islands or the California mainland. Utilization of other frequently recorded kelps as food by black abalone likely involves spatial subsidies as well, but on smaller scales of distance (∼10–100 m for E. arborea ; ∼0–100 m for E. menziesii ). In the context of the imperiled status of black abalone, recovery actions may include outplants of captive-reared animals or transplantation of wild animals from other populations. For such actions, data from SNI suggest a need for consideration of scales of separation among release locations and nearby populations of the three apparently predominant kelp species in black abalone diet.

California↗

USGS2020

Distribution and transport of Olympia oyster, Ostrea lurida, larvae in northern Puget Sound, Washington, USA

As efforts for restoring Olympia oyster (Ostrea lurida) populations have expanded, there is an increased need to understand local factors that could influence the long-term success of these projects. To address concerns over potential limitations to recruitment at a restoration site in northern Puget Sound, Washington, USA, a study was developed to characterize physical processes governing larval transport in conjunction with larval abundance and environmental factors. Larval presence was not associated with tide cycle, season, or a combination of tide cycle and season. In terms of location, larvae were more likely to be present at offshore and intertidal sites versus the estuarine lagoon where the adult population resides. Larval density was higher during late summer ebbs versus early summer floods. Across sampling dates and locations, larval sizes ranged from 184 to 263 µm, indicating that larvae were released into the water column throughout the reproductive season and retained in the embayment for at least ~16 days. Throughout different tidal cycles in Skagit Bay, acoustic Doppler current profilers were used to measure current direction and velocities, concurrent with plankton sampling. Surface currents in the study area alternated between a clockwise and counterclockwise gyre during initial ebb and flood tides, respectively. Larvae exported from the source population during initial to mid-ebbs are swept into a northward gyre, and potentially retained at intertidal sites alongshore. These results will provide resource managers attempting to restore native bivalves with the ability to expand populations by identifying optimal areas for habitat enhancement through natural recruitment.

Washington↗

USGS1962

Crustal structure along the coast of California from seismic-refraction measurements

Two reversed seismic-refraction profiles were recorded between Los Angeles and San Francisco in 1961. The three shotpoints were located in Santa Monica Bay, offshore near San Francisco, and at Camp Roberts, about halfway between Los Angeles and San Francisco. The velocity of P g along these profiles is 6.1 ± 0.1 km/sec, with possible exceptions near San Francisco and near Los Angeles, where the scatter in the arrival times indicates complex near-surface velocity variations. The velocity of P n between Los Angeles and Camp Roberts is 8.2 ± 0.1 km/sec, and between Camp Roberts and San Francisco 8.0 ± 0.2 km/sec. There is no indication of an intermediate crustal layer in the traveltimes of first arrivals. Computed depths to the Mohorovicic discontinuity, if the crust consists of a single layer, are: 35 km at Los Angeles, 23 km at Camp Roberts, and 23 km at San Francisco. Refractions from crustal layers of intermediate velocity need not appear as first arrivals, and in the extreme, the depth to the Mohorovicic discontinuity may be one-third greater than the thickness of a one-layer crust.

California↗

USGS2005

Population status of Kittlitz's Murrelet Brachyramphus brevirostris along the southern coast of the Alaska Peninsula

The Kittlitz's murrelet ( Brachyramphus brevirostris ) is a rare seabird that nests in alpine terrain and generally forages near tidewater glaciers during the breeding season. An estimated 95% of the global population breeds in Alaska, with some unknown proportion breeding in the Russian Far East. A global population estimate using bestavailable data in the early 1990s was 20,000 individuals. However, recent survey data from two core areas (Prince William Sound and Glacier Bay) suggest that populations have declined by 75-90% during the past 10-20 years. In response to these declines, a coalition of environmental groups petitioned the USFWS in May 2001 to list the Kittlitz’s murrelet under the Endangered Species Act (ESA), and in 2004 Kittlitz’s Murrelet was declared a candidate species under the ESA. In 2005, BirdLife International classified the species as “critically endangered”. In 2002, we began a three-year project to examine population status and trend of Kittlitz’s Murrelets in areas where distribution and abundance were poorly known. Results from the 2002 field season, focused on the south coast of the Kenai Peninsula, suggested that the local population of Kittlitz’s Murrelets has declined by ca. 74% since 1986, with a current population of ca. 500 individuals. Here we present results from the 2003 field season when we surveyed Kittlitz’s Murrelets along the southern coast of the Alaska Peninsula. This is a large region that encompasses a substantial portion of the known range of the Kittlitz’s Murrelet, yet has never been surveyed rigorously for murrelets or any other non-colonial marine birds. During four weeks of surveys, we established a set of nearshore and offshore transects (over 825 linear kilometers in total) with a stratified sample design, combining random and systematically selected transects. From a total of 123 individuals seen on transects, we estimate a total population of 2265 (95% CI 1165-4405) Kittlitz’s Murrelets along the south coast of the Alaska Peninsula. For comparison, we estimate the population size of the congeneric Marbled Murrelet ( Brachyramphus marmoratus ). We discuss broad-scale murrelet habitat relationships and species comparisons, and present recommendations for management and future work. Other species of marine birds and mammals were also surveyed; summarized information is included as an appendix.

Alaska↗

USGSDate not supplied

Seafloor studies of Mamala Bay, Honolulu, Hawaii

No comprehensive study of the effects of disposal of dredge spoils has been conducted to determine if the environment has suffered. The U.S. Army Corps of Engineers (USACE) has regularly dredged the shipping channels of Honolulu Harbor and Pearl Harbor for commercial and military purposes. The 5-year frequency for new dredging activity has led to the formation of extensive offshore wide deposits of relatively coarse sediments being created in a sedimentary environment that naturally collects much finer-grained materials. At the same time, the rapid growth of Honolulu and its suburban region over the past 3 decades has added nutrient-enriched sewage outfall to the artificially-heavy sediment load. The combined effects of dredge-spoil disposal and contaminant loads are not well-documented, and are poorly understood. The U.S. Geological Survey (USGS) began a comprehensive study to characterize the seafloor of Mamala Bay. In 1993, the USGS initiated a program sponsored by the USACE and the U.S. Environmental Protection Agency that produced a detailed map of the seafloor, especially of the distribution of dredge spoils, and a preliminary analysis of the environmental effects of dredge-spoils disposal and the chemical effects of the introduction of nutrient-rich outfall. Part of the work would involve sediment sampling, biological testing, and evaluation. Of particular interest is tissue sampling and analysis of endemic shrimp to determine the rates of absorption of toxic elements, such as heavy metals, that are easily brought into the food chain.

Hawaii↗

USGS2006

Estimating Potential Effects of Hypothetical Oil Spills on Polar Bears

Much is known about the transport and fate of oil spilled into the sea and its toxicity to exposed wildlife. Previously, however, there has been no way to quantify the probability that wildlife dispersed over the seascape would be exposed to spilled oil. Polar bears, the apical predator of the arctic, are widely dispersed near the continental shelves of the Arctic Ocean, an area also undergoing considerable hydrocarbon exploration and development. We used 15,308 satellite locations from 194 radiocollared polar bears to estimate the probability that polar bears could be exposed to hypothetical oil spills. We used a true 2 dimensional Gausian kernel density estimator, to estimate the number of bears likely to occur in each 1.00 km2 cell of a grid superimposed over near shore areas surrounding 2 oil production facilities: the existing Northstar oil production facility, and the proposed offshore site for the Liberty production facility. We estimated the standard errors of bear numbers per cell with bootstrapping. Simulated oil spill footprints for September and October, the times during which we hypothesized effects of an oil-spill would be worst, were estimated using real wind and current data collected between 1980 and 1996. We used ARC/Info software to calculate overlap (numbers of bears oiled) between simulated oil-spill footprints and polar bear grid-cell values. Numbers of bears potentially oiled by a hypothetical 5912 barrel spill (the largest spill thought probable from a pipeline breach) ranged from 0 to 27 polar bears for September open water conditions, and from 0 to 74 polar bears in October mixed ice conditions. Median numbers oiled by the 5912 barrel hypothetical spill from the Liberty simulation in September and October were 1 and 3 bears, equivalent values for the Northstar simulation were 3 and 11 bears. In October, 75% of trajectories from the 5912 barrel simulated spill at Liberty oiled 9 or fewer bears while 75% of the trajectories affected 20 or fewer polar bears when we simulated an October spill at the Northstar site. Northstar Island is nearer the active ice flaw zone than Liberty. Simulations suggested that oil spilled at Northstar would spread more effectively and more consistently into surrounding areas. Also, polar bear densities are consistently higher near Northstar. Oil spills simulated for the Liberty site were more erratic in the areas they covered and the numbers of bears impacted, and numbers of bears hypothetically exposed were usually smaller. Methods described here are broadly applicable to other dispersed marine wildlife. Key words: Arctic, Beaufort Sea, clustering, kernel, management, oil spill, polar bears, population delineation, radiotelemetry, satellite, smoothing, Ursus maritimus

Report↗

USGS1953

The "Clinton" sands in Canton, Dover, Massillon, and Navarre quadrangles, Ohio

The Canton, Dover, Massillon, and Navarre quadrangles cover about 880 square miles in eastern Ohio. Canton is the largest city in the mapped area. In these four quadrangles, the well drillers generally recognize three "Clinton" sands - in descending order, the "stray Clinton", the "red Clinton", and the "white Clinton". The Clinton sands of Ohio are of early Silurian age and probably correlate with the middle and upper part of the Albion sandstone in the Niagara gorge section in western New York. The study of drillers' logs and examination of well samples show that of the three so-called Clinton sands, the red is most readily recognized. The "Packer shell", a probable equivalent of the Clinton formation of New York, and the Queenston shale - the drillers' "red Medina" - are also good units for short distance correlations. Each of the Clinton sands consists of a thin layer that contains long narrow lenses of thicker sand. Although the pattern of the trend of the lenses varies for each of the Clinton sands, the trend generally is westward across the mapped area. It is thought that these lenses represent deposition in channels, probably offshore from a large delta. Production of gas and oil from the so-called Clinton apparently is closely related to the sorting, porosity, and permeability of the sand. Stratigraphic traps contain the oil or gas, and structure appears to be relatively unimportant in localizing the accumulation of the petroleum. East of the mapped area, the Clinton sands have not produced oil or gas in commercial quantities. Several parts of the mapped area may hold additional amounts of gas.

Ohio↗

USGS1978

A gravity study of the northern part of the Arctic National Wildlife Range, Alaska

Interpretation of all publicly available onshore gravity data provides a basis for outlining regional elements of the basement and overlying structure in the northern part of the Arctic National Wildlife Range. Major post-Carboniferous sedimentary basins whose centers lie offshore on the Beaufort Shelf extend onshore in the northern part of the study area. There is strong evidence that dense basement rocks beneath these onshore extensions rise to relatively shallow depths along the northeast Alaskan coast. Geologic studies and interpretation of the gravity data indicate that the gross structural and stratigraphic framework of this area is similar to that of the Prudhoe Bay region to the west. A high potential for petroleum accumulation in the Wildlife Range is indicated by the surface presence of oil seeps, oil sands, and outcrops of reservoir and source rocks along the southern margins of the onshore basinal areas. Present data suggest that the greatest potential for onshore accumulations of hydrocarbons lies in the area south and east of Barter Island and that possibilities exist there for commercially substantial reserves. The potential of this area cannot be fully assessed, however, until the subsurface extent of several major unconformities is known. Unfortunately, gravity data alone are of little use in such determinations. South of the mountain front, large granitic intrusions are marked by a sharply defined gravity minimum. The subsurface distribution of these bodies could probably be mapped by detailed gravity surveys. Detailed aeromagnetic surveys should serve equally well for such a purpose.

Alaska↗

USGS2001

Petroleum geology and resources of the North Caspian Basin, Kazakhstan and Russia

The North Caspian basin is a petroleum-rich but lightly explored basin located in Kazakhstan and Russia. It occupies the shallow northern portion of the Caspian Sea and a large plain to the north of the sea between the Volga and Ural Rivers and farther east to the Mugodzhary Highland, which is the southern continuation of the Ural foldbelt. The basin is bounded by the Paleozoic carbonate platform of the Volga-Ural province to the north and west and by the Ural, South Emba, and Karpinsky Hercynian foldbelts to the east and south. The basin was originated by pre-Late Devonian rifting and subsequent spreading that opened the oceanic crust, but the precise time of these tectonic events is not known. The sedimentary succession of the basin is more than 20 km thick in the central areas. The drilled Upper Devonian to Tertiary part of this succession includes a prominent thick Kungurian (uppermost Lower Permian) salt formation that separates strata into the subsalt and suprasalt sequences and played an important role in the formation of oil and gas fields. Shallow-shelf carbonate formations that contain various reefs and alternate with clastic wedges compose the subsalt sequence on the 1 basin margins. Basinward, these rocks grade into deep-water anoxic black shales and turbidites. The Kungurian salt formation is strongly deformed into domes and intervening depressions. The most active halokinesis occurred during Late Permian?Triassic time, but growth of salt domes continued later and some of them are exposed on the present-day surface. The suprasalt sequence is mostly composed of clastic rocks that are several kilometers thick in depressions between salt domes. A single total petroleum system is defined in the North Caspian basin. Discovered reserves are about 19.7 billion barrels of oil and natural gas liquids and 157 trillion cubic feet of gas. Much of the reserves are concentrated in the supergiant Tengiz, Karachaganak, and Astrakhan fields. A recent new oil discovery on the Kashagan structure offshore in the Caspian Sea is probably also of the supergiant status. Major oil and gas reserves are located in carbonate reservoirs in reefs and structural traps of the subsalt sequence. Substantially smaller reserves are located in numerous fields in the suprasalt sequence. These suprasalt fields are largely in shallow Jurassic and Cretaceous clastic reservoirs in salt dome-related traps. Petroleum source rocks are poorly identified by geochemical methods. However, geologic data indicate that the principal source rocks are Upper Devonian to Lower Permian deep-water black-shale facies stratigraphically correlative to shallow-shelf carbonate platforms on the basin margins. The main stage of hydrocarbon generation was probably in Late Permian and Triassic time, during deposition of thick orogenic clastics. Generated hydrocarbons migrated laterally into adjacent subsalt reservoirs and vertically, through depressions between Kungurian salt domes where the salt is thin or absent, into suprasalt clastic reservoirs. Six assessment units have been identified in the North Caspian basin. Four of them include Paleozoic subsalt rocks of the basin margins, and a fifth unit, which encompasses the entire total petroleum system area, includes the suprasalt sequence. All five of these assessment units are underexplored and have significant potential for new discoveries. Most undiscovered petroleum resources are expected in Paleozoic subsalt carbonate rocks. The assessment unit in subsalt rocks with the greatest undiscovered potential occupies the south basin margin. Petroleum potential of suprasalt rocks is lower; however, discoveries of many small to medium size fields are expected. The sixth identified assessment unit embraces subsalt rocks of the central basin areas. The top of subsalt rocks in these areas occurs at depths ranging from 7 to 10 kilometers and has not been reached by wells. Undiscovered resources of this unit did not rec

Bulletin↗

USGS2003

Petroleum geology and resources of the West Siberian Basin, Russia

The West Siberian basin is the largest petroleum basin in the world covering an area of about 2.2 million km2. The basin occupies a swampy plain between the Ural Mountains and the Yenisey River. On the north, the basin extends offshore into the southern Kara Sea. On the west, north, and east, the basin is surrounded by the Ural, Yenisey Ridge, and Turukhan-Igarka foldbelts that experienced major deformations during the Hercynian tectonic event and the Novaya Zemlya foldbelt that was deformed in early Cimmerian (Triassic) time. On the south, the folded Caledonian structures of the Central Kazakhstan and Altay-Sayan regions dip northward beneath the basin?s sedimentary cover. The basin is a relatively undeformed Mesozoic sag that overlies the Hercynian accreted terrane and the Early Triassic rift system. The basement is composed of foldbelts that were deformed in Late Carboniferous?Permian time during collision of the Siberian and Kazakhstan continents with the Russian craton. The basement also includes several microcontinental blocks with a relatively undeformed Paleozoic sedimentary sequence. The sedimentary succession of the basin is composed of Middle Triassic through Tertiary clastic rocks. The lower part of this succession is present only in the northern part of the basin; southward, progressively younger strata onlap the basement, so that in the southern areas the basement is overlain by Toarcian and younger rocks. The important stage in tectono-stratigraphic development of the basin was formation of a deep-water sea in Volgian?early Berriasian time. The sea covered more than one million km2 in the central basin area. Highly organic-rich siliceous shales of the Bazhenov Formation were deposited during this time in anoxic conditions on the sea bottom. Rocks of this formation have generated more than 80 percent of West Siberian oil reserves and probably a substantial part of its gas reserves. The deep-water basin was filled by prograding clastic clinoforms during Neocomian time. The clastic material was transported by a system of rivers dominantly from the eastern provenance. Sandstones within the Neocomian clinoforms contain the principal oil reservoirs. The thick continental Aptian?Cenomanian Pokur Formation above the Neocomian sequence contains giant gas reserves in the northern part of the basin. Three total petroleum systems are identified in the West Siberian basin. Volumes of discovered hydrocarbons in these systems are 144 billion barrels of oil and more than 1,300 trillion cubic feet of gas. The assessed mean undiscovered resources are 55.2 billion barrels of oil, 642.9 trillion cubic feet of gas, and 20.5 billion barrels of natural gas liquids. The largest known oil reserves are in the Bazhenov-Neocomian total petroleum system that includes Upper Jurassic and younger rocks of the central and southern parts of the basin. Oil reservoirs are mainly in Neocomian and Upper Jurassic clastic strata. Source rocks are organic-rich siliceous shales of the Bazhenov Formation. Most discovered reserves are in structural traps, but stratigraphic traps in the Neocomian clinoform sequence are pro-ductive and are expected to contain much of the undiscovered resources. Two assessment units are identified in this total petroleum system. The first assessment unit includes all conventional reservoirs in the stratigraphic interval from the Upper Jurassic to the Cenomanian. The second unit includes unconventional (or continuous), self-sourced, fractured reservoirs in the Bazhenov Formation. This unit was not assessed quantitatively. The Togur-Tyumen total petroleum system covers the same geographic area as the Bazhenov-Neocomian system, but it includes older, Lower?Middle Jurassic strata and weathered rocks at the top of the pre-Jurassic sequence. A Callovian regional shale seal of the Abalak and lower Vasyugan Formations separates the two systems. The Togur-Tyumen system is oil-prone; gas reserves are insignificant. The principal o

Bulletin↗

USGS2006

Oligocene–Miocene Maykop/Diatom Total Petroleum System of the South Caspian Basin Province, Azerbaijan, Iran, and Turkmenistan

The South Caspian Basin encompasses the southern extension of the Caspian Sea, including land areas in eastern Azerbaijan, western Turkmenistan, and northern Iran. The region is endowed with abundant petroleum resources, and oil and gas production has played an important commercial role in the region for more than 150 yr, especially in Azerbaijan and to a lesser extent in Turkmenistan. Major oil reserves are concentrated in 2,500–3,500 m of shallow-marine, deltaic to lacustrine deposits of middle Pliocene age. To date, some 620 oil and gas fields have been discovered in strata ranging in age from Miocene to Quaternary; however, less than a dozen fields produce from both Miocene and Quaternary reservoirs. The principal reserves and targets for future exploration are in the middle Pliocene Productive Series. The South Caspian Basin is unusual in several respects: sediment accumulated at exceptionally high rates (as high as 4.5 km/m.y.); sediment accumulation in each of three depocenters was as great as 20 km (5 km of Pliocene sedimentary deposits); there was low sediment compaction; geothermal gradients are relatively low (1.5°C/100 m); and abnormally high pressures exist in some basin areas. In this depositional environment, good reservoir porosities and permeabilities could be preserved to depths as great as 12 km. Analysis of source rock samples collected from outcrops, cores, and mud-volcano ejecta shows total organic carbon contents to range from 1.2 to more than 10 percent, the richest being in the middle part of the Oligocene–Miocene Maykop Series. Source rocks, with thicknesses ranging from 100 m to more than 2,500 m, form the cores of many anticlines in the basin. Reservoir rocks, consisting of fluviodeltaic clastic deposits ranging in texture from mudstone to conglomerate, are mostly in the middle Pliocene Productive Series, but some Miocene and lower Pliocene reservoirs are also present. Reservoir seals are formed by interbedded shales. Hydrocarbon traps developed mainly during the late Pliocene and early Pleistocene. The Oligocene–Miocene Maykop/Diatom Total Petroleum System within the South Caspian Basin is separated into five hydrocarbon assessment units: Apsheron-Pribalkhan Zone, Lower Kura Depression and Adjacent Shelf, Gograndag-Okarem Zone, Central Offshore, and Iran Onshore-Nearshore.

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USGS2001

Total petroleum systems of the Pelagian Province, Tunisia, Libya, Italy, and Malta; the Bou Dabbous, Tertiary and Jurassic-Cretaceous composite

Undiscovered conventional oil and gas resources were assessed within total petroleum systems of the Pelagian Province (2048) as part of the U.S. Geological Survey World Petroleum Assessment 2000. The Pelagian Province is located mainly in eastern Tunisia and northwestern Libya. Small portions of the province extend into Malta and offshore Italy. Although several petroleum systems may exist, only two ?composite? total petroleum systems were identified. Each total petroleum system comprises a single assessment unit. These total petroleum systems are called the Bou Dabbous?Tertiary and Jurassic-Cretaceous Composite, named after the source-rock intervals and reservoir-rock ages. The main source rocks include mudstone of the Eocene Bou Dabbous Formation; Cretaceous Bahloul, Lower Fahdene, and M?Cherga Formations; and Jurassic Nara Formation. Known reservoirs are in carbonate rocks and sandstone intervals throughout the Upper Jurassic, Cretaceous, and Tertiary sections. Traps for known accumulations include fault blocks, low-amplitude anticlines, high-amplitude anticlines associated with reverse faults, wrench fault structures, and stratigraphic traps. The estimated means of the undiscovered conventional petroleum volumes in total petroleum systems of the Pelagian Province are as follows: [MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids] Total Petroleum System MMBO BCFG MMBNGL Bou Dabbous?Tertiary 667 2,746 64 Jurassic-Cretaceous Composite 403 2,280 27

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