Gamma-ray Values Research Articles

Well logging responses of UHP metamorphic rocks from CCSD main hole in Sulu terrane, eastern central China

The lithologies of the Chinese Continental Scientific Drilling main hole (CCSD-MH) are mainly comprised of orthogneiss, paragneiss, eclogite, amphibolite, and ultramafic rocks. The statistical results of logs of CCSD-MH indicate that ultramafic rocks are characterized by very high CNL (neutron log) and very low GR (gamma ray log) and RD (resistivity log); eclogites are characterized by high DEN (density), V P (P-wave velocity) and PE (photoelectric absorption capture cross section); orthogneiss and paragneiss are characterized by high GR, U (uranium content), Th (thorium content), K (potassium content) and RD, and low DEN, PE, and CNL; logging values of amphibolite are between the logging values of eclogites and paragneiss. In addition, the logs could reflect the degree of retrograde metamorphism of eclogites. The upper section (100–2 000 m) shows higher DEN, PE, V P, and lower GR, U, Th, K, RD than the lower section (2 000–5 000 m). Most logs of the upper section are more fluctuant than those of the lower section. This indicates that the upper section has more heterogeneities than the lower section. The cross plots of logs indicate that DEN, GR, K, and CNL are more powerful in identifying ultrahigh pressure metamorphic (UHPM) rocks at the CCSD-MH. GR value of the rocks from CCSD-MH shows obviously an increasing trend from ultramafic rock (the most mafic rocks at CCSD-MH) to orthogneiss (the most acid rocks at CCSD-MH). On the contrary, DEN value decreases from the ultramafic rock to the orthogneiss. CNL log is a good indicator of the content of structure water in crystalline rocks.

Uranium enrichment in lacustrine oil source rocks of the Chang 7 member of the Yanchang Formation, Erdos Basin, China

The oil source rocks of the Chang 7 member of the Yanchang Formation in the Erdos Basin were deposited during maximum lake extension during the Late Triassic and show a remarkable positive uranium anomaly, with an average uranium content as high as 51.1 μg/g. Uranium is enriched together with organic matter and elements such as Fe, S, Cu, V and Mo in the rocks. The detailed biological markers determined in the Chang 7 member indicate that the lake water column was oxidizing during deposition of the Chang 7 member. However, redox indicators for sediments such as S 2− content, V/Sc and V/(V + Ni) ratios demonstrate that it was a typical anoxic diagenetic setting. The contrasted redox conditions between the water column and the sediment with a very high content of organic matter provided favorable physical and chemical conditions for syngenetic uranium enrichment in the oil source rocks of the Chang 7 member. Possible uranium sources may be the extensive U-rich volcanic ash that resulted from contemporaneous volcanic eruption and uranium material transported by hydrothermal conduits into the basin. The uranium from terrestrial clastics was unlike because uranium concentration was not higher in the margin area of basin where the terrestrial material input was high. As indicated by correlative analysis, the oil source rocks of the Chang 7 member show high gamma-ray values for radioactive well log data that reflect a positive uranium anomaly and are characterized by high resistance, low electric potential and low density. As a result, well log data can be used to identify positive uranium anomalies and spatial distribution of the oil source rocks in the Erdos Basin. The estimation of the total uranium reserves in the Chang 7 member attain 0.8 × 10 8 t.

Late Quaternary stratigraphic analysis of the Lake Malawi Rift, East Africa: An integration of drill-core and seismic-reflection data

Lake Malawi contains a long continuous sedimentary record of climate change in the southern hemisphere African tropics. We develop a stratigraphic framework of this basin over the last ∼ 150 ka by integrating several vintages of seismic-reflection data with recently acquired drill cores. In the seismic-reflection data set, we document three lake-level cycles where progradational delta seismic facies and erosional truncation surfaces mark the basal boundary of each sequence. The clinoform packages and their down-dip, time-equivalent surfaces can be mapped throughout each basin, where each major lowstand surface was followed by a transgression and highstand. On several occasions, lake level dropped as much as 500 m below present lake level (BPLL) in the North Basin and 550 m BPLL in the Central Basin, resulting in a 97% reduction of water volume and 89% reduction of water surface area relative to modern conditions. Evidence for these lake-level fluctuations in the drill cores includes major changes in saturated bulk density, natural gamma ray values, and total organic carbon. During lowstands, density values doubled, while total organic carbon values dropped from ∼ 5% to 0.2%. Coarse-grained sediment and organic matter flux into the basin were higher during transgressions, when precipitation, runoff, sediment supply, and nutrient input were high. This sedimentation pattern is also observed in seismic-reflection profiles, where coarse-grained seismic facies occur at the bases of sequences, and in the drill-core data where the highest total organic carbon values are observed immediately above lowstand surfaces.

Extending NMR data for permeability estimation in fine-grained sediments

We combine nuclear magnetic resonance (NMR) transverse relaxation time data and gamma ray data to estimate lithology-dependent permeability in silt- and clay-rich sediments. This approach extends the utility of the Schlumberger-Doll Research (SDR) permeability equation from reservoirs to aquicludes and seals, and thus improves the value and robustness of NMR data. Data from Keathley Canyon, northern Gulf of Mexico show that NMR data can be used to define permeability from 10 −18 to 10 −14 m 2 (0.001–10 millidarcies) as calibrated and tested by direct measurements on core samples. We performed uniaxial, constant rate-of-strain consolidation experiments on sediments from Keathley Canyon to determine core-scale permeability. Permeabilities from these experiments were compared to permeabilities calculated from logging-while-drilling data. A better fit between log-derived permeability and laboratory-measured permeability was obtained using the SDR equation with a variable coefficient A, rather than a constant A as is typically used. We show how A is a function of lithology and can be modeled from gamma ray data. The relationship between A and gamma ray values suggests that variations in A are caused by platy clay minerals and the effect they have on the pore system. Our results provide improved means for permeability estimation for application in basin flow modeling, hydrocarbon migration modeling, and well completion design.

Geophysical and geological signatures of relative sea level change in the upper Wheeler Formation, Drum Mountains, West-Central Utah: A perspective into exceptional preservation of fossils

The Middle Cambrian Wheeler Formation in Utah is renowned for its exceptionally preserved fossils. Herein, geophysical techniques (gamma ray spectrometry and magnetic susceptibility), carbonate analyses, lithofacies, and fossils are used to characterize limestones and calcareous mudrocks of the upper portion of the middle Wheeler Formation and the upper Wheeler Formation in the Drum Mountains, West-Central Utah. This succession was deposited on a mixed carbonate-siliciclastic ramp within the House Range Embayment, an eastward incursion of deeper water, created and bounded on the southeast by a normal fault. Geophysical data clearly reveal variations in carbonate versus clay content. Observed patterns among outcrop geophysical data, lithofacies, and fossils are interpreted within the contexts of relative sea level changes and a sequence stratigraphic model. Above a basal interval of rhythmically-bedded limestones interpreted to have been deposited on the middle part of the ramp during a period of relatively low sea level, lithofacies, rising gamma ray and rising magnetic susceptibility track a transgression, which culminates in a maximum flooding surface (MFS). This MFS is near the base of a “hot zone”: an ~ 6-m-thick interval of exceptional preservation ( Konservat-Lagerstätte), highest magnetic susceptibility, highest gamma ray values, highest clay content, and with a bulk carbonate content of ~ 4–20%. The basal layer of this zone contains abundant, fully articulated specimens of agnostids and tiny polymerid trilobites. Exceptionally preserved fossils, such as non-trilobite arthropods, priapulid and annelid worms, hyolithids, phyllocarid arthropods with soft parts, algae, and fragile sponges, are much more abundant within this hot zone than in other strata of the succession. These specimens are preserved in very thin-bedded to laminated mudrock, only rarely interrupted by macroscopic signs of bioturbation. Stratigraphically above the hot zone is a thick unit of mudrock in which both gamma ray and magnetic susceptibility decrease, gradually at first and then precipitously to very low levels; in concert, few soft-bodied specimens have been recovered from this upper interval. The gradual decrease in geophysical responses is interpreted to be the result of gradual shallowing during highstand, caused mainly by progradation of the carbonate factory; it was followed by an abrupt regression. Mudrocks are capped by burrow-mottled grainstone and stromatolitic boundstone, representing a lowstand or the earliest transgressive phase of the next cycle. These observations lead to the following conclusions: (1) the upper Wheeler Formation represents most of a third or fourth-order cycle of relative sea level change on a mixed carbonate-siliciclastic ramp, rather than a period of shallow-water lagoonal sedimentation as previously proposed; (2) superimposed on this overall cycle are several (perhaps many) higher-order fluctuations in relative sea level; (3) when the present results are coupled with results of earlier workers, who interpreted the lower and middle Wheeler Formation as a major transgression and regression, the Wheeler Formation of the Drum Mountains involves two major sea level cycles, rather than one; (4) the Konservat-Lagerstätte was preserved in the deepest-water, early-highstand portion of the upper Wheeler sequence; and (5) these results support earlier hypotheses that Konservat-Lagerstätten in mixed carbonate-siliciclastic successions are most likely to occur during late transgression to early highstand, given that low energy, anoxic conditions prevailed.

Identification of early Llandovery (Silurian) anoxic palaeo-depressions at the western margin of the Murzuq Basin (southwest Libya), based on gamma-ray spectrometry in surface exposures

ABSTRACT Following the melting of the Gondwanan icecap and the resulting postglacial sea-level rise, organic-rich shales were deposited in shelfal palaeo-depressions across North Africa and Arabia during the latest Ordovician to earliest Silurian. The unit is absent on palaeohighs that were flooded only later when the anoxic event had already ended. The regional distribution of the Silurian black shale is now well-known for the subsurface of the central parts of the Murzuq Basin, in Libya, where many exploration wells have been drilled and where the shale represents the main hydrocarbon source rock. On well logs, the Silurian black shale is easily recognisable due to increased uranium concentrations and, therefore, elevated gamma-ray values. The uranium in the shales “precipitated” under oxygen-reduced conditions and generally a linear relationship between uranium and organic content is developed. The distribution of the Silurian organic-rich shales in the outcrop belts surrounding the Murzuq Basin has been long unknown because Saharan surface weathering has commonly destroyed the organic matter and black colour of the shales, making it complicated to identify the previously organic-rich unit in the field. In an attempt to distinguish (previously) organic-rich from organically lean shales at outcrop, seven sections that straddle the Ordovician-Silurian boundary were measured by portable gamma-ray spectrometer along the outcrops of the western margin of the Murzuq Basin. It was found that the uranium content of the shales remained largely unaltered by the weathering processes and could therefore be used as a valid proxy parameter to distinguish between pre-weathering organically rich and lean shales. It is now possible to identify and map-out the thickness and approximate organic richness of the black shale using measurement of uranium radiation. Five of the newly measured sections are characterised by uranium-enriched intervals, representing areas of earliest Silurian palaeo-depressions. Major uranium peaks are absent in the spectral gamma-ray curves of two other sections, which are interpreted to mark earliest Silurian palaeo-highs. The new data on the distribution of Silurian black shales from the outcrop belt was integrated with subsurface data from the Murzuq Basin. The resulting map of the distribution of black shales may help with predictions of the occurrence of this unit in less well-explored areas of the basin. Graptolite biostratigraphic data suggests that the anoxic event centred on the middle Rhuddanian, with more oxygenated conditions and onset of deposition of organically leaner shales having commenced sometime during the late Rhuddanian. The presence of anoxic palaeo-depressions during the earliest Silurian within the Ghat outcrop belt indicates that the Tihemboka High at the western margin of the Murzuq Basin could not have been a positive structure during this time.

A study of charge density in copper

Quantitative convergent-beam electron diffraction (QCBED) experiments allow absolute scale measurements of low-order structure factors with very high accuracy. In this paper, eight low-order structure factors for copper measured by QCBED have been combined with the higher-order gamma-ray structure factors in order to obtain a larger highly accurate experimental data set. The gamma-ray values were relativistically corrected and rescaled. The new data set was then used for studying the charge distribution in copper. Charge deformation maps have been produced and both a maximum-entropy and a multipole analysis have been applied to the data. The result is compared to density functional theory calculations. An almost spherical charge depletion is found around the atomic sites showing typical metal bonding in copper.

Frasnian organic-rich shales in North Africa: regional distribution and depositional model

Abstract During the Frasnian, organic-rich shales were deposited across much of North African, most notably in parts of Morocco, Algeria, southern Tunisia, western Libya and the Western Desert of Egypt. They are estimated to be the origin of about 10% of all Palaeozoic-sourced hydrocarbons in North Africa. The depositional, palaeoecological and geochemical characteristics of this black shale unit can be best studied in the eastern Algerian Berkine (i.e. western Ghadames) Basin where the thickest and organically richest ‘hot shales’ occur. In wireline logs, the Frasnian hot shales are marked by high gamma-ray values, often in excess of 300–400 API, which, according to gamma-ray spectrometry, almost exclusively originate from an elevated uranium content. Comparison with total organic carbon (TOC) data shows that the gamma-ray curve can be used as a proxy for the TOC content of the Frasnian shales, with 150 API correlating approximately with TOCs of about 3% in eastern Algeria. The hot shale unit usually consists of high-frequency, high-amplitude, metre-scale gammaray cycles; however, especially in the thicker hot shale units, the lower frequency envelope curve of the high-frequency gamma-ray cycles has a gradual, bell-shaped form. The gradual increase and subsequent decrease in organic richness over time may be interpreted as evidence for a gradual rise and subsequent fall of the oxygen minimum zone (OMZ), with invasion of oxygen-depleted waters onto the North African shelf. The rise of the OMZ may have been triggered by the Early Frasnian transgression, which has been described in detail from Morocco, where it is now well-dated by conodonts and is associated with characteristic black shales and carbonates. Additional high-resolution biostratigraphic data are still needed in order to better correlate the Frasnian hot shales of Algeria, Tunisia and western Libya with other Late Devonian dysaerobic/anaerobic facies in Morocco, western Egypt, Europe, South and North America.

Determining the inputs to the Mariana Subduction Factory: Using core-log integration to reconstruct basement lithology at ODP Hole 801C

[1] Calculating elemental mass balance across subduction zones enhances our understanding of global geochemical budgets and large-scale Earth processes. However, to accurately constrain the input flux, it is critical to know the lithological diversity and chemical characteristics of the downgoing oceanic plate. The west Pacific altered ocean crust that was drilled during Ocean Drilling Program (ODP) Leg 185 represents a significant component of the input to the Mariana Subduction Factory. The lithological sequence in Hole 801C consists of aphyric basalt, occurring as thick massive units, pillow units, and breccia units. The shallowest basalts are intercalated with sediments and two hydrothermal deposits. Core recovery was good for a basement hole (average 47%); however, over half the lithological section was unaccounted for. Downhole logging data provide a continuous record of physical, chemical, and structural properties of the rocks at the borehole wall, thus, when calibrated using available cored material, they can be used to reconstruct lithology in unrecovered intervals. Core-log integration results reveal a significant bias in core recovery, with infrequent retrieval of delicate breccia units and preferential recovery of more massive, competent, and less altered flow units. This is important because the breccia units are host to many of the key tracer elements used in mass balance calculations. The massive basalts exhibit high density, resistivity and velocity values and low porosity and gamma ray values. Formation MicroScanner (FMS) images of massive basalts are bright (reflecting their resistive nature) with a homogenous texture and regular fracture pattern. Breccia or pillow basalts are characterised by low resistivity, density and velocity, and high porosity and gamma ray values and unrecovered intervals displayed these same characteristics. The reconstructed log-based lithological sequence consists of thick massive flow units (27.4%), pillow units (33%), breccia units (31%), sediments (1.4%), and hydrothermal deposits (1.3%), with 5.9% unclassified due to unreliable tool response in intervals where hole conditions were poor. These findings have a significant bearing on the Subduction Factory recycling equation. The proportion of pillow basalts doubled and the amount of breccia increased six-fold from that reported using core description alone, demonstrating convincingly that core-log integration is essential to provide an accurate representation of the input flux. The log-based stratigraphy reconstructed for Hole 801C represents the first example of Jurassic fast-spread (160 km/m.y.) ocean crust and provides constraints on the relationships between crustal structure, age, alteration, and spreading rate.

Facies Architecture of an Early Jurassic Carbonate Platform Slope (Jbel Bou Dahar, High Atlas, Morocco)

The plateau of Jbel Bou Dahar (High Atlas, Morocco) represents an outstanding example of a well-preserved early Mesozoic carbonate platform. Field observations of the large-scale sedimentary structures and the arrangement of lithofacies associations within the mid- and lower slope were combined with detailed microfacies analyses and gamma-ray measurements. The investigations were carried out in order to (1) resolve the sequence stratigraphic architecture of the slope, and (2) reconstruct the sediment export patterns and depositional processes related to high-frequency sea-level variations. The slope strata are arranged into two different types of lithofacies associations that are bounded by characteristic discontinuities and show a specific occurrence along the investigated slope section. The steep (8-18°) mid-slope is dominated by thick- to medium-bedded limestone successions that were able to maintain a high angle of repose due to their detrital composition. The sediment packages are bounded by basal low-angle unconformities and wedge out within the lower slope or thin into the adjacent basin as single carbonate layers. The sediments are characterized by coarse-grained and poorly sorted floatstones to rudstones, which show low gamma-ray values and contain components derived mainly from the platform edge. The facies associations were interpreted as debris-flow deposits that were exported during sea-level lowstands when the flattened platform top and extended parts of the margin were exposed and sediment production was restricted mainly to the outer margin and upper slope section. In contrast, the gently dipping lower slope (4-18°) is characterized by rhythmic limestone-marl alternations that show increasing thicknesses with increasing distance from the platform. From the lower part of the mid-slope down, the successions onlap the debris-flow deposits, forming a characteristic interfingering pattern of different lithofacies associations. The medium- to thin-bedded carbonates of the limestone-marl alternations comprise two different microfacies types: (1) mudstones to wackestones with high gamma-ray values and a mixed shallow-marine to deep-marine microfacies, overlain by (2) well-sorted, arenitic packstones to grainstones with low gamma-ray values and a shallow-marine component composition. The latter points to the platform interior and margin as primary source areas. The shallow-marine material was exported during sea-level highstands when the carbonate factory included the entire platform margin and the flooded lagoons of the platform top. During these periods, the exported fine-grained sediments could maintain only low angles of repose. Thus they bypassed the steep upper and mid-slope sections, developed into turbidites during downslope transport, and were finally redeposited on the gently dipping lower slope and in the adjacent basin. The architecture of the investigated slope area shows a characteristic interfingering of different facies associations that can be interpreted as genetic highstand and lowstand stratigraphic sequences. The arrangement of these sequences along the slope, as well as their composition, were controlled by high-frequency sea-level fluctuations, affecting the primary sediment production (source area), export patterns, and sedimentary processes, resulting in a shift of the depositional centers during different sea-level stands.

Organic matter evaluation of the Nkporo Shale, Anambra basin, from wireline logs.

A modified form of Schmoker's formation density log technique has been used in evaluating the total organic carbon content TOC of the continuous shale sections of the Nkporo Shale in three exploratory wells (Alade – 1, Anambra River – 1, and Anambra River – 3) in the Anambra Basin. Weight percent TOC was found to range from 0.2 – 4.9%; with an average of 2.8% in Alade – 1 well; 0.2 – 4.3%, with an average of 1.8% in the Anambra River – 1 well; and 0.2 – 5.3%, with an average of 3% in the Anambra River – 3 well. The average TOC values obtained in this study are similar to the average TOC values obtained from organic geochemical analysis of the Nkporo Shale in the Anambra Basin, the Afikpo Syncline, and the Calabar Flank. Crossplots of the formation density and the gamma ray logs show a generally positive correlation between them; although a few intervals have high gamma ray values corresponding with low formation density. The positive correlation between the formation density and the gamma ray logs suggests a dominantly terrestrial source for the organic matter. Intervals with low formation density (high TOC) corresponding with high gamma ray values are interpreted as source beds with significant marine organic matter. Global Journal of Pure and Applied Sciences Volume , No 1 January (2001) pp. 103-110 KEY WORDS: Organic carbon, wireline logs, source rocks, shale, basins.

Refinement of the Messinian APTS from sedimentary cycle patterns in the lacustrine Lava section (Servia Basin, NW Greece)

A high-resolution cyclostratigraphy and magnetostratigraphy is presented for the Messinian lacustrine Lava section from the Servia Basin in NW Greece, constraining more precisely the absolute ages of magnetic polarity subchrons C3An.1n and C3An.2n. The section contains 15 distinct sedimentary cycles of alternating dark- and light-coloured marls, while the gamma-ray attenuation record reveals an additional five to six cycles. The cycles in the lower half of the section are on average 5.3 m thick, as opposed to the cycles in the upper part, which have an average thickness of 3.1 m. Palynological results define the lithological alternations in both the lower and upper cycles in terms of periodic changes in humidity, where the light marls represent the humid periods and the dark marls the relatively dry periods. Changes in cycle thickness and shifts in average gamma-ray values suggest a rather abrupt decrease in sedimentation rate at ∼60 m in the section. This is confirmed by the magnetostratigraphy, which recorded four reversals, which – given the biostratigraphic constraints from the Lava locality – could be correlated unambiguously to subchrons C3An.1n and C3An.2n of the geomagnetic polarity time scale. With this magnetostratigraphic time control, the average duration of the cycles can be calculated to be constant in the entire section, and similar to precession. The astronomical origin of the cycles is confirmed by the results of spectral analyses of gamma-ray and susceptibility time series. The sedimentary cycles in the upper part of the Lava section are unambiguously tuned to insolation using the typical clustering of the cycles that follows the eccentricity cycle. The filtered gamma-ray record centred at 41 kyr confirms the tuning in the upper part and allows tuning of the lower part. The tuning results in accurate ages for the sedimentary cycles and polarity reversals that confirm the astronomical tuning of [Krijgsman et al., Nature 400 (1999) 652–655], but define more precisely the astronomical polarity time scale.

Characterisation and high resolution sequence stratigraphy of storm-dominated braid delta and shoreface sequences from the Basal Grit Group (Namurian) of the South Wales Variscan peripheral foreland basin

Sedimentological studies of Basal Grit successions indicate that they were deposited in braid deltas that prograded into a storm-dominated, shallow-marine, ramp-type environment. High resolution sequence stratigraphy data and concepts have been used to subdivide the strata into fourth-order systems tracts and longer period, third-order sequence sets. Falling stage and lowstand deposits of forced regressions are recorded by a variety of sharp-based/incised sandstone bodies, transgressive phases by thin marine bands with high gamma ray values and highstands by coarsening upward parasequences. Incised, lowstand sandstone bodies are particularly well represented and these have frequently removed underlying highstand and transgressive deposits. Their predominance is considered to be related to high sediment flux and slow basin subsidence, the latter frequently being balanced or outpaced by glacio-eustatic sea level falls resulting in the creation of limited to negative accommodation space. Synsedimentary tectonics have frequently overprinted the effects of glacio-eustatic sea level changes. The complex interplay between these allogenic processes was a direct result of repeated Gondwana glaciations (slow sea level falls) and melt-outs (rapid sea level rises) and the location of the depositional site on the cratonic side of a Variscan foreland basin. The tectono-eustatic depositional model proposed in this case study is considered as an analogue for a number of hydrocarbon-bearing sequences.

Influences of depositional environment and diagenesis on geophysical log response in the South Carolina Coastal Plain: effects of sedimentary fabric and mineralogy

Interpretations of depositional environments and hydrologic units are made routinely from the study of geophysical well logs. Spontaneous potential (SP) and resistivity logs can be used as indicators of textural parameters. Gamma-ray logs denote lithologic zones based on the presence of radioactive material, particularly in fine-grained sediments. On the South Carolina Coastal Plain, surficial fluvially derived cobbles, sands and clays are characterized by very low radioactivity. Weathered paleosurfaces enriched in hydroxy-interlayered vermiculite (HIV) and associated mica produce higher gamma-ray responses at discrete intervals. The underlying sequence of siliciclastic shelf sands, deposited under conditions of eustatic sea-level rise, has high concentrations of smectite and low gamma-ray signatures. Small-scale variability of gamma radiation reflects temporal changes of sub-environments; higher values are recorded in finer-grained, organic-rich nearshore sediments. Calcareous Ecocene sediments are characterized by highly variable signals on the gamma-ray, SP and resistivity logs. Phospharic material, smectite and disseminated organic matter produced very high gamma-ray values; corresponding low SP readings are a function of low macro- to microscale porosity resulting from the fine grain size and authigenic pore-filling cement. Intervals that resemble massively bedded chalks at the macroscale are characterized by low gamma-ray and resistivity values and high SP readings. High SP values result from microposity preserved in biogenic tests rather than intergranular porosity that would be expected for a permeable sand. These observations indicate the importance of incorporating sedimentologic techniques with well-log data for a comprehensive evaluation of subsurface lithologic units.

High-Resolution Surface and Subsurface Sequence Stratigraphy of Late Middle to Late Ordovician  (Late Mohawkian-Cincinnatian) Foreland Basin Rocks, Kentucky and Virginia

The late Middle to Late Ordovician (late Mohawkian to Cincinnatian) supersequence in Kentucky and Virginia was deposited in a tectonically active foreland basin during a transition from the ice-free Early Ordovician to the glacial Late Ordovician. This sequence was deposited in less than 12 m.y., is 250-500 m thick, and is composed of four large third-order sequences (each 40-150 m thick) that are regionally correlative in outcrops, cores, and gamma-ray logs. Smaller scale third-order parasequence sets (up to 20 m thick) and component parasequences (1-8 m thick) make up the larger sequences, and are only locally correlative. Subtidal-dominated parasequences comprise the basal part of each sequence, whereas shallower subtidal- or peritidal-dominated parasequences compose upper parts of sequences. Each large, third-order sequence is asymmetric and marked by lowstand, transgressive, and highstand systems tracts with unique lithologic and gamma-ray log response characteristics. Ramp margin wedges (RMW) are poorly developed and consist of marine siltstone or grainstone/packstone sheets extending into deep ramp settings; these sheets have high gamma-ray values in their base and lower, blockier gamma-ray responses in their tops. Transgressive systems tracts (TST) are thin; composed of high-energy, locally phosphatic grainstone ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received February 7, 1996; revised manuscript received May 27, 1997; final acceptance June 16, 1997. 2Department of Geology, Virginia Tech, Blacksburg, Virginia 24061-0420. Present address: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. 3Department of Geology, Virginia Tech, Blacksburg, Virginia 24061-0420. This study was supported by NSF Grant EAR-9316057 to J. F. Read, a grant from Mobil Oil Company, and grants-in-aid of research from the Geological Society of America and Appalachian Basin Industrial Associates to Mike Pope. We thank John Haynes, Brian Keith, and Rick Major for thorough and insightful reviews of an earlier version of this manuscript.

Reservoir Characterization Utilizing a Low Cost Resistivity-Porosity Crossplot and an Interactive Spreadsheet: ABSTRACT

In mature oil and gas producing provinces it is necessary to find a low cost, but reliable method to evaluate oil/gas field reservoirs for improving recovery. If a field has very limited core data, and production is commingled, the reservoir study will often need to rely on wireline logs of many types and from different logging companies. The challenge is to fully evaluate each well with a technique in which reservoir characterization is comparable and consistent so as not to [open quotes]build[close quotes] anomalies that are not really present. A reliable technique is the [open quotes]Super[close quotes] Pickett plot that is an extension of the classical log-log porosity/resistivity plot. The [open quotes]Super[close quotes] Pickett plot is a pattern recognition tool where points are linked successively by depth and annotated with lines corresponding to bulk volume water and permeability in addition to the standard water saturation, porosity, and resistivity lines. Subtle trends and patterns of these depth-linked series of points are used to characterize each reservoir. Color is used to display additional attributes such as gamma ray values, photo electric values, capillary pressure and lithology (RHOMAA/UMAA). The computational power and flexibility of a spreadsheet computer program is utilized with a colormore » graphic display to expedite the characterization of complex reservoirs. Key reservoir parameters derived from this analysis are mapped using the software to portray interwell heterogeneities and evaluate continuity. This log analysis technique can be tailored to varied, complex heterogeneous reservoirs both siliciclastics and carbonate lithology. Results provide the means to promote communication among geologists and engineers. The use of this technique is illustrated by a study of the Schaben Field in Ness County, Kansas, which is being supported by a Class II DOE grant.« less

Use of Variograms to Quantify Distribution of Barriers to Flow in Incised Valley-Fill Newcastle Sandstone, Wyoming: ABSTRACT

Abstract The objective of this paper is to quantify the distribution of potential flow units within valley-fill outcrop deposits by utilizing geostatistical parameters, especially variograms. The Newcastle (Muddy) Sandstone, which is a prolific producer in the Powder River Basin of Wyoming, is the subject of our outcrop reservoir characterization analysis which compares the relationships of detailed geological (sedimentological) interpretations of facies, detailed in situ permeability measurements, and outcrop gamma-ray log surveys. Much of the Newcastle (Muddy) Sandstone, which crops out near Newcastle, Wyoming was deposited by tidal currents backfilling valleys formed during a Lower Cretaceous sea level drop. The Skull Creek Shales which underlies and is lateral to the Newcastle sandstone forms lateral and bottom seals. We concurrently measured the permeabilities and gamma-ray responses of beds on three vertical transects with a grid extending normal to the bedding and three lateral grids which are as nearly parallel to the bedding as possible. We generated geostatistical parameters from these data sets. In all cases, Dykstra-Parsons coefficients are in the neighborhood of 0.35 for the single-unit data and above 0.8 for the multi-unit measurements. Coefficients of variation are below 0.5 for single units and above 0.8 for multi-unit data sets. Multi-unit variograms show hole effect and often strong trend, while single unit variograms always indicate nugget effect with a weak trend. The correlation between the plots generated from measured permeability and gamma-ray responses are impressive. Permeability and gamma-ray variograms are similar. The variograms in the vertical and horizontal directions are quite different. Therefore, they cannot substitute one for another. We also concluded that relative semi-variograms are a better way of presenting formation heterogeneities. We observed that abrupt variations in outcrop gamma-ray values are also excellent indicators of bed contacts. We used the generated geostatistical parameters in a simulated annealing program to predict permeabilities of lateral profile A, and concluded that variogram, mean and standard deviation are required but not sufficient parameters to predict the distribution of reservoir heterogeneity.

Application of Petrology and Fracture Analysis to Characterize Reservoir Heterogeneity of the Middle Pennsylvanian Marmaton Reservoir at Pshigoda-Desmoinesian Field, Ochiltree County, Texas Panhandle: Is a Horizontal Well Appropriate?

Pshigoda-Desmoinesian field, located in north-central Ochiltree County, Texas panhandle, has produce 1.94 MMBO and 5 bcf of gas from a stratigraphic trap in the Middle Pennsylvanian, Desmoinesian, Marmaton Group. The Marmaton reservoir interval is composed of complexly interlayered, fine to medium grained arkosic sandstones, calcareous bioclastic sandstones, and sandy bioclastic limestones. Sandstone and limestone layers typically are 2.5-60 cm thick. Arkosic sandstones in the reservoir interval have core analysis porosities of 5.5-15% and horizontal permeabilities ranging from 0.06 to 1.78 md. Porosity is mainly intergranular and, secondarily, leached feldspar. Intergranular porosity has been occluded partly by mechanical/chemical compaction, calcite cementation and precipitation of authigenic clay minerals and quartz and feldspar overgrowths. Calcareous bioclastic sandstones and sandy bioclastic limestones of the reservoir interval average 4.9 and 3.9% porosity, respectively. Horizontal permeabilities to air range from 0.003 to 0.02 md. An increase in total carbonate, predominantly ferroan calcite, correlates with a decrease in detrital feldspar and clay mineral content and lower gamma-ray log values. Low porosity and permeability in sandy bioclastic limestone and calcareous bioclastic sandstone layers in the Marmaton reservoir interval at Pshigoda-Desmoinesian field may act as partial barriers to the vertical migration of fluids and gases in the reservoir. Naturalmore » fractures in the reservoir interval are relatively rare, short in length, poorly connected, restricted to sandy limestone layers and partly calcite cemented, suggesting they do not enhance significantly the vertical movement of fluids or gases through the reservoir. Thus, the strongly layered and weakly fractured Marmaton reservoir at Pshigoda-Desmoinesian fields is a poor candidate for a horizontal well.« less

Puzzling questions about excited superdeformed rotational bands of atomic nuclei are answered by the two-revolving-cluster model.

The two-revolving-cluster model provides explanations of several questions about excited superdeformed bands: restriction to the lanthanons and the Hg-Tl-Pb region and to the smaller values of the neutron number for each element, truncation of the gamma-ray cascades, differences in shape of the lanthanon and Hg-Tl-Pb bands, alignment of quantified spins, and the existence of pairs of bands with nearly identical gamma-ray sequences. A previously unrecognized kind of pairing (intercalation of gamma-ray values) is also reported and a discussion is given of the values of electric quadrupole moments.

Are molecular clouds in virial equilibrium?

A correlation between CO luminosities and virial theorem masses of Galactic molecular clouds has been claimed to prove conclusively that molecular clouds are in gravitational virial equilibrium. This correlation is an artifact of the size-line width relation exhibited by molecular clouds, which does not require clouds to be virialized. Virial theorem masses do not provide an estimate of N(H2)/I(co). Comparison of the virial theorem-derived values of N(H2)/I(co) with the gamma-ray value shows that the masses of lower luminosity clouds are considerably overestimated by virial theorem analyses; these clouds are probably strongly influenced by nongravitational forces. 24 refs.