Gamma-ray Profiles Research Articles

Petrophysical Parameters and Electrofacies Models of the Itararé Group Applying Multi-Resolution Graph-Based Clustering Method, Paraná Basin

Volume shale (Vsh) and effective porosity (Φe) were calculated and partially applied as petrophysical input parameters to generate electrofacies models using the Multi-Resolution Graph-Based Clustering method in sedimentary successions of the Itararé Group. The petrophysical parameters and electrofacies models were determined from geophysical data from two wells located in the eastern portion of the Paraná Basin. The stratigraphic intervals in each well (analogues of hydrocarbon reservoirs) allowed petrophysical analysis and the determination of electrofacies models based on gamma ray profiles, apparent density and neutron porosity, together with lithological data. The petrophysical parameters were calculated by different procedures, and the effective porosity results were applied as input parameters for electrofacies modeling. The electrofacies models were correlated to lithological data and patterns of gamma ray profiles and apparent density, as well as different types of porosity. The generated models provide differences related to the type of porosity, the method applied to calculate the effective porosity and present probable relationships with the lithological units of the wells.

Petroleum Potential Prospectivity of Muglad Basin, South Sudan: A Case of Source Rocks Characterisation in the Drilled Kaikang West-1 Well

This study focuses on Prospectivity of Petroleum Potential of Muglad basin, South Sudan, with the help of petroleum source rocks characterisation in the drilled Kaikang West-1 Well based on the organic matter quantity, quality, and the thermal maturation generation capability of the organic matter disseminated in the analysed rock samples. Muglad basin is a major petroliferous Western and Central African Rifts System (WCARS) member, with estimated reserves of 2,053 mm bbl. Muglad Basin comprises thick sandstones of Aradeiba and Bentiu Formations, considered to be the main petroleum prospectivity targets, with trapping mechanism being structural faults and lithological anticlines. The organic matter contents were determined directly from laboratory analyses of the source rock samples with the help of seismic and gamma ray profiles, total organic carbon (TOC wt %), maturity indices (OI, HI and PI), temperature maximum (Tmax °C), porosities and other sedimentological parameters. This study involved a series of analytical geochemistry and petro-physical studies to ascertain a number of effective source rock samples from the well cores which were then analysed in terms of TOC, OI, PI, HI, S2, S1 and Tmax to determine oil/gas prone samples (resource areas). These were then distinguished from strata with very high organic matter samples as well as very high TOC to help identify for possible source rock characterizations within the lithology pertaining to the well in terms of potential source-reservoir-seals combinations. The Rock-Eval pyrolysis data were useful in assessing and evaluating the type of organic matter, thermal maturity, and the generation capability of source rocks for hydrocarbon exploration rationale. The analyses revealed that some strata, within the sampled well data, have high hydrocarbon generation potential with the existence of commercial hydrocarbon production

Gamma-ray, stable carbon and oxygen isotope chemostratigraphy and sequence stratigraphy of the Lower Mahil Formation (KS-1 Khuff-Equivalent), Northern Oman

This research presents findings from a study focused on the Lower Triassic (Induan) Lower Mahil KS-1 Formation, situated on a homoclinal carbonate platform in Northern Oman. The sequence stratigraphy of this formation is characterized by a considerable thickness variation, slumps, and breccia deposits related to active normal faults coupled with intra-basin growth faults. The main objective was to establish a reliable stratigraphic framework for the Lower Mahil KS-1 Formation by integrating high-resolution carbon isotope data along with high-resolution spectral and total gamma-ray data. To achieve this, whole-rock samples were analyzed for δ13C and δ18O isotopes. Spectral and total gamma-ray records were obtained for the formation. Isotope sampling is conducted every 0.5 m in the Saiq Plateau and Wadi Sahtan sections. Furthermore, spectral gamma-ray measurements were taken at intervals of 10 cm from the logged sections. Within the third-order sequence, the spectral gamma-ray data revealed a distinct sea-level trend, leading to the division of KS1 into two different parts. Five fourth-order depositional sequences were identified by analyzing stable carbon isotopes, uranium, and total gamma-ray profiles. Four of these sequences displayed complete patterns, reflecting transgression and regression phases, while the fifth sequence was incomplete and solely comprised a transgressive phase. An essential outcome of the study is the correlation of the δ13C curve of the Lower Mahil KS-1 Formation with other similar formations around the Tethys region. This correlation indicates that the Lower Mahil KS-1 Formation captures the near-primary signal of carbon isotope variations in coeval seawater. As a result, it holds promise as a reference section for future investigations and studies in this field. Compared to the prior investigation, this study utilizes data with higher precision, capturing spectral gamma-ray measurements at 10 cm intervals and isotope measurements at 50 cm intervals. Furthermore, the study’s focus is confined explicitly to KS1.

Identification and Characteristics of Source Rocks in Lt-1 Well, Northern Kenya

This study on the hydrocarbon source rocks identification of LT-1 well in a Tertiary rift basin, northern Kenya, which had oil and gas shows was based on the organic matter quantity, quality, and the thermal maturation generation capability of the organic matter disseminated in the analysed rock samples. The organic matter contents were determined directly from laboratory analyses of the source rock samples with the help of seismic and gamma-ray profiles, total organic carbon (TOC wt%), maturity indices (HI and PI), temperature maximum (Tmax °C), porosities and other sedimentological parameters. This study involved a series of analytical geochemistry and petro-physical studies in ascertaining a number of effective source rock samples from the well cores which were then analysed in terms of TOC, PI, HI, S2, S1 and Tmax to determine oil/gas prone samples (resource areas) and distinguish them from strata with very high organic matter content. The samples with very high TOC were identified for possible source rock characterisations of the lithology pertaining to the well in terms of potential source-reservoir-seals associations. The Rock-Eval pyrolysis data were useful in assessing and evaluating the type of organic matter, thermal maturity, and the generation capability of source rocks for hydrocarbon exploration rationale. The analyses revealed that some strata within the sampled well data have high hydrocarbon generation potential with the existence of commercial hydrocarbon production. In conclusion, after an in-depth comparison and study of organic carbon content, hydrogen and production indices, type of organic matter content and maturity of organic matter, we confirmed that the studied resource areas are favourably considered for medium- to large-size hydrocarbon discoveries.

Interpretation of the puzzling gamma-ray spectrum of the Geminga halo

The gamma-ray halo around Geminga is formed owing to the slow diffusion of the electrons released by the Geminga pulsar wind nebula (PWN). The latest observations from the HAWC and H.E.S.S. Collaborations exhibit complex features in the TeV gamma-ray spectrum of the Geminga halo. We first show that the new results cannot be interpreted by the commonly used simple model, where a single power-law injection spectrum and an energy index of $\ensuremath{\delta}=1/3$ for the diffusion coefficient are assumed. We then propose a two-population electron injection model based on the x-ray observations of the Geminga PWN, which consists of a population of freshly accelerated electrons escaping from the PWN through rapid outflows and a population trapped longer inside the PWN before escaping. The two-population model interprets the HAWC and H.E.S.S. data well, and the goodness of fit improves significantly compared with the single power-law injection model. It also predicts a different energy dependency of the gamma-ray profile from the single power-law injection model, which could be tested by the Large High Altitude Air Shower Observatory in the coming future. We note that a $\ensuremath{\delta}$ slightly larger than 1 is needed to fit the HAWC and H.E.S.S. data consistently. We also discuss the possible improvements by adopting the two-zone diffusion model.

The Upper Cretaceous Rock Springs Formation of northwest Colorado

The Upper Cretaceous Rock Springs Formation of the Mesaverde Group in northwestern Colorado, southwestern Wyoming, and northeastern Utah is composed of fluvial, deltaic, and marine sediments that record the regression of the Western Interior Seaway during the Early to Middle Campanian. Contemporaneous deposits are present along the eastern and southeastern margins of the Greater Green River Basin in Wyoming, but correlation across the basin is challenging. Analysis of a small (1-km-long), understudied outcrop in northwestern Colorado assists in bridging that gap. The outcrop consists of distal and proximal deltaic deposits, overlain by distributary-channel complexes within delta-plain deposits. Correlation panels based on subsurface wireline logs and outcrop gamma-ray profiles show that the deposits are younger than lithostratigraphically equivalent strata of the Rock Springs Formation in Utah and Wyoming. Regional nomenclature is introduced for the area, and it is shown that these deposits differ from better-documented, older Rock Springs Formation deposits in Utah and Wyoming by having a higher net sandstone percentage due to the presence of substantial distributary-channel complexes. This study benefits subsurface exploration efforts in the Greater Green River Basin by providing outcrop analogs of reservoir distribution and quality.

Follow-up of 27 radio-quiet gamma-ray pulsars at 110–190 MHz using the international LOFAR station FR606

Context. The Fermi Large Area Telescope has detected over 260 gamma-ray pulsars. About one quarter of these are labeled as radio-quiet, that is they either have radio flux densities < 30 μJy at 1400 MHz, or they are not detected at all in the radio domain. In the population of nonrecycled gamma-ray pulsars, the fraction of radio-quiet pulsars is higher, about one half. Aims. Most radio observations of gamma-ray pulsars have been performed at frequencies between 300 MHz and 2 GHz. However, pulsar radio fluxes increase rapidly with decreasing frequency, and their radio beams often broaden at low frequencies. As a consequence, some of these pulsars might be detectable at low radio frequencies even when no radio flux is detected above 300 MHz. Our aim is to test this hypothesis with low-frequency radio observations. Methods. We have observed 27 Fermi-discovered gamma-ray pulsars with the international LOw Frequency ARray (LOFAR) station FR606 in single-station mode. We used the LOFAR high band antenna band (110−190 MHz), with an average observing time of 13 h per target. Part of the data had to be discarded due to radio frequency interference. On average, we kept 9 h of observation per target after the removal of affected datasets, resulting in a sensitivity for pulse-averaged flux on the order of 1−10 mJy. Results. We do not detect radio pulsations from any of the 27 sources, and we establish stringent upper limits on their low-frequency radio fluxes. These nondetections are compatible with the upper limits derived from radio observations at other frequencies. We also determine the pulsars’ geometry from the gamma-ray profiles to see for which pulsars the low-frequency radio beam is expected to cross Earth. Conclusions. This set of observations provides the most constraining upper limits on the flux density at 150 MHz for 27 radio-quiet gamma-ray pulsars. In spite of the beam-widening expected at low radio frequencies, most of our nondetections can be explained by an unfavorable viewing geometry; for the remaining observations, especially those of pulsars detected at higher frequencies, the nondetection is compatible with insufficient sensitivity.

Constraining millisecond pulsar geometry using time-aligned radio and gamma-ray pulse profile

Context. Since the launch of the Fermi Gamma-Ray Space Telescope, several hundred gamma-ray pulsars have been discovered, some being radio-loud and some radio-quiet with time-aligned radio and gamma-ray light curves. In the second Fermi Pulsar Catalogue, 117 new gamma-ray pulsars have been reported based on three years of data collected by the Large Area Telescope on the Fermi satellite, providing a wealth of information such as the peak separation Δ of the gamma-ray pulsations and the radio lag δ between the gamma-ray and radio pulses. Aim. We selected several radio-loud millisecond gamma-ray pulsars with period P in the range 2–6 ms and showing a double peak in their gamma-ray profiles. We attempted to constrain the geometry of their magnetosphere, namely the magnetic axis and line-of-sight inclination angles for each of these systems. Method. We applied a force-free dipole magnetosphere from the stellar surface up to the striped wind region – well outside the light cylinder – to fit the observed pulse profiles in gamma-rays, consistently with their phase alignment with the radio profile. In deciding whether a fitted curve is reasonable or not, we employed a least-square method to compare the observed gamma-ray intensity with that found from our model, emphasising the amplitude of the gamma-ray peaks, their separation, and the phase lag between radio and gamma-ray peaks. Results. We obtained the best fits and reasonable parameters in agreement with observations for ten millisecond pulsars. Eventually, we constrained the geometry of each pulsar described by the magnetic inclination α and the light-of-sight inclination ζ. We found that both angles are larger than approximately 45°.

Reservoir architecture and heterogeneity distribution in floodplain sandstones: Key features in outcrop, core and wireline logs

AbstractExploration and production from formations deposited in low‐gradient fluvial systems is typically associated with a high degree of uncertainty; a reflection of the inherent characteristics of these environments, notably the dominance of non‐reservoir floodplain fines, rapid lateral facies variations and associated heterogeneities at different scales. However, for a field development to be successful it becomes crucial to know the location, geometry, dimensions and connectivity of the most permeable facies, related to the main channel and the associated proximal overbank deposits (crevasse‐splay complexes). With the aim of addressing this problem, a multi‐disciplinary study is presented, combining outcrop data, high‐resolution sedimentological descriptions and advanced visualization techniques based on Digital Outcrop Models. This is compared with subsurface data from behind the outcrop (core, gamma ray and borehole image logs). The Mudstone–Sandstone Unit of the Triassic Red Beds of Iberian Meseta formation in south‐central Spain was selected for the present study. The unit is characterized by the lateral and vertical stacking of four architectural elements: (i) channelized sandstone bodies; (ii) asymmetrical sigmoidal‐shaped sandstone bodies; (iii) lobe‐shaped to sheet‐like sandstone bodies; and (iv) sheet‐like mudstones. These elements represent meandering channel, crevasse‐channel‐splay and floodplain sub‐environments, comprising a distal, low‐gradient meandering fluvial system. Together with well‐documented outcrop and core facies, calibrated log responses are also presented for the channel bodies (bell‐shape Gamma Ray profile, random azimuths and low to high dip angles), the crevasse‐splay bodies (funnel‐shape Gamma Ray profile, unidirectional azimuths and low dip angles) and the floodplain deposits (serrated Gamma Ray profile, unidirectional azimuths and very low dip angles). The full integration of outcrop and subsurface datasets has enabled generation of a robust conceptual model with predictive potential when establishing the three‐dimensional stacking of facies, distribution of heterogeneities, and the connectivity between reservoir rock geobodies of primary (channel) and secondary (crevasse complex) interest in this type of fluvial reservoir.

Water Absorption Curves versus Gamma-Ray Attenuation Profiles: A Comparative Analysis of Hygric Permeance Results

The knowledge of moisture transfer in multi-layered building materials and components using numerical simulations is fundamental to predict the behaviour of that building materials and components when in contact with moisture and to avoid some possible future pathology guarantying a correct performance.This paper describes a comparative analysis of experimental values of Hygric Permeance (HP) obtained by water absorption curves and gamma-ray attenuation profiles, for perfect contact interface. ARTICLE REVISED ON 25.03.2022

Tectonic and eustatic impact on depositional features in the upper Cretaceous Austin Chalk Group of south-central Texas, USA

The upper Cretaceous Austin Chalk Group in south-central Texas, USA, displays vertical and lateral variations in thickness and facies resulting from an irregular paleotopography of the seafloor and variable preservation of 3rd-order sea-level cycles. During the Coniacian to early Campanian, the tectonic evolution of the San Marcos Arch, a prominent basement feature in the study area, impacted facies and sequence architecture within the Austin Chalk Group. Outcropping sections of the Austin Chalk Group along the margin of the Edwards Plateau in south-central Texas stratigraphically transition between inner and outer ramp environments. The cyclic stacking pattern of facies is recorded in vertical stratigraphic succession as 3rd-order cycles separated by discontinuity surfaces and condensed deposits. Inner ramp facies consist of Thalassinoides-burrowed bivalve wackestone to packstone with minor occurrences of skeletal grainstone. Outer ramp facies comprise Planolites-burrowed foraminiferal chalk and marl with fine skeletal debris. Condensed facies associated with transgressive re-working are characterized by glauconitic packstone with high concentrations of coarse skeletal debris and lithoclasts. Gamma-ray profiles of outcrops investigated in the study area are correlated to subsurface wireline logs of wells drilled along the outcrop and subsurface trend, in order to construct a stratigraphic framework across the San Marcos Arch. This stratigraphic framework is biostratigraphically well constrained and correlated to the type section of the Austin Chalk Group farther north in the vicinity of Austin. The integration of sedimentological and stratigraphic information enables the identification of four regional 3rd-order depositional sequences within the Austin Chalk Group. Thickness maps of each depositional sequence illustrate the tectonic evolution of the San Marcos Arch and its influence on the deposition of the Austin Chalk Group, which is judged as negligible during most of the lower Austin Chalk Group (Coniacian), and started to trigger lateral thickness variations and pinch-outs in the Santonian and early Campanian. Syndepositional tectonic movement played an active role in the geometry of Austin Chalk depositional sequences and should be accounted for in sequence stratigraphic models developed in chalk depositional settings to explain the occurrence of coarser and cleaner facies.

Lithology, stratigraphy, chemostratigraphy, and depositional environment of the Mississippian Sycamore rock in the SCOOP and STACK area, Oklahoma, USA: Field, lab, and machine learning studies on outcrops and subsurface wells

Abstract This paper presents an interpreted depositional environment, developed 2nd order sequence stratigraphy framework, and detailed lithofacies identification from two Mississippian Sycamore outcrops (I-35 Sycamore and Speake Ranch) and subsurface wells in the SCOOP (South Central Oklahoma Oil Province). Then these rocks calibrate the rock properties with wireline log responses to identify the best landing zones. Qualitative and quantitative techniques of field, laboratory, and machine learning studies were conducted. For the field studies, we measured the complete 450 ft of the outcrop stratigraphic section, another separate 50 ft outcrop, examined the underlying Woodford Shale and overlying Caney Shale boundary contacts, documented sedimentary structures, constructed an outcrop gamma-ray profile, and developed a sequence stratigraphic framework. Laboratory studies included petrographic analyses, detailed X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). For machine learning studies, a principal component analysis (PCA), elbow method, and self-organizing map (SOM) were used to analyze the electrofacies and chemofacies from the outcrop and a subsurface uncored well. The outcrop hand-held gamma ray profile was obtained and correlated with subsurface wells. Five major outcrop lithofacies and chemofacies, within six stratigraphic units of alternating siltstone and shale strata, were identified from the wireline logs. A Maximum Flooding Surface (MFS), and two major 2nd order Sequence Boundaries (SB) were recognized at the outcrop and a nearby subsurface well. Bouma sequences and repetitive cycles of sedimentary structures indicated sediment gravity flow deposition on a marine slope setting. This study provides geologic insights to better understanding the depositional environment and the lithology of the Sycamore rocks. The bioturbated siliceous shale and/or the sandy siltstone can be potential target zones due to their reservoir quality, lithology, bed continuity, and brittleness. This information can be of direct benefit to the exploration and development programs of many companies in the SCOOP area, particularly in the Anadarko and Ardmore basins in Oklahoma.

Applications of real-time chemical stratigraphy in support of the safe drilling of HPHT wells: examples from the Shearwater Field, Central North Sea, UK

X-ray fluorescence (XRF) data from a series of high-pressure–high-temperature (HPHT) wells drilled in the Shearwater Field, UK Continental Shelf (UKCS) are presented. These data comprise a total of 789 samples across a series of eight study wells. Chemical stratigraphy was utilized in conjunction with other geological data (i.e. well logs, ditch cutting descriptions and biostratigraphic analyses) to aid real-time stratigraphic placement. Systematic changes in geochemical composition were identified and used to build a chemostratigraphic zonation to aid delineation of lithostratigraphic boundaries. The application of chemical stratigraphy provided a valuable source of data, particularly through critical ‘drill-in-liner’ sections that are devoid of conventional logging while drilling (LWD) data. The identification of key geochemical zones and provision of a synthetic gamma-ray (Synth GR) profile were utilized in the placement of key casing points and identification of challenging top reservoir picks (Fulmar Formation). These data also served as a backup during LWD tool failure resulting from excessive temperatures. The successive addition of geochemical data accumulated with each well-site exercise permitted continued refinement of the zonation framework, with derived Synth GR profiles exhibiting an extremely close relationship with subsequent wireline logs. Chemical stratigraphic data are shown to be robust and repeatable within the Shearwater Field, enabling close stratigraphic control through critical sections, whilst also providing a low-cost backup in the event of conventional logging tool failures.

Surface to subsurface correlation of the Middle-Upper Triassic Shublik Formation within a revised sequence stratigraphic framework

Recent work on cores and outcrops of the Middle-Upper Triassic Shublik Formation has facilitated surface to subsurface correlation of depositional sequences across the Alaska North Slope. Five transgressive-regressive depositional sequences have been defined within three large-scale stratigraphic units. Outcrop spectral gamma-ray (GR) profiles were used to correlate observed stacking patterns with nearby exploration wells, and GR logs from 161 exploration wells were used to correlate the three large-scale stratigraphic units across the North Slope and nearby offshore. Interpretations of depositional facies and sequence stratigraphy in cores from 26 wells were used to corroborate regional correlations. Isochore maps constructed for each of the three stratigraphic units illustrate the influence of accommodation on depositional patterns and suggest reactivation of several older tectonic elements during Shublik deposition. An isochore map of the richest, oil-prone interval of the Shublik Formation reveals a thick pod south of Harrison Bay, the eastern part of which lies beneath a recent giant Shublik-sourced oil discovery in the Cretaceous Nanushuk Formation. In addition, when integrated with thermal maturity, this isochore map may provide leads for areas that are optimal for unconventional resource exploration.

Logging and Lithostratigraphic Study of the Cenomanian-Santonian Reservoirs of Four Oil Wells MSP1, MSP2, MSP3 and MSP4 of the Margin of San-Pedro (Côte d’Ivoire)

The logging and petrophysical study of four oil wells, MSP1, MSP2, MSP3 and MSP4 from San-Pedro margin of the Ivorian sedimentary basin has made it possible to evaluate the reservoir characteristics of the Cenomanian-Santonian age formations. Lithostratigraphically, this study has shown that this interval consists of clay and sandstone deposits interspersed with frequent past carbonate.
 At the logging, ten (10) sandstone reservoirs are highlighted with effective porosities ranging from 16% to 21% and permeabilities from 100 mD to 1100 mD (millidarcy).
 These reservoirs have very good petrophysical characteristics however their high water saturation show that they are rather aquifers. The various log gamma ray profiles of the intervals considered highlight a fluvial and marine deposition environment. Sedimentation would have started in a Cenomanian-type fluvial environment and would have continued in a marine environment marked by the accumulation of sandstone and clay under the influence of transgression and regression phases in the Turonian and Lower Senonian.

Controls On Sedimentation and Cyclicity of the Boquillas and Equivalent Eagle Ford Formation from Detailed Outcrop Studies of Western and Central Texas, U.S.A.

AbstractFacies of the Boquillas and equivalent Eagle Ford Formation can be assigned to two associations: a hydrodynamically recycled association and a pelagic association. The hydrodynamically recycled association is composed of lenticular to continuous cross-laminated lime packstone–grainstone, heterolithic facies, and intraclastic grainstone conglomerate with intercalated calcareous mudrock. Sharp bedding contacts, abundant scours, ripple cross-lamination, oscillatory-current indicators, hummocky cross-bedding, and rounded clasts indicate recycling of the bottom sediments by storms and bottom currents above storm wave base (SWB). The pelagic association is composed of continuous lime packstone to grainstone and chalk beds with intercalated calcareous mudrock. Gradational contacts and planar lamination indicate dominant deposition by pelagic rain, but scour surfaces, coarse-fine alternations, and ripple cross-laminae indicate the influence of bottom currents below SWB.Vertical shifts between the hydrodynamically recycled association, interpreted to represent storm agitation of the seafloor during falls in sea level, alternate with the pelagic association and define four sequence boundaries and three depositional sequences with a lower-order periodicity (third-order periodicity, ca. average 2.4 My). Sequence boundaries correspond approximately to member boundaries recognized by gamma-ray profiles. Spectral analysis reveals higher-frequency mudrock–carbonate cycles prominent in the pelagic association, likely resulting from Milankovitch climate forcing of planktonic carbonate productivity versus clay flux. Comparison of gamma-corrected spectra with absolute age constraints reveals that the Ernst Tinaja section (in Big Bend National Park) is less than 28% complete. This is interpreted to result from abundant depositional hiatuses and erosional gaps that occur at the bed scale and at member boundaries within and bounding the Eagle Ford Formation.We demonstrate that deep-marine pelagic deposits in epicratonic settings may be subject to a complex interaction of depositional processes including pelagic rain and recycling by bottom currents and wave-generated currents modulated by low-frequency sea-level fluctuation and high-frequency fluctuations in sediment supply.

Oscillations of global sea-level elevation during the Paleogene correspond to 1.2-Myr amplitude modulation of orbital obliquity cycles

The mechanisms involved in the formation of 1- to 2-Myr-long third-order stratigraphic sequences through the Phanerozoic have been extensively debated, but the main underlying forcings remain uncertain. In this study, we investigated third-order sequences of Paleocene-Eocene age in the East China Sea Shelf Basin, identifying a prominent ∼1.2-Myr periodicity in the 405-kyr-tuned gamma ray profile of the WP-1 borehole. We infer that this ∼1.2-Myr cycle, which corresponds to long-period amplitude modulation of obliquity, was the main driver of third-order sea-level changes and sequence development. This modulation entails variation in the range of obliquity amplitude between a maximum of 2.4° (i.e. 24.5°–22.1°) and a minimum of 0.3° (i.e. 23.3°–23.0°) as a result of interactions among the three primary parameters of Earth's orbit (i.e., eccentricity, obliquity, and precession). We anchored our floating astronomical time scale (ATS) to the T42 seismic horizon, a major sequence boundary between the Lin Feng and Mingyuefeng formations that corresponds to the top of Nannofossil Zone NP8 (reported age of 57.7 Ma). This yields a fixed ATS for the late Paleocene-to-middle Eocene interval spanning 60.84 to 48.71 Ma. Finally, we evaluated our findings relative to the third-order sequence stratigraphy of the East China Sea Shelf Basin and to records of early Cenozoic eustasy and global events.

Evolução estratigráfica dos depósitos cretáceos da porção norte da Bacia de São Luís-Grajaú (NE do Brasil)

Os depósitos cretáceos da Bacia de São Luís-Grajaú são recorrentemente organizados, nos trabalhos prévios, em três sequências deposicionais, consolidadas desde o Neoaptiano até uma idade incerta no Neocretáceo, abrangendo as formações Grajaú e Codó, como também o Grupo Itapecuru. Este trabalho objetivou aplicar a estratigrafia de sequências, com base na correlação de perfis litológicos e de raios gama, na análise dos depósitos cretáceos localizados na parte norte da bacia. Para esse propósito, esta pesquisa utilizou dados de cinco poços, interpretados em uma etapa 1D, que posteriormente serviram de base para a construção de uma seção estratigráfica, interpretada em uma fase 2D. Além disso, trabalhos prévios acerca dos sistemas deposicionais envolvidos na sedimentação desses depósitos cretáceos foram cotejados para propor um modelo evolutivo para o intervalo estudado. A partir da integração dessas interpretações, individualizou-se o intervalo em duas sequências deposicionais (A e B), sendo cada uma delas composta por três tratos de sistemas: de nível baixo (TSNB), transgressivo (TST) e de nível alto (TSNA). A Sequência A foi, primariamente, dominada por processos flúvio-deltaicos correspondentes ao TSNB. Posteriormente, representando o TST, houve a ampliação das fácies lacustres, paralelamente à retração do sistema flúvio-deltaico desenvolvido durante a fase anterior. Finalmente, caracterizando o TSNA, ocorreu uma continuação dos processos suspensivos vistos no TST, porém com a influência de fluxos hiperpicnais. No tocante à Sequência B, inicialmente instalou-se um sistema deltaico representativo do TSNB. Subsequentemente, no TST, esse delta deu lugar a um estuário dominado por ondas. Por fim, em um estágio atribuído ao TSNA, um sistema deltaico novamente se estabeleceu, encerrando assim a história evolutiva do intervalo analisado.

Field gamma-ray patterns and stratigraphic reinterpretation of offshore well-log data from Lower Jurassic organic-rich units of the Lusitanian Basin (Portugal)

Abstract Lower Jurassic (Sinemurian–Pliensbachian) successions cropping out in onshore areas of the Lusitanian Basin provide detailed information of three well characterized organic-rich intervals: i) the Unit F from the Coimbra Formation, ii) the Polvoeira Member of the Agua de Madeiros Formation, and the iii) Marly Limestones with Organic-rich Facies (MLOF) Member of the Vale das Fontes Formation. Despite decades of research, these source rocks have not yet been properly identified and characterized in terms of extension and thickness in the basin's offshore. This paper compares detailed gamma-ray (GR) profiles from onshore outcrops (Coimbra, Montemor-o-Velho, Figueira da Foz, Monte Real, Sao Pedro de Moel and Peniche regions) with seven offshore wells. Our objective is to recognise Lower Jurassic organic-rich intervals in offshore areas of the basin, highlighting their lateral extension and improving the overall stratigraphic definition of the Lower Jurassic in the Lusitanian Basin. GR field measurements indicate that the highest GR values and U contents are consistently observed in the organic-rich intervals, reaching up to 134 API and 9.0 ppm of U. In contrast, the lowest API and U contents were determined in dominantly calcareous units, ranging between 5-34 API and 0.1-1.4 ppm, respectively. Wireline gamma-ray logs show a good correspondence between field patterns and offshore well logs, recognizing the organic-rich intervals in offshore areas. The highest GR values are interpreted to be linked with the 2nd- and 3rd-order maximum flooding intervals previously recognised in the Sinemurian–Pliensbachian successions in the western Iberian margin. Detailed GR maps linked to maximum flooding intervals highlight the variation of the vertical and lateral distributions of the organic-rich units in offshore areas of the Lusitanian Basin.

STRATIGRAPHIC CORRELATION OF THE LATE SILURIAN-DEVONIAN OF THE AMAZON BASIN (BRAZIL) BASED ON GENETIC STRATIGRAPHIC SEQUENCE APPROACH

The current study consists in the stratigraphic analysis of Late Silurian-Devonian interval in the Amazon Basin (Brazil) using the concepts of the Modern Stratigraphy, more specifically the genetic stratigraphic sequence, proposed by Galloway. that uses the marine flooding surface as a boundary of sequence. This methodology is since the sediments of studied interval are related to marine transgressions which were part of the basin paleogeographic context. Thus, the maximum flood surfaces, which are presented in four sections (AA ', B-B', CC 'and D-D') throughout the basin, represent chronostratigraphic events, which stand out in gamma ray profiles and are used as correlation data in thirteen exploration wells. Analysis of these sections allowed the identification of four fourth-order sequences (AB, BC, CD and DE) bounded by maximum flooding surfaces on the top and the base. Each sequence consists of asymmetric regressive-transgressive cycles, represented by highstand systems tract and transgressive systems tract. The analysis of these sections integrated with the stratigraphic maps (isopach and isolith) allowed the identification of the depocenter and two main source areas of sandy sediments (west and south). Besides, it was possible to infer that the marine transgression during the Paleozoic induced the deposition of pelitic sediments and followed a trend from northeast to southwest.