Low Gamma-ray Research Articles

Effect of low dose gamma ray on the plasmonic behavior of gold nanoparticle

The aim of this paper is to study the effect of very low dose gamma radiation exposure on the plasmonic behavior of gold nanoparticles. For this purpose, gold nanoparticles were exposed to three different dose rates of radiation, with the absorbed doses of 0.33, 0.56 and 0.75 milligray. Subsequently, the effect of radiation exposure on the color of the solution, which is due to the accumulation of nanoparticles in the presence of salt, was investigated using visible spectroscopy. The results show that the presence of radiation can change the behavior of gold nanoparticles in the presence of salt for a specified period, in a way that the spectrum is similar to the spectrum of nanoparticles in the absence of salt. Based on these observations, a simple way of detecting ionizing radiation is introduced.

PERBAIKAN VIGOR BENIH JABON PUTIH SETELAH PENYIMPANAN 4,5 TAHUN MENGGUNAKAN IRADIASI SINAR GAMMA

Low dosage gamma ray iradiation has a potency to improve the seed germination by increasing of enzimatic activities, cell division, stimulating of responsive genes to auksin and improving of seed metabolism. The aim of the research was to identify seed storability of white jabon ( Neolamarckia cadamb a ) and to find out the effective gamma ray irradiation dosages to increase the seed vigor. Seeds were collected from 4 populations (Alas Puwo, Kampar, Batu Hijau, dan Pomalaa ) and were stored for 4,5 years. Randomized completely design was used to analysis seed storability and the effect of irradiation dosages ( (0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 Gy) on the parameters of seed germination and seedling growth. The result showed that seed storage for 4.5 years generally caused the decrease of seed viability and vigor, except for seeds from Batu Hijau.Seed moisture content decrease d significantly to 4.08-4.87percent. Gamma ray irradiation provided d ifferent responses on the seed origin .Irradiation was only effetive to improve germination with an initial seed germination more than 40 percent. Overall, dose of 40 Gy was able to improve seed vigor and seedling growth so that it can be applied to increase vigor of white jabon seeds.

Abnormal open-hole natural gamma ray (GR) log in Baikouquan Formation of Xiazijie Fan-delta, Mahu Depression, Junggar Basin, China

Abstract Based on large amounts of cores, open-hole conventional logs and mineral components analysis, abnormal natural gamma ray (GR) log showing high values in conglomerates and low values in fine-grained sediments, are described and explained in Baikouquan Formation of Xiazijie Fan-delta, Mahu Depression, Junggar Basin. After observing cores, normalizing the GR log and correcting depth errors between both, the GR log values of individual grain-sized lithology are extracted and counted. When grain-size decreases, the average GR values of different sized grains increase generally. The GR values of conglomerates are mostly between 50 and 80 API, while the values of fine-grains are mainly between 70 and 100 API. However, abnormal GR log features exist in the cores and wells of Baikouquan Formation prevalently. A great deal of high radioactive intermediate-acid volcanic minerals, such as volcanic tuff, felsite, andesite, granite, rhyolite, et al., distribute widely in the conglomerates, which results in abnormal high GR values in conglomeratic intervals. Low radioactive quartz components exit widely in high percentage in mudstones, which is the primary mineral explanation for the abnormal low GR values in reddish-brown siltstones and sandstones intervals.

Dielectric and gamma shielding properties of sodium silicate glassy structure modified with tungsten

ABSTRACTIn this study, (x)%Na2Si3O7/(100 − x)%W glassy structured composites where x = 100, 25 and 60 were prepared for the investigation of their dielectric and gamma-ray shielding properties for the first time. The surface morphology of the samples was imaged by Scanning Electron Microscope. The room temperature complex impedance and dielectric function analysis of the samples revealed that the Na2Si3O7/40%W composite has a promising potential for the decoupling capacitor and electromagnetic shielding applications. From this point of view, the gamma-ray shielding performance of the samples including pure Na2Si3O7 was tested by Ba-133 radioactive point source for transmission of the gamma rays at 81 and 356 keV photon energies. The mass attenuation coefficients (μ/ρ) of the composites were measured at these energies by using NaI(Tl) scintillation detector. The experimental half-value layer (HVL), tenth-value layer (TVL) and mean free path (λ) were also calculated. The theoretical values of the (μ/ρ), HVL, TVL and λ were determined by using Monte Carlo N-Particle simulation code and WinXCom for the comparison. A good agreement between the experimental and theoretical results for the gamma-ray shielding parameters was obtained. While the mass attenuation coefficient increased considerably with an increasing tungsten concentration doping, it decreased with increasing gamma-ray energy. It was observed that the mass attenuation coefficient of Na2Si3O7/40%W composite is higher than that of lead at 81 keV. In addition, HVL, TVL and λ parameters were found to decrease with increasing W doping. Since there is a good agreement between MNCP results and experimental data, the gamma radiation shielding performance of the samples for some low and high energetic gamma-ray sources such as Am-241, I-131, Cs-137 and Co-60. Ultimately, Na2Si3O7/40%W composite can be suggested as a new alternative non-toxic radiation shielding material for low energetic gamma rays.

Cross plot Analysis of Rock Properties from Well Log Data for gas detection in Soku Field, Coastal Swamp Depobelt, Niger Delta Basin

The cross plotting of rock properties for fluid and lithology discrimination was carried out in a Niger Delta oil field using well data X-26 from a given oil field in the coastal swamp depobelt. The data used for the analysis consisted of suites of logs, including gamma ray, resistivity, sonic and density logs only. The reservoir of interest Horizon 1, was identified using the available suite of logs on the interval where we have low gamma ray, high resistivity and low acoustic impedance specifically at depths 10,424ft (3177.24m) to 10 724ft (3268m). We first obtained other rock attributes from the available logs before cross plotting. The inverse of the interval transit times of the sonic logs were used to generate the compressional velocities and the S-wave data was generated from Castagna´s relation. Employing rock physics algorithm on Hampson Russell software (HRS), rock attributes including Vp/Vs ratio, Lambda-Rho and Mu-Rho were also extracted from the well data. Cross plotting was carried out and Lambda Rho (λρ) versus MuRho (μρ) crossplots proved to be more robust for lithology identification than Vp versus Vs crossplots, while λρ Versus Poisson impedance was more robust than Vp/Vs versus Acoustic impedance for fluid discrimination, as well as identification of gas sands. The crossplots were consistent with Rock Physics Templates (RPTs). This implies the possibility of further using the technique on data points of inverted sections of various AVO attributes within the field in areas not penetrated by wells within the area covered by the seismic.

Detection of persistent VHE gamma-ray emission from PKS 1510–089 by the MAGIC telescopes during low states between 2012 and 2017

Context. PKS 1510–089 is a flat spectrum radio quasar strongly variable in the optical and GeV range. To date, very high-energy (VHE, > 100 GeV) emission has been observed from this source either during long high states of optical and GeV activity or during short flares. Aims. We search for low-state VHE gamma-ray emission from PKS 1510–089. We characterize and model the source in a broadband context, which would provide a baseline over which high states and flares could be better understood. Methods. PKS 1510–089 has been monitored by the MAGIC telescopes since 2012. We use daily binned Fermi-LAT flux measurements of PKS 1510–089 to characterize the GeV emission and select the observation periods of MAGIC during low state of activity. For the selected times we compute the average radio, IR, optical, UV, X-ray, and gamma-ray emission to construct a low-state spectral energy distribution of the source. The broadband emission is modeled within an external Compton scenario with a stationary emission region through which plasma and magnetic fields are flowing. We also perform the emission-model-independent calculations of the maximum absorption in the broad line region (BLR) using two different models. Results. The MAGIC telescopes collected 75 hr of data during times when the Fermi-LAT flux measured above 1 GeV was below 3 × 10−8 cm−2 s−1, which is the threshold adopted for the definition of a low gamma-ray activity state. The data show a strongly significant (9.5σ) VHE gamma-ray emission at the level of (4.27 ± 0.61stat) × 10−12 cm−2 s−1 above 150 GeV, a factor of 80 lower than the highest flare observed so far from this object. Despite the lower flux, the spectral shape is consistent with earlier detections in the VHE band. The broadband emission is compatible with the external Compton scenario assuming a large emission region located beyond the BLR. For the first time the gamma-ray data allow us to place a limit on the location of the emission region during a low gamma-ray state of a FSRQ. For the used model of the BLR, the 95% confidence level on the location of the emission region allows us to place it at a distance > 74% of the outer radius of the BLR.

Geophysical strata rating (GSR) as an aid in carbonate reservoir characterisation: an example from the South Pars gas field, Persian Gulf Basin

In this study, the geophysical strata rating (GSR) is calculated from petrophysical data using the equations developed for clastic rocks. The region being investigated is the South Pars gas field in the Persian Gulf Basin, where the Permian–Triassic Dalan and Kangan reservoirs host the largest accumulations of gas in the world. A 3D GSR model is estimated from 3D poststack seismic data by using a probabilistic neural network model. In this study, two methodologies are used to obtain GSR values at the different scales of wireline logs and 3D seismic data.Strong correlations between neural network predictions and actual GSR data at a blind well prove the validity of the intelligent model for estimating GSR. The GSR results are also in good agreement with porosity and elastic moduli of these carbonate rocks. Discrimination between the reservoir and non-reservoir shaly units can easily be obtained by comparing GSR and well logs. Very low GSR values with high gamma ray log responses indicate shaly intervals. These can cause washouts, casing collapse and other related drilling problems. Intervals with low GSR values and low gamma ray log responses indicate the presence of good reservoir units.

Rock physics analysis and Gassmann’s fluid substitution for reservoir characterization of “G” field, Niger Delta

This study investigates the relationship between rock physics and petrophysical parameters in reservoir characterization. The data used for the analysis consist of suites of three wells, intercalated with sand and shale. Two reservoir sands were mapped, and petrophysical analysis was done to evaluate the hydrocarbon potential of “G” field. Elastic parameters such as compressional velocity (Vp), shear wave (Vs), acoustic impedance (AI), LambdaRho (λρ), MuRho (μρ), and bulk modulus (k) were generated and cross-plotted against petrophysical parameters to infer the Facies types. The sand lithology gave a low gamma ray, high resistivity, and low acoustic impedance in contrast to the shale. For the three wells, two reservoirs were observed each, at average depth ranging between 1546 and 1605 m and between 2010 and 2168 m. Cross plots such as Vp/Vs versus AI using gamma ray as indicator, λρ versus μρ using density as indicator, and volume of shale (Vsh) versus water saturation (Sw) using porosity as indicator were carried out for each reservoir to determine and discriminate their porosity and fluid content respectively. Well 2 was observed to be the most productive due to its high hydrocarbon saturation and reservoir characteristics. Gassmann’s fluid substitution was also carried out to understand the behavior of the dry rock; the pores were filled and superimposed on a rock physics template to extrapolate the general reservoir behavior of the field. It was observed that Vp and Rho decrease with increase in Vs for 100% gas saturation, while Vp and Rho increase as Vs remains constant with 100% water saturation.

Measurement of the cross sections for 238U(n, γ)239U reaction in the energy range of 14.1–14.8 MeV using neutron activation method

The neutron capture reaction cross sections of 238U were measured at the neutron energies of 14.1, 14.5 and 14.8 MeV using the activation method and off-line γ-ray spectrometric technique. The induced gamma-ray activities were measured using a low background gamma ray spectrometer equipped with a high resolution HPGe detector. The excitation functions of the 238U(n,γ)239U reaction were also calculated theoretically using the TALYS-1.8 program and are in general agreement with experimental data. The experimentally determined cross sections were compared with the data in the literature, and the evaluated data of ENDF/B-VIII.0, JENDL-4.0, and CENDL-3.1, as well as calculated results from TALYS-1.8.

Evaluation of capsaicinoid profile and antioxidant properties in dried Korean red pepper (Capsicum annuum L.) as affected by variable dose rates of electron beam and gamma ray irradiation.

The application of food irradiation is based on the parameters including energy (MeV), dose rate (kGy/s or kGy/h), and absorbed dose (kGy). Red pepper powders were treated with different dose rates of 1, and 5kGy/s electron beam (EB) as well as 1.8 and 9kGy/h gamma ray (GR) in separate experiments. Constant absorbed dose (3kGy) was maintained to observe whether different dose rates affect the product quality. Total and individual capsaicinoids increased at low EB and GR dose rates. Vitamin C content decreased at all dose rates, except at low GR dose rate, where no significant difference was observed. Low EB dose rate led to a marked increase (21%) in the total phenolics than high dose rate (13%). In contrast, high GR dose rate increased the total phenolics than low dose rate. Maximum antioxidants (1.36mg TE/mL) were observed at high EB dose rate, although low dose rate also increased the antioxidant activity. Results indicated that different dose rates delivering the same absorbed dose had significant impact on the product quality and that the efficacy of absorbed dose is dependent on applied dose rates. In other words, a constant absorbed dose applied to the product is influenced by dose rate of the irradiation treatment.

High resolution sequence stratigraphic reconstruction of mud-dominated systems below storm wave base; A case study from the Middle to Upper Devonian Horn River Group, British Columbia, Canada

The application of sequence stratigraphy to deep-water mud-dominated systems is more challenging than in shallow water siliciclastic settings, largely because mudstones display subtle contrasts in grain size and lithology that make identification of facies and recognition of major stratigraphic surfaces difficult. However, the integration of sedimentology, ichnology and geochemistry and an extensive suite of cores from the Middle to Upper Devonian Horn River Group shales allow us to better understand how deep-water mud-dominated systems respond to fluctuations in the relative sea level, manifested in sequence stratigraphic relationships. The Horn River Group consists of, in ascending order, the Evie Member and Otter Park Member of the Horn River Formation and the Muskwa Formation. Four systems tracts, including highstand, transgressive, lowstand and falling stage systems tracts (HST, TST, LST, FSST, respectively) are identified and mapped. Highstand and transgressive systems tracts are both represented by dominantly massive mudstone lithofacies, sparse bioturbation, high Mo/Al ratios and high TOC values (avg. 3.1 and 3.4 wt%, respectively), as well as low Al2O3 concentrations. They can be distinguished by upward increasing gamma ray values for transgressive systems tracts and upward decreasing values for highstand systems tracts. The falling stage and low stand systems tracts display more heterolithic and laminated lithofacies, sparse to intense bioturbation, lower Mo/Al ratio and lower TOC values (avg. 1.2 and 1.7 wt%, respectively) and high Al2O3, concentrations and relatively low gamma ray readings. Sequence boundaries are represented by a transition from unbioturbated units to moderately or intensely bioturbated units, suggesting increased bottom water oxygen levels, which we infer to represent shallower waters. They are gradational and clearly conformable contacts in distal parts of the basin. They are sharp in more proximal settings, but no erosional surfaces were identified that would indicate unconformable contacts. Maximum flooding surfaces are characterized by the highest TOC contents and an increased gamma ray values. Our results suggest that the Otter Park Member belongs to a long-period sea level lowstand (e.g., 2nd order cycle), whereas the Evie Member and Muskwa Formation belong to long-period sea level highstands.

Study on the large area MCP-PMT glass radioactivity reduction

The Jiangmen Underground Neutrino Observatory (JUNO) will install approximately 18,000 20-inch Photomultiplier Tubes (PMTs) in the central detector. From the full detector Monte Carlo (MC) simulation, the PMT glass has one of the largest contribution to the natural radioactive background due to its large mass. Various techniques have been developed to reduce radioactivities introduced in the raw material and during the production process. Efforts such as low background raw material selection, dust and dirty water isolation and continuous production monitoring were taken to reduce the glass background. The radioactivity in the glass sample was measured using a low background gamma ray spectrometer equipped with a high-purity germanium (HPGe) detector. The average level of 238U, 232Th and 40K in total MCP-PMT glass now reach 2.5 Bq/kg for 238U, 0.5 Bq/kg for 232Th and 0.5 Bq/kg for 40K, at least a factor of 2.5 lower than other type of 20-inch PMTs.

Well log data analysis for lithology and fluid identification in Krishna-Godavari Basin, India

Well log analysis provides the information on petrophysical properties of reservoir rock and its fluid content. The present study depicts interpretation of well log responses such as gamma ray, resistivity, density and neutron logs from six wells, namely W-1, W-2, W-9, W-12, W-13 and W-14 under the study area of Krishna-Godavari (K-G) basin. The logs have been used primarily for identification of lithology and hydrocarbon-bearing zones. The gamma ray log trend indicates deposition of cleaning upward sediment. Coarsening upward, clayey-silty-sandy bodies have been evidenced from the gamma ray log. Gas-bearing zones are characterised by low gamma ray, high deep resistivity and crossover between neutron and density logs. Total 14 numbers of hydrocarbon-bearing zones are identified from wells W-9, W-12, W-13 and W-14 using conventional log analysis. Crossplotting techniques are adopted for identification of lithology and fluid type using log responses. Crossplots, namely P-impedance vs. S-impedance, P-impedance vs. ratio of P-wave and S-wave velocities (Vp/Vs) and lambda-mu-rho (LMR), have been analysed to discriminate between lithology and fluid types. Vp/Vs vs. P-impedance crossplot is able to detect gas sand, brine sand and shale whereas P-impedance vs. S-impedance crossplot detects shale and sand trends only. LMR technique, i.e. λρ vs. μρ crossplot is able to discriminate gas sand, brine sand, carbonate and shale. The LMR crossplot improves the detectability and sensitivity of fluid types and carbonate lithology over other crossplotting techniques. Petrophysical parameters like volume of shale, effective porosity and water saturation in the hydrocarbon-bearing zones in these wells range from 5 to 37%, from 11 to 36 and from 10 to 50% respectively. The estimated petrophysical parameters and lithology are validated with limited core samples and cutting samples from five wells under the study area.

X-Ray Study of Variable Gamma-Ray Pulsar PSR J2021+4026

Abstract PSR J2021+4026 showed a sudden decrease in the gamma-ray emission at the glitch that occurred around 2011 October 16, and a relaxation of the flux to the pre-glitch state at around 2014 December. We report X-ray analysis results of the data observed by XMM-Newton on 2015 December 20 in the post-relaxation state. To examine any change in the X-ray emission, we compare the properties of the pulse profiles and spectra at the low gamma-ray flux state and at the post-relaxation state. The phase-averaged spectra for both states can be well described by a power-law component plus a blackbody component. The former is dominated by unpulsed emission and probably originated from the pulsar wind nebula as reported by Hui et al. The emission property of the blackbody component is consistent with the emission from the polar cap heated by the back-flow bombardment of the high-energy electrons or positrons that were accelerated in the magnetosphere. We found no significant change in the X-ray emission properties between two states. We suggest that the change of the X-ray luminosity is at an order of ∼4%, which is difficult to measure with the current observations. We model the observed X-ray light curve with the heated polar cap emission, and we speculate that the observed large pulsed fraction is owing to asymmetric magnetospheric structure.

On the gamma spectrometry efficiency of reference materials and soil samples

The relative discrepancies between the gamma spectrometry efficiency of RGU, RGTh, RGK reference materials and some soil samples have been studied using a MCNP model of a real HPGe detector. It has been shown that, in a specified geometry, efficiencies differences depend on the sample elemental composition. The elemental compositions of RGU-1, RGTh-1 reference materials and a soil sample have been evaluated using X-Ray fluorescence (XRF) method and used in the MCNP simulation along with RGK-1 and six other soil samples with different elemental compositions to calculate their efficiencies in different gamma ray energies. To estimate the maximum relative efficiencies differences between soil samples and reference materials, five soil samples with higher attenuation properties were selected from a large data set of soils elemental compositions. The results show that the efficiency differences between soil samples and reference materials are almost ignorable for more than 100 KeV gamma energies. It strongly depends on the sample attenuation factor in the lower energies, so use of a self-attenuation correction is essential for radionuclide counting in low energies gamma rays. Results show about 8 percent discrepancy between RGU and two soil samples efficiencies in 63.2 KeV energy.

Mechanism and Distribution of Calcareous Interbeds in Songtao Uplift and Its Periphery of Qiongdongnan Basin

Based on core and casting slice observation, well drilling and logging data, the source of carbonate materials, lithology together with electric properties, types, genesis mechanism and distribution of calcareous interbeds in Songtao uplift and its periphery of Qiongdongnan Basin have been thoroughly analyzed. Results show that typical features have been appeared from well logging curves, containing low gamma-ray, low acoustic travel time, low neutron value, high density, as well as bright white calcium nodules or bands in electrical imaging well-logging curves. Drilling results reveal that calcareous interbeds developed mostly in high position of paleostructures and their distribution was controlled by the combined effects of macroscopic and microscopic factors. Macroscopically, calcareous interbeds relate to paleogeomorphology together with the combination of sandstone and mudstone. They are also controlled microscopically by the source of carbonate cements and pore space. Under normal circumstances, with regard to the same sand, the closer to the mudstone and the thicker of mudstone, the more conducive to the formation of calcareous interlayer. Low compaction strength, high content of rigid particles, coarse grain size, well-sorted sandstone and large pore space during carbonate cementation are favorable for the development of calcareous interbeds.

Study of uranium toxicity using low-background gamma-ray spectrometry

The natural activity of 238U, 232Th and 40K present in soil, fertilizer and palaeosols besides animal fossils was determined using low background gamma ray spectrometry. The highest uranium activity level were found in animal fossils of geogenic origin compared to palaeosols, soils and fertilizers, a result of post-mortem uranium assimilation in these fossils via diagenetic processes. Hence, geogenic mobilization is likely a major cause of elevated uranium levels in Malwa region of Punjab, also known for higher than normal cancer incidence.

An application of variogram modelling for electrical resistivity soundings to characterize depositional system and hydrogeology of Bannu Basin, Pakistan

This study describes a method of combining geostatistical analysis with geophysical inversion of 1D electrical resistivity data acquired in the Bannu Basin, northwestern Himalayas of Pakistan. The data has been integrated and broadened from single dimension resistivity data into a 2D model that can be fully visualized and interpreted in a spatial sense. By interpreting and calibrating the electrical resistivity curves with the lithologies and geophysical logs of boreholes, it has been possible to identify unique sedimentary accumulations that occur throughout the basin. Through the use of variogram modelling the spatial extent of these accumulations can be identified and associated with measured hydraulic properties. It has become possible to map the extents and facies of the alluvial systems of the Bannu Basin formed during periods of heightened tectonic activity. The coarser sediments are associated with higher levels of resistivity as measured in the electrical surveys, whereas the finer sediments exhibit characteristically lower resistivities. Thus, the zones of high and low resistivity values become indicative of sediments associated with alluvial fans and lacustrine environments, respectively. The sediments of alluvial fans show relatively low gamma ray levels and higher transmissivities, whereas those of the piedmont deposits as well as lacustrine and sandy plains tend to show medium to high gamma ray values and lower transmissivities. Gross transmissivities were estimated by studying regression relationships between transmissivities measured in boreholes and the apparent formation factors of sediments. These relationships indicate an exponential increase in transmissivity with increasing apparent formation factor. These trends suggest a contrast in transmissivities between alluvial fan and other environments where lacustrine and sandy plains are characterized by low transmissivities. These estimated transmissivities do not represent absolute values but are merely indicative of any high or low trends in transmissivity of sediments, throughout the basin.

Gas hydrate accumulation and saturations estimated from effective medium theory in the eastern Pearl River Mouth Basin, South China Sea

Pore- and fracture-filling gas hydrates were identified from the core samples at several sites during the second Guangzhou Marine Geological Survey (GMGS2) expedition. Well logs indicated that gas hydrate occurred in three distinct layers at site GMGS2-08. The gas hydrate saturations calculated from well-log data and the seismic responses for the three gas hydrate-bearing layers, especially within the middle carbonate layer, were poorly known. We estimated gas hydrate saturations using isotropic and anisotropic models based on the mineral composition of the sediments and the effective medium theory. In the upper and lower gas hydrate-bearing layers, saturations estimated from anisotropic models are close to those estimated from pressures cores and chlorinity data. The average saturation using an anisotropic model in the upper (fracture-dominated) hydrate layer is approximately 10% with a maximum value of 25%. In the lower (fracture-dominated) layer, the horizontal and vertical gas hydrate-filled fractures and visible gas hydrate were formed with a maximum saturation of approximately 85%. For the middle layer, well logs show high P-wave velocity, density, high resistivity as well as low gamma ray, porosity, and drilling rate, together indicating a carbonate layer containing gas hydrate. The hydrate saturations calculated from isotropic models assuming hydrate formed at grain contacts are less than 20%, which fit well with two values calculated from chlorinity data for this layer. The upper gas hydrate layer shows no clear seismic response and probably consisted of small fractures filled with gas hydrate. The middle carbonate and lower fracture-filled gas hydrate-bearing layers show pull-up reflections, with the carbonate layer exhibiting relatively higher amplitudes. Pore-filling gas hydrate was also identified just above the depth of the bottom-simulating-reflector (BSR) from the GMGS2-05 drill site. Below the BSR, the push-down reflections, polarity reversal, and enhanced reflections indicate the occurrence of free gas in the study area.

Integrated core-log facies analysis and depositional model of the gas hydrate-bearing sediments in the northeastern continental slope, South China Sea

Host sediments may exert a significant influence on the formation of gas hydrate reservoirs. However, this issue has been largely neglected in the literature. In this study, we investigated the types, characteristics and the depositional model of the fine-grained gas hydrate-bearing sediments in the northeastern margin of the South China Sea by integrating core visual observations and logging-while-drilling downhole logs. The gas hydrate-bearing sediments consist dominantly of muddy sediments formed in the inter-canyon ridges of the upper continental slope, including hemipelagites, debrites (mud with breccia) and fine-grained turbidites. Cold-seep carbonates and associated slumping talus, muddy breccia debrites, as well as coarse-grained turbidites, may locally occur. Four classes and six sub-classes of log facies were defined by cluster analysis. Core-log correlation indicates that gas hydrates are majorly distributed in fine-grained sediments with high resistivity and low acoustic transit time (AC) log responses, which are easily differentiated from the fine-grained background sediments of high gamma-ray (GR), high AC, and low resistivity log values, and the seep carbonates characterized by low GR, high resistivity, high density, low AC and low porosity log values. The primary host sediments consist of fine-grained hemipelagic sediments formed by deposition from the nepheloid layers of river material and from the microfossils in seawater column. Most of the hemipelagic sediments, however, might have been extensively modified by slumping and associated gravity flow processes and were re-deposited in the forms of debrites and turbidites. Locally developed seep carbonates associated with gas hydrate dissociation and leakage provided additional sources for the gravity flow sediments.