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1.
High-quality logging-while-drilling (LWD) downhole logs were acquired in seven wells drilled during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II in the spring of 2009. Well logs obtained in one of the wells, the Green Canyon Block 955 H well (GC955-H), indicate that a 27.4-m thick zone at the depth of 428 m below sea floor (mbsf; 1404 feet below sea floor (fbsf)) contains gas hydrate within sand with average gas hydrate saturations estimated at 60% from the compressional-wave (P-wave) velocity and 65% (locally more than 80%) from resistivity logs if the gas hydrate is assumed to be uniformly distributed in this mostly sand-rich section. Similar analysis, however, of log data from a shallow clay-rich interval between 183 and 366 mbsf (600 and 1200 fbsf) yielded average gas hydrate saturations of about 20% from the resistivity log (locally 50−60%) and negligible amounts of gas hydrate from the P-wave velocity logs. Differences in saturations estimated between resistivity and P-wave velocities within the upper clay-rich interval are caused by the nature of the gas hydrate occurrences. In the case of the shallow clay-rich interval, gas hydrate fills vertical (or high angle) fractures in rather than filling pore space in sands. In this study, isotropic and anisotropic resistivity and velocity models are used to analyze the occurrence of gas hydrate within both the clay-rich and sand dominated gas-hydrate-bearing reservoirs in the GC955-H well.  相似文献   

2.
Gas hydrates have been identified from two-dimensional (2D) seismic data and logging data above bottom simulating reflector (BSR) during China’s first gas hydrate drilling expedition in 2007. The multichannel reflection seismic data were processed to be preserved amplitudes for quantitatively analyzing amplitude variation with offset (AVO) at BSRs. Low P-wave velocity anomaly below BSR, coinciding with high amplitude reflections in 2D seismic data, indicates the presence of free gas. The absolute values of reflection coefficient versus incidence angles for BSR range from 0 to 0.12 at different CMPs near Site SH2. According to logging data and gas hydrate saturations estimated from resistivity of Site SH2, P-wave velocities calculated from effective media theory (EMT) fit the measured sonic velocities well and we choose EMT to calculate elastic velocities for AVO. The rock-physics modeling and AVO analysis were combined to quantitatively assess free gas saturations and distribution by the reflection coefficients variation of the BSRs in Shenhu area, South China Sea. AVO estimation indicates that free gas saturations immediately beneath BSRs may be about 0.2 % (uniform distribution) and up to about 10 % (patchy distribution) at Site SH2.  相似文献   

3.
During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient (a), cementation factor (m) and saturation exponent (n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 (Sh = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (Sh = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km2. Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.  相似文献   

4.
《Marine and Petroleum Geology》2012,29(10):1768-1778
During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient (a), cementation factor (m) and saturation exponent (n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 (Sh = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (Sh = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km2. Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.  相似文献   

5.
Through the use of 3-D seismic amplitude mapping, several gas hydrate prospects were identified in the Alaminos Canyon (AC) area of the Gulf of Mexico. Two locations were drilled as part of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) in May of 2009 and a comprehensive set of logging-while-drilling (LWD) logs were acquired at each well site. LWD logs indicated that resistivity in the range of ∼2 ohm-m and P-wave velocity in the range of ∼1.9 km/s were measured in the target sand interval between 515 and 645 feet below sea floor. These values were slightly elevated relative to those measured in the sediment above and below the target sand. However, the initial well log analysis was inconclusive regarding the presence of gas hydrate in the logged sand interval, mainly because large washouts caused by drilling in the target interval degraded confidence in the well log measurements. To assess gas hydrate saturations in the sedimentary section drilled in the Alaminos Canyon 21 B (AC21-B) well, a method of compensating for the effect of washouts on the resistivity and acoustic velocities was developed. The proposed method models the washed-out portion of the borehole as a vertical layer filled with sea water (drilling fluid) and the apparent anisotropic resistivity and velocities caused by a vertical layer are used to correct the measured log values. By incorporating the conventional marine seismic data into the well log analysis, the average gas hydrate saturation in the target sand section in the AC21-B well can be constrained to the range of 8–28%, with 20% being our best estimate.  相似文献   

6.
Through the use of 2-D and 3-D seismic data, several gas hydrate prospects were identified in the Ulleung Basin, East Sea of Korea and thirteen drill sites were established and logging-while-drilling (LWD) data were acquired from each site in 2010. Sites UBGH2–6 and UBGH2–10 were selected to test a series of high amplitude seismic reflections, possibly from sand reservoirs. LWD logs from the UBGH2–6 well indicate that there are three significant sand reservoirs with varying thickness. Two upper sand reservoirs are water saturated and the lower thinly bedded sand reservoir contains gas hydrate with an average saturation of 13%, as estimated from the P-wave velocity. The well logs at the UBGH2–6 well clearly demonstrated the effect of scale-dependency on gas hydrate saturation estimates. Gas hydrate saturations estimated from the high resolution LWD acquired ring resistivity (vertical resolution of about 5–8 cm) reaches about 90% with an average saturation of 28%, whereas gas hydrate saturations estimated from the low resolution A40L resistivity (vertical resolution of about 120 cm) reaches about 25% with an average saturation of 11%. However, in the UBGH2–10 well, gas hydrate occupies a 5-m thick sand reservoir near 135 mbsf with a maximum saturation of about 60%. In the UBGH2–10 well, the average and a maximum saturation estimated from various well logging tools are comparable, because the bed thickness is larger than the vertical resolution of the various logging tools. High resolution wireline log data further document the role of scale-dependency on gas hydrate calculations.  相似文献   

7.
珠江口盆地神狐海域是天然气水合物钻探和试验开采的重点区域,大量钻探取心、测井与地震等综合分析表明不同站位水合物的饱和度、厚度与气源条件存在差异。本文利用天然气水合物调查及深水油气勘探所采集的测井和地震资料建立地质模型,利用PetroMod软件模拟地层的温度场、有机质成熟度、烃源岩生烃量、流体运移路径以及不同烃源岩影响下的水合物饱和度,结果表明:生物成因气分布在海底以下1500 m范围内的有机质未成熟地层,而热成因气分布在深度超过2300 m的成熟、过成熟地层。水合物稳定带内生烃量难以形成水合物,形成水合物气源主要来自于稳定带下方向上运移的生物与热成因气。模拟结果与测井结果对比分析表明,稳定带下部生物成因气能形成的水合物饱和度约为10%,在峡谷脊部的局部区域饱和度较高;相对高饱和度(>40%)水合物形成与文昌组、恩平组的热成因气沿断裂、气烟囱等流体运移通道幕式释放密切相关,W19井形成较高饱和度水合物的甲烷气体中热成因气占比达80%,W17井热成因气占比为73%,而SH2井主要以生物成因为主,因此,不同站位甲烷气体来源占比不同。  相似文献   

8.
Mass-transport-deposits (MTDs) and hemipelagic mud interbedded with sandy turbidites are the main sedimentary facies in the Ulleung Basin, East Sea, offshore Korea. The MTDs show similar seismic reflection characteristics to gas-hydrate-bearing sediments such as regional seismic blanking (absence of internal reflectivity) and a polarity reversed base-reflection identical to the bottom-simulating reflector (BSR). Drilling in 2007 in the Ulleung Basin recovered sediments within the MTDs that exhibit elevated electrical resistivity and P-wave velocity, similar to gas hydrate-bearing sediments. In contrast, hemipelagic mud intercalated with sandy turbidites has much higher porosity and correspondingly lower electrical resistivity and P-wave velocity.At drill-site UBGH1-4 the bottom half of one prominent MTD unit shows two bands of parallel fractures on the resistivity log-images indicating a common dip-azimuth direction of about ∼230° (strike of ∼140°). This strike-direction is perpendicular to the seismically defined flow-path of the MTD to the north-east. At Site UBGH1-14, the log-data suggest two zones with preferred fracture orientations (top: ∼250°, bottom: ∼130°), indicating flow-directions to the north-east for the top zone, and north-west for the bottom zone. The fracture patterns may indicate post-depositional sedimentation that gave rise to a preferred fracturing possibly linked to dewatering pathways. Alternatively, fractures may be related to the formation of pressure-ridges common within MTD units.For the interval of observed MTD units, the resistivity and P-wave velocity log-data yield gas hydrate concentrations up to ∼10% at Site UBGH1-4 and ∼25% at Site UBGH1-14 calculated using traditional isotropic theories such as Archie's law or effective medium modeling. However, accounting for anisotropic effects in the calculation to honor observed fracture patterns, the gas hydrate concentration is overall reduced to less than 5%. In contrast, gas hydrate was recovered at Site UBGH1-4 near the base of gas hydrate stability zone (GHSZ). Log-data predict gas hydrate concentrations of 10–15% over an interval of 25 m above the base of GHSZ. The sediments of this interval are comprised of the hemipelagic mud and interbedded thin sandy turbidites, which did contain pore-filling gas hydrate as identified from pore-water freshening and core infra-red imaging. Seismically, this unit reveals a coherent parallel bedding character but has overall faint reflection amplitude. This gas-hydrate-bearing interval can be best mapped using a combination of regular seismic amplitude and seismic attributes such as Shale indicator, Parallel-bedding indicator, and Thin-bed indicator.  相似文献   

9.
2D and 3D seismic reflection and well log data from Andaman deep water basin are analyzed to investigate geophysical evidence related to gas hydrate accumulation and saturation. Analysis of seismic data reveals the presence of a bottom simulating reflector (BSR) in the area showing all the characteristics of a classical BSR associated with gas hydrate accumulation. Double BSRs are also observed on some seismic sections of area (Area B) that suggest substantial changes in pressure–temperature (P–T) conditions in the past. The manifestation of changes in P–T conditions can also be marked by the varying gas hydrate stability zone thickness (200–650 m) in the area. The 3D seismic data of Area B located in the ponded fill, west of Alcock Rise has been pre-stack depth migrated. A significant velocity inversion across the BSR (1,950–1,650 m/s) has been observed on the velocity model obtained from pre-stack depth migration. The areas with low velocity of the order of 1,450 m/s below the BSR and high amplitudes indicate presence of dissociated or free gas beneath the hydrate layer. The amplitude variation with offset analysis of BSR depicts increase in amplitude with offset, a similar trend as observed for the BSR associated with the gas hydrate accumulations. The presence of gas hydrate shown by logging results from a drilled well for hydrocarbon exploration in Area B, where gas hydrate deposit was predicted from seismic evidence, validate our findings. The base of the hydrate layer derived from the resistivity and acoustic transit-time logs is in agreement with the depth of hydrate layer interpreted from the pre-stack depth migrated seismic section. The resistivity and acoustic transit-time logs indicate 30-m-thick hydrate layer at the depth interval of 1,865–1,895 m with 30 % hydrate saturation. The total hydrate bound gas in Area B is estimated to be 1.8 × 1010 m3, which is comparable (by volume) to the reserves in major conventional gas fields.  相似文献   

10.
The passive northern continental margin of the South China Sea is rich in gas hydrates, as inferred from the occurrence of bottom-simulating reflectors (BSR) and from well logging data at Ocean Drilling Program (ODP) drill sites. Nonetheless, BSRs on new 2D multichannel seismic reflection data from the area around the Dongsha Islands (the Dongsha Rise) are not ubiquitous. They are confined to complex diapiric structures and active fault zones located between the Dongsha Rise and the surrounding depressions, implying that gas hydrate occurrence is likewise limited to these areas. Most of the BSRs have low amplitude and are therefore not clearly recognizable. Acoustic impedance provides information on rock properties and has been used to estimate gas hydrate concentration. Gas hydrate-bearing sediments have acoustic impedance that is higher than that of the surrounding sediments devoid of hydrates. Based on well logging data, the relationship between acoustic impedance and porosity can be obtained by a linear regression, and the degree of gas hydrate saturation can be determined using Archie’s equation. By applying these methods to multichannel seismic data and well logging data from the northern South China Sea, the gas hydrate concentration is found to be 3–25% of the pore space at ODP Site 1148 depending on sub-surface depth, and is estimated to be less than values of 5% estimated along seismic profile 0101. Our results suggest that saturation of gas hydrate in the northern South China Sea is higher than that estimated from well resistivity log data in the gas hydrate stability zone, but that free gas is scarce beneath this zone. It is probably the scarcity of free gas that is responsible for the low amplitudes of the BSRs.  相似文献   

11.
The Gas Hydrate Research and Development Organization (GHDO) of Korea successfully accomplished both coring (hydraulic piston and pressure coring) and logging (logging-while-drilling, LWD, and wireline logging) to investigate the presence of gas hydrate during the first deep drilling expedition in the Ulleung Basin, East Sea of Korea (referred to as UBGH1) in 2007. The LWD data from two sites (UBGH1-9, UBGH1-10) showed elevated electrical resistivity (>80 Ω-m) and P-wave velocity (>2000 m/s) values indicating the presence of gas hydrate. During the coring period, the richest gas hydrate accumulation was discovered at these intervals. Based on log data, the occurrence of gas hydrate is primarily controlled by the presence of fractures. The gas hydrate saturation calculated using Archie’s relation shows greater than 60% (as high as ∼90%) of the pore space, although Archie’s equation typically overestimates gas hydrate saturation in near-vertical fractures. The saturation of gas hydrate is also estimated using the modified Biot-Gassmann theory (BGTL) by Lee and Collett (2006). The saturation values estimated rom BGTL are much lower than those calculated from Archie’s equation. Based on log data, the hydrate-bearing sediment section is approximately 70 m (UBGH1-9) to 130 m (UBGH1-10) in thickness at these two sites. This was further directly confirmed by the recovery of gas hydrate samples and pore water freshening collected from deep drilling core during the expedition. LWD data also strongly support the interpretation of the seismic gas hydrate indicators (e.g., vent or chimney structures and bottom-simulating reflectors), which imply the probability of widespread gas hydrate presence in the Ulleung Basin.  相似文献   

12.
《Marine and Petroleum Geology》2012,29(10):1979-1985
The Gas Hydrate Research and Development Organization (GHDO) of Korea successfully accomplished both coring (hydraulic piston and pressure coring) and logging (logging-while-drilling, LWD, and wireline logging) to investigate the presence of gas hydrate during the first deep drilling expedition in the Ulleung Basin, East Sea of Korea (referred to as UBGH1) in 2007. The LWD data from two sites (UBGH1-9, UBGH1-10) showed elevated electrical resistivity (>80 Ω-m) and P-wave velocity (>2000 m/s) values indicating the presence of gas hydrate. During the coring period, the richest gas hydrate accumulation was discovered at these intervals. Based on log data, the occurrence of gas hydrate is primarily controlled by the presence of fractures. The gas hydrate saturation calculated using Archie’s relation shows greater than 60% (as high as ∼90%) of the pore space, although Archie’s equation typically overestimates gas hydrate saturation in near-vertical fractures. The saturation of gas hydrate is also estimated using the modified Biot-Gassmann theory (BGTL) by Lee and Collett (2006). The saturation values estimated rom BGTL are much lower than those calculated from Archie’s equation. Based on log data, the hydrate-bearing sediment section is approximately 70 m (UBGH1-9) to 130 m (UBGH1-10) in thickness at these two sites. This was further directly confirmed by the recovery of gas hydrate samples and pore water freshening collected from deep drilling core during the expedition. LWD data also strongly support the interpretation of the seismic gas hydrate indicators (e.g., vent or chimney structures and bottom-simulating reflectors), which imply the probability of widespread gas hydrate presence in the Ulleung Basin.  相似文献   

13.
Gas hydrate stability conditions restrict the occurrence of gas hydrate to unconsolidated and high water-content sediments at shallow depths. Because of these host sediments properties, seismic and well log data acquired for the detection of free gas and associated gas hydrate-bearing sediments often require nonconventional analysis. For example, a conventional method of identifying free gas using the compressional/shear-wave velocity (Vp/Vs) ratio at the logging frequency will not work, unless the free-gas saturations are more than about 40%. The P-wave velocity dispersion of partially gas-saturated sediments causes a problem in interpreting well log velocities and seismic data. Using the White, J.E. [1975. Computed seismic speeds and attenuation in rocks with partial gas saturation. Geophysics 40, 224–232] model for partially gas-saturated sediments, the difference between well log and seismic velocities can be reconciled. The inclusion of P-wave velocity dispersion in interpreting well log data is, therefore, essential to identify free gas and to tie surface seismic data to synthetic seismograms.  相似文献   

14.
This article provides new constraints on gas hydrate and free gas concentrations in the sediments at the margin off Nova Scotia. Two-dimensional (2-D) velocity models were constructed through simultaneous travel-time inversion of ocean-bottom seismometer (OBS) data and 2-D single-channel seismic (SCS) data acquired in two surveys, in 2004 and 2006. The surveys, separated by ∼5 km, were carried out in regions where the bottom-simulating reflection (BSR) was identified in seismic reflection datasets from earlier studies and address the question of whether the BSR is a good indicator of significant gas hydrate on the Scotian margin. For both datasets, velocity increases by 200–300 m/s at a depth of approximately 220 m below seafloor (mbsf), but the results of the 2006 survey show a smaller velocity decrease (50–80 m/s) at the base of this high-velocity layer (310–330 mbsf) than the results of the 2004 survey (130 m/s). When converted to gas hydrate concentrations using effective medium theory, the 2-D velocity models for both datasets show a gas hydrate layer of ∼100 m thickness above the identified BSR. Gas hydrate concentrations are estimated at approximately 2–10% for the 2006 data and 8–18% for the 2004 survey. The reduction in gas hydrate concentration relative to the distance from the Mohican Channel structure is most likely related to the low porosity within the mud-dominant sediment at the depth of the BSR. Free gas concentrations were calculated to be 1–2% of the sediment pore space for both datasets.  相似文献   

15.
During the China’s first gas hydrate drilling expedition -1 (GMGS-1), gas hydrate was discovered in layers ranging from 10 to 25 m above the base of gas hydrate stability zone in the Shenhu area, South China Sea. Water chemistry, electrical resistivity logs, and acoustic impedance were used to estimate gas hydrate saturations. Gas hydrate saturations estimated from the chloride concentrations range from 0 to 43% of the pore space. The higher gas hydrate saturations were present in the depth from 152 to 177 m at site SH7 and from 190 to 225 m at site SH2, respectively. Gas hydrate saturations estimated from the resistivity using Archie equation have similar trends to those from chloride concentrations. To examine the variability of gas hydrate saturations away from the wells, acoustic impedances calculated from the 3 D seismic data using constrained sparse inversion method were used. Well logs acquired at site SH7 were incorporated into the inversion by establishing a relation between the water-filled porosity, calculated using gas hydrate saturations estimated from the resistivity logs, and the acoustic impedance, calculated from density and velocity logs. Gas hydrate saturations estimated from acoustic impedance of seismic data are ∼10-23% of the pore space and are comparable to those estimated from the well logs. The uncertainties in estimated gas hydrate saturations from seismic acoustic impedances were mainly from uncertainties associated with inverted acoustic impedance, the empirical relation between the water-filled porosities and acoustic impedances, and assumed background resistivity.  相似文献   

16.
During the India National Gas Hydrate Program (NGHP) Expedition 01 in 2006 significant sand and gas hydrate were recovered at Site NGHP-01-15 within the Krishna–Godavari Basin, East Coast off India. At the drill site NGHP-01-15, a 5–8 m thick interval was found that is characterized by higher sand content than anywhere else at the site and within the KG Basin. Gas hydrate concentrations were determined to be 20–40% of the pore volume using wire-line electrical resistivity data as well as core-derived pore-fluid freshening trends. The gas hydrate-bearing interval was linked to a prominent seismic reflection observed in the 3D seismic data. This reflection event, mapped for about 1 km2 south of the drill site, is bound by a fault at its northern limit that may act as migration conduit for free gas to enter the gas hydrate stability zone (GHSZ) and subsequently charge the sand-rich layer. On 3D and additional regional 2D seismic data a prominent channel system was imaged mainly by using the seismic instantaneous amplitude attribute. The channel can be clearly identified by changes in the seismic character of the channel fill (sand-rich) and pronounced levees (less sand content than in the fill, but higher than in surrounding mud-dominated sediments). The entire channel sequence (channel fill and levees) has been subsequently covered and back-filled with a more mud-prone sediment sequence. Where the levees intersect the base of the GHSZ, their reflection strengths are significantly increased to 5- to 6-times the surrounding reflection amplitudes. Using the 3D seismic data these high-amplitude reflection edges where linked to the gas hydrate-bearing layer at Site NGHP-01-15. Further south along the channel the same reflection elements representing the levees do not show similarly large reflection amplitudes. However, the channel system is still characterized by several high-amplitude reflection events (a few hundred meters wide and up to ~ 1 km in extent) interpreted as gas hydrate-bearing sand intervals along the length of the channel.  相似文献   

17.
Gas hydrates in the western deep-water Ulleung Basin, East Sea of Korea   总被引：1，自引：0，他引：1  
Geophysical surveys and geological studies of gas hydrates in the western deep-water Ulleung Basin of the East Sea off the east coast of Korea have been carried out by the Korea Institute of Geoscience and Mineral Resources (KIGAM) since 2000. The work included a grid of 4782 km of 2D multi-channel seismic reflection lines and 11 piston cores 5–8 m long. In the piston cores, cracks generally parallel to bedding suggest significant in-situ gas. The cores showed high amounts of total organic carbon (TOC), and from the southern study area showed high residual hydrocarbon gas concentrations. The lack of higher hydrocarbons and the carbon isotope ratios indicate that the methane is primarily biogenic. The seismic data show areas of bottom-simulating reflectors (BSRs) that are associated with gas hydrates and underlying free gas. An important observation is the numerous seismic blanking zones up to 2 km across that probably reflect widespread fluid and gas venting and that are inferred to contain substantial gas hydrate. Some of the important results are: (1) BSRs are widespread, although most have low amplitudes; (2) increased P-wave velocities above some BSRs suggest distributed low to moderate concentration gas hydrate whereas a velocity decrease below the BSR suggests free gas; (3) the blanking zones are often associated with upbowing of sedimentary bedding reflectors in time sections that has been interpreted at least in part due to velocity pull-up produced by high-velocity gas hydrate. High gas hydrate concentrations are also inferred in several examples where high interval velocities are resolved within the blanking zones. Recently, gas hydrate recoveries by the piston coring and deep-drilling in 2007 support the interpretation of substantial gas hydrate in many of these structures.  相似文献   

18.
印度国家天然气水合物计划(NGHP01)于2016年实施第1次钻探,证实了天然气水合物在印度大陆边缘的广泛分布。选择位于克里希纳-戈达瓦里盆地(KG盆地)的NGHP01-07D和NGHP01-15A钻孔,基于测井数据和岩心样品估算天然气水合物饱和度,分析天然气水合物赋存状态并探讨其形成机制。基于各向同性介质模型利用电阻率和声波测井计算NGHP01-15A钻孔的天然气水合物饱和度为0. 2%～33. 0%,平均值为9. 6%,在NGHP01-07D钻孔利用电阻率计算获得的天然气水合物饱和度高于岩心氯离子异常和气体释放获得的结果,但是基于各向同性岩石物理模型利用声波测井计算的天然气水合物饱和度与岩心结果一致,平均值为5. 0%。前人研究认为NGHP01-10D钻孔中天然气水合物以相对较高饱和度富集在高角度裂隙中。结合前人研究结果推断在克里希纳-戈达瓦里盆地存在3种不同的天然气水合物储层,即泥岩中砂质夹层各向同性储层、泥质/粉砂质高角度低连通性的低饱和度裂隙储层和泥岩中高角度高连通性的高饱和度裂隙储层,并提出对应的3种天然气水合物储层模型。  相似文献   

19.
A mound related to a cold vent in a columnar seismic blanking zone (CSBZ) was formed around site UBGH1-10 in the central Ulleung Basin (2077 m water depth), East Sea, Korea. The mound is 300–400 m wide and 2–3 m high according to multi-beam bathymetry, 2–7 kHz sub-bottom profiler data, and multi-channel reflection seismic data. Seafloor topography and characteristics were investigated using a remotely operated vehicle (ROV) around site UBGH1-10, which is located near the northern part of the mound. The origin of the mound was investigated through lithology, mineralogy, hydrate occurrence, and sedimentary features using dive cores, piston cores, and a deep-drilling core. The CSBZ extends to ∼265 ms two-way traveltime (TWT) below the seafloor within a mass-transport deposit (MTD) unit. Gas hydrate was entirely contained 6–141 m below the seafloor (mbsf) within hemipelagic deposits intercalated with a fine-grained turbidite (HTD) unit, characteristically associated with high resistivity values at site UBGH1-10. The hydrate is commonly characterized by veins, nodules, and massive types, and is found within muddy sediments as a fracture-filling type. Methane has been produced by microbial reduction of CO2, as indicated by C1/C2+, δ13CCH4, and δD4CH analyses. The bowl-shaped hydrate cap revealed at 20–45 ms TWT below the seafloor has very high resistivity and high salinity, suggesting rapid and recent gas hydrate formation. The origin of the sediment mound is interpreted as a topographic high formed by the expansion associated with the formation of the gas hydrate cap above the CSBZ. The lower sedimentation rate of the mound sediments may be due to local enhancement of bottom currents by topographic effects. In addition, no evidence of gas bubbles, chemosynthetic communities, or bacterial mats was observed in the mound, suggesting an inactive cold vent.  相似文献   

20.
Drilling at the site UBGH1-9, offshore Korea in 2007, revealed varied gas-hydrate saturation with depth and a wide variety of core litholgies, demonstrating how the variations in the lithology are linked with those in gas-hydrate saturation and morphology. Discrete excursions to low chlorinity values from in situ background chlorinity level occur between 63 and 151 mbsf. In this occurrence zone, gas-hydrate saturations estimated from the low chlorinity anomalies range up to 63.5% of pore volume with an average of 9.9% and do not show a clear depth-dependent trend. Sedimentary facies analysis based on grain-size distribution and sedimentary structures revealed nine sediment facies which mainly represent hemipelagic muds and fine- to medium-grained turbidites. According to the sedimentary facies distribution, the core sediments are divided into three facies associations (FA): FA I (0–98 mbsf) consisting mainly of alternating thin- to medium-bedded hemipelagic mud and turbidite sand or mud beds, FA II (98–126 mbsf) dominated by medium- to very thick-bedded turbidite sand or sandy debris flow beds, and FA III (126–178 mbsf) characterized by thick hemipelagic mud without intervening discrete turbidite sand layers. Thermal anomalies from IR scan, mousse-like and soupy structures on split-core surfaces, non-destructive measurements of pressure cores, and comparison of gas-hydrate saturations with sand contents of corresponding pore-water squeeze cakes, collectively suggest that the gas hydrate at the site UBGH1-9 generally occurs in two different types: “pore-filling” type preferentially associated with thin- to medium-turbidite sand beds in the FA I and “fracture-filling” type which occurs as hydrate veins or nodules in hemipelagic mud of the FA III. Gas-hydrate saturation in the FA II is generally anomalously low despite the dominance of turbidite sand or sandy debris flow beds, suggesting insufficient methane supply.  相似文献   

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