Rock physics modeling for assessing gas hydrate and free gas: a case study in the Cascadia accretionary prism

We investigate the estimation of gas hydrate and free gas concentration using various rock physics models in the Cascadia accretionary prism, which is one of the most intensively studied regions of natural gas hydrate occurrences. Surface seismic reflection data is the most useful and cost-effective in deriving seismic velocity, and hence estimating gas hydrate and free gas across a BSR with depth, if a proper background (without gas hydrate and free gas) velocity is chosen. We have used effective medium theory of Helgerud et al. (EMTH) and, a combination of self-consistent approximation and differential effective medium (SCA-DEM) theory coupled with smoothing approximation for crystalline aggregate. Using the SCA-DEM (non-load-bearing) and EMTH (load-bearing) modeling, we calculate the average saturations of gas hydrate as 17 and 19%, respectively within ~100 m thick sedimentary column using velocity, derived from the surface seismic data. The saturations of gas hydrate are estimated as 15 and 18% using the SCA-DEM, and 20 and 25% using EMTH from the logging-while-drilling and wire-line sonic velocities, respectively. Estimations of gas hydrate from Poisson’s ratio are in average 50% for EMTH and 10% for SCA-DEM theory. We obtain the maximum saturation of free gas as 1–2% by employing the SCA-DEM theory either to seismic or sonic velocities, whereas the free-gas saturation varies between 0.1 and 0.4% for EMTH model. The gas hydrate saturation estimated from the sonic velocity and the free gas saturation derived from both the seismic and sonic velocities using the SCA-DEM modeling match quite well with those determined from the pressure core data in the study region.     

Reflectivity template,a quantitative intercept-gradient AVO analysis to study gas hydrate resources – A case study of Iranian deep sea sediments

Presence of gas hydrate and free gas in Iranian part of Makran accretionary prism changes the elastic properties of unconsolidated sediments and produces sharp bottom simulating reflectors (BSRs) which are observed on the 2-D seismic data. Different methods have been applied to estimate the gas hydrate and free gas saturations in marine sediments based on seismic measurements. Most of these methods are based on relating the elastic properties to the hydrate and free gas saturations and remotely estimating their concentration. In this regard, using the effective medium theory (EMT) which was developed for different modes of hydrate distribution is more considered among other rock physics theories. The main concern about saturation estimations based on EMT is that the velocities of the hydrate-bearing sediments primarily depend on how they are distributed within the pore space. Therefore, understanding the modes of hydrate distribution (at least cementing or non-cementing modes) is necessary to decrease the estimation uncertainties.The first intention of paper is to investigate amplitude variation versus offset (AVO) analysis of BSR to determine the hydrate distribution modes. The results from the probable saturation revealed that if the hydrate cements the sediment grains, BSR would show the AVO class IV and if hydrate does not cement the sediment grains, then BSR would show either the AVO class II or class III depending on the free gas saturation just beneath the BSR. The second intention of paper is to introduce some templates called reflectivity templates (RTs) for quantitative study of hydrate resources. These templates are provided based on the EMT to quantify the hydrate and free gas near the BSR. Validation of this approach by synthetic data showed that a reliable quantification could be achieved by intercept-gradient RTs, only if these attributes are determined with a high accuracy and good assumptions are made about the mineralogical composition and porosity of the unconsolidated host sediments. The results of this approach applied to a 2-D marine pre-stack time migrated seismic line showed that less than 10% of the gas hydrate accumulated near to the BSR in anticlinal-ridge type structure of Iranian deep sea sediments. The free gas saturation near to the BSR by assuming a homogeneous distribution was less than 3% and by assuming patchy distribution was about 3–10%.     

Evidence of gas hydrate accumulation and its resource estimation in Andaman deep water basin from seismic and well log data

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.     

Seismic velocities on the Nova Scotian margin to estimate gas hydrate and free gas concentrations

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.     

A seismic study to investigate the prospect of gas hydrate in Mahanadi deep water basin,northeastern continental margin of India

The presence of gas hydrates, one of the new alternative energy resources for the future, along the Indian continental margins has been inferred mainly from bottom simulating reflectors (BSR) and the gas stability zone thickness mapping. Gas hydrate reserves in Krishna Godawari Basin have been established with the help of gas-hydrate related proxies inferred from multidisciplinary investigations. In the present study, an analysis of 3D seismic data of nearly 3,420 km² area of Mahanadi deep water basin was performed in search of seismic proxies related with the existence of natural gas hydrate in the region. Analysis depicts the presence of BSR-like features over a large areal extent of nearly 250 km² in the central western part of the basin, which exhibit all characteristics of a classical BSR associated with gas hydrate accumulation in a region. The observed BSR is present in a specific area restricted to a structural low at the Neogene level. The coherency inversion of pre-stack time migration (PSTM) gathers shows definite inversion of interval velocity across the BSR interface which indicates hydrate bearing sediments overlying the free gas bearing sediments. The amplitude versus offset analysis of PSTM gathers shows increase of amplitude with offset, a common trend as observed in BSR associated with gas hydrate accumulation. Results suggest the possibility of gas hydrate accumulation in the central part of the basin specifically in the area of structural low at the Neogene level. These results would serve as preliminary information for selecting prospective gas hydrate accumulation areas for further integrated or individual study from geophysical, geological, geochemical and microbiological perspectives for confirmation of gas hydrate reserves in the area. Further, on the basis of these results it is envisaged that biogenic gas might have been generated in the region which under suitable temperature and pressure conditions might have been transformed into the gas hydrates, and therefore, an integrated study comprising geophysical, geological, geochemical and microbiological data is suggested to establish the gas hydrate reserves in Mahanadi deep water basin.     

Seismic Insights into Bottom Simulating Reflection (BSR) in the Krishna-Godavari Basin,Eastern Margin of India

The multichannel seismic data along one long-offset survey line from Krishna-Godavari (K-G) basin in the eastern margin of India were analyzed to define the seismic character of the gas hydrate/free gas bearing sediments. The discontinuous nature of bottom simulating reflection (BSR) was carefully examined. The presence of active faults and possible upward fluid circulation explain the discontinuous nature and low amplitude of the BSR. The study reveals free gas below gas hydrates, which is also indicated by enhancement of seismic amplitudes with offsets from BSR. These findings were characterized by computing seismic attributes such as the reflection strength and instantaneous frequency along the line. Geothermal gradients were computed for 18°C and 20°C temperature at the depth of BSR to understand the geothermal anomaly that can explain the dispersed nature of BSR. The estimated geothermal gradient shows an increase from 32°C/km in the slope region to 41°C/km in the deeper part, where free gas is present. The ray-based travel time inversion of identifiable reflected phases was also carried out along the line. The result of velocity tomography delineates the high-velocity (1.85–2.0 km/s) gas hydrate bearing sediments and low-velocity (1.45–1.5 km/s) free gas bearing sediments across the BSR.     

Amplitude-versus-offset modeling of the bottom simulating reflection associated with submarine gas hydrates

Velocity analysis of multi-channel seismic (MCS) data and amplitude-versus-offset (AVO) modeling provides an efficient way of identifying gas hydrate and free gas, and therefore the nature of the bottom-simulating reflector (BSR). Additionally, AVO modeling also yields estimates of the hydrate concentration and free gas saturation across the BSR in terms of velocity distribution. In the present study, we apply directivity correction in order to accentuate the AVO behavior. Modeling for AVO pattern of the observed BSR over the Kerala–Konkan Offshore Basin may provide the probable velocity distribution across the BSR and thereby infer whether hydrate or hydrate/free gas model governs the AVO observations. Initial results indicate the possible presence of free gas underlying the gas hydrates-saturated sediments in this region.     

Appraisal of gas-hydrate/free-gas from VP/VS ratio in the Makran accretionary prism

Existence of gas-hydrate in the marine sediments elevates both the P- and S-wave seismic velocities, whereas even a small amount of underlying free-gas decreases the P-wave velocity considerably and the S-wave velocity remains almost unaffected. Study of both P- and S-wave seismic velocities or their ratio (V_P/V_S) for the hydrate-bearing sediment provides more information than that obtained by the P- or S-wave velocity alone for the quantitative assessment of gas-hydrate. We estimate the P- and S-wave seismic velocities across a BSR (interface between gas-hydrate and free-gas bearing sediments) using the travel time inversion followed by a constrained AVA modeling of multi channel seismic (MCS) data at two locations in the Makran accretionary prism. Using this V_P/V_S ratio, we then quantify the amount of gas-hydrate and free-gas based on two rock-physics models. The result shows an estimate of 12–14.5% gas-hydrate and 4.5–5.5% free-gas of the pore volume based on first model, and 13–20% gas-hydrate and 3–3.5% free-gas of the pore volume based on the second model, respectively.     

Variability of the bottom-simulating reflector (BSR) and its association with tectonic structures in the Chilean margin between Arauco Gulf (37°S) and Valdivia (40°S)

Multichannel seismic reflection data recorded between Arauco Gulf (37°S) and Valdivia (40°S), on the Chilean continental margin, were processed and modeled to obtain seismic images and sub-surface models, in order to characterize the variability of the bottom-simulating reflector (BSR), which is a geophysical marker for the presence of gas hydrates. The BSR is discontinuous and interrupted by submarine valleys, canyons, as well as by faults or fractures. The BSR occurrence is more common south of Mocha Island due to moderate slopes and greater organic matter contribution by rivers in that area. Tectonic uplift and structural instability change the stability gas hydrate zone and consequently the BSR position, creating in some cases missing or double BSRs. Our modeling supports the presence of gas hydrate above the BSR and free gas below it. Higher BSR amplitudes support higher hydrate or free gas concentrations. In the study area, gas hydrate concentration is low (an average of 3.5%) suggesting disseminated gas hydrate distribution within the sediments. Also higher BSR amplitudes are associated with thrust faults in the accretionary prism, which serve as conduits for gas flow from deeper levels. This extra gas supply produces a wider thickness of gas hydrates or free gas.     

Tomographic approach for gas hydrate investigation in Kerala-Konkan region,India

Seismic tomography is an effective means of estimating velocity and structure from multichannel seismic (MCS) reflection data. In this study we have followed a 2D approach to arrive at the probable velocity field configuration from multichannel seismic data and infer the presence of gas hydrates/free-gas in the offshore Kerala-Konkan region, along the eastern part of a seismic line on which a bottom simulating reflector (BSR) has previously been identified. Tomographic modeling consists of the identification of reflection phases and picking of respective travel times for various source-receiver positions. These picks were then utilized to arrive at a 2D velocity field following a forward and inversion approach using a ray tracing technique. The modeling for the first time brought out the finer scale velocity structure under the region of investigation. Modeling through the 2D approach shows lateral variation in velocity field along the studied segment of the seismic line. The results indicate a thin (∼50–60 m) sedimentary cover with velocity ranging from 1,770 to 1,850 m/s. A sedimentary layer with high P-wave velocity 1,980–2,100 m/s below the sea floor was interpreted as the hydrate layer. The thickness of this layer varies between 110 and 140 m. The hydrate layer is underlain by a low-velocity layer having velocities in the range 1,660–1,720 m/s. This low velocity may represent a free gas layer, whose thickness varies between 50 and 100 m located below the hydrated layer. The investigation suggests the occurrence of gas hydrate underlain by free gas in some parts of the Kerala-Konkan offshore region.     

Seismic character of gas hydrates on the Southeastern U.S. continental margin

Gas hydrates are stable at relatively low temperature and high pressure conditions; thus large amounts of hydrates can exist in sediments within the upper several hundred meters below the sea floor. The existence of gas hydrates has been recognized and mapped mostly on the basis of high amplitude Bottom Simulating Reflections (BSRs) which indicate only that an acoustic contrast exists at the lower boundary of the region of gas hydrate stability. Other factors such as amplitude blanking and change in reflection characteristics in sediments where a BSR would be expected, which have not been investigated in detail, are also associated with hydrated sediments and potentially disclose more information about the nature of hydratecemented sediments and the amount of hydrate present.Our research effort has focused on a detailed analysis of multichannel seismic profiles in terms of reflection character, inferred distribution of free gas underneath the BSR, estimation of elastic parameters, and spatial variation of blanking. This study indicates that continuous-looking BSRs in seismic profiles are highly segmented in detail and that the free gas underneath the hydrated sediment probably occurs as patches of gas-filled sediment having variable thickness. We also present an elastic model for various types of sediments based on seismic inversion results. The BSR from sediments of high ratio of shear to compressional velocity, estimated as about 0.52, encased in sediments whose ratios are less than 0.35 is consistent with the interpretation of gasfilled sediments underneath hydrated sediments. This model contrasts with recent results in which the BSR is explained by increased concentrations of hydrate near the base of the hydrate stability field and no underlying free gas is required.     

Evidence for gas hydrate occurrences in the Canadian Arctic Beaufort Sea within permafrost-associated shelf and deep-water marine environments

The presence of a wedge of offshore permafrost on the shelf of the Canadian Beaufort Sea has been previously recognized and the consequence of a prolonged occurrence of such permafrost is the possibility of an underlying gas hydrate regime. We present the first evidence for wide-spread occurrences of gas hydrates across the shelf in water depths of 60–100 m using 3D and 2D multichannel seismic (MCS) data. A reflection with a polarity opposite to the seafloor was identified ∼1000 m below the seafloor that mimics some of the bottom-simulating reflections (BSRs) in marine gas hydrate regimes. However, the reflection is not truly bottom-simulating, as its depth is controlled by offshore permafrost. The depth of the reflection decreases with increasing water depth, as predicted from thermal modeling of the late Wisconsin transgression. The reflection crosscuts strata and defines a zone of enhanced reflectivity beneath it, which originates from free gas accumulated at the phase boundary over time as permafrost and associated gas hydrate stability zones thin in response to the transgression. The wide-spread gas hydrate occurrence beneath permafrost has implications on the region including drilling hazards associated with the presence of free gas, possible overpressure, lateral migration of fluids and expulsion at the seafloor. In contrast to the permafrost-associated gas hydrates, a deep-water marine BSR was also identified on MCS profiles. The MCS data show a polarity-reversed seismic reflection associated with a low-velocity zone beneath it. The seismic data coverage in the southern Beaufort Sea shows that the deep-water marine BSR is not uniformly present across the entire region. The regional discrepancy of the BSR occurrence between the US Alaska portion and the Mackenzie Delta region may be a result of high sedimentation rates expected for the central Mackenzie delta and high abundance of mass-transport deposits that prohibit gas to accumulate within and beneath the gas hydrate stability zone.     

Reservoir characterization of gas hydrate in the Northwestern part of the Ulleung Basin,East Sea

An analysis of 3D seismic data from the northwestern part of the Ulleung Basin, East Sea, revealed that the gas hydrate stability zone (GHSZ) consists of five seismic units separated by regional reflectors. An anticline is present that documents activity of many faults. The seismic indicators of gas hydrate occurrence included bottom simulating reflector (BSR) and acoustic blanking in the gas hydrate occurrence zone (GHOZ). By the analysis of the seismic characteristics and the gradient of the sedimentary strata, the GHOZ was divided into four classes: (1) dipping strata upon strong BSR, (2) dipping strata below strong BSR, (3) parallel strata with acoustic blanking, and (4) parallel strata below weak BSR. Seismic attributes such as reflection strength and instantaneous frequency were computed along the GHOZ. Low reflection strength and high instantaneous frequency were identified above the BSR, indicating the occurrence of gas hydrate. A remarkably high reflection strength and low instantaneous frequency indicated the presence of free gas below the BSR. Considering the distribution of the gas hydrate and free gas, two gas migration processes are suggested: (1) stratigraphic migration through the dipping, permeable strata and (2) structural migration from below the GHSZ along faults.     

Geothermal modeling of the gas hydrate stability zone along the Krishna Godavari Basin

A wide-spread bottom simulating reflector (BSR), interpreted to mark the thermally controlled base of the gas hydrate stability zone, is observed over a close grid of multichannel seismic profiles in the Krishna Godavari Basin of the eastern continental margin of India. The seismic data reveal that gas hydrate occurs in the Krishna Godavari Basin at places where water depths exceed 850 m. The thickness of the gas hydrate stability zone inferred from the BSR ranges up to 250 m. A conductive model was used to determine geothermal gradients and heat flow. Ground truth for the assessment and constraints on the model were provided by downhole measurements obtained during the National Gas Hydrate Program Expedition 01 of India at various sites in the Krishna Godavari Basin. Measured downhole temperature gradients and seafloor-temperatures, sediment thermal conductivities, and seismic velocity are utilized to generate regression functions for these parameters as function of overall water depth. In the first approach the base of gas hydrate stability is predicted from seafloor bathymetry using these regression functions and heat flow and geothermal gradient are calculated. In a second approach the observed BSR depth from the seismic profiles (measured in two-way travel time) is converted into heat flow and geothermal gradient using the same ground-truth data. The geothermal gradient estimated from the BSR varies from 27 to 67°C/km. Corresponding heat flow values range from 24 to 60 mW/m². The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths.     

Possible link between weak bottom simulating reflections and gas hydrate systems in fractures and macropores of fine-grained sediments: Results from the Hikurangi Margin,New Zealand

Small amounts of free gas in interstitial sediment pores are known to significantly lower compressional (P-) wave velocity (Vp). This effect, combined with moderately elevated Vp from the presence of gas hydrates, is usually thought to be the cause for the often observed strong negative reflection coefficients of bottom simulating reflections (BSRs) at the base of gas hydrate stability (BGHS). At several locations however, weak BSRs have been observed, which are difficult to reconcile with a presence of gas in sediment pores. We here present a rock physics model for weak BSRs on the Hikurangi Margin east of New Zealand. Thin sections of a fine-grained mudstone sample from a submarine outcrop in the vicinity of a weak BSR show macroscopic porosity in the form of fractures and intrafossil macropores. We apply the Kuster-Toksöz theory to predict seismic velocities for a rock with water-saturated interstitial micropores and gas or hydrates in macroscopic pore space simulating fractures or compliant macropores. We match field observations of a weak BSR with a reflection coefficient of −0.016 with two end-member models; (1) rocks with gas hydrate-filled voids with a concentration of <4% of bulk sediment overlying water-filled voids, or (2) fully gas-saturated voids at a concentration of <2% beneath water-filled voids. A natural system is likely to consist of a combination of these end-members and of macroporosity filled with a mixture of water and gas or hydrate. Our results suggest weak BSRs may be caused by gas hydrate systems in fractures and macropores of fine-grained sediments with fully water-saturated interstitial pore space. Gas may be supplied into the macroscopic pore space by diffusion-driven short-range migration of methane generated within the gas hydrate stability field or, our favoured model based on additional geologic considerations, long-range advective migration from deeper sources along fractures.     

Canyon-infilling and gas hydrate occurrences in the frontal fold of the offshore accretionary wedge off southern Taiwan

We utilized reflection seismic and bathymetric data to infer the canyon-infilling, fold uplift, and gas hydrate occurrences beneath the frontal fold at the toe of the accretionary wedge, offshore SW Taiwan. The lateral migrating paleo-Penghu canyons has cut across the frontal fold with six distinct canyon/channel incisions marked by channel infills. The longitudinal bathymetric profile along the modern canyon course shows a knickpoint of ~300 m relief at this frontal fold, indicating that the rate of fold uplift is greater than that of canyon incision. The age for the initial thrusting of this fontal fold is around 240 kyr ago, as estimated by using the maximum thickness of growth strata of this fold divided by the sedimentation rate obtained from a nearby giant piston core. Bottom simulating reflector (BSR) on seismic sections indicates the base of gas hydrate stability zone. Beneath the frontal fold, there is a widespread occurrence of BSRs, suggesting the highly probable existence of substantial quantities of gas hydrates. A seismic flat spot and a few push-down reflectors below BSR are found lying beneath the anticlinal axis with bathymetric four-way dip closure. The flat spot, cutting across a series of dipping reflections beneath BSR, may indicate the contact between free gas and its underlying formation water. The push-down reflectors beneath BSRs are interpreted to result from abundant free gas hosted beneath the gas hydrate stability zone. The multiple paleo-canyon infills seen along and beneath the frontal fold and above BSRs may provide thick porous sands to host gas hydrates in the frontal fold.     

Multi-frequency seismic study of gas hydrate-bearing sediments in Lake Baikal,Siberia

In this paper we present and discuss the frequency-dependent behaviour of the acoustic characteristics of methane hydrate-bearing sediments in Lake Baikal, Siberia. Five different types of seismic sources (airgun-array, two types of single airguns, watergun and sparker) are used, encompassing a frequency bandwidth from 10 up to 1000 Hz. On low-frequency airgun-array data, the base of the hydrate stability zone (HSZ) is observed as a high-amplitude bottom-simulating reflection (BSR) with reversed polarity. The amplitude and continuity of the BSR decrease or even disappear on medium- to high-frequency data, a feature explained in terms of vertical and horizontal resolution. The increasing reflection amplitude of the BSR with increasing offset, the calculated reflection coefficient of the BSR and the occurrence of enhanced reflections below the BSR suggest the presence of free gas below the HSZ. The observation of some enhanced reflections extending above the BSR may be interpreted as an indication for free gas co-existing with hydrates within the HSZ. Amplitude blanking above the BSR is highly variable while the BSR itself appears to act as a low-pass frequency filter for medium- to high-frequency data.New single-channel airgun profiles provide the first seismic information across the Baikal Drilling Project (BDP-97) deep drilling site, at which hydrate-bearing sediments were retrieved at about 200 m above the base of the local HSZ. At the drilling site there are no seismic characteristics indicative of the presence of hydrates. Combination of the drilling and seismic information has allowed us to make a rough estimation of the volume of hydrates and carbon stored in the sediments of Lake Baikal, which lead us to conclude that the Lake Baikal gas hydrate reservoirs do not form a prospective energy resource.     

东海天然气水合物的地震特征

使用中国科学院海洋研究所“科学一号”调查船于2001年以及20世纪80年代在东海地区采集的多道地震资料,以海域天然气水合物研究为目的,对这些资料进行了数据处理并获得了偏移地震剖面。通过对地震剖面的解释,在6条剖面上确定了6段异常反射为BSR,均有振幅强、与海底相位相反的特点。6段BSR基本上都没有出现和沉积地层相交的现象。分析认为,这与东海地区第四纪以来的沉积特征有关,并不能由此否认这些异常反射是BSR。6段BSR出现的水深为750~2 000 m,埋深在0.1~0.5 s(双程时间)之间。随着海底深度的增大,BSR埋深有增大的趋势。计算结果显示,6段BSR所处的温度和压力条件都满足水合物稳定赋存所需要的温度和压力条件。本文的BSR主要与北卡斯凯迪亚盆地以及智利海域水合物的温度、压力条件相似,而与日本南海海槽、美国布莱克海台等海域水合物的温度、压力条件相差比较大。在地震剖面上,6段BSR所处的局部构造位置都和挤压、断层有关,有利于水合物的发育;在空间上,它们主要分布在东海陆坡近槽底的位置以及与陆坡相近的槽底。在南北方向上,除分布在吐噶喇断裂和宫古断裂附近外,还与南奄西、伊平屋和八重山热液活动区相邻。热液活动和水合物虽然没有直接的成因关系,但岩浆活动为水合物气源的形成提供了热源条件,为流体和气体的运移、聚集提供了通道条件,从而有利于水合物的发育与赋存。根据地震剖面反射特征推断,剖面A1A2和A14A23发育BSR的位置应该有气体或者流体从海底流出,可能是海底冷泉发育的位置。剖面A14A23上BSR发育处,振幅比的异常增大和BSR埋深的降低是相关联的。这种关联支持该处发育海底冷泉的推测。     

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

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 (S_h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (S_h = 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.     

天然气水合物BSR的识别与地震勘探频率

地震勘探是调查天然气水合物广泛使用而有效的方法,而BSR是水合物赋存的主要标志.通过对实际调查资料的分析对比,结合国外的调查研究成果,探讨了地震勘探频率在BSR识别中的影响和作用,提出了在我国海洋天然气水合物的地震调查中有利于BSR识别的合适的频率范围.