SEISMIC DETECTION AND QUANTIFICATION OF GAS HYDRATES -APPLICATION TO INDIAN CONTINENTAL MARGIN

Kalachand Sain

Gas-hydrate Group, National Geophysical Research Institute, Uppal Road, Hyderabad –500007 (Council of Scientific & Industrial Research)

INDIA

ABSTRACTGlobal interest in gas hydrates has spread due to their natural occurrences and huge potential as a feasible major energy resource of future. Many countries are pursuing research and development

for the exploration and safe production of this new treasure of energy. The bathymetry, seafloor temperature, rate of sedimentation, sedimentary thickness, geothermal gradient and total organic carbon content indicate good prospects of gas hydrates along the Indian margin. By analysing available seismic data, we have identified bottom simulating reflector or BSR, the main marker for gas hydrates, in the Krishna-Godavari, Mahanadi, Andaman, Saurashtra and Kerala-Konkanbasins. The drilling and coring by Indian National Gas Hydrates Program have validated the

ground truth in the Bay of Bengal (eastern Indian margin). From drilling at one location, we can not categorically rule out the possibility of gas hydrates in the entire Arabian Sea (western Indian margin). Of late, we have acquired multi-channel and ocean-bottom seismic data in deep waters of the Krishna-Godavari and Mahanadi basins. The analysis exhibits wide-spread occurrences of BSRs that coincide with the base of updated gas hydrates stability thickness map, and reveals new prospective zones for gas hydrates in both the basins.

We have demonstrated that seismic attributes like the reflection strength, blanking, instantaneous frequency, and attenuation (Q-1) can characterize the sediments containing gas hydrates and underlying free gas. For quantification of gas hydrates, we have employed the rock physics modeling to seismic velocities, derived from the traveltime tomography, full-waveform inversion, or amplitude versus offset modeling. As gas hydrates increases and underlying free gas decreases

the seismic velocities, the velocity anomaly against the background trend has been utilized for delineating the zones of gas hydrates- and free gas-bearing sediments. We will present these approaches with their application to seismic data in the Bay of Bengal and the Arabian Sea.

Keywords: gas hydrates, BSR, identification, quantification, Indian margin

Corresponding author: Phone: +91 040 23434799 Fax +91 040 23434651 E-mail: kalachandsain@yahoo.com

NOMENCLATUREAVO amplitude versus offsetBSR bottom simulating reflectorEEZ exclusive economic zoneGHSZ gas hydrate stability zone

KG Krishna-GodavariKK Kerala-KonkanMCS multi channel seismicNGHP national gas hydrate program

OBS ocean bottom seismicQ quality factorSCS single channel seismic STP standard temperature and pressureTCM trillion cubic meters

TOC total organic carbon

INTRODUCTION

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

Gas hydrates are crystalline form of methane and water, and are formed at high pressure and low temperature when methane concentration exceeds the solubility limit. Depletion of fossil fuels which fulfills ~80% of global energy requirement, andthe overwhelming demand of energy necessitates

looking for an alternate source of energy for sustainable growth of any country. Gas hydrates have attracted the scientific community due to their natural occurrences in shallow sediments of the permafrost and outer continental margins of

the world. Gas hydrates seem to be a viable major energy resource of future particularly for energy starving countries like India, Japan etc. The bathymetry, seafloor temperature, TOC content, sedimentary thickness, rate of sedimentation, geothermal gradient indicate good prospects of gas

hydrates in both the Bay of Bengal and the Arabian Sea (Sain and Gupta, 2008).Gas-hydrates are mostly recognized by identifying an anomalous reflector, known as the BSR, on seismic section based on its characteristics of mimicking the shape of seafloor, cross-cutting the

inclined sedimentary strata and exhibiting large amplitude reverse polarity with respect to the seafloor reflection (Fig.1). By filling the pores of sediments, gas hydrates reduces the permeability and hence trap free gas underneath. Thus, the BSR

is a physical interface between the gas hydrate-bearing sediments above and free gas- saturated sediments below, and is often associated with the base of GHSZ.

Fig.1: A specimen seismic section showing BSR (Sain et al., 2000).

A total volume of 1894 TCM of methane gas, stored in the form of gas hydrates, has been predicted within the Indian vast EEZ. This volume of gas is more than 1500 times of India’s present

natural gas reserve. It is envisaged that 10% recovery from this natural storehouse can satisfy India’s irresistible energy requirement for about a century. Since methane is the cleanest of all hydrocarbon fuels, its usage can have less impact

to the climate change. Besides the energy potential, gas hydrates may be responsible for environment hazard such as the global warming orslope failure, if dissociated in an uncontrollable manner by some reason.Gas hydrate exploration in India was first initiated

by the Gas Authority of India Limited in 1996 under the aegis of the Ministry of Petroleum & Natural Gas, Govt. of India. NGHP was formulated in which various industries, research and academic institutes, and universities joined to

carry out specific activities for evaluating the recourse potential of gas hydrates in India. As a constituent member, NGRI created a dedicated gas hydrate group. First, we prepared the GHSZ map along the Indian margin (Rao et al., 1998). We then scrutinized more than 50,000 line km of

analog SCS records, and noticed probable sites for gas hydrates occurrences (Gupta et al., 1998). Lot of new data sets has been generated during the last twelve years. By incorporating these data from published literature and available documents, Sain et al. (2011) have recently modified the Indian

GHSZ map (Fig.2) to fill the gap at many places.Since the BSR often coincides with the base of GHSZ, this map provides guidance for identifyingBSRs on seismic sections.

Fig.2: Gas-hydrate stability thickness map along the Indian continental margin (Sain et al., 2011)

IDENTIFICATION OF GAS-HYDRATESBy analyzing the accessible MCS data, we have

identified BSRs in the KK and Saurashtra basins in the western Indian margin, and in the KG,Mahanadi and Andaman regions in the eastern margin of India (Sain and Gupta, 2008; Sain and Ojha, 2008). The drilling and coring of Indian NGHP Expedition-01 (Collett et al., 2008) have

validated the ground truth where the gas hydrates were predicted from the surface seismic data in the Bay of Bengal. This has boosted our efforts to

advance research for the exploration of gas hydrates along the Indian margin and technology development for their validation, and subsequently aiming for production. We have brought out specific character of BSR near mud diapir, gas escape features (faulting or gas-chimney), seafloor

pockmarks etc (Shankar and Sain, 2007) and geophysical proxies (Shankar et al., 2008) for the investigation of gas hydrates in the western Indian margin. We have computed the attenuation (Q-1), reflection strength, blanking, and instantaneous

frequency, and demonstrated that these attributes can be used for characterizing the sediments containing gas hydrates and underlying free gas (Satyavani et al., 2008; Ojha and Sain, 2009; Sain et al., 2009; Sain and Singh, 2011). Fig.3 shows a three-layered Q-structure of submarine shallow

sediments, which shows almost uniform interval-Q for the first layer. However, a large lateral change in interval-Q from 191 to 223 to 117 is observed for the second layer at locations where the BSR is moderate, strong and absent. The sudden increase in interval-Q from 108 to 223 followed by large

drop to 107 near the strong BSR suggests good amount of gas hydrates and free gas across the BSR at this location.

Fig.3: Seismic gathers at CDPs 4286, 4372 and 4524, respectively, where BSR is moderate, strong and almost absent along the seismic line shown in Fig.1, each superimposed with interval-Q (Sain and Singh, 2011).

Besides investigating gas-hydrates, the estimated Q can be used for designing an inverse Q-filter to compensate the effects of attenuation for producing clear structural image including the BSR and base of free gas zone. The improved

image may delineate features like faults or fractures that might can act as paths for fluids migrating from below, and help to understand the genesis of gas hydrates. As presence of gas hydrates increases and underlying free gas decreases the velocity, the 2-D velocity anomaly

can be used for delineating the gas hydrates- and free gas- bearing sediments across the BSR (Fig.4)along a seismic line. Likewise, if we derive velocity from 3-D seismic data, we can demarcatethe areal and vertical extension of gas hydrate and free gas potential zones.

Fig.4: Seismic velocity anomalies depicting the lateral and vertical distribution of gas hydrates and free gas (Ojha and Sain, 2009) along the seismic line shown in Fig.1.

QUANTIFICATION OF GAS-HYDRATESFor quantification of gas hydrates, we need to estimate accurate seismic velocity by traveltime

tomography of large-offset OBS data or AVO modeling or full-waveform inversion of MCS data. The velocity anomaly across the BSR can then be translated using a suitable rock physics in terms of saturations of gas hydrates and underlying free gas. By employing the AVO modeling to the MCS

data in the western Indian margin, Ojha and Sain (2007) derived the seismic velocities, and estimated 30% gas hydrate by rock physics modeling. The modeling is not complete unless we assess the result. One of the ways for quantitative assessment is to apply two or more different

techniques to the same data sets and compare the result. We have employed two different techniques to the same MCS data, and appraised 15.5% gashydrates and 4.5% free gas using cooperative traveltime inversion followed by AVO modeling

(Ojha and Sain, 2008) and 20% gas hydrates and 2.8% free-gas from effective-medium modeling (Ghosh and Sain, 2008) in the sand-dominated Makran accretionary prism. The comparable results imply that the estimation is reasonable. Using the AVO A-B crossplot, coupled with the

rock-physics modeling, Ojha et al. (2010) have recently estimated the saturations of gas hydrates and free gas (Fig.5) at BSR along a seismic line, and the result brings out gas hydrates as varying

from 4.5% to 15%, and free gas from 2.0% to 3.5%, respectively.

Fig.5: Assessment of gas hydrates and free gas

(Ojha et al., 2010) at BSR along the seismic line shown in Fig.1.

Gas hydrates have been recovered in fractured

shale in the KG basin by the coring of the NGHP Expedition-01. Quantification of gas hydrates in fractured shale has been a challenge. To overcome this problem, we have employed the effective medium modeling, and evaluated the saturation of gas hydrates as 33-41% of total porosity, varying

vertically between 60 to 140 m below sea floor(Ghosh et al., 2010). The result for anisotropic background (due to fractures) matches quite well with the pressure core data. To demonstrate the application of the effective medium modelingelsewhere, Sain et al. (2010) have estimated the

saturations of gas hydrates and free gas in the Cascadia accretionary prism from the seismic velocity, derived by the full-waveform inversion of MCS data.Encouraged by the drilling and coring results of

the NGHP Expedition-01, we have collected a large volume of MCS and OBS data in the KG and Mahanadi basins in 2010 under the sponsorship of the Ministry of Earth Sciences, Govt. of India. The preliminary analysis shows wide-spread occurrences of BSRs, and thus reveals new

prospective zones of gas hydrates occurrences. All the approaches mentioned above are being employed to the new data set with an aim for delineating the gas hydrate-bearing sediments and quantifying the amount of gas hydrates present therein.

The entire velocity anomaly can be tied with the rock physics modeling, and volume of gas hydrates and underlying free gas can be determined. Likewise, the interval-Q can also be

translated in terms of saturations of gas hydrates and underlying free gas to complement the results

for evaluating the resource potential in the study region.

ACKNOWLEDGEMENTWe are grateful to the Director, NGRI for his permission to publish this work. The Ministry of

Earth Sciences, Govt. of India is acknowledged for financial support to pursue research on gas hydrates.

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