Indian Journal of Marine Sciences Vol. 38(1), March 2009, pp. 110-115

Heat flow variation from bottom simulating reflector in the Kerala- basin of the western continental margin of

Uma Shankar and Kalachand Sain National Geophysical Research Institute, Uppal Road, Hyderabad 500 606, India (Council of Scientific and Industrial Research, New Delhi) [E-mail: [email protected]; [email protected]]

The base of the gas-hydrate stability field, representing the bottom simulating reflector or BSR, is observed over a closely spaced grid of multichannel seismic data in the Kerala-Konkan (KK) basin of the western continental margin of India (WCMI). The data reveal that gas-hydrates occur in the KK basin at places where water depth exceeds 1500 m. The thickness of the gas-hydrate stability field, inferred from BSR on seismic data, ranges between 190 and 340 m. The geothermal gradient, estimated from BSR, ranges from 40 to 60°C/km. The corresponding heat flow values vary between 36 to 54 mW/m 2. The result shows a seaward increase in geothermal gradient in the KK basin and brings out relatively high heat flow to the north and low heat flow in the south of the study area. The high heat flow distribution is explained by the decrease of sediment thickness proximal to the ocean/ boundary. [Keywords: WCMI, Kerala-Konkan, Gas-hydrates, BSR, Geothermal gradient, Heat flow]

Introduction in detail 4. The phase diagram for methane in pure Gas-hydrates are ice-like solid belonging to the water and in seawater have been compared with the class of clathrate compound in which the gas (mainly laboratory data and in-situ measurements of the Deep methane) molecules are trapped within cages of water Sea Drilling Project (DSDP) and Ocean Drilling molecules. Gas-hydrates are considered as the future Program (ODP) sites 5,6. The seafloor is the upper limit major energy resource for our country 1. They also of the gas-hydrate stability zone where water depth is play role in global climate change and submarine more than 300 m. The lower limit for the hydrate hazards 2. Seismic evidence of gas-hydrates was first stability field depends mainly on the bathymetry, observed over the Blake outer ridge 3. Since then, geothermal gradient and the seafloor temperature. numerous studies have been carried out to infer gas- The purpose of this study is to characterize the base hydrates at many places in the permafrost and outer of gas-hydrates stability field and to derive the continental margins, particularly in the accretionary geothermal gradient and heat flow map in the KK wedges. Gas-hydrates are mainly detected by basin of the WCMI with a view to shed light on mapping an anomalous reflector, known as the BSR geothermal aspects. To obtain these physical using the seismic experiment. This reflector is properties, we utilize the BSR observed on the identified based on its characteristic features of multichannel seismic (MCS) data at water depth mimicking the shape of seafloor reflection event; between 500 to 3000 7. cutting across dipping sedimentary strata and having opposite polarity with respect to the seafloor Materials and Methods reflection event. The BSR is often associated with the Geological setting base of the gas-hydrate stability zone. The WCMI has evolved as a consequence of the Gas-hydrates are formed at shallow sediments in breakup of the eastern Gondwanaland, specifically the moderately low temperature and high pressure separation of Madagascar and later Seychelles from regime. Temperature, pressure, methane India about 80-65 ma ago. It is a typical continental concentration, pore fluid salinity, and molecular margin of the Atlantic type, with sedimentation composition of gas are the main factors that influence accompanying subsidence in several areas. The the formation and dissociation of gas-hydrates. The margin is characterized by the Chagos-Laccadive stability criteria of gas-hydrates have been discussed Ridge (CLR), Pratap, Lakshmi and Basement ridges UMA SHANKAR AND KALACHAND SAIN: HEAT FLOW VARIATION IN THE KERALA-KONKAN BASIN 111

and offshore sedimentary basins like KK, Bombay prevailing fluid expulsion process, sediment and Saurasthra. This renders the WCMI a complex thickening and the nature of heat transport ridge-graben tectonic regime 8. These basins have mechanisms (i.e. whether advective or conductive) in evolved through two phases of rifting during the late an area. Several such studies have been reported for Cretaceous. The earliest being the Kutch offshore various margins of the world oceans. Baikal Rift basin followed by the KK and the Bombay offshore Zone 15, Blake Ridge and Nankai Trough 16, basins. The KK basin forms the southern part of this 17,18, Cascadia 19,20,21, Barbados 22, Oregon 23 and margin. Six contiguous tectonic elements have been Chile Triple Junction 24. Present study is limited to the identified as shelfal horst-graben complex, Kori- estimation of geothermal gradient and heat flow from Comorin depression, Kori-Comorin ridge, Laxmi- the BSRs in the KK basin of the WCMI. Laccadive depression, Laxmi-Laccadive ridge and Arabian abyssal plain 9. The Indus fan sediments, Data and analysis which is the youngest, have also contributed The MCS reflection data used in this study were significantly to the sedimentation history along the originally collected over the WCMI in 1993 as a part margin. Sediment thickness varies from 1-3 km in the of exploration programme for hydrocarbon southern part of the WCMI near the study area and explorations by ONGC Limited. The data were made increases to more than 6 km in the north 10 . The available to NGRI by the Gas Authority of India presence of the Bombay High structure, a potential oil Limited (GAIL) with a view to reprocess with bearing structure, together with other oil and gas suitable parameters for identifying possible locations bearing structures are indicators for the hydrocarbon of gas-hydrate-bearing horizons. Good quality MCS potential in the WCMI. data were collected along 12 seismic lines (Fig. 1) covering ~5000 km in the study area. The data set The surfacial sediments of the Arabian Sea are consists of 96 channels with 48 fold coverage. They characterized by high sedimentation rates 0.44-0.88 were collected with air-gun array source tuned with a mm/yr, and total organic carbon (TOC) concentration total volume of 1382 cubic inches and a streamer of 2-4% with a good degree of preservation 11 . These length of 2375 m. The seismic source had a frequency parameters indicate favorable conditions for the range from 3.5 to 128 Hz. Processing of seismic data generation of methane and formation of gas-hydrates was carried out using the commercial seismic data along the WCMI. Based on multi-disciplinary processing software (ProMAX) installed on SUN geophysical data (swath bathymetry, echo sounding, workstations using suitable parameters that include chirp sonar, side scan sonar and sub-bottom profiler); predictive deconvolution and careful band pass geochemical anomalies (sulphate reduction, methane enrichment and chloride depletion) and microbiological proxies (sulphate and nitrate reducing bacteria and fermenters), Ramana et al .12 indicate that both margins (especially the KK basin in the west and the Krishna-Godavari basin in the east) are promising for gas-hydrates occurrences. Various proxies 13 like pockmarks, seeps, venting of gas, blanking and diapir-like features also showed favorable conditions for the formation of gas-hydrates in the western margins of India. In fact, BSRs, the most important proxies for gas-hydrates are observed on both single- and multi-channel seismic data along the WCMI 7,14. Evolution of the continental margin can be better understood from the knowledge of its geothermal structure. Heat flow estimated from BSR depths is significant for inaccessible areas, where probe Fig. 1—Solid lines represent the multi-channel seismic lines in the measurement is risky. The BSR derived heat flow vicinity of the KK basin of the WCMI. Shaded portion of seismic values can be used to prepare the heat flow map that lines are identified BSRs on seismic sections 7. Contours represent provides input to understand the geothermal structure, the bathymetry over the in meters. 112 INDIAN J. MAR. SCI., VOL. 38, NO. 1, MARCH 2009

filtering to help remove the receiver ghost reflection. parameters such as pressure, temperature, salinity and A band pass filter (8-10-60-70 Hz) was applied to the gas molecular composition, which in turn, control the data. True amplitude recovery was done at 6 dB/s. depth of BSR. Fig. 4 shows the schematic phase The BSR, associated with the base of the gas-hydrates diagram based on gas-hydrate phase equilibrium stability zone, is shown at ~2950 ms two way travel studies of pure-water-methane and sea-water-methane time (TWTT) in one representative seismic stack hydrate system with 4% pore water salinity. The section (Fig. 2) in the study area. Presence of gas- choice of 4% salinity depends on earlier studies on hydrates changes the acoustic impedance resulting in core samples from the WCMI which have a range of blanking (reduction in seismic amplitudes) 3.84-4.59% 31. The BSR temperatures in the DSDP- phenomenon. Since the bedding planes are parallel to ODP drill holes from other margin 20 are also plotted the seafloor, the cross-cutting phenomenon is not in Fig. 4. This shows that most sites are falling on the manifested. Since the P-wave velocity of pure gas- lower side of pure water-methane hydrate curve and hydrates is much higher than that of the normal close to the sea water-methane hydrate curve. oceanic sediments, higher velocities for gas-hydrates- bearing sediments, typically varying between 2000 to 4000 m/s, are generally observed above the BSR at many continental margins of the world 25,26,27. The average velocity of the hydrates-bearing sediment upto the BSR is found to be 2000 m/s 28. The sound velocities of sonobuoys 29for the sediments in the Arabian Sea show the velocity variation from 1794- 1826 m/s 30 , which are considered as the background velocity i.e. the velocity of the sediments without gas- hydrates. Therefore, we convert the TWTT of various BSRs into the depths using the 2000 m/s average velocity of sediments. As BSR represents the base of gas-hydrates stability zone, we prepare the gas- hydrates stability thickness map (Fig. 3) in the KK region. The map shows that the stability thickness varies widely between 190 to 340 m.

Stability of gas-hydrates in marine sediments can be better understood from studying gas-hydrates Fig. 3—Gas-hydrates stability thickness map in the KK basin of phase equilibrium 4. In a given region, phase the WCMI. NGHP drilling site is indicated by ☼ equilibrium curve is defined by the physical

Fig. 2—A representative multi-channel seismic stack section showing the bottom simulating reflector (BSR) and hydrate Fig. 4—Phase diagram of methane hydrate stability field for pure stability zone (HSZ). water-methane and sea water-methane hydrate system 4. UMA SHANKAR AND KALACHAND SAIN: HEAT FLOW VARIATION IN THE KERALA-KONKAN BASIN 113

The sea floor temperatures (T 0) in the study area decreases the thermal conductivity of sediments and can be known either from the measured values or its variation exhibits no systematic pattern 22. To from the hydrothermal charts. Using the BSR depth, calculate the heat flow of the region, we should know we determine the temperature at BSRs (T z) from Fig. the thermal conductivities in the area. The thermal 4, and subsequently calculate the geothermal gradient conductivity measured recently by the Indian NGHP at various points. The heat flow (Q) of a region can be Expedition 01 34 at a location (Figs 1, 3, 6 and 7) to the calculated by multiplying the geothermal gradient northwest of the study region shows wide variation of with the thermal conductivity (k) of the region 22 as 0.439 to 1.174 W/m °C from the sea floor to 256 m below the sea floor. The average thermal conductivity Q = -k(T z – T 0)/Z … (1) is calculated as 0.88 W/m °C. The average value of global thermal conductivity is also observed as The accuracy of this estimation depends upon the velocity information to convert the TWTT into depths, pore water salinity, gas molecular composition, choice of hydrate system and type of conductive regime considered. As the effects of other parameters such as pore pressures, sediment density, grain size and variation in sea floor temperature on heat flow estimations are negligible 22; these were ignored for the computations.

Results and Discussion The values of sea floor temperatures (T 0) have been taken from the appropriate hydrothermal charts 32,33 and we show the sea bottom temperature contour map (Fig. 5) in the WCMI. We determine the temperatures at various BSRs (T z) and then calculate the geothermal gradients at respective points in the KK basin up to the water depth of 3000 m. The Fig. 6—Geothermal gradient map, derived from the identified geothermal gradient map (Fig. 6) shows the variation BSRs, in the KK basin of the WCMI. NGHP drilling site is from 40 to 60°C/Km. Formation of gas-hydrates indicated by ☼.

Fig. 5—Sea bottom temperature (in °C) contour map for the WCMI. Inbox shows the study area with seismic lines and Fig. 7—Heat flow map, derived from the identified BSRs, in the identified BSRs (solid circles). KK basin of the WCMI. NGHP drilling site is indicated by ☼. 114 INDIAN J. MAR. SCI., VOL. 38, NO. 1, MARCH 2009

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