Internal structure of hydrated sediments and BSR as seismic evidence of free gas trapping by hydrate zone
Authors: Kumar Sanjay 
Published: October 2008

The gas hydrate zone (GHZ) is identified on a seismic section by appearance of anomalous acoustic reflector which runs parallel to sea bed (Ref. 1). This bottom simulating reflector (BSR) is a result of velocity contrast between overlying gas hydrate (high velocities) and underlying sediments possibly containing free gas. In conventional model we assume that pore space is filled with hydrate and hence velocity contrast give rise to strong BSR. Ecker2 et al (1996) proposed two models of hydrate formation depending upon hydrate location within rock frame. In first model, hydrate cement grain contacts (reinforcing sediments) while in other model hydrate is located away from grain contacts. The density and seismic velocity in hydrate zone will depend upon percentage of hydrate, water and gas in pore space. The permeability will depend upon cementation of grain contacts by hydrates.

Introduction

Gas hydrates are special kind of clathrate formed from mixture of natural gas and water. Clathrate are inclusion compounds in which two or more chemical compounds form a stable crystalline mixture. The building blocks of gas hydrates consist of a gas molecule surrounded by a cage of water molecules that are stabilized by hydrogen bonding. Many gases have molecular size suitable to form hydrates, including such naturally occurring gases as carbon dioxide, hydrogen sulphide and several low carbon number hydrocarbons. Two different lattice structures, Structure-I & Structure-II are identified by X-ray diffraction (Fig 1). In fully saturated Structure-I methane hydrate, 1 m3 methane hydrate can contain up to 164m3 of methane gas. High pressure and low temperature are required for formation of gas hydrate and these conditions restrict natural occurrence of gas hydrates to permafrost region and deep ocean bottom sediments(Ref. 3). Thus gas hydrates form shallow reservoir and this fact has generated interest of scientists in study of gas hydrates as a potential (a) future energy source, (b) submarine geo-hazard and (c) factor in global energy change (Ref. 4). Being in field of hydrocarbon exploration, we are more interested in gas hydrate as source of energy. Occurrence of gas hydrate in nature is controlled mainly by temperature, pressure, their inter relationship, composition of gases etc. In marine environment such conditions exist within continental margins sediments at water depth approximately greater than 500m (Fig. 2). Within sediments temperature increases with depth, thus limiting the zone at which gas hydrate can occur. In hydrate formation, seismic wave travel fast. StollS et al. (1971) showed that seismic velocities can increase from 1850 m/s. to as high as 2700 m/s. This increase in velocity may result in impedance contrast at the bottom of gas hydrate zone and thereby giving birth to seismic marker known as bottom simulating reflector (Ref. 6). The presence of BSR on a seismic section is taken as evidence of presence of gas hydrate and free gas trapped below hydrate zone. In recognizing the need for ground truth in situ information, the safety panel of ODP (Ocean Drilling Programme) in 1991, approved the penetration of promising BSRs on cascadia accretionary wedge where drilling through strong BSRs at three different locations was accomplished as ODP leg 146 without any gas kick or other hazardous event occurring (Ref. 7).