1. Introduction
As global energy demands rise, it becomes increasingly critical to identify unconventional sources of hydrocarbons. One alternative is natural gas hydrates. These are ice-like crystalline substances, composed of gas and water molecules that exist at relatively high pressures (>30 bar) and low temperatures (<20 °C) [
1]. The exact pressure and temperature at which hydrates form is however dependent on the composition of the gas and the ionic impurities in the water. At suitable conditions of pressure and temperature the water molecules arrange themselves to form cages, with each cage enclosing one gas molecule. Naturally occurring hydrates are found in a relatively narrow zone of ideal pressure and temperature that parallels and underlie: (a) the terrestrial surface in permafrost areas and (b) the seabed surface in deep-water oceanic sediments. The dominant hydrocarbon gas discovered in hydrate form is methane; as such, natural gas hydrates are increasingly viewed as a potential economic resource.
Globally, the distribution of natural gas hydrates has mainly been inferred from geophysical observations, the most widely used being the high-amplitude bottom-simulating reflector (BSR). This positive indicator was previously used to confirm the presence of gas hydrates in Block 27, offshore Trinidad [
2]. In that study, it was also concluded that hydrate-bearing sediments may extend northward into the adjoining area of interest, Block 26 (
Figure 1).
Table 1 is a summary of published values of acoustic properties in pure hydrates, water saturated sediments, hydrate-bearing sediments and gas bearing sediments [
3]. As indicated, water saturated and gas bearing sediments have acoustic velocities in the order of 1.6–2.5 and 0.16–1.45 km/s respectively. The presence of hydrates in sediments however increases their average acoustic velocity to the order of 2.05–4.5 km/s.
Table 1.
Summary of acoustic properties of hydrates [
3].
Table 1.
Summary of acoustic properties of hydrates [3].
Parameter | Water-Saturated | Hydrate-bearing | Pure hydrate | Gas-bearing |
---|
Compressional Wave _Vp (km/s) | 1.6–2.5 | 2.05–4.5 | 3.25–3.6 | 0.16–1.45 |
In the case of hydrate-bearing sediments, the high-amplitude BSR is the response of the seismic signal to the change from solid hydrate-bearing sediments to underlying sediments which may contain water and free gas. As a result, there is a decrease in the acoustic impedance as the seismic wave travels from the overlying high-acoustic velocity hydrate bearing zone, to the low acoustic velocity zone [
1].
The depth of the BSR generally coincides with the base of the Gas Hydrate Stability Zone (GHSZ). This zone is defined by
in-situ temperature and pressures, and its base is generally interpreted as representing the phase boundary between overlying gas hydrates and underlying free gas zone. As the base of the GHSZ is defined by pressure and temperature, the BSR generally runs parallel to the seafloor and cuts the dominant stratigraphy. The BSR is also generally recognized by its reversed polarity relative to the seafloor [
1]. On occasion, bright reflectors have also been observed at the top of a hydrate-bearing layer [
4].
Figure 1.
Location map for blocks 26 and 27 offshore Trinidad.
Figure 1.
Location map for blocks 26 and 27 offshore Trinidad.
The Columbus Basin, located off the east coast of Trinidad in the Atlantic Continental Margin (
Figure 1), is a sediment-rich depositional centre, formed as a result of the tectonic interaction of the Caribbean, South American and Atlantic Plates. The primary structural elements of the basin include: (1) transpressional northeast-southwest trending ridges and (2) northwest-southeast oriented, down to the northeast, normal faults.
The basin contains Cretaceous to recent sediments, where more than 12,200 m are Plio-Pleistocene sediments deposited by the Orinoco Delta system which now extends from Venezuela to southeast Trinidad [
5,
6]. It also contains several conventional oil and gas sandstone reservoirs trapped both in anticlinal ridges and against the northwest-southeast trending faults to produce structural closure. These conventional reservoirs can be found at depths from 1200 m to greater than 4000 m, and exhibit high porosities (20 to 30%) and permeabilities (10 to 1000 mD) [
7,
8]. In the study area, the conditions for hydrate formation are ideal where pressures are greater than 50 bars and temperatures are less than 10 °C.
Gas composition and isotope data indicate that both biogenic methane and thermogenic gas are present in the Columbus Basin [
9]. The source of the biogenic gas is microbial methanogenesis that occurs as a result of the large influx of organic matter from the Orinoco Delta and at shallow depths [
5]. Thermogenic gas comes from the breakdown of organic matter at depths great enough to prohibit bacterial action. This gas then migrates upwards through various pathways, such as faults and mud volcanoes.
In this study, three-dimensional (3D) seismic data for Block 26 were evaluated to determine if there are potential hydrate-bearing sediments. This forms part of a wider study aimed at characterizing potential hydrate deposits in the Atlantic Continental Margin offshore Trinidad and Tobago.
4. Conclusions
Subsea pressure and temperature data indicate that the sediments in Block 26 exist at conditions that favour hydrate formation. Also, given that the east coast of Trinidad is a sediment-rich depositional centre and a prolific hydrocarbon province, the probability is high that the hydrocarbon gas needed for the formation of hydrates is present and in sufficient quantities.
The south-eastern corner of Block 26 offshore Trinidad contains a seismic event that has been interpreted to be a BSR associated with the presence of gas hydrates. Where present, these BSR surfaces were observed to parallel the seafloor and cut across reflectors, except in the immediate vicinity of mud volcanoes where they generally migrate upwards.
The areal extent of the mapped BSR is approximately 520 km2 which accounts for approximately 43% of Block 26, and appears to continue from that previously mapped in the southern Block 27. The BSR observed in Block 26 also extends to the northern and eastern limits of the block, and suggests that hydrates may be present in the blocks immediately to the north and east of Block 26.