OS4A-6:GAS HYDRATE PERSPECTIVE AS A POTENTIAL RESOURCE IN THE CARIBBEAN SEA FROM EXPLORATION POINT OF VIEW

Andrés Eduardo Calle OCHOA, reddy Mauricio Niño RODRIGUEZ
Instituto Colombiano del Petróleo, Ecopetrol.

    The potential of gas hydrates as a source of hydrocarbons, methane gas, in the Colombian Caribbean Sea has been revised several times in the past, Krason (1994), Gómez and León (2000), López (2005), Rodolfo and Arthur D'little (2008), ANH - Vargas (2012), however, these estimations have high uncertainty due to the limited information of data. In addition to this fact, the previous works are difficult to compare because its differences in the extent of the area of interest. In the report of the mining and energy planning unit of Colombia (UPME 2010) was expressed the need to increase gas reserves as demand will overcome in the short term. These two elements are the motivator for the beginning of a project focused on the exploration of potential gas hydrate reservoirs in the Colombian Caribbean Sea by the Colombian Petroleum Institute. The main activities will be related to: the use of geophysical exploration techniques to estimate the saturation, to delineate the most prospective areas and to validate the information by drilling.

    Results of the preliminary investigation helped to identified well-defined needs in the path of a successful achievement of project objectives, as well as the generation of new knowledge to the work team. The first activity was to determine the work area, natural to any project, but primarily for the variety of previous works areas that will serve as reference. The whole scenario includes the Colombian Caribbean margin (Sinú and Guajira Basin) and Colombia basin, and then it was imperative to delimit and review the available information. The first instrument was the construction of so called “theoretical stability zone”, Peláez and Calle 2012, which made, on the one hand, the reduction of the potential study area of over 300,000 km2 to 70,000 km2 between water tables 500-3000 m, on the other hand, the imperative need of oceanographic studies to generate hard data or measurements in the water column and seabed.

    The Colombian Caribbean Sea is, in general terms, a frontier basin highlighting the existence of more than 1000 lines 2D and several 3D seismic volumes, the latter are where the greatest concentration of data is available, although designed, acquired and processed for the purpose of searching conventional hydrocarbons. Here again was foresee other needs such as reprocessing of seismic data and detailed review of the piston core campaigns, although in the latter there is no much information recoverable, since there was any particular analysis for the presence or formation of hydrates.

    In the 3D seismic volumes were created "surfaces" as result of BSR´s identification, with the classic features: i) Reverse polarity relative to the water bottom interface, ii) Mimicking the shape of seafloor, iii) Cutting across the dipping strata. Continuity of these surfaces, along with, geological & geophysical analogies with worldwide gas hydrates studies, were the criteria to define the most prospective areas, a total of five were pre-selected. In these areas, with a total of 1400 km2 and BSR's presence of up to 80%, several seismic attributes were calculated helping to highlight geophysical and geological events (Calle 2013). According to Calle and Vesga 2013, through the estimation of geothermal gradient maps, using BSR´s surfaces, new evidences of anomalies arise, likely associated with fluid migration to the gas hydrates stability zone.

    From the viewpoint of sedimentologic, the interval near the BSR´s surface is characterized by three units. The lower one starts with a clearcut contact involving an abrupt facies change, and is defined by a large complex of channel levees that maintains stable the channel position, with a very pronounced aggradational character. The inner facies of the channel display high amplitude, discontinuous reflections, while the levee facies increase the slope upward, and are wedged outwards on both sides. Both in the channels and in the levee area there are intraformational unconformities, having a widespread downlap. They mark distinct stages of the system evolution and can be traced from the levee, separating distinctive groups of reflections forming the channel fill. The intermediate unit is onlap disposed on the levees surface and represents the row stage of the channel fill, in transition to overbank wedge deposits (slope fan). This subunit is strongly distorted by methane hydrate levels marked by high amplitude reflections, although its primary features disclose mean amplitude subparallel reflections and local development of mass-transport deposit and minor channels. The upper unit is characterized by a dull seismic facies, although minor channels can be identified in many uncertain cases which are difficult to pin down.