The Utilisation of SeaSeep Surveys (a Defense / Hydrography Spin-Off) to Identify and Sample Hydrocarbon Seeps in Offshore Frontier Basins
Authors: Orange Daniel L. , Teas Philip A. , Decker John , Baillie Peter , GIlleran Paul , Levey Matthew D. 
Published: December 2008

The goal of SeaSeep exploration is to rapidly and cost effectively evaluate an offshore area for hydrocarbon prospectivity. The tools that are used to do this, multibeam sonar and piston coring - in addition to the more traditional 2D seismic, gravity, magnetics, and heat flow - have been around for decades, yet it is only through recent advances in all aspects of the survey system that data relevant to petroleum systems can now be acquired.

Natural hydrocarbon seepage out of the seafloor can alter the physical and biological characteristics of the sediment-water interface. The presence of seep-related biologic communities and authigenic carbonate or gas hydrate generates anomalously strong backscatter sonar returns in an otherwise monotonous background. Seepage can also lead to local bathymetric highs (mounds, mud volcanoes), and lows (pock marks), and seepage may occur along bathymetrically expressed fault lineaments.

The ability to map the seafloor at very high resolution - both bathymetrically and with acoustic backscatter - allows for the identification of anomalous seafloor features that may be related to seepage. To interrogate these potential seep sites, we deploy a standard 6m - 9m piston corer equipped with a Ultra-Short BaseLine (USBL) positioning beacon so that the core can be tracked in three dimensions all the way to the seafloor target.

Through survey design, system calibration, execution, and the proper attention to detail to every aspect of the acquisition system, we are able to image seafloor features on the order of 10–100 meters in lateral dimension, and &lt;5m vertically on the seafloor in up to 3000m of water. Over 1000 sq. km. of quality data can be acquired on a daily basis, with final seafloor maps and mosaics delivered upon the completion of a cruise leg. This allows for rapid integration with existing data, and identification of targets relevant to the petroleum system.

The SeaSeep exploration technique was recently applied to a large frontier basins program offshore Indonesia, acquiring 400,000 sq. km. of multibeam bathymetry and backscatter data, 100,000 line km of gravity/magnetics and 1,188 piston cores, as well as 34,000 line km of new 2D seismic data. The results of a geochemical suite of analyses on the cores showed that 12% contain evidence of migrated liquid petroleum, and 44% contain thermogenic gas. Over 400 isotope pairs, and 20 biomarker (molecular fingerprinting) suites provide insight into the maturity and source of multiple gas and oil petroleum systems. The success of this program significantly impacted the exploration risk through demonstration of source, maturity and migration. The SeaSeep program results, coupled with interpretation of potential leads on the 2D seismic data, allowed BGE to high-grade areas for further exploration, focusing efforts (and money) on the most prospective areas within the larger survey program. Black Gold's recent exploration activities in Indonesia demonstrate the business case for SeaSeep as a cost effective tool for rapid risk reduction in frontier exploration.

Introduction

SeaSeep surveys achieve offshore what many early explorationists achieved onshore: the rapid identification of basins that have hydrocarbon charge and a working petroleum system. The SeaSeep methodology comprises high-resolution multibeam sonar, targeted sea-bottom coring, gravity, magnetics, 2D seismic and hydrocarbon geochemistry data to explore a basin via traditional detection of oil and gas seeps juxtaposed with robust structures. The goal of a SeaSeep program is to rapidly and efficiently delineate and high-grade prospects and prospective areas in previously under-explored basins.