Oil and gas seeps are of great significance to modern offshore petroleum exploration. Foremost, they indicate the presence of generative hydrocarbon source rocks, without which there can be no accumulations. Sediment cores retrieved using a ship-based piston core device can be analyzed in the laboratory to detect hydrocarbons derived from seeping oil and gas. This approach has been employed successfully in various offshore petroliferous basins including the northern Gulf of Mexico, West Africa and in parts of Latin America such as Trinidad and Colombia. In deep water where exploration costs are high and few wells have been drilled, the occurrence and nature of hydrocarbon seeps is one of the few available means for assessing prospective areas.
Piston coring has several advantages over other surface geochemical techniques. In addition to being one of the most cost-effective approaches, sediments record an integrated seep signal over time and typically yield sufficient hydrocarbon material to perform conventional geochemical analyses used to identify oil properties, thermal maturity and source rock type. This information is integral to proper play concepts necessary for a successful exploration program.
A piston corer uses a "free fall" of the coring rig to achieve a greater initial force on impact than gravity coring, and a sliding piston inside the core barrel to reduce inside wall friction with the sediment and to assist in the evacuation of displaced water from the top of the corer.
The piston-coring rig is comprised of a trigger assembly, the coring weight assembly, core barrels, tip assembly, and piston. The core barrels are in lengths of 6 and 9 meters. Expendable (used only once) butyrate tubes line the core-barrel and contain the sample.
As soon as the core is retrieved on-deck, the core liner is divided and labeled into 20-cm sections. The core sections are then capped and taken to a "clean" laboratory for processing. Each core section is extruded and the top and bottom 1-cm of sediment is discarded. Generally, three core sections from near the bottom of the core are sampled for the various analytical requirements, and a fourth section is saved as an archive. The exact core sections sampled is dependent on the core recovery (see figure below). All samples are stored at -15 degrees to -20 degrees C. Detailed records are kept for each core, describing subsampling procedures and noting any unusual features in the sample (visible oil, hydrogen sulfide odor, etc). Water depth, date, time, and WGS 84 latitude and longitude are also recorded for each sample.
