Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.  All Rights Reserved.
 

BP Exploration Company (Colombia) Ltd. 
Santafe de Bogota, Colombia

The risk on petroleum charge in acreage in the South Caribbean, offshore Colombia, has been assessed using integrated seepage detection techniques, including seabed coring. Eight offshore wells have been drilled but have only discovered limited volumes of dry gas. Oil seepage is well known onshore in the basin, and mud diapirs can be seen on seismic profiles offshore. Seepage detection and sampling techniques can be used to better define exploration risk by determining if the oil seepage extends into the offshore area, and if so, what is the most likely source of the oil.

An airborne survey located two main areas of possible offshore oil seepage and was followed by a shallow coring survey that retrieved oil-bearing cores from an offshore mud volcano. Other cores collected on mud diapirs and shallow faults identified from seismic did not retrieve readily identifiable oil. The oil extracted from the offshore mud volcano core was correlated with oil samples collected from onshore seepage sites. Biomarker analysis led to identification of two oil families in onshore seeps, one mainly from a Cretaceous marine carbonate and the other from a Tertiary mixed marine-terrestrial source. Tertiary oils are common in northwestern South America, but are seldom volumetrically important; a Cretaceous marine carbonate source is likely to have much more potential. The offshore seep correlated with the Tertiary oil.

The other shallow cores did not contain readily identifiable oil, but appeared to have abundant recent organic material (ROM) masking any potential thermogenic signature on gas chromatography or gas chromatography-mass spectrometry (GC-MS) results. Even cores with sufficient oil to leave a slick on the sea surface during retrieval were overprinted by ROM on the GC-MS hopane and sterane traces. High resolution GC-MS, however, did show that the cores contained aromatic biomarkers in the relative proportions expected in thermogenic oils but not in ROM. The thermal maturities derived from these biomarkers made a good match with the crudely modeled maturity of a middle Tertiary source rock. The extensive ROM overprint is probably due to the present-day depositional environment. Most of the mud diapirs and shallow faults identified on seismic profiles do not break the surface, but are covered by a veneer of recent sediments; any petroleum leaking into this is diluted by the abundant ROM.

The results of the seabed core analysis alone were ambiguous, but once integrated with geologic models and other seepage data, they improved the definition of the exploration risk on petroleum charge. Cores with a wide geographic spread offshore do contain seeped thermogenic petroleum, but this is most likely from a Tertiary source rather than the potentially more prolific Cretaceous source identified onshore.