James A. Deckelman1,
Bradley J. Huizinga2
(1) ARCO Latin America, Inc, Plano, TX
(2) ARCO International Oil and Gas Company, Plano, TX
Abstract: Seabed piston coring: A cost-effective means of assessing petroleum charge, offshore Colombia
In 1998, ARCO and Amoco carried out a Seabed Piston Coring Program in the Los Galeones/La Fragata Contract Areas, offshore Lower Magdalena Basin, Colombia. The purpose of the program was to determine if a working liquid petroleum system was present in these areas and, if so, to determine if the petroleum was generated by Cretaceous, Tertiary, or a combination of both sources.
Sediment soluble extracts from 76 cores were analysed using total scanning fluorescence (TSF), C15+ gas chromatographic, GC-MS, and headspace-gas data. These data indicated that 12 cores contained thermogenic gas and/or gas condensate, four cores contained strong shows of oil, and 16 cores contained weak shows of oil.
The oils or condensates were moderately to highly biodegraded and, in most cases, seepage of gas, oil and condensate was associated with mud diapirs or faults. Cores containing gas and gas condensate were concentrated in the northern part of the survey area; cores containing oil were concentrated in the southern part of the survey area.
Biomarker data indicate that Tertiary strata solely or dominantly sourced the oil in these cores. Cumulative expulsion modeling based on geochemical data from the adjacent onshore area indicates that Tertiary source rocks are likely to generate gas and minor amounts of oil, whereas Cretaceous source rocks are more oil-prone and generate substantially less gas. This is consistent with production from onshore fields in that those fields with known Tertiary (Oligo-Miocene) sources produce gas and gas condensate with subordinate amounts of oil. If the Tertiary sequence is the dominant source of petroleum in the offshore domain and if onshore-to-offshore source facies variation is minimal, oil will be present, but in quantities that are volumetrically insignificant with respect to gas, in the offshore Lower Magdalena Basin.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana