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Abstract: Integrated Surface/Subsurface Geochemical Exploration Surveys Identify Active Petroleum System in Deepwater Angola

Cameron, Nick R. and Craig F. Schiefelbein - GeoMark Research, Inc.; Mário G. P. Brandão - Sonangol; James M. Brooks - TDI-Brooks Int'l Inc.

Surface geochemical exploration (SGE) studies have been instrumental in defining active Petroleum Systems on the west African continental margin. These studies have shown that active oil generation and migration characterizes offshore central Angola. Upward negation along faults and other conduits produces macroseepage of oil and gas in near-surface seafloor sediments that can express itself visually as oil-staining within or throughout a core. In addition to the widespread macroseepage of oil and, microseepage of oil and gas identified through geochemical analyses of the cores is also common.

Two SGE studies have recently been performed in the deep and ultra deepwater regions of offshore Angola. The initial purpose of regional reconnaissance studies was to define the aerial distribution of oil, condensate and gas seepage on the continental margin. These studies high graded areas and prospects by defining areas of active oil migration and charge through routine gas and high molecular weight hydrocarbon screening methods. A comprehensive suite of analytical techniques was used on the cores to define the regional hydrocarbon seepage patterns within the study areas. The higher molecular weight, migrated hydrocarbons were determined by total scanning fluorescence (TSF) and capillary gas chromatography of the normal C15+ hydrocarbons (C15+GC) in hexane sediment extracts. The cores are also analyzed for free-hydrocarbon gas (methane to pentanes) using standard gas chromatographic techniques with flame ionization detection.

On a secondary level, the SGE coring studies were used to define the distribution of off sources, families and maturities in a region. This goal required accompanying oil and rock data for cross correlation with the seep data. The regional SGE program on the Angolan margin utilized biological marker and isotope correlations of macroseep samples with approximately 120 regional oils. The macroseep and representative oils samples were both characterized using detailed molecular and isotopic analyses. Multivariate statistical techniques (cluster and principal component analyses) using isotopic and key biomarker ratios are used to aid in grouping oils/seeps which share a common source. In addition, relative thermal histories of the oils/seeps are evaluated based on certain biomarker distributions.

The results of these studies showed that there is significant macroseepage of oil in parts of the deep and ultra deepwater regions of Angola. Migrant oil was predominantly of Post-Salt origin, but Pre-Salt oils were also identified.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil