Integrated High-Resolution Palynomorph Biostratigraphy and Wireline Log Analysis as a Tool for Basin Analysis and Depositional Environment Interpretation

NORTON, NORMAN J., Chevron Overseas Petroleum Inc., San Ramon, CA

In Cabinda, Angola, serious correlation problems are encountered in thick Cenomanian and Albian successions of mixed clastics and carbonates. Rapid lateral and vertical facies changes result from the close proximity of diverse depositional environments, ranging from shallow marginal marine to deep open marine. Lithological miscorrelations

can result from reliance solely on wireline logs. Foraminiferal and nannofossil distributions are strongly modified by these facies, but palynomorphs, particularly pollen and spores and some microplankton, are less subject to facies control. A stratigraphic framework within which the facies relationships can be correctly resolved is provided by a high-resolution palynomorph biozonation.

Palynomorph assemblages from 30 offshore wells were analyzed. Biozones are defined on selected first downhole occurrences (tops), concurrent ranges, and relative abundances of regional key zone forms. Palynomorphs were selected by the criteria: readily recognizable morphology, relative abundance, stratigraphic range, and known stratigraphic value. Fifteen biozones are defined: four are Cenomanian, ten are Albian, and one is Aptian. Specific environments that were identified in conventional cores produced characteristic populations of environmentally sensitive palynomorphs. These distinctive assemblages can now be used to identify open marine, upper shoreface to foreshore, shallow lagoon, and tidal flat environments in cuttings.

Application of the high-resolution palynomorph biozonation is illustrated in a depositional setting characterized by rapid facies changes. Correlation using sonic, density, gamma ray, and resistivity logs is improved when constrained by the biozonal framework.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)