Cobàn Formation: An Example of a Carbonate Petroleum Source Rock

Barry Katz
ChevronTexaco, Bellaire, TX

Deposition of the Cobàn Formation of Guatemala occurred within a shallow marine carbonate platform. The unit, which may exceed 3000 meters in thickness, is composed of alternating beds of limestones, including black micritic interbeds, dolomites, and evaporites. An Aptian to Santonian age has been assigned to the formation. The black micrites are organic-rich and largely restricted to the lower dolomitic member of the Cobàn. These interbeds account for only a few percent of the gross interval. The organic-rich interbeds are associated with the more transgressive periods. These periods of higher sea-level tended to improve organic preservation efficiency, through changes in both sea water density and chemistry. Deposition of these organic-rich interbeds occurred under anoxic conditions. The organic carbon contents of these black micrites may exceed 5%, with total pyrolytic hydrocarbon generation yields ranging upward to about 31 mg HC/g rock. The organic matter is strongly oil-prone, plotting on a van Krevelen-type diagram between the type I and II reference curves. The hydrogen indices ranged between 450 and 620 mg HC/g TOC. The oils generated from this sequence are aromatic-rich, with significant amounts of asphaltenes and resins. The API gravity values of these oils tends to be low, typically less than 20^o, but the oils largely lack any evidence of biodegradation (i.e., they generally display a full suite of n-alkanes and the isoprenoid/n-alkane ratios are low). The low API gravity values are a result of the high sulfur content (>2%), which is thought to be primary and an artifact of limited availability of iron within the carbonate/evaporite depositional setting.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005