Abstract: Oxfordian-Oxfordian Petroleum System (!) from Offshore Campeche, México

Romero I., M. A.; J. Ruiz M.; L. M. Medrano M.; J. Durán G. - Pemex

The existence of an Oxfordian-Oxfordian Petroleum System was postulated in 1991, due to the discovery of the light oil producing Ek-Balam field, where hydrocarbons are being produced from Oxfordian sands. Its very existence was confirmed, at a later date, through research done by CHEVRON (1993) and CENPES-IMP-PEMEX (1994), using rock-oil biomarker sand isotopy.

Up to now, the most important reservoir rock in this System is represented by quartzitic sands deposited in coastal dunes and beach environments during the Oxfordian. These rocks with a porosity varying from 10 to 22% are considered to be equivalent to the Norphlet Formation. Likewise, light oil has been recovered from carbonates composed of ooides and pelletoids, that were deposited in a shallow platform environment, that probably formed incipient oolitic banks during the Late Oxfordian. The porosity of these rocks ranges from 3 to 6%, and they could be equivalent to the Smackover Formation, which likewise the Norphlet Formation, produces liquid and gaseous hydrocarbons in the north coast region of the Gulf of México.

The traps are a combination of stratigraphic and structural types, and two different tectonic events contributed to their formation. The former, of extensional nature, occurred at the end of the Late Jurassic, generating elongated structures with an almost North-South orientation, corresponding to a block system of a «salt roller» type, whereas, the latter event, of transpressional nature, occurred in the Middle Miocene and formed anticline type structures, with a NW-SE orientation, whose cores are affected by salt intrusions. The traps, generally, have an upper anhydrite seal.

Up to date, two hydrocarbon source horizons have been identified in the Oxfordian column. One of them is located in the upper part, whereas the other seems to correspond chronostratigraphically to the same age sands. Both of them consist of calcareous shales with interbeds of limestones and algae bodies containing oil-prone type I-II kerogen of very good quality, whose charge capacity ranges from moderate to low. The oils are characterized by a high sulfur content, low gas-oil ratio, biomarkers indicating hypersaline depositional environments, and a gravity range from 23 to 48 API.

The fields are generally located outside the perimeter of the hydrocarbon expulsion zones, so it can be assumed that a lateral short distance migration occurred about 10 to 0 million years ago, whereas the latest trap formation event took place approximately 11 million years ago, favoring the synchronization process.

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