Abstract: Reservoir Geochemistry Applied to Taratunich Oil Field Offshore Campeche, Mexico

Maldonado, R.V. - Pemex and L.A.F. Trindade - Petrobras/Cenpes

The Taratunich oil field is located in La Sonda de Campeche offshore Mexico, which is part of the Southeast Mexican basins that comprise 80% of the known reserves in the Gulf of Mexico. It involves a stratigraphic section from middle Jurassic to Recent which corresponds to an elongated anticlinal limited by two inverse faults parallel to the main structural axis to the Northeast and Southwest. The oil was generated by the Tithonian source rocks and has accumulated in two main reservoirs: Kimmeridgian oolitic packstone and grainstone with shale intercalations ranging from 3500 to 4250 m., and late Cretaceous lithoclastic breccia formed by dolomited packstone-grainstone intercalated with mudstone ranging from 3,000 to 3,600 m. According to seismic interpretation, the Taratunich field has been divided into three structural blocks.

The oil in the Taratunich field presents °API gravity between 25.3 and 36.5 °API and sulfur between 1.21% and 1.83%. X-20 well located in the Northeast block is a condensate that shows a sulfur percentage less then 0.15% and a gravity of 53.6 °API. D13 values are between -27.34 to -28.14 %. Together with biomarker data these results indicate a marine carbonate source rock.

The chromatographic characteristics present a big similarity among them. Detailed correlation between smaller branched cyclic compounds allows to establish small differences between the Jurassic and the Cretaceous oils.

The study of the light hydrocarbons leads to calculate parafinicity index and aromaticity index in the hydrocarbons giving useful key tools to evaluate the processes that are changing the original composition of the hydrocarbons through evaporative fractionation processes. Oils in the Taratunich field show Heptane value (H) and lsoheptane value (1) that classify them as supermature oils, giving an idea about the catagenetic grade of the sediments from where they were originated. The oils accumulated in the Cretaceous reservoirs have a higher thermal evolution than that for the stored oil in the deeper Jurassic reservoir. Cretaceous oils are also more affected by water washing than the Jurassic oils, except for the oil in the Occidental block which was not affected.

To establish the direction reservoir filling, the ratio between the shielded pyrrolic hydrogen carbazole with the exposed and partially shielded compounds were used. These parameters were plotted with the distance from each well to the bordering faults that can work as the possible migration routes. These plots show that the fault bordering the field in the Northeast area may have acted as migration route for the oils accumulated in the Cretaceous reservoir, whereas for the Jurassic reservoir, the fault on the Southeast area seems to be the migration route.

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