PACROFI VI - Electronic Program

Preliminary Results Documenting Conditions of Fluid Migration in an Exhumed Petroleum Reservoir, The Abra Limestone (Mid Cretaceous), NE Mexico

Sean T. Brennan

University of Kansas, Department of Geology, 120 Lindley Hall, Lawrence KS 66045

The study of hydrocarbon inclusions in diagenetic cements has become an important tool for the investigation of petroleum migration and petroleum reservoirs (Jensenius and Burruss, 1990, Bodnar, 1990, McLimans and Videtich, 1987, Burruss, et al, 1985). Documentation of hydrocarbon inclusion types coupled with their relative timing can help to decipher the evolution of hydrocarbons within a reservoir. This type of study could lead to characterization of ancient petroleum reservoirs, which would then be used as analogs for modern reservoirs. One example of an ancient petroleum reservoir is found in the mid Cretaceous Abra Limestone of the Sierra del Abra, east of Valles, Mexico. Quarries in this region expose the platform-margin facies of the Abra limestone of the Valles-San Luis Potosi Platform (Carrillo-Bravo, 1971, Aguayo, 1978, Minero et al, 1983). Locally, the walls of the quarries exhibit extensive oil stains which indicate that the exposures are of an exhumed petroleum reservoir (Minero et al, 1983). This study uses samples from one of these quarries (the "El Abra" quarry) to document the fluids that were present, where those fluids fit into the paragenesis, and to determine temperatures using microthermometric analysis of fluid inclusions.

Thin section petrography was used to determine the paragenetic sequence (Fig. 1). Charging the reservoir with hydrocarbons is not the final diagenetic event, it is postdated by megaquartz and a later equant calcite cement. Inclusions occur in the early equant calcite, the fibrous cement and the megaquartz cement.

Based on fluid inclusion petrography three fluid inclusion types were documented in the Abra Limestone. One of the inclusion types consists of clear, two-phase, liquid-rich, secondary, petroleum inclusions. These inclusions have a fairly consistent vapor-to-liquid ratio of about 1:6 to 1:7 (~85% liquid), and homogenize to liquid at temperatures ranging from 74 to 116oC (Fig. 2). There are two distinct populations of these inclusions; in UV epi-fluorescence illumination, one population fluoresces blue, and the other fluoresces yellow. Another inclusion type consists of clear, two-phase, vapor-rich, secondary, non-fluorescent, gas-condensate, hydrocarbon inclusions. These inclusions have fairly consistent vapor-to-liquid ratios of about 10:1 to 7:1 (~10 - 15% liquid), and homogenize to vapor from 70 to 79oC. A third type of inclusion consists of amber to brown/black, single-phase, non-fluorescent, hydrocarbon inclusions which are present as secondary as well as primary inclusions. The primary fluid inclusions define growth bands in the megaquartz cement.

The fluid inclusions present in these samples indicate a complex fluid history for the Abra Limestone. Hydrocarbons are only found as secondary inclusions in the early calcite cement, suggesting that the charging of the reservoir post-dates precipitation of the early equant calcite. The hydrocarbon inclusions in the calcite may record the evolution of the hydrocarbons in the reservoir. The reservoir was most likely charged with volatile-rich petroleum which constitutes the liquid-rich, fluorescent, two-phase inclusions. As the hydrocarbons in the reservoir evolved, the volatile hydrocarbons may have separated from the heavier hydrocarbons which could have resulted in the gas condensate inclusions (light hydrocarbons) and the amber to brown/black inclusions (heavy hydrocarbons). Later, most of the volatiles would have been lost from the reservoir. The final stage of the reservoir was the precipitation of the megaquartz cement, which incorporated the amber to brown/black heavy hydrocarbons as primary inclusions.

Microthermometric analyses yield homogenization temperature values which range from 70 to 116oC. Within individual fluid inclusion assemblages the range of homogenization temperature values is typically only within 10oC, arguing against significant thermal reequilbration or other alteration of the fluid inclusions. The minimum estimate for the maximum temperature of the reservoir is 116oC.

The record of hydrocarbon fluid inclusions in the rocks from the El Abra quarry provide a detailed history of the various hydrocarbons that existed within an exhumed petroleum reservoir. When combined with compositional data on the hydrocarbons for modeling PVT relationships, this record could provide a new insight into migration and compositional evolution of hydrocarbons in petroleum reservoirs.