Porosity in Vuggy Platform Carbonates Measured Over Six Orders of Magnitude

Abstract

Understanding pore networks in prolific vuggy carbonate reservoirs has long been a buried treasure of hydrocarbon exploitation. Documenting porosity at four scales ranging over six orders of magnitude can provide a preliminary treasure map. Our field laboratory is the extensive quarry exposures of the mid-Cretaceous El Abra limestone in the Sierra El Abra, San Luis Potosí State, Mexico. These expose the exhumed windward margin of the vast Valles-San Luis Potosí isolated, shallow-water platform, long recognized as the best outcrop analogue of the giant Golden Lane fields of the Tampico embayment. The platform margin consists of two distinctive facies: Rudist reefs and skeletal sand shoals of the outer margin; and inner-margin tidal deposits and sand/rubble islands. Cyclic subaerial exposure of the more elevated inner-margin sediments produced a more complicated diagenetic history and ultimately lower porosity. Both facies belts record a late, pervasive episode of dissolution, apparently in a regional meteoric aquifer, that produced molds, vugs, channels, and, rarely, caverns. Porosity in these facies was measured at four scales: Micropores (10-6-10-3 m) with standard 1-in plugs, mesopores (10-4-10-3 m) with thin-section point counts, megapores (<10-1 m) with 4-in plugs, and larger megapores with outcrop photopans. This represents a range of pore sizes spanning more than six orders of magnitude. Permeability was measured with 1- and 4-in plugs. Both inner and outer margin rocks have low matrix porosity (micropores) and permeability, 3% and 0.03 md, respectively, and negligible mesoporosity. Measured megaporosity was 9% for the inner margin and 20% for the outer margin, with maximum measured permeability of 553 md. This contrast of values between scales of measurement and between facies underscores the need to measure porosity and permeability in vuggy carbonate rocks over the widest feasible range of scales.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017