GENETIC PORE TYPES AND A DIAGENESIS LOG FOR CARBONATES AND MICROBIALITE RESERVOIRS (UPPER CAMBRIAN MICROBIAL REEFS, MASON COUNTY, CENTRAL TEXAS)

Microbial carbonate build-ups are major oil and gas reservoirs around the world, from the Aptian pre-salt offshore Brazil and Angola to the Tertiary on the Arabian Peninsula. The effects of diagenetic alteration on the microbial build-ups and their contemporaneous inter-reef sediments make these reservoirs very complex. Very few microbialite exposures can be studied in the field that outcrops in three dimensions. A series of world class outcrops in Mason County Texas (Upper Cambrian) have been recently discovered with well-organized microbial build-ups and inter-reef sediment that are 10-12m-thick and exposed in vertical outcrop and on a pavement.

This workflow can be used to construct a diagenesis and permeability log that is built free of influence from well logs and requires a detailed diagenetic history established with thin section petrography and high (0.001mm) and low resolution (0.5mm) Computed Tomographic (CT) image volumes. The Diagenesis Log tracks the extent of diagenetic alteration of both minerals and pore space.

Dual Energy CT scanning is a novel tool used to derive mineralogy (calcite and dolomite) and porosity logs from whole core which is validated by thin section petrography. The extent of diagenetic alteration is scaled up from the high resolution CT to the whole core to tune and normalize the diagenesis log. This log tracks the extent of diagenetic alteration volumetrically in whole core at 0.5mm resolution and is based on the assumption that dolomite is dominantly diagenetic and that porosity types and different diagenetic stages of calcite can be distinguished in CT and volumetrically measured.

The permeability log requires diagenetic rock types that are established from the diagenesis log. Genetic pore typing is a uniquely well suited rock typing approach because it clusters based on the genesis of the pore space. A porosity versus permeability trend is measured at the pore scale and for each rock type, which is directly used to create a permeability log.

A diagenesis log, built free of open hole well logs, has never been done before and can save money by downscaling a coring program and it can reduce uncertainty for characterizing reservoir potential if Density, Resistivity, and Spectral Gamma Ray open hole logs are available.