Diagenesis of Zeolite-Cemented Sands in the Frio Formation, Perdido Fold Belt, Western GoM

Oligocene-age Frio sands represent a significant HC resource in the western GOM Perdido Fold Belt. Oil-bearing reservoirs include volcanic ash-rich sands that contain significant amounts of pumice and glass-rich material in which shards are still preserved. These sands are characterized by high macro-porosity and good permeabilities. By contrast, zeolite-cemented sands tend to be micro-porous and to have significantly lower permeabilities. Permeability loss is attributed to the diagenetic reaction of volcanic ash to clinoptilolite, a common zeolite mineral.

All of the ash-rich sands were originally deposited as mass flow deposits in seawater. Detailed petrologic study of cuttings and sidewall cores from several Frio sands shows that the presence of ash and zeolite are almost mutually exclusive. The presence of stacked ash-rich and zeolite-rich sands presents a technical challenge in risking the presence of hydrocarbons and predicting reservoir quality. It also debunks the hypothesis that the ash to zeolite transition is primarily a function of temperature (depth).

Based on our geochemical, petrological and detailed petrographic study of Frio sands, we have established a thermodynamic model for the ash to zeolite transformation that is consistent with brine and clinoptilolite compositions. Results of our study show that in every instance, clinoptilolite is more sodic than its precursor ash and that there is a regional increase from Na-poor and K-rich ash in the south, to almost a completely Na-bearing clinoptilolite in the North. This regional increase in the Na content of clinoptilolite is mimicked by the composition of coexisting brines.

We conclude that the most likely cause for the selective transformation of ash-rich sands is due to progressive infiltration of brine that is more concentrated than present day seawater (Ionic Strength up to 2× SW), along certain ash-rich horizons. The infiltration of the concentrated brine is mediated by favourable fault transmissibility both on a regional and local scale. Alteration along a particular horizon apparently terminates when the ash is completely altered to zeolite, resulting in an effective decrease in brine permeability that inhibits any further infiltration of brine.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California