BASALTIC GLASS ALTERATION AND ITS RESERVOIR IMPORTANCE

Introduction

Basaltic glass readily alters to phyllosilicates and zeolites. This can significantly alter reservoir properties by reducing permeability and porosity. Increased understanding on authigenic clay formation is key knowledge to predict reservoir properties, and e.g. recoverable oil and gas reserves.

Purpose

In this project the alteration of basaltic glass in various temperature and hydrous regimes have been analyzed. This is done by studying natural samples and performing controlled hydrothermal experiments to increase the understanding on alteration mechanisms of basaltic glass. Samples and experimental material are collected from the Paleocene – Eocene deep marine Balder Fm. located on the Norwegian Continental Shelf. Additional material is collected from Quaternary basalts and sandstones from the Canary Islands. Hydrothermal alteration experiments are performed on the sampled glass under varying experimental conditions. Detailed petrographic studies using whole rock and clay XRD, SEM-EDS, XRF, thin section and microprobe analysis is performed on all collected and experimentally formed material.

Preliminary results

Preliminary studies of borehole cores and outcrops show varying degrees of basalt alteration and generation of secondary phases, largely reflecting difference in original rock and fluid composition and temperature variations during alteration. Core logging of the Balder Fm. between 1620 – 1770 m MD show alternating turbidic and injected sandstones interfingering hemipelagic mud and tuff. The former sandstone is consolidated and tight whereas the latter is loose and hydrocarbon filled. Three outcrops on the Canary Islands have been studied and sampled. The beach sandstone of Montaña Amarilla is unconsolidated and made up of volcanic rock fragments, mafic minerals and minor quantities of basaltic glass. The Bc. Tamaraceite sandstone shows small scale hummocky cross stratification and soft sediment deformation formed by the overlying sampled hyaloclastite. It consists of altered volcanic glass, feldspars and ooids. These stands in contrast to the green hydrothermally altered phonolite of the Azulejos outcrop.