Petrophysical Properties of Fresh to Mildly Altered Hyaloclasitie Tuffs

Abstract

In Iceland a significant part of reservoir rocks of cold and thermal systems are basaltic hyaloclastite tuffs. Hyaloclastites are dominantly formed in sub-glacial eruptions. They are fragmental and glassy rocks as they are formed during a rapid magma quenching in the glacial melt water. Hyaloclastites are highly permeable due to their clastic and very porous structure and should therefore be an important source rock for both thermal and fluid systems. It is known that the capacity and longevity of a reservoir is greatly controlled by the petrophysical properties of the host rocks. The purpose of the paper is to describe the progress results of a petrophysical study of hyaloclastites. About 80 samples of hyaloclastites were collected in the Neovolcanic zone and upper Quaternary rocks of S and SW Iceland and then analyzed with respect to chemical and mineral composition and structure (thin- and polished sections, X-Ray and microprobe analysis) and petrophysical properties among which are density, effective and total porosity, hygroscopy, gas permeability, sonic velocity, axial strength and magnetic characteristic. The petrophysical analysis done to date shows extremely variable characteristics: density (1,2-2,34 g/cm3), hygroscopy (0,4-13%), porosity (14-57%), permeability (10-3-6,4*103 mD), axial strength (1,8-111 MPa), Vp (0,9-4,05 km/sec), magnetic susceptibility (0,3-39 10-3 SI). Statistical calculation of data including cluster analysis was done and correlation between petrophysical parameters was estimated. The wide dispersion of properties appears to be highly dependent on the alteration of the hyaloclastites. Basaltic glass is a highly sensitive material and reacts relatively rapidly in cold and hot groundwater environments resulting in chemical and mineral alteration, progressive consolidation and significant changes of petrophysical characteristics. The mineralogical studies allow a classification of alteration stages to be defined - from fresh or slightly palagonized to altered hyaloclastites with zeolites and clay minerals developed in intra-fragmental space and partially in glass. Mineral alteration is correlated with consequent petrophysical changes. The paper considers also the compaction and consolidation of hyaloclastites during regional burial process. A progressive increase in density, sonic velocity and strength and a decrease in permeability occur during gradual burial and increasing age although some exceptions are found.