Investigating the petrophysical properties of volcanic reservoir analogues through the use of micro-focus x-ray computed tomography

Abstract

Volcanic rocks can host significant hydrocarbon resources but are poorly understood in terms of their reservoir properties: especially their porosity and permeability characteristics. Basaltic lavas and volcaniclastic outcrops around Tenerife and basaltic lava flows from the Large Igneous Provinces of the Deccan Traps and the Faroe Islands were studied as potential “reservoir analogues” to compare with existing hydrocarbon discoveries in volcanic rocks.

These studies together with the analysis of subsurface samples from South America permitted the geochemical and petrophysical properties of several volcanic reservoir analogues to be examined. Field observations of flow morphology and continuity of defined lava flow facies, were integrated with conventional core porosity and permeability measurements and the results of a micro-focus X-ray tomography (µCT) study to characterize the petrophysical properties of various volcanic rocks.
Application of µCT allows the quantification and 3D visualisation of the pore space down to a µm scale and thus provides unprecedented insights into pore morphologies. When combined with traditional petrographic observations, this provides a powerful tool with which to analyse porosity development. Vesicles and fractures are the principle controls upon effective porosity development with permeability often controlled by the degree of vesiculation. Samples with vesicle densities greater than 20 % show significantly higher permeabilities and higher effective porosities due to the increased degree of vesicle coalescence forming connected networks.

Individual lava flows can be divided into base, core and top facies, with lava piles comprising repeated cycles of these distinct facies. The best reservoir quality occurs in basalt flow tops (mean µCT ? = 21.85%) where vesicular porosity dominates. Reservoir quality significantly decreases in the tight flow cores, where primary porosity is controlled by cooling joints (fracture porosity) and inter-crystalline micro-porosity (mean µCT ? = 2.32%). Flow bases show variable reservoir potential due to the presence of breccia and/or vesiculation, depending upon the eruption environment (e.g. subaerial v subaqueous) and original geochemistry (e.g. acidic v basaltic) of the lava (mean µCT ? = 9.70%). The most porous horizons are the flow tops of each successive lava flow, whilst connectivity (permeability) between these horizons is via primary cooling joints or secondary fractures. Volcaniclastic lithologies of, tuff, ignimbrite, scoria and pumice, have the highest porosity and permeability (mean µCT ? = 48.09%).

Distribution of the various volcanic facies is influenced by the magma chemistry, cooling rate and mode of eruption. The primary porosity and the permeability of the rocks may be subsequently modified either to create additional porosity and permeability by tectonic fracturing and dissolution during burial or weathering, or occlude porosity through the precipitation of secondary minerals and the alteration of primary minerals.

Given volcanic rocks can exhibit high porosity (especially within flow top internal zones and to as lesser extent flow bases) these lithologies can form viable reservoirs. The stacking of successive flows results in a layer cake stratigraphy, with the more porous and permeable flow tops and bases separated by relatively impervious flow cores. Should sufficient fractures and cooling joints exist, then these reservoir horizons will be in communication with each other, permitting charging of hydrocarbons. The conclusion of this thesis is that despite low permeabilities, volcanic rocks are good hydrocarbon prospects, and understanding the distribution of these key internal zones and their characteristic porosities and permeabilities will enhance hydrocarbon exploration within these unconventional reservoirs.

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ORCID for Stephen Roberts: orcid.org/0000-0003-4755-6703

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