USE OF X-RAY COMPUTED TOMOGRAPHY TO QUANTIFY THE PETROPHYSICAL PROPERTIES OF VOLCANIC ROCKS: A CASE STUDY FROM TENERIFE, CANARY ISLANDS
C. Couves1*, S. Roberts2, A. Racey3, I. Troth4 and A. Best1
1 National Oceanography Centre Southampton, University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZH.
2 Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Waterfront Campus, Southampton, SO14 3ZH.
3 Andy Racey Geoscience, 5 Norris Close, Whitehill, Bordon, Hampshire, GU35 9EG.
4 BG Group, 100 Thames Valley Park Drive, Reading, Berkshire, RG6 1PT.
* Corresponding author, email: C.Couves@noc.soton.ac.uk
Basaltic lava flows and ignimbrite units around Puerto de Santiago, SW Tenerife, were investigated as potential analogues for hydrocarbon-bearing volcanic reservoir rocks. Conventional helium porosity and air permeability measurements together with the results of a micro-focus X-ray computed tomography study were integrated with field observations on flow morphology and the continuity of defined internal lava flow zones.
Individual lava flows typically comprise distinct tops, cores and bases, with lava piles showing repeated cycles of these three internal zones. Reservoir quality is best in the flow tops (mean ϕ= 23.14%, k = 5.622mD) where vesicular porosity dominates. Flow cores are relatively tight with primary porosity mainly controlled by cooling joints, fractures and intercrystalline microporosity (mean ϕ = 2.40%, k = 0.001mD). Flow bases show variable reservoir potential due to the presence of breccia and/or vesiculation (mean ϕ = 11.77%, k = 0.001mD). By contrast, ignimbrites show the highest porosities but have low permeabilities (mean ϕ = 35.64%, k = 0.0056mD). In all cases, the primary porosity and permeability may have been modified to create additional secondary porosity and permeability as a result of fracturing and mineral dissolution during burial or weathering, although porosity may also be occluded through the precipitation of secondary minerals or the alteration of primary minerals.
The new porosity data presented demonstrates that pyroclastics and basaltic lava flow tops have the best reservoir properties, with the flow tops having sufficient porosity and permeability to transmit fluids and gas. In contrast, flow cores are relatively tight (impermeable), and would act as seals to potential hydrocarbon accumulations when fractures and cooling joints are absent. The reservoir potential can be high where vesicles and fractures are present (i.e. in lava flow tops), although there is uncertainty whether flow tops could be connected to each other vertically to form a potentially exploitable hydrocarbon reservoir. Although in some areas such volcanic rocks can be primary targets for hydrocarbon exploration (e.g. onshore China), in many others they are considered to be a secondary target, adding incremental resources to hydrocarbons produced from more conventional sandstone and carbonate reservoirs.
Key words: Tenerife, volcanic reservoir rocks, reservoir quality, lava flows, porosity permeability, micro-focus X-ray tomography
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