Controls on primary porosity and permeability development in igneous rocks

Abstract

Some of the more important processes leading to the development of primary igneous porosity due to the cooling and crystallization of magma are reviewed. A distinction is made between volcanic and plutonic rocks, and crystalline and granular volcanic material. Porosity in each rock type is classified according to a proposed effective length scale and geometry into diffusive (Class D) and macroscopic flow (Class F) features. Estimated ranges in values of porosity and permeability are given for a wide selection of igneous rock types, and comparison is made with permeability variations (Δk) derived for both the continental and oceanic crust. While fracture porosity is dominant in most crystalline materials, primary porosity development may play an important role in the final (total) porosity in igneous basement. Some types of primary porosity and permeability in igneous rocks will be strongly time- and scale-dependent due to thermal effects associated with the emplacement and cooling of magmas and volcanic material. Tectonic reworking of the primary petrophysical properties of basement-forming igneous rocks may be significant in the development of regions of anisotropy and enhanced porosity.

Porosity and Permeability of Round Top Mountain Rhyolite (Texas, USA) Favor Coarse Crush Size for Rare Earth Element Heap Leach

“…This percentage varies dramatically with rock type and individual occurrence; some sedimentary rocks can exhibit 40% or more porosity, whereas igneous rocks, such as granite and rhyolite, typically have porosities in the low single digit range [15][16][17][18][19][20]. For most crystalline rocks (i.e., igneous and metamorphic groups), porosity values are low due to formation of the rock at high temperatures and/or pressures.…”

Fracturing of doleritic intrusions and associated contact zones: Implications for fluid flow in volcanic basins

“…The matrix of crystalline igneous rocks is typically tight, with sub-milliDarcy permeability (e.g., 59 Sruoga et al, 2004) and primary porosity commonly less than 0.5-1% (e.g., Petford, 2003;van 60 Wyk, 1963). Fractured igneous aquifers may nonetheless be considered as potential groundwater 61 reservoirs (e.g., Gustafson and Krásný, 1994;Woodford and Chevallier, 2002) which is part of the regional Karoo Supergroup (Fig.…”

“…Cooling fractures perpendicular to the contact are often viewed as primary permeability 438pathways (e.g.,Rateau et al, 2013), and the multi-sided pervasive jointing exposes much of the 439 intrusion to percolating fluids. However, as pointed out byPetford (2003), the geometry of 440 columnar joints lacks the fracture connectivity required for high levels of permeability. In441 essence, the columnar joints may locally provide permeability pathways across the intrusion, 442 provided that they are open, but permeability is thought to be higher parallel to the intrusion 443 contacts.…”

A fresh approach to ditch cutting analysis as an aid to exploration in areas affected by large igneous province (LIP) volcanism

“…The point is that the cuttings quality may be influenced by many factors related to both lithology and drilling, and all must be checked as thoroughly as possible prior to and during analysis. The loss in circulation at around 8400 ft is inferred to relate to the highly porous and permeable zone at the top of a simple flood basalt flow that has not undergone significant secondary mineralization (Freeze & Cherry 1979;Petford 2003;McGrail et al 2006). The similar facies lower in the sequence are inferred to have secondary minerals filling all the vesicular pore-space and thus do not represent potential loss zones.…”