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Felsic

Felsic describes igneous rocks and minerals that are rich in silicon, aluminum, and alkali metals (sodium and potassium), with a relatively low density and a light color in hand specimen. The word is a portmanteau of feldspar and silica, the two mineral groups that dominate felsic composition. Granite is the most familiar felsic intrusive rock; rhyolite is its fine-grained volcanic equivalent. Felsic rocks stand at the opposite end of the compositional spectrum from mafic rocks (derived from magnesium and iron), which are darker, denser, and represented by basalt on the volcanic side and gabbro on the intrusive side.

Key Takeaways

  • Felsic rocks contain more than 65 percent silica (SiO₂) by weight. They are dominated by quartz, orthoclase feldspar, plagioclase feldspar (sodium-rich varieties), and muscovite mica. They are typically light gray, pink, or white in fresh exposure.
  • The felsic-to-mafic spectrum is continuous. Rocks at the boundary (roughly 52 to 65 percent SiO₂) are called intermediate (andesite, diorite). Below 45 percent SiO₂ are the ultramafic rocks (peridotite, komatiite).
  • Felsic magmas are more viscous than mafic magmas. This is why felsic volcanic eruptions (like the 1980 Mount St. Helens eruption in Washington State) tend to be more explosive than basaltic ones, which flow quietly as lava.
  • Continental crust is predominantly felsic. Oceanic crust is predominantly mafic (basalt). This is why continental crust is thicker and lighter than oceanic crust, which in turn drives plate tectonics: denser oceanic crust subducts under lighter continental crust at convergent margins.
  • In petroleum geology, felsic rocks appear as arkosic sandstone reservoirs (quartz and feldspar grains eroded from granite), as naturally fractured basement reservoirs producing from granite (Cuu Long Basin offshore Vietnam, Bohai Bay in China), and as rhyolitic tuff reservoirs where fractures provide permeability through otherwise tight volcanic rock.

What Does Felsic Mean?

Hold a piece of granite up to the light. The white and pink minerals are feldspar. The glassy, translucent grains are quartz. The shiny flat flakes are mica. Everything you are looking at is felsic: light in color, relatively low in density (granite typically runs 2.65 to 2.75 grams per cubic centimetre), and rich in silicon and aluminum. Now hold a piece of basalt next to it. Basalt is dark gray to black, denser (2.8 to 3.0 g/cm³), and composed of pyroxene, calcium-rich plagioclase, and olivine. That is the felsic-mafic contrast in a single comparison.

The distinction matters in geology because it tells you a great deal about how the rock formed. Felsic magma is viscous and silica-rich. It forms either deep in the crust where it cools slowly into granite, or at arc volcanoes (like the Cascade Range in British Columbia and Washington) where crustal material is melted back and reprocessed into eruptions of andesite and rhyolite. Mafic magma is runny and silica-poor. It erupts quietly at mid-ocean ridges and hot spots (like Hawaii or Iceland) and forms flat, thick lava flows.

Fast Facts

The Canadian Shield, which underlies most of central Canada from Labrador to the Mackenzie Delta, is predominantly felsic granite and gneiss ranging from 1 to 4 billion years old. These ancient rocks have been eroded to their roots over billions of years of exposure. They contain economically significant gold, copper, nickel, and uranium mineralization but are not themselves petroleum source or reservoir rocks. However, oil and gas fields on the margins of the Shield (Alberta Basin, Williston Basin, Hudson Bay Basin) produce from younger sedimentary rocks that were deposited on top of or adjacent to the Shield during the Paleozoic.

Felsic Rocks in the Petroleum Context

Most oil and gas reservoirs sit in sedimentary rocks, not in igneous ones. But felsic rocks show up in several petroleum geology contexts.

Arkose sandstones are sedimentary rocks composed largely of feldspar grains weathered from felsic granite or gneiss. The Rotliegend sandstone in the southern North Sea, a major gas-producing reservoir under the Netherlands, Germany, Denmark, and the UK sector, contains significant arkosic feldspar derived from uplifted granite. The dissolution of feldspar during diagenesis creates secondary porosity that can improve reservoir quality. Conversely, kaolinite clay produced by feldspar dissolution can occlude pore throats and reduce permeability.

Rhyolite and felsic tuffs (volcanic ash beds) can form tight but fractured reservoirs. In some Argentine Neuquén Basin wells and in the Cuu Long Basin offshore Vietnam, volcanic units interbedded with marine sediments have been tested as production targets. The rhyolitic tuff in the Neuquén Basin is highly fractured because of its brittle mechanical properties, and some wells produce from these units at commercial rates through the natural fracture network.

In geochemistry, the felsic-mafic classification guides assessments of source rock provenance (where the sediment came from) and diagenetic risk (how the rock's original mineral composition controls porosity and permeability evolution during burial).

The antonym in standard usage is mafic. Related terms include mafic (igneous rocks and minerals dominated by magnesium and iron; basalt and gabbro are the common examples; denser and darker than felsic equivalents), silica (silicon dioxide, SiO₂, the most abundant compound in the Earth's crust and the primary determinant of magma viscosity and rock classification on the felsic-to-mafic spectrum), granite (the coarse-grained felsic intrusive rock that forms the core of most continental cratons; an important basement reservoir in some hydrocarbon provinces where natural fractures provide permeability), arkose (a sandstone containing more than 25 percent feldspar grains, typically derived from erosion of granitic or metamorphic felsic rocks; a common reservoir type in basins adjacent to ancient felsic cratons), and diagenesis (the physical and chemical changes that sedimentary rocks undergo after deposition; feldspar dissolution and kaolinite precipitation during diagenesis are particularly important controls on reservoir quality in felsic-rich sandstones).

Why a Felsic Tuff Layer Changed a Gas Field's Development Plan in the Neuquén Basin

An operator drilling an appraisal well in the Neuquén Basin of Argentina encountered a 40-metre interval of rhyolitic tuff interbedded between two marine shale source rock intervals at 2,700 metres. The tuff was not in the original well plan as a target. Core analysis showed the tuff had matrix porosity below 5 percent, typically sub-commercial, but a logging run showed natural fracture frequency of four to six open fractures per metre, confirmed by image log interpretation.

A drill stem test of the tuff interval flowed gas at 280,000 cubic metres per day through the natural fracture network. The matrix was irrelevant; the fractures did all the work. Stable isotope analysis confirmed the gas was sourced from the marine shale directly above and below the tuff, which had been cracking gas into the adjacent fractured felsic layer for millions of years.

The operator redesigned the field development to target the tuff interval with horizontal wells drilled parallel to the dominant fracture orientation identified on the seismic data. Three horizontal wells drilled through the tuff produced at combined rates that exceeded the initial vertical well test by a factor of four. The felsic composition of the tuff, brittle compared to the surrounding shales, was what created the fracture density that made the reservoir productive.

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