Metamorphic rocks

Igneous rocks

Igneous rocks are the primary form of rocks as they are formed from the molten material of the Earth’s interior (from the mantle or the asthenosphere). They may be extrusive igneous rocks, formed by expulsion from volcanoes onto the Earth’s surface (e.g., lavas, tuffs), or intrusive igneous rocks, where the melts (magma) have been intruded at a depth within the Earth’s crust (e.g., granites, basalt sills) and solidified there. Igneous rocks rarely form drilling targets for petroleum - however, very occasionally, fractured granites and lavas can be productive for hydrocarbons.

Sedimentary rocks

Sedimentary rocks are formed by the erosion of pre-existing rocks and subsequent transport of the resulting particles by water or air, and their subsequent deposition (sandstones, mudstones). They can also be precipitated directly from sea water (limestones and evaporites) and include those altered by relatively low temperature chemical changes (e.g., dolomites). Sedimentary rocks form the large majority of petroleum reservoirs and source rocks.

Metamorphic rocks

Metamorphic rocks are formed by the heating - often accompanied by deformation-of pre-existing rocks (igneous or sedimentary) at depth within the Earth’s crust (e.g., schist, marble, gneiss). Ultimately metamorphic rocks will be melted and re-cycled as igneous rocks. Metamorphic rocks are rarely drilling targets for hydrocarbons.

The difference between a rock and mineral:

1) A rock is an aggregate of one or more minerals (e.g., granite, marble or sandstone) or a body of undifferentiated mineral matter (e.g., obsidian) or organic matter (e.g., coal). Note that no self-respecting Geologist uses the term “stone” other than as a suffix - sandstone, mudstone, etc.

2) A mineral is a naturally occuring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form and physical properties.

3) The number of different minerals is vast and only those most commonly occurring in sedimentary rocks, considered most relevant to the petroleum engineer, are consid-ered at this time.

Quartz [SiO₂]: The dominant mineral in sandstones and important in many igneous rocks (especially granite). A hard (cannot be scratched with a steel blade), transparent or white mineral which is not easily dissolved.

Calcite [CaCO₃]: The dominant mineral in limestones(carbonates) and an important cement in sandstones. A fairly soft (can be scratched with a knife), transparent or cloudy mineral which is fairly easily dissolved in weak acids

Dolomite [CaMg(CO₃)₂]: A primary mineral in evaporite (i.e., those formed by chemical precipitation from a saturated brine) sediments and also a common alteration product of limestone. Hard, white mineral, more resistant to acid than limestone