There are five types of sedimentary rocks that are important in the
production of hydrocarbons:
Sandstones
Sandstones are clastic sedimentary rocks composed of mainly sand size
particles or grains set in a matrix of silt or clay and more or less firmly
united by a cementing material (commonly silica, iron oxide, or calcium
carbonate).
The sand particles usually consist of quartz, and the term “sandstone”,
when used without qualification, indicates a rock containing about 85-90%
quartz.
Carbonates, broken into two categories, limestones
and dolomites.
Carbonates are sediments formed by a mineral compound characterized by a
fundamental anionic structure of CO3-2.
- Calcite and aragonite CaCO3, are examples of carbonates.
- Limestones are sedimentary rocks consisting chiefly of the mineral calcite (calcium carbonate, CaCO3), with or without magnesium carbonate. Limestones are the most important and widely distributed of the carbonate rocks.
- Dolomite is a common rock forming mineral with the formula CaMg(CO3)2. A sedimentary rock will be named dolomite if that rock is composed of more than 90% mineral dolomite and less than 10% mineral calcite.
Shales
Shale is a type of detrital sedimentary rock formed by the consolidation
of fine-grained material including clay, mud, and silt and have a layered or
stratified structure parallel to bedding. Shales are typically porous and
contain hydrocarbons but generally exhibit no permeability. Therefore, they typically
do not form reservoirs but do make excellent cap rocks. If a shale is
fractured, it would have the potential to be a reservoir.
Evaporites
Evaporites do not form reservoirs like limestone and sandstone, but are
very important to petroleum exploration because they make excellent cap rocks
and generate traps. The term “evaporite” is used for all deposits, such as salt
deposits, that are composed of minerals that precipitated from saline solutions
concentrated by evaporation. On evaporation the general sequence of
precipitation is: calcite, gypsum or anhydrite, halite, and finally bittern
salts.
Evaporites make excellent cap rocks because they are impermeable and,
unlike lithified shales, they deform plastically, not by fracturing.
The formation of salt structures can produce several different types of traps. One type is created by the folding and faulting associated with the lateral and upward movement of salt through overlying sediments. Salt overhangs create another type of trapping mechanism.
