BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Deposits of cementing material between grains of sediment, whether compacted or uncompacted, will
bind grains together. Sediments that are tightly cemented produce hard, coherent rocks, thus
solidifying the formation. Since this solidity reduces both porosity and permeability characteristics of
rock, the formation loses its potential as a productive reservoir. Should this be the case, fracturing of
very hard limestones and sandstones can greatly enhance their intrinsically low reservoir potential.
Carbonates
An abundant rock that often contains oil is limestone. Sometimes the limestone contains substantial
amounts of magnesium, replacing calcium, and it then becomes dolomite. It has become customary in
the oil business to call both limestone and dolomite carbonates to avoid making a distinction.
Oil was first discovered in carbonate rocks in Ontario in the 1850’s and later (in the early 1900’s) in
the Tampico region of Mexico. In the 1920’s, the carbonates of West Texas became important. By
the 1930’s and 1940’s, the great oilfields of Iran and Saudi Arabia were found in the Asmari limestone
of Miocene age and the Jurassic limestone, respectively. It has been estimated that half the world’s oil
reserves are in carbonates, although there are numerically fewer carbonate than sandstone reservoirs
outside the Middle East.
Carbonates differ in many respects from sandstones. They are mostly formed from the remains of
animals (shellfish) and plants (algae); they are therefore found in nearly the same place where they
originated and were not transported and then deposited like sandstones. Calcium carbonate can easily
be dissolved in water solutions, so that solution and re-crystallization of the carbonates after their
deposition (diagenesis) is very common. This solution forms some of the cavities that can store oil.
Limestones are much more brittle than sandstones, and as a result of folding or faulting they may
break, leaving open fractures that serve as routes of fluid flow.
Modern Carbonate Depositional Environments
Calcium carbonate is precipitated from seawater by many types of organisms. Mollusks such as
oysters and clams make their shells of calcium carbonate, but many other animals and plants also form
shells which are called exoskeletons. Some animals, notably the corals, live in large colonies and
form sturdy build-ups or reefs. These are attacked by waves and fish, producing fine calcareous mud
that washes down the sides of the reefs.
Naming Carbonate Rock Types
The interpretation of carbonate facies to define zones of good reservoir properties has been held back
by the lack of a generally accepted system for naming the different rock types. Two different
geologists describing the same core might very well use different words.
In 1913 A. W. Grabau proposed the words calcilucite for consolidated lime mud, calcarenite for
limestones whose particles are sand size, and calcirudite for limestones with pebble size grains. These
terms are simple, descriptive, and still widely used.
Beginning about 25 years ago, it became obvious to oil company geologists that some uniformity in
the terms used to describe carbonates was desirable. Several major oil companies developed
classifications about the same time. The most widely used have been those of Robert Folk of the
University of Texas, Robert Dunham of Shell, and Leighton and Pendexter of Exxon. The latter is
similar to Folk’s. Perhaps the most practical and easily understood classification is a modification of
Folk’s suggested by G. M. Friedman.
Folk said that a carbonate rock consists of three textural components: grains, matrix and cement. The
cement is clear calcite that filled or partially filled the pores after the original deposition. There are
several different kinds of grains, of which four are the most important. These are (1) shell fragments
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-11