LITHOLOGY BASICS
In oil field applications of logs, interest is primarily directed to definition of the amount and type of fluids in the formations. These determinations require that matrix effects be defined and accounted for through appropriate assumptions about the mineralogy of the reservoir or by combinations of logging measurements that automatically compensate for mineral effects. In addition, we have found that a knowledge of the mineral composition of the reservoir aids in understanding its depositional environment, porosity distribution, production characteristics, and exploitation potential. So lithology or mineralogy results from petrophysical analysis are worthwhile pursuits in their own right.

In coal, evaporite, and mineral exploration, the primary interest is in the identification of the minerals - porosity is usually negligible or less important. Mathematically, the oil-field and mining situations are identical, so the methods described here apply to both disciplines equally.

Before proceeding, we need to define the nature of rocks more clearly.

An element is a primary component of a chemical compound. Familiar elements are iron (Fe), calcium (Ca), carbon (C), and oxygen (O).

A mineral is a naturally occurring inorganic compound with a specific chemical formula and a defined crystal structure. Many naturally occurring minerals are impure, so their chemical makeup varies slightly. Familiar mineral compounds are quartz (SiO2) and calcite (CaCO3).

A rock is made from a mixture of minerals, although one mineral may dominate. For example, some sandstones are mostly quartz (SiO2) but many other minerals may also be present. Other sandstones may be mostly feldspar with little quartz. Limestone is a rock containing mostly calcite (CaCO3) but other minerals may be mixed with it. Most rocks have a wide range of minerals and the fraction of each mineral in a rock may vary widely from one sample to another.

Minerals are often described and identified by, their hardness, magnetic response, colour, luster, streak, cleavage, crystal form, specific gravity, reaction to acid, or even their taste and smell. These terms are useless for petrophysical log analysis, which relies on physical properties that can be measured remotely in a well bore, such as density, acoustic velocity, neutron and gamma ray response, or electrical resistivity.

Minerals are classified into groups and sub-groups. Silicate minerals have silicon and oxygen in their composition. There are four types of silicate minerals:

  • Single chain silicate (eg. augite)
  • Double chain silicate (eg. hornblende)
  • Sheet silicate (eg. micas and clays)
  • 3-D framework silicate (eg. feldspars, quartz)

Silicates are also divided into two groups based on their color and density. Light (nonferromangesian) silicates are light in color and have a specific gravity around 2.7. Light silicates contain various amounts of aluminum, potassium, calcium and sodium. Dark (ferromagnesian) silicates are dark in color and have a specific gravity ranging from about 3.2 to 3.6. They contain mostly iron and magnesium.

All other minerals are put into the non-silicate group, then broken down into six subgroups:

  • Carbonates - minerals that contain carbon and oxygen
  • Oxides - minerals with an oxygen base
  • Sulfides - minerals that contain sulfur
  • Sulfates - minerals that contain sulfur and oxygen
  • Halides - minerals that contain a metal and a halogen element
  • Native metals - copper, silver, gold, zinc, iron, and nickel


ROCK CLASSIFICATION
Because the earth has an active surface, minerals (in the form of rocks) are under constant change. Molten rock from the interior of the earth can be exposed at the surface from volcanoes or mid-ocean ridges. When molten these rocks are called lava flows and when cool they are called igneous rocks.

As igneous rocks are eroded by weather and water, they become loose grains or dissolved in water. When deposited, they become soil or sediment, and later under the pressure of overburden, turn into sedimentary rocks.

If sedimentary rocks are forced deep enough, heat and pressure modify the rock structure. These are called metamorphic rocks.

All three kinds of rocks can contain porosity that can hold economic quantities of oil and gas, although sedimentary reservoirs are much more common. Any of these rock types can re-enter the mantle and become molten again, by subduction at the edges of tectonic plates. This cycle of igneous – sedimentary – metamorphic is called the rock cycle.

Sedimentary rocks are an accumulation of fragments of many pre-existing rocks. Weathering is a process by which rocks are broken down into sediments. There are two types of weathering:

  • Mechanical - weathering in which physical process such as frost wedging and unloading break down rocks.
  • Chemical - weathering in which chemical processes such as oxidation break down rocks.

Transport describes the process by which sediments are moved across the surface. Types of transport include fluvial, glaciers, wind (aolean), and gravity.

Depositional environments describes where sediment comes to rest, The three main groups however are:

  • Continental - deserts, lakes, river beds, swamps, and caves
  • Continental and Marine - deltas
  • Marine - ocean

Lithification is the process by which sediments come together to form a sedimentary rock. There are three ways in which this is done:

  • Compaction – the intense weight and compression caused by the weight of overburden welds sediments together to form a sedimentary rock
  • Cementation - sediments are cemented together by precipitation.
  • Crystallization - process where an existing solution creates a sedimentary rock.

Texture of a rock is based on the size, shape, and arrangement of the grains and other parts of the rock. Sedimentary rocks can be broken down into five different textures:

    • Clastic - consists of broken fragments of pre-existing rock.
    • Bioclastic - consists of the remains of organic material.
    • Crystalline (Nonclastic) - minerals are in a pattern of interlocking crystals.
    • Amorphous - no crystal structure .
    • Oolitic - made of small round particles of calcium carbonate.

Mineral composition in sedimentary rocks varies widely.

  • Silicates
  • Carbonates
  • Clay Minerals
  • Organic Matter
  • Evaporites
  • (Volcanic) Rock Particles
  • Heavy Minerals
  • Feldspar

Many descriptive terms are used to define rock samples, most cannot be determined directly from petrophysical logs. Shape, sorting, bedding type and bed thickness are common terms. Size of the sedimentary particles is a semi-quantitative approach to sample description and assists the petrophysicist in understanding the rock texture. Terms used are:

  • Clay - <1/256 mm
  • Silt - 1/256mm – 1/16 mm
  • Sand - 1/16mm – 2 mm
  • Pebble- 2mm – 64 mm
  • Cobble - 64mm – 256 mm
  • Boulder - >256 mm

WELL LOGS AND LITHOLOGY
Most logs are useful in identifying mineralogy. Sonic, density, neutron, natural gamma ray, photo electric, spectral natural gamma ray, induced gamma ray (elemental capture spectroscopy), and rarely resistivity logs are used individually or in combinations to calculate mineral abundance in a rock.

The kinds of rocks we can identify with well logs depend on the logging tools that have been run in the well bore, the rock mixtures present, and local zone knowledge. In clastic and carbonate sections, we can usually identify quartz, shale, limestone, dolomite, anhydrite, coal, pyrite or glauconite or siderite or other heavy minerals, salt, potash, trona, sulphur, gypsum, and a few rarer minerals like fluorite or barite, provided the minerals occur as mixtures of only a few components and we have a full modern log suite.

Shale minerals, such as montmorillonite, illite, and chlorite, can be distinguished if we have additional logs. Kaolinite and feldspars can also be defined under certain conditions, as can mica. Although not discussed in this Chapter, hardrock minerals and uranium deposits can be evaluated with well logs.

The mineralogy of unconventional reservoir rocks, such as granite, metamorphic, and volcanic rocks, can be evaluated with the techniques described here, provided the list of minerals is small and their physical properties can be determined.

In most carbonate reservoirs, the lithology is usually reasonably well known from sample descriptions or can be determined from log response. This is not true in sandstones because the mineral makeup of the sand is NOT usually described in much detail. There is a universal trend to give sandstones the physical properties of pure quartz, but this is almost universally NOT appropriate. Most sandstones contain other minerals such as mica, volcanic rock fragments, calcite, dolomite, anhydrite, and ferrous minerals, as well as the shale and clay described above. All of these minerals have different density, acoustic, and neutron properties than quartz. If a sandstone is assumed to be pure quartz when it is not, the commonly used properties of quartz will provide pessimistic porosity answers.

Thus, authors and service company manuals that present mineral properties for “sandstone” are misleading their audience into believing these properties are constant. In more than 40 years of petrophysical analysis, I have never seen a thin section or XRD report that gave an assay of 100% quartz in any petroleum reservoir. A 100% quartz sand is very rare. If anyone doubts this statement, look at the PEF curve. If it reads more than 1.8, you have “quartz plus other things” in your sandstone.

There is a story (it may even be true) that reserves for the early North Sea discoveries were seriously underestimated because the mica in the sands was not accounted for properly. The engineers used density log porosity without correcting for the real matrix density. If true, good engineering practice would have undersized all the offshore equipment and early cash flow and rate of return on investment would have been significantly reduced. If the myth that sandstone is pure quartz is perpetuated, there will be more economic blunders of this type.

Well logging literature is full of other inconsistencies by mixing the names of minerals with the names of rocks. Sometimes the words are synonymous, sometimes not. For example:

      MINERAL NAME    ROCK NAME
      Quartz                  Sandstone
      Calcite                  Limestone
      Dolomite              Dolostone
      Illite                     Shale
Check any service company chartbook and see how often the rock names are used as mineral names or vice versa.


To further confuse the uninitiated, many logs show data on a "porosity" scale. These log curves are transforms of some measured physical property to an approximate porosity, based on some arbitrary parameters. Examples are density, neutron, or sonic porosity on so-called Sandstone, Limestone, or Dolomite porosity scales. Porosity as defined by these transforms is only directly useful if there is no shale, the scale matches the rock mineralogy. and there are no accessory minerals. Real reservoirs are rarely this simple. DO NOT use these porosity transforms without further analysis unless all the arbitrary assumptions used to create them match exactly the rock you are analyzing.

Some people call these porosity curves an “interpretation”. They are not. They are merely a transform of the raw data to a more attractive scale. The difference between a transform and an interpretation is critical. Interpretation infers some intelligent thought went into creating and understanding the result. The service company running the log does not provide interpretations. YOU are the interpreter.

There are endless cases where a transform to an inappropriate porosity scale has caused millions in losses due to poorly informed analysts who see “gas cross over” when there is no gas, or who read porosity directly from the transform and either seriously over estimate or under estimate reservoir effective porosity.

In spite of these comments, a number of charts and tables in this Chapter and elsewhere in this Handbook show the word "sandstone' when they really should say "quartz". I have not edited the charts and tables taken from common sources, such as service company chart books, so the common usage of incorrect terminology is repeated even here.

It should be noted also that this book uses the term "matrix rock" to mean the solid, non-shale portion of a porous or non-porous rock. In petrographic descriptions, "matrix" is the clay between rock grains.
 

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