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Reservoir rock properties

By Donatien Ishimwe posted 09-29-2014 07:10 PM

  1. Content
  1. Reservoir Rock properties significance on petroleum system.
  2. Types of reservoir rocks and their description.
  3. Factors affecting some of the reservoir rocks properties.
  4. Methods and technics determining rock properties.
  5. Reservoir rock properties calculation formulae.



  1. According to Society of Petroleum Engineers Glossary, a reservoir rock is a rock containing porosity, permeability, sufficient hydrocarbon accumulation and a sealing mechanism to form a reservoir from which commercial flows of hydrocarbons can be produced. Porosity and permeability are the reservoir rock most significant physical properties.  A fundamental property of a reservoir rock between them is porosity. However, for explorationists, an effective reservoir rock, the most fundamental reservoir rock property is its permeability. Both of them are geometric properties are the result of its lithological, structural and compositional behavior (composition). These physical compositions of a rock and the textural properties are geometric such as sizes and shapes of the rock grains, their arrangement system and packaging. The efficiency of reservoir rock account on different important properties, however in this paper discussing on reservoir rocks properties, porosity and permeability are main topics to focus on. As discussed in last paper (Assignment I, 2014) petroleum system is made of different elements which encompass reservoir rock. Moreover, it is found in a sedimentary basin whereby explorationists are able to study its stratigraphy and its sedimentology (rock history) to determine if there is a likeliness of the existence of petroleum system. After they (explorationists) come up with concluding the existence of a petroleum system, they go for further scientific tests-collecting different data and analyzing them to be able to decide about drilling and completion of a petroleum system. Significant part of their scientific studies on particular petroleum system is on estimating the possible quantity of hydrocarbons (crude oil) that may be present there. The reservoir content is estimated by means studying rock properties which can be determined in a direct way or indirectly. The indirect are done through laboratory measurements on core samples of reservoir rock of interest which constitutes direct methods of porosity data acquisition. This is done by measuring a bulk and their pore (empty spaces in a rock). Its bulk volume is gravimetrically determined when a core-sample is having an irregular shape. A petroleum system may have one or more reservoir rocks, and reservoir rocks may have different property basing on their types. Types of reservoir rock depend on kinds of their contents, composition, morphology and sedimentology.

  2. Types of reservoir rocks.

    As a rock to be named a reservoir has to be a porous and permeable lithological structure. It encompasses sedimentary rocks. These sedimentary rocks may be made of sandstones (quartz sand or arksosic sandstone), carbonates mud or dolomite. Dolomites mostly form good reservoirs because the common reason behind it is that there is Mg, 13% smaller than Ca in a way that during dolomitization, there is a total decrease in volume of the material by 13%, here by 13% porosity is gained.



  1. Sandstone reservoir rocks

    The term sand refers to a specific grain with sizes between (62 µm - 2 mm). The performance of the sandstone as a reservoir rock is described by its combination of porosity and permeability depending on the degree to which the sand dominates its. The favorable texture is depicted by packaging of similar sized grains, not a combination of coarse and fine grained composition. The best sandstone reservoirs are those that are composed mainly of quartz grains of sand size of nearly equal sizes or silica cement, with minimal fragmented particles. Sandstone reservoirs are generally 25 meters thick.

  2. Carbonate reservoir rocks.

    The most fascinating aspects of carbonate reservoir rocks are their content. Carbonates are usually made of fossils which “range from the very small single cell to the larger shelled animals”. Most carbonate rocks are deposited at or in very close neighborhood to their site of creation. The "best-sorted" carbonate rocks are Oolites in which encompass grains of the same size and shapes even though Oolites are poorly sorted.




    1. Reservoir rock properties, interpretations and their significance on a petroleum system.
  1. Porosity of reservoir is the property that tells how porous a rock is. It is also defined as a measure of the capacity of reservoir rocks to contain or store fluids. The porosity is genetically classified basing on standard sedimentologic description of reservoir rock; there are primary and secondary porosity.

    (a) The primary porosity types are:

    i) Inter-particle- In this type by which rock content was quickly lost in muds and carbonate sands through compaction and cementation respectively. This type is mostly found as siliciclastic sands.

    ii) Intra particle porosity by which the porosity is made of interiors of carbonate skeletal grains.

    1. Secondary porosity, the porosity formed after deposition leads to other couple of reservoirs types.

      i) Dissolution porosity type is made of carbonate dissolution and leaching. It is also called carbonate reservoirs.

      ii) Fracture porosity which is characterized by not being voluminous.

      Porosity can also be classified basing on rock morphology. There are three types of morphologies to the pore spaces which are:

      a)         Caternary in which the pore open to more than one throat passage

      b)         Cul-de-sac in which the pore open to only one throat passage

      c)         Closed pore in which there is no connection with other pores.

  2. Permeability is a measure of the ability of a fluid to pass through its porous medium. Permeability is one of important to determine the effective reservoir. Porosity and permeability are two properties describing the reservoir rock capacity with regard to the fluid continence. Moreover, a reservoir rock can be porous without being permeable. For example it is said to be permeable if and only if the pores “communicate”. Hence for explorationists, knowing reservoir rock permeability is a key mile stone because it is important for being used to determine if it really has sufficient commercial accumulation of oil, indeed measuring it is very difficult. The measuring of permeability can differently be understood basing on two different ways. When the porous medium is completely saturated by a single fluid, the permeability will be described absolute, become described as effective permeability when its porous medium is occupied by more than one fluid.


    1. Other factors affecting the volume of the reservoir rocks.
      1. Grain size and pattern arrangement: Apart from the arrangement pattern of grains size which effect rock properties, the actual size of the grains does not affects the permeability of a neither reservoir rock nor porosity.
      2. Shape of the grains: grains with high sphericity tend to pack themselves well to make a minimum pore space, the fact which increases angularity and hence pore space volume increases.
      3. Sorting or uniformity of size of the grains: size of grains has an effect on reservoir properties; the more uniform the grains are sized, the great proper volume of voids spaces. Thereby mixing grains of different sizes tends to decrease total volume of void space.
      4. Subsequent action to the sediments (compaction): The more grains are compacted, more the volume of void spaces decreases. However the compaction of sand is less effective than the way clay does.
      5. How the grains were formed.


        1. Methods determining rock properties.

          Reservoir rock properties such as porosity and permeability are directly or indirectly measured. The direct methods consists of measuring the core sample taken from the parallel lithological area of the reservoir rock to assess them while the indirect methods consist of using data collection, well logs, seismic, production tests, etc., the porosity data are used in the basic reservoir to evaluate volumetric calculation of fluids in the reservoir and calculating fluid saturations and geologic characterization of the reservoir

        2. Reservoir rock properties calculation formula.



          Porosity: ф=   , Vp=VB-VS    ,

          Ф symbolizes porosity, Vp (volume of all pores), Vs represents volume occupied by other particles (Matrix materials), VB represents the total volume, ƿ represents their respective densities.

          Permeability: the permeability of a reservoir reckons on the determining the flow of a fluid which depends on constan, k , Darsey constant. With q representing flow rate, A the area section of pores, µ represents  viscosity constant of fluid and dp/ dL represents the infinitesimal change of flowing pressure.

           q= -




BATES, R. L. AND J. A. JACKSON, 1980, Glossary of Geology: American Geological Institute, Falls Church, VA., 2nd Ed., 749 pp.

BERGER, B. D. and K. E. Anderson, 1992, Modern Petroleum - A Basic Primer of the Industry: PennWell Books, 3rd Ed., Tulsa, OK., 517 p.

DOTT, R. H., and M. J. REYNOLDS, 1969, Source book for Petroleum Geology: Am. Assoc. Petrol. Geol., Mem. No. 5, 471 pp.

NORTH, F. K., 1985, Petroleum Geology: Allen & Unwin, Inc., Winchester, MA., 607 pp.

Other sources:

1 comment



11-09-2014 02:25 PM

It is well known that the permeability scale is logarithmic, do you know what are the principles that led to such state?
Best regards