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For analysis of hydrocarbon potential, rocks are defined as aggregates or mixtures of minerals plus pores. Any analysis of the extent to which hydrocarbons are trapped in the formation and whether those hydrocarbons can be removed from the rock pores and produced has to begin with an analysis of the rocks themselves. There are three general rock types — igneous, metamorphic, and sedimentary. Although hydrocarbon reservoirs have been found in all three rock types, this page will consider primarily sedimentary rocks, by far the most common rocks associated with hydrocarbons.
Minerals are defined as naturally occurring solids; they have a definite structure, composition, and suite of properties that are either fixed or vary systematically within a definite range. Although there are dozens of elements and hundreds of described minerals available in the Earth’s crust, the actual number that we must concern ourselves with for reservoir engineering purposes is remarkably small. Classification can be broken into:
- Siderite (less common)
In addition, "solid" organic mixtures such as coal or bitumen can be abundant.
For most of purposes, reservoir engineers can restrict their attention to a working knowledge of quartz, feldspars, clays, calcite, dolomite and anhydrite.
The properties of primary engineering interest are often controlled more by the rock fabric than by the bulk composition. The "holes" that can contain hydrocarbons are usually more important than the mineral frame. The following examples show many of the most common sedimentary rock forms and textures. Numerous attempts have been made to extract rock properties from images of the rock and pore space. These techniques often work well, but depend on the:
- Observation scale
- Representative nature of the image
- Internal heterogeneity
A thin section of clean sandstone is shown in Fig. 1. Under plane-polarized light, quartz grains appear white and pores are stained blue. This is a high-porosity, friable sample that has not undergone substantial consolidation. Silica cement can be seen coating the individual grains and bonding the largely unchanged, rounded quartz grains. Grain-to-grain stress is indicated by the fractures radiating from points of grain contact. Although these fractures have a relatively small volume, they have a disproportionately large influence on the mechanical properties, particularly the pressure dependence. With continued diagenesis, quartz grains typically would become intergrown, and large amounts of cement would develop, reducing the pore volume.
A scanning electron microscope (SEM) image of another sandstone is seen in Fig. 2. A higher degree of compaction is indicated here by the intergrown, sutured contacts of the quartz grains (gray areas). A grain undergoing alteration as well as some of the matrix quartz, contain isolated, ineffective porosity. Fractures are again present, particularly near point of grain contact. Many of these fractures, however, may be caused by stress relief as the sample was cored, or by the cutting and polishing. The most obvious features are the contorted and rotated mica grains. These micas were crushed due to compaction, and now host numerous sets of parallel fractures. Some diagenetic clays are also beginning to grow in the pore spaces and act as a cement.
Fig. 2 – Scanning electron microscope (SEM) image of sandstone AT49 showing numerous compaction features. Some grains are either altering (a) or have internal, ineffective porosity (b). Fractures (c) cut numerous grains. Mica plates (d) are rotated and crushed, forming parallel sets of microfractures.
A cementation "front" is visible in Fig. 3. Cements come in a wide variety of forms. Open pores are black in the SEM image. In this case, the lighter gray calcite has filled the pores in the lower portion of the image. Unlike the dispersed silica and clay cements seen in the previous figures, the calcite is deposited with an abrupt front. This kind of texture is common for carbonate cements in sands and is probably caused by the availability of crystal nucleation sites available to a slightly supersaturated pore fluid. We would obviously expect vastly different properties of the uncemented vs. cemented portions separated by only a few grain diameters. This rock is an example of the extreme heterogeneity that can frequently occur even within the same small geologic unit of the same formation.
Carbonates can have extremely complex textures resulting form the mixture of fossils and matrix building the rock. In Fig. 4, an optical image demonstrates the multitude of forms that can be present. Shell fragments appear as crescent shapes in cross section. Much of the material between fragments can be filled with carbonate mud, reducing the porosity substantially. In this sample, bulk porosity is dominated by the larger disconnected vugs. Such vugs can occur as parts of fossils or as a result of chemical dissolution after deposition. Here, a coating of crystals has grown on the vug surfaces. Because of the wide range of sizes, shapes, and compositions that can occur in carbonate rocks, they are often difficult to characterize with core or even log sampling.
Dolomites are usually formed by recrystallization of original aragonite or calcite crystals in sediments. Magnesium in the pore fluids replace some of the calcium, forming a Mg-Ca carbonate structure. Because of the greater density of dolomite, this transformation can include a porosity increase. Sometimes, the replacement can be subtle, and original sedimentary structures and fossil forms can be preserved. Often, however, the recrystalliztion largely destroys the original rock fabric and rhombohedral dolomite crystals appear, as at (a) in Fig. 5. The other intergrown dolomite crystals form porosity that is polygonal. In this sample, many of the pores are coated (b) with pyrobitumin, a complex organic material similar to coal. This pyrobitumen is sometimes incased within dolomite crystals. In this case, it will lower the apparent grain density and strength of the rock.
Clays represent an entire family of minerals with widely differing properties. Clays are among the most abundant minerals and can influence or control physical properties to a major degree. many clays are sensitive to the environment and will change properties and forms under different conditions (e.g., in-situ pressure, temperature, chemical environment). An example of such "sensitive" clay fabrics is shown in Fig. 6. Note that the scale is much finer here than in previous figures. In Fig. 6a, chlorite originally coats the quartz grains. On top of the chlorite, a smectite coating was developed. This core sample was allowed to dry, and the smectite collapsed, forming long slender columns in the pore space. Resaturating the rock with distilled water allowed the smectite coating to expand and fill the pore space (Fig. 6b). The closed pores will obviously have different fluid-flow characteristics. In this case, we cannot assume the mineral in is a passive, inert solid. This rock will change properties according to pore fluid chemistry. The tendency to change with conditions has led to a bias against clays when describing rocks. A "clean" sand, for example, is one that has little or no clay. "Dirty" sandstones or limestones have significant amounts of clay. Clays and their influence on rock properties remain poorly understood and continue to be an area requiring intensive research.
Shales and silts
The most common sedimentary rock types are shales and silts. In Fig. 7, white quartz grains float in the surrounding clay matrix. Black organic material in thin layers indicates the horizontal bedding. As a result, this rock has properties that vary strongly with direction and are thus anisotropic. This material could serve as both a source rock and reservoir seal. This sample demonstrates how a mudstone or shale could have a complex composition. Although clays typically make up a large portion of fine-grained rocks, terms such as "clay" and "shale" are not synonymous.
Common sedimentary rock textures
The rock images shown in the figures above are meant to convey a feel for the types of textures common in sedimentary rocks that can influence physical properties. These few images can in no way be considered a complete description of rock textures. For a more thorough treatment, the reader should consult one of the standard petrography texts or pertinent papers.
Sedimentary rock porosities
Most sedimentary rocks have porosities under 0.50 (fractional). This is easy to understand, particularly with coarser clastic sediments, in which open grain packings that can support a matrix framework have maximum porosities around 0.45. Exceptions to this and other generalizations can occur, and an example is shown in Fig. 8. This globigerina "ooze" is composed largely of the small shells or tests of organisms. The matrix mud fills the region between tests, but interiors remain empty. In addition, the tests themselves are porous. As a result, porosities can be as high as 0.8. Despite these huge porosities, because of the isolated nature of the pores, permeability can be in the microdarcy range. A similar situation often occurs in shallow clay-rich sediments where the open clay plate structure results in initial very high porosities. However, these types of sediments are less common.
- Moore, C.H. 2001. Carbonate Reservoirs: Porosity, Evolution and Diagenesis in a Sequence Stratigraphic Framework, No. 55. Amsterdam, The Netherlands: Developments in Sedimentology, Elsevier Science.
- Berryman, J.G. and Blair, S.C. 1987. Kozeny-Carman relations and image processing methods for estimating Darcy's constant. J. Appl. Phys. 62 (6): 2221-2228. http://dx.doi.org/10.1063/1.339497.
- Blair, S.C., Berge, P.A., and Berryman, J.G. 1993. Two-point correlation functions to characterize microgeometry and estimate permeabilities of synthetic and natural sandstones. Technical Report UCRL-LR--114182, ON: DE93019878, DOE Contract No. W-7405-ENG-48, Lawrence Livermore National Laboratory, Livermore, California (01 August 1993), http://www.osti.gov/bridge/servlets/purl/10182383-BaMnom/.
- Keehm, Y., Mukerji, T., and Nur, A. 2001. Computational rock physics at the pore scale: Transport properties and diagenesis in realistic pore geometries. The Leading Edge 20 (2): 180-183. http://dx.doi.org/10.1190/1.1438904.
- Williams, H., Turner, F.J., and Gilbert, C.M. 1954. Petrography: An Introduction to the Study of Rocks in Thin Sections. San Francisco, California: W.H. Freeman.
- Bloss, F.D. 1961. An Introduction to the Methods of Optical Crystallography. New York: Holt, Rinehart, and Winston.
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