Manual Reservoir Sandstones

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Den Norske Stats Oljeselskap Statoil microscopy. We focus on diagenetic clays because, are acknowledged for supporting publication of this paper. Analytical Methods Used to Characterize subject to damage during sample preparation. Difficulties may exist in achieving complete impregnation of detrital clay clasts, in addition to the textural modifications of influential in affecting the petrophysical character- fibrous clays.

Geological evidence Regions of the petrographic section exhibiting demonstrates that authigenic chlorite and kaolinite the clay textures of interest were examined, and may have significant effects on formation conduc- BSE images were acquired. Details of samples are tivity Hurst and Archer, ; Hurst, a. Typically, the area of analysis for Here, we present the application of clay micro- clay microporosity is less than 1 mm 2 Hurst and porosity data obtained from petrographic analysis Nadeau, , but varies depending on the clay to estimate water saturation in sandstones.

The texture examined; for example, coarse-grained, methodology uses the relationship between clay pore-filling clays have larger areas available for anal- microporosity, the volume of clay-bound water, ysis than grain-coating or fibrous clays. The digital and irreducible water saturation, and is comple- analysis of the images allowed pixel gray level clas- mentary to more conventional wireline-log and sification Dilks and Graham, such that the special core analysis data. The samples analyzed area occupied by the clay and that occupied by the contain good examples of commonly occurring microporosity epoxy could be distinguished and clay minerals, kaolinite, chlorite, and illite, with a quantified.

Actual quantification of the microporosity Nadeau Despite the awareness of the variability and and Hurst, is accomplished by successive importance of textural variations of diagenetic clay minerals, it appears that no analytical methods are routinely applied to the problem. Mineralogical Table 2. Sample Description analyses used in reservoir description typically comprise a combination of methods Table 1 Formation Lithology Depth m Clay Type where thin-section petrography and x-ray diffrac- tion XRD analyses are normally the most com- Etive Fine-grained Scanning electron microscopy SEM data sandstone usually augment the basic data set.

Thin-section analysis gives a volume percent mineralogy, but Garn Medium-grained The percentage of gray tones deleted equals the microporosity of the region of interest. Analytical precision is substantiated by the approximately constant cutoff level found suitable for represent- ing complete deletion of micropores for a given clay type Nadeau and Hurst, Figure 2.

Between 5 and 52 observations were acquired for the three varieties of clay materials studied.


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Diagenetic kaolinite and chlorite proved to be particularly amenable to this method of analysis Nadeau and Hurst, From the stereo-pair images, estimates of b the dimensions and number of illitic fibers were made within a sample volume for which the depth of view was estimated visually. Then we could esti- mate the solid mineral volume within the total vol- ume mineral plus microporosity. Comparison of microporosity estimates from illitic clays analyzed by BSE imaging and stereo-pair SEM shows that results of BSE images underestimate clay micropo- rosity.

A summary of the BSE microporosity data is given in Table 3. Several textural varieties of kaolinite may be present in any one sample. In our experi- ence, chlorite usually has limited textural variation in any one sample. If no precautions are made to preserve the com- monly delicate fibrous textures, illite may be diffi- cult to characterize by this method. Analysis of stereo- images. Although reasonable estimates of V m may be derived directly from mineralogical data given as b weight percent, to make an accurate evaluation correction for density and porosity are required. In a reservoir, one needs to consider possible associated with the alteration of volcanic fragments capillary pressure characteristics of microporosity in mineralogically immature sandstones, also may because the water preserved in reservoir pore sys- be susceptible to collapse of delicate clay textures tems is a function of what the buoyant force of the when air dried.

For example, in tight gas sandstones, clay micropores may store significant quantities of Clay-Bound Water gas Schulz-Rojahn and Stuart, Independent of which phase is saturating the micropores, quan- Evaluation of clay-bound water or irreducible tification of clay microporosity provides valuable water can be made using microporosity measure- information for evaluating f luid saturation and ments. If clay-bound water Vcbw is approximated reservoir performance characteristics. Hurst and Nadeau Table 3. Clearly, the diagenetic clays, Chlorite 44—58 51 4.

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A review of deformation bands in reservoir sandstones: geometries, mechanisms and distribution

Log evaluation and geological analyses two and three orders of magnitude reduction of of clay content measure only the weight percent of permeability, respectively Figure 7. Such reduc- clay present and not its textural characteristics. The sample shown in Figure 5b has a higher Saturation Evaluation weight percent of illite and smectite than the sam- ple shown in Figure 5a. A well-established equation for eval- acteristics, the Figure 5b sample would be assumed uating saturation is the Waxman-Smits equation for to make the greatest contribution to the matrix which an expression of formation conductivity may conductivity.

In the samples sity log calibrated against core data. In the Figure 5a sample, which contains The CEC of clay minerals, however, is assumed to diagenetic clays, a fourfold increase of Ve for clay is be strongly dependent on the clay mineral type and produced by correcting for microporosity. The grain size Ormsby et al. Clay minerals that effect per volume percent clay on the reservoir usually are interpreted to have negligible effect on characteristics.

Computer Classification of Reservoir Sandstones

BSE images provide a rapid identifi- formation conductivity, such as kaolinite and chlo- cation of clay minerals and their texture, and thus rite, which generally have low CEC values Frost allow selection of the most appropriate values of and Fertl, ; Serra, , may have significant CECshale for the mineral present. Normally, when mum textural variations for specific clay minerals, porosity is classified from petrographic sections, i.

Commonly, this is give an order of magnitude difference in Qv Table simply because of the limited resolution obtained 5. Similarly, if two clay minerals occur together, from a petrographic image. Although in general terms micro- porosity may be characterized as pores with pore- aperture radii of less than 0.

Our experience is that clay microporosity is very similar to mercury porosi- meter measurements of microporosity Hurst and Nadeau, Efforts should always be made to ensure that the images acquired are consistent with this definition and representative of the fea- tures displayed by the specimens.

The inf luence of the clay microtextures on reservoir characteristics can only be inferred from the textural data. For example, the micropo- rosity of coarse-grained and fine-grained vermicu- lar kaolinite are similar, but the micropore size distributions are very different, and therefore should have different petrophysical characteris- tics. BSE images are amenable to image-process- ing techniques for further analysis of pore-size distribution.

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Table 3 may not be representative for clay miner- als in other sandstones. Mineralogy is given as weight percent. Porosi- ty is derived from a core-plug cali- brated log evaluation. Table 4.


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Kaolinite, which may be the most common clay Quartz 77 2. The same also holds true for diagenetic smectite. If diagenetic kaolinite was to give an equiva- sandstones, a belief that is readily validated by lent reduction of microporosity, It is recommended that eral volume of illite would be needed. There may be several or only one textural matically reduce permeability. Hurst and Nadeau Figure 6—Comparison of weight percent and volume percent of sandstone min- eralogy from Figure 5. Data are from Table 4.

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Because it is impossible to reconstitute damaged clay fibers, one cannot ascertain if the original fibrous textures had a substantially different micro- porous character. SEM analysis of critical-point dried material shows that the fibrous illitic clay has similar microporous texture throughout. We con- cluded that air drying and epoxy impregnation may have varied effects on fibrous, microporous tex- ture. Bil, J.

Kantorowicz, and A. Dicker, , Petrophysical core analysis of sandstones containing deli- Mineral Qv meq cm—3 cate illite: Log Analyst September—October , p. Dilks, A. Graham, , Quantitative mineralogical char- Kaolinite 0.

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Smectite 0. Kennedy, S. Crabtree, and R. Cannon, , Illlite 0. Analysis of reservoir pore com- plexes: Journal of Sedimentary Petrology, v. Frost, E. Fertl, , Integrated core and log analysis concepts in shaly clastic reservoirs: Log Analyst, v. Heaviside, J. Langley, and N. Pallatt, , The permeability characteristics of Magnus reservoir rock: 8th European mode of occurrence in the subsurface.

Studies of modern environments of deposition and associated Holocene deposits provide a basis for understanding petroleum fields and predicting reservoir distribution and behavior. The Rocky Mountain province of the United States contains structural and stratigraphic traps from which petroleum is produced from all types of sandstone petroleum is produced from all types of sandstone reservoirs ranging in age from Cambrian to the Eocene. Three large typical stratigraphic traps, where reservoirs are of Cretaceous age, are described.

The Cut Bank field, Montana produces from alluvial point bar sandstones; Patrick Draw field, Wyoming produces from marine shoreline sandstones; and, Hartzog Draw field, Wyoming produces from marine shelf sandstones. The majority of petroleum reserves in the world is found in ancient sandstones which have porosity and permeability. When sandstone contains petroleum that can be extracted by known technology, it is referred to as a sandstone reservoir. Sandstone reservoirs are normally composed of stable minerals e. The quantity of pore volume and the nature of the interconnections pore volume and the nature of the interconnections between pores may be related to the primary processes under which the sandstone accumulated, or they may be related to secondary changes diagenesis that are post-depositional.

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The origin and distribution of a reservoir rock are controlled primarily by the processes by which the sand was deposited. Scientists concerned with the origin of sedimentary rocks are fortunate because they can observe the conditions and processes by which sediments accumulate today and use their observations to interpret the origin of ancient sediment. By using the areas of modern environments of deposition as natural laboratories, scientists seek to improve the quality of their work.

A massive effort has been underway for the past 30 years by industry, government and university scientists throughout the world to investigate modern environments of deposition and to interpret the results of the processes in Holocene sequences. Concepts have evolved from these studies that have revolutionized scientific thought about sedimentary rocks.

The results of these investigations clearly reaffirm the time-honored concept that processes within the environment of deposition control the type and distribution of lithology and associated primary porosity and permeability. Therefore, newly porosity and permeability. Therefore, newly deposited stratigraphic units can be described as processcontrolled genetic units. Sediments with a common processcontrolled genetic units. Sediments with a common genesis have similar lithologies because the physical, biologic and chemical processes within the environment are similar.

For convenience, the phrase is shortened to genetic unit. The genetic unit is defined as a deposit resulting primarily from the physical, chemical and biological processes operating within the environment at the time of sedimentation. The approach is to recognize lithologic differences that can be related to a stratigraphic level at which a unique set of processes began to influence sedimentation and when they processes began to influence sedimentation and when they stopped e.