Petrographic analysis

 Petrographic analysis identifies the origin, whether igneous, sedimentary, or metamorphic, and the mineral content for the classification of the rock.

This usually comprises the description of the macroscopic aspects of the rock, such as fabric, color, grain size, and other relevant characteristics that may be visually observed in hand specimen or in outcrops, and chiefly the identification and description of microscopic characteristics of the studied material in thin sections such as mineral composition, texture, grain size, and evidence of alterations and/or deformation.

Petrographic analysis provides a detailed description of the texture (grain size, sorting, and grain contacts), sedimentary structures (laminations, bioturbation), framework grain composition, authigenic minerals, and types and distribution of macroporosity seen in a thin section. Detailed petrographic techniques can be used in porosity modeling programs and analysis with ultraviolet light can be useful in delineating features that are too small to be easily recognized with standard petrographic analysis.

Petrographic Analysis for Burial History Modeling

A highly detailed petrographic analysis can be used as petrographic input for specific burial history modeling programs and reservoir characterization models. This type of modal analysis includes a point count (minimum of 300 points) of over 150 categories of framework grains, cements, and matrix, estimation of amounts of grain coats (typically 50 grains), and grain measurements (generally a minimum of 100 grains).

Petrographic analysis can be used to evaluate the pore system in a reservoir rock. The occurrence and distribution of pore types can be identified, and pore structure analysis can determine the ratio of primary intergranular pores to secondary leached pores and provide critical data for evaluation the efficiency of the pore network.

Epifluorscence Petrography

Epifluorscence petrography uses ultraviolet light to emphasize features in a thin section that are difficult to observe with standard petrographic techniques. This technique is especially useful in identifying and describing microfractures and microporosity in shale thin sections.

1. Cast thin section 

Rock pore configuration, area–volume ratio, fracture index, fracture density and width, pore throat coordination number, and so on can be measured accurately by cast thin section. Combined with ordinary polaroid, cathodeluminescence thin section, fluorescence thin section, thin section staining techniques, and so on, the cast thin section can be used for determining the types and occurrences of frame grains, matrix, cement, and other sensitive minerals, describing type and origin of pores, and estimating strength and structural stability of rock. This is very important for reservoir protection, sand control, and acidization designs during well completion. 

2. X-ray diffraction 

X-ray diffraction is the most widely used, effective technique of identifying crystalline minerals and is especially useful for fine dispersed clay mineral and its inner structure analyses. An X-ray diffraction instrument can be used to determine the various types of clay minerals, which also include the interlayer clay minerals forming during diagenesis. XRD can also be used for determining the proportion of montmorillonite in interlayer minerals (e.g., illite–montmorillonite interlayer minerals). In addition, the structural type of clay minerals can be further determined. In general, the XRD analysis technique is very important for determining the absolute content, type, and relative content of clay minerals, which are the basic parameters necessary for the performance of reservoir protection from damage in well completion process and also are helpful in the analysis of type of scales in tubing and perforations and in the analysis of corrosion products. 

 The XRD analyses determined the type and semi-quantitative composition of the minerals present in the samples

3. Scanning electron microscope 

The type of sensitivity and the degree of damage are closely related to the composition, content, and occurrence distributions of sensitive minerals. The aforementioned XRD is particularly suitable for identifying the composition and content of sensitive minerals, whereas SEM is especially applicable to visually identifying mineral grain size and occurrence, pore configuration, throat size, and grain surface and pore throat wall configuration fast and effectively (Table 1-10). In addition, SEM can also be used for observation of the pore throat plugging status after contact between the rock and the foreign fluids. Combined with the energy spectrometer, SEM can be further used for conducting element analysis, such as ferric ion identification related to formation damage. Therefore, the results of SEM analysis are also important data needed by performing the reservoir protection in the well completion process. In recent years, an environment scanning electron microscope (ESEM) has been used for studying the formation damage and observing the swelling process of clay minerals and the microscopic structure of polymer in pore throats. Samples can be observed in the wet state by using ESEM. This is its most outstanding superiority. 

SEM using thin sections was carried out. For this purpose, they stained the thin sections with potassium ferricyanide and alizarine red to distinguish and detect the carbonate phases. Petrographic characteristics and attributes, such as grain composition and size, cementing agents, porosity types, and reservoir quality, were determined.

Scanning electron microscopy (SEM) with energy dispersive X-ray microbeam (EDX) analysis. This is referred to as the SEM/EDX analysis. For this purpose, the samples were sputter-coated with gold and palladium. The SEM analyses identified the pore-lining minerals and the EDX analysis determined the elemental composition.

4. Electron probe analysis 

Electron probe X-ray microanalysis is such a spectral analysis that high-speed minute electron beams are used as a fluorescent X-ray exciting source. The electron beams, which are as minute as a needle, can perform analysis of the microscopic area in a sample and penetrate into a sample by 1–3 mm without disrupting the chemical composition of the measured microscopic area. The information provided by this analysis includes fine mineral composition, crystal structure, diagenetic environment, and type and degree of formation damage.