Precise Petrographic Analysis for Reliable Geological Insights

Sandstone petrography

Optical thin section petrography of sandstones is an essential ingredient to understand and characterize sandstone reservoirs on different scales (from field to basin-scale), whether as a stand-alone method or in combination with other techniques. Information gained can be used for numerous purposes. Some examples are listed here:

Reservoir characterization

Thin section analysis is commonly used in combination with other microscopic techniques like scanning electron microscopy (SEM), X-ray diffraction (XRD), and cathodoluminescence (CL) to describe the reservoir lithology and to assess the quality of the reservoir. These methods allow to determine pore types and distribution, the extent of reservoir enhancement or degradation through the effects of diagenesis, and the influence of depositional textures on reservoir quality. Furthermore, reservoir homogeneities/heterogeneities may be analyzed and documented. A major advantage of thin section petrography is that the lithological variation on a lamina-scale and its influence on petrophysical properties can be determined.

Pore network assessment

Thin section petrography in combination with SEM is routinely used to evaluate the pore system in a reservoir rock. This approach can be limited to the identification of various pore types and their distribution what provides critical data for the evaluation of pore interconnectivities and thus the effectiveness of the pore network. Moreover, as part of an “advanced” petrographic workflow the pore geometry, pore size, and pore throat diameters can be defined in 2D or 3D. This state-of-the-art approach uses modern image analysis tools (delivering pore area, diameter, perimeter, length, width, and aspect ratio) in combination with classical petrographic analysis and provides estimates of porosity and permeability values as well as models for capillary pressure curves especially for samples that are not suitable for core analysis, such as cuttings, percussion sidewall cores, and unconsolidated core samples.

Diagenesis evaluation and reservoir quality prediction

Explaining and modeling the diagenetic overprint through the effect of temperature, pressure or fluids needs a profound description of the inventory of a reservoir rock. Detailed thin section descriptions will identify the major and minor diagenetic processes and their impact on reservoir quality control. The timing and significance of mineral reactions caused by fluids or temperature/pressure changes can be documented in form of a paragenetic sequence which ideally can be linked to specific burial conditions. As a consequence, concepts and diagenetic trends become apparent and predictions of reservoir properties on different scales (from field to basin-scale) may be possible.

Log calibration / Petrophysical evaluation

To understand the behavior of fluids in the reservoir and to correctly interpret routine core analysis (RCAL) and special core analysis (SCAL) data as well as well log response, the petrophysicist needs information on the mineralogical composition and textural variations in the sandstone reservoir. There are many occasions when rock properties based on results of thin section petrography can be used for calibration of log responses, e.g.: NMR – Pore size distribution, Thin section porosity vs log derived porosity, Effective porosity evaluation (identification and quantification of microporosity, moldic porosity etc.), Identification of radioactive minerals that may affect GR logs, Determination of Vclay, clay mineral type and clay mineral distribution (disperse, laminated) in thin section, Vclay estimation from thin section (Vcltx) vs. Vclay from log (scaling issue!), ELAN and density logs – mineral composition

Support for rock typing approaches

Thin section petrography is an important tool for the distinction of rock types. Different rock types exhibit different properties depending on their lithology, mineralogy, density, porosity and permeability. By using petrographical methods mineralogical and textural parameters can be measured, identified and clustered in order to describe each individual rock type and explain observed petrophysical properties.

Provenance analysis / paleogeographic reconstruction

A more detailed sandstone petrography can be a valuable tool for the reconstruction of the paleogeography in almost all clastic depositional settings. Especially, a closer look at e.g. varieties of quartz grains (undulose, non undulose, polycrystalline, 2-3 crystals, > 3 crystals per grain), feldspar composition, heavy mineral/opaque types and shape, and lithic clast varieties provides useful information for the identification of different provenances, sediment transport processes and/or paleoclimate conditions (see also textural and compositional maturity index).

Formation damage & Understanding mineral/fluid interactions during EOR/Completion/Injection/Production

Thin section analysis in combination with SEM can also be used for identifying potential hazards, e.g. near wellbore permeability reduction by fines migration, swelling clays etc. Valuable information are provided through the identification of types, quantity, and location of clays and other minerals which may interact with drilling, completion, stimulation, or enhanced recovery fluids. Such analyses are extremely important for the assessment of potential risks of formation damage before  reactive stimulation fluids are injected into the reservoir (e.g. acidizing). In matrix stimulation during enhanced oil recovery, one potential risk is the dissolution of mineral surfaces in contact with the stimulation fluid. A petrographic study often helps anticipating how a rock reacts to the injection of certain fluids. The reactivity of a given mineral depends on surface area, chemical composition and temperature. Especially, the investigation of clays with a greater (reactive) surface area compared to other matrix minerals, is highly advisable. 

Further subjects where a petrographic analysis is beneficial: