core image, Oil & Gas Glossary

What Is a Core Image?

Core image (also called core photography or digital core documentation) is the acquisition of high-resolution digital photographs, X-ray computed tomography (CT) scans, and hyperspectral or SEM images of conventional and sidewall core samples to document sedimentary structures, fractures, lithology, and mineralogy at scales ranging from the whole core tray to the pore scale. Core imaging creates a permanent digital record that supports geological description, petrophysical calibration, and fracture characterization without consuming the physical sample, enabling multiple analysts to access the same data simultaneously and preserving information from core sections that may later be destroyed by plug extraction or mechanical testing.

Key Takeaways

Whole-core tray photography at 100 to 200 pixels per inch produces a continuous photomosaic used for geological logging; standard practice on all conventional core programs regardless of reservoir type.
Medical CT scanners resolve internal core structure at 1 to 5 mm voxel size; industrial micro-CT systems achieve 0.1 to 0.001 mm resolution for pore-scale imaging of plug samples.
X-ray CT scanning is non-destructive and reveals fractures, vugs, laminations, and density heterogeneity invisible on the core surface, enabling informed sample selection before any cutting or plugging begins.
Hyperspectral core imaging captures reflectance spectra across 400 to 2500 nm wavelengths, identifying clay minerals, carbonates, and organic content at millimeter-scale spatial resolution without laboratory digestion or thin sections.
SEM imaging resolves pore throats at the micrometer to nanometer scale, identifying clay mineral morphology (kaolinite booklets, illite fibers, chlorite coatings) that controls irreducible water saturation and permeability in tight reservoirs.

Core Imaging Methods and Their Applications

Whole-core photography is the baseline documentation step performed immediately after cores are laid out in trays at the laboratory. A calibrated linear scanner or line-scan camera traverses the length of each tray under controlled lighting, capturing both dry and wet surfaces (wetting with water or solvent reveals sedimentary structures that are invisible on dry, dusty surfaces). The resulting photomosaic provides a continuous color image strip at 100 to 200 dpi across the full cored interval, typically archived as TIFF or high-resolution JPEG files with embedded depth annotations. After the core is slabbed longitudinally, the archive half and working half are photographed separately under white light and ultraviolet (UV) light. UV illumination causes crude oil residues to fluoresce, highlighting hydrocarbon-stained laminae, residual oil in partially flushed zones, and oil-bearing fractures that show no visible staining under white light.

X-ray CT scanning of whole-core sections before slabbing has become standard practice on high-value reservoir core programs. A medical CT scanner passes the core through the gantry at 10 to 20 mm slice intervals, producing a stack of cross-sectional density images. Denser minerals such as pyrite, barite, or calcite cementation appear bright (high Hounsfield units), while open fractures, vugs, and low-density organic matter appear dark. The CT images allow the geologist and petrophysicist to identify intact versus fractured sections, select optimal plug locations away from natural fractures that would compromise routine core analysis measurements, and map the internal architecture of complex heterolithic or vuggy carbonates before any core is consumed. Industrial micro-CT systems operating at 100 to 1000 kV image small plugs (25 mm diameter, 50 mm length) at voxel sizes from 100 micrometers down to 1 micrometer, producing three-dimensional pore network models from which single-phase permeability and capillary pressure curves can be computed without flow experiments.

Scanning electron microscopy (SEM) imaging addresses the pore-scale features that control reservoir quality in tight sandstones, shales, and carbonates. A polished thin section or a freshly broken surface is coated with a conductive layer of carbon or gold and imaged in the SEM at magnifications from 50x to 50,000x. Secondary electron (SE) images reveal surface topography at nanometer resolution, showing the three-dimensional morphology of clay mineral crystals growing in pore spaces. Backscattered electron (BSE) images show atomic number contrast, distinguishing quartz (silicon, atomic number 14) from feldspar (aluminum, 13), carbonates (calcium, 20), and pyrite (iron and sulfur, 26 and 16) without special staining. Energy-dispersive X-ray spectroscopy (EDS) attached to the SEM provides elemental compositions of individual minerals, enabling clay mineral identification and quantification in zones where X-ray diffraction of bulk samples would miss thin mineral coatings on grain surfaces.

Fast Facts: Core Image

Whole-core photo resolution: 100 to 200 dpi standard; 400 dpi for thin-bedded reservoirs requiring lamina-scale documentation
Medical CT voxel size: 0.5 to 5 mm; adequate for fracture detection and bulk heterogeneity mapping
Micro-CT voxel size: 1 to 100 micrometers; resolves individual pore throats in sandstones and carbonates
Hyperspectral wavelength range: 400 to 2500 nm (visible-NIR-SWIR); clay minerals identified by OH-stretch absorption bands near 1400, 1900, and 2200 nm
SEM magnification range: 50x to 50,000x; pore throat imaging typically at 500x to 5000x
UV fluorescence: Crude oil fluoresces at 340 to 400 nm excitation; peak emission wavelength shifts from blue (light oil) to yellow-green (heavy oil)
Storage requirement: Full CT dataset for a 100-foot core run: 5 to 50 GB depending on slice thickness and matrix size
Industry standard: API RP 40 covers core photography and documentation requirements for conventional core analysis programs

Core Description Tip:

Run the CT scan before opening or slabbing the core. Natural fractures in tight gas or unconventional reservoirs are mechanically weak and often pop open when the confining stress is removed during core retrieval, creating apparent fractures that did not exist in the subsurface. The pre-slabbing CT image records the true state of the core on arrival and distinguishes drilling-induced petal fractures and stress-relief fractures from natural open fractures. This distinction is critical for fracture characterization studies that feed into hydraulic fracture stimulation designs.

Core image is also referred to as:

Core photography — the traditional term, now broadened to include digital scanning and CT imaging alongside conventional photography
Digital core documentation — emphasizes the archival and data-management function of the imaging workflow in integrated core-to-log studies
Core scanning — used interchangeably in laboratory practice to describe the automated line-scan photography or CT scanning workflow
Core CT scan — specifically refers to X-ray computed tomography imaging of whole-core or plug samples, the most technically demanding component of the core imaging suite

Frequently Asked Questions About Core Images

How does X-ray CT scanning of core differ from a hospital CT scan?

Both use the same basic principle of rotating X-ray source and detector, but the applications differ in energy level, object size, and purpose. Medical CT scanners operate at 80 to 140 kV and are designed for human tissue; they handle standard core tubes (4-inch diameter) well and are widely used in core laboratories because they are relatively affordable and widely available. Industrial core CT systems operate at 150 to 450 kV, providing better penetration through dense minerals such as pyrite-cemented sandstones or metallic tool joints. Micro-CT systems for plug samples operate at 40 to 230 kV with geometric magnification, achieving sub-micron voxel sizes by placing the sample close to the X-ray source. The interpretive output from core CT is a three-dimensional density map of the rock, rather than the tissue-contrast images used in medical diagnostics.

Can core images replace physical core description by a geologist?

Core images are a complement to physical description, not a replacement. A geologist examining core physically can assess grain texture by touch, smell hydrocarbon odor, use a hand lens at variable angles, and apply field tests such as dilute hydrochloric acid to detect carbonate. These observations are not captured by photography or CT scanning. However, digital core images extend the value of physical description by creating a permanent record accessible to analysts worldwide, enabling consistent re-logging years after the original description, and providing the geometric framework for integrating log and core data. On high-value wells, physical description and digital imaging are conducted simultaneously, with the geologist annotating a digital image on a tablet while examining the physical slab.

What is a digital rock physics workflow and how does it use core images?

Digital rock physics (DRP) uses three-dimensional micro-CT images of core plugs as the input geometry for numerical simulation of fluid flow, electrical resistivity, and elastic wave propagation through the pore network. The micro-CT image is segmented into pore space and mineral phases using threshold algorithms or machine learning classifiers, then a lattice-Boltzmann or finite-element solver computes absolute permeability, formation factor, and elastic moduli from the segmented geometry. The computed properties are compared against laboratory measurements on the same plugs to validate the segmentation and simulation methodology. Once validated, DRP can simulate conditions that are difficult to reproduce in the laboratory, such as reservoir temperature and pressure, multiphase flow at irreducible saturations, or the effect of clay swelling on permeability.

Why Core Images Matter in Oil and Gas

Core images are the permanent visual record of the reservoir as it existed at the time of drilling, before production, diagenesis, or mechanical alteration changes the rock fabric. As physical core is consumed by plug extraction, fluid injection tests, and mechanical property measurements, the digital image archive becomes the only evidence of features that no longer exist in the sample. In complex reservoirs with natural fractures, thin beds, or heterogeneous mineralogy, the imaging suite provides the spatial context that transforms a set of isolated plug measurements into a coherent reservoir description. Operators who invest in comprehensive core imaging programs recover that investment through improved geological models, better-calibrated petrophysical equations, and more accurate fracture characterization for stimulation design, all of which reduce well-level reserve uncertainty and improve capital allocation decisions across a development program.

Core Image: Digital Photography, CT Scanning, and Microscopy for Reservoir Characterization