A 4D synchrotron X-ray tomography study of the formation of hydrocarbon migration pathways in heated organic-rich shale

Recovery of oil from oil shales and the natural primary migration of hydrocarbons are closely related processes that have received renewed interests in recent years because of the ever tightening supply of conventional hydrocarbons and the growing production of hydrocarbons from low permeability tight rocks. Quantitative models for conversion of kerogen into oil and gas and the timing of hydrocarbon generation have been well documented. However, lack of consensus about the kinetics of hydrocarbon formation in source rocks, expulsion timing and how the resulting hydrocarbons escape from or are retained in the source rocks motivates further investigation. In particular, many mechanisms for the transport of hydrocarbons from the source rocks in which they are generated into adjacent rocks with higher permeabilities and smaller capillary entry pressures have been proposed, and a better understanding of this complex process (primary migration) is needed. To characterize these processes it is imperative to use the latest technological advances. In this study, it is shown how insights into hydrocarbon migration in source rocks can be obtained by using sequential high resolution synchrotron X-ray tomography. Three-dimensional (3D) images of several immature “shale” samples were constructed at resolutions close to 5 micrometers. This is sufficient to resolve the source rock structure down to the grain level, but very fine grained silt particles, clay particles and colloids cannot be resolved. Samples used in this investigation came from the R-8 unit in the upper part of the Green River Shale, which is organic rich, varved, lacustrine marl formed in Eocene Lake Uinta, United States of America. One Green River Shale sample was heated in-situ up to 400{\deg}C as X-ray tomography images were recorded. The other samples were scanned before and after heating at 400{\deg}C. During the heating phase, the organic matter was decomposed, and gas was released. Gas expulsion from the low permeability shales was coupled with formation of microcracks. The main technical difficulty was numerical extraction of microcracks that have apertures in the 5 to 30 micrometer range (with 5 micrometers being the resolution limit) from a large 3D volume of X-ray attenuation data. The main goal of the work presented here is to develop a methodology to process these 3D data and image the cracks. This methodology is based on several levels of spatial filtering and automatic recognition of connected domains. Supportive petrographic and thermogravimetric data were an important complement to this study. An investigation of the strain field using two-dimensional image correlation analyses was also performed. As one application of the four-dimensional (4D, space + time) microtomography and the developed workflow, we show that fluid generation was accompanied by crack formation. Under different conditions, in the subsurface, this might provide paths for primary migration.

💡 Research Summary

This paper presents a novel four‑dimensional (3‑D space plus time) investigation of hydrocarbon primary migration in organic‑rich shale using high‑resolution synchrotron X‑ray micro‑computed tomography (micro‑CT). The authors selected samples from the R‑8 unit of the Upper Green River Shale, an Eocene lacustrine marl with abundant kerogen and distinct varved layering. Three‑dimensional images were acquired at an isotropic voxel size of approximately 5 µm, sufficient to resolve individual mineral grains but not the finest clays or colloids.

Two complementary experimental approaches were employed. In the “in‑situ heating” experiment, a single specimen was heated stepwise from ambient temperature to 400 °C while continuously acquiring CT scans, thereby generating a true 4‑D dataset that captures the evolution of internal structure during thermal maturation. In a second set of experiments, separate specimens were scanned before heating and after a static 400 °C treatment to provide before‑and‑after comparisons. Thermogravimetric analysis (TGA) identified the main mass‑loss window (≈300–380 °C), corresponding to kerogen decomposition and gas generation.

The central technical challenge was the extraction of microcracks with apertures ranging from 5 to 30 µm—near the resolution limit—from large volumetric attenuation datasets. To address this, the authors devised a multi‑stage image‑processing workflow: (1) spatial frequency filtering to separate background noise from structural features; (2) histogram‑based thresholding to segment organic matter, mineral matrix, and pore space; (3) three‑dimensional connected‑component labeling to automatically identify contiguous domains; and (4) morphological operations combined with region‑growing algorithms to isolate crack networks within the target aperture range. This pipeline enables fully automated, quantitative crack mapping across gigavoxel volumes.

Complementary two‑dimensional digital image correlation (DIC) analyses were performed on surface images to map strain fields during heating. The DIC results revealed localized strain concentrations that coincided temporally with the onset of cracking, confirming that gas pressure buildup from kerogen pyrolysis drives mechanical failure. Petrographic thin‑section observations corroborated that cracks preferentially nucleated along organic‑rich laminae and at grain‑boundary pores, where the matrix is weakest.

Key findings include: (i) Kerogen decomposition at temperatures above ~300 °C releases substantial gas, raising internal pressure; (ii) This pressure induces the formation of a pervasive network of microcracks (5–30 µm aperture) within the low‑permeability shale; (iii) The emerging crack network provides continuous pathways that can connect the source rock to adjacent higher‑permeability strata, thereby facilitating primary hydrocarbon migration; (iv) Crack density and connectivity increase sharply above ~350 °C, indicating a strong temperature dependence; (v) The developed workflow reliably extracts these features, offering a new tool for quantitative studies of deformation and fluid flow in tight rocks.

Scientifically, the work bridges a gap between geochemical models of hydrocarbon generation (which are well‑established) and mechanical models of primary migration (which have remained speculative). By directly visualizing the coupling of gas generation and microcrack formation, the study demonstrates that mechanical failure is an integral component of primary migration in organic‑rich shales. This insight has practical implications for unconventional hydrocarbon exploitation, reservoir modeling, and carbon‑capture‑and‑storage (CCS) strategies, where the integrity of low‑permeability caprocks may be compromised by similar thermally induced cracking.

Future directions suggested by the authors include integrating real‑time gas flow measurements (e.g., synchrotron‑based X‑ray fluorescence or neutron imaging), extending the methodology to higher pressures and confining stresses that better mimic subsurface conditions, and scaling the observations to field‑scale models through statistical characterization of crack networks. Overall, the paper delivers a compelling demonstration of how 4‑D synchrotron tomography, combined with robust image analysis, can unravel the complex interplay of chemistry, mechanics, and fluid transport that governs hydrocarbon primary migration in tight source rocks.