Comment: Mudrock cement and the importance of basic research

by Kitty Milliken
Wednesday, August 17, 2016

Red mudrock in the Ragged Reef Formation ( Pennsylvanian), Cumberland Basin, Nova Scotia. Credit: Michael C. Rygel, CC BY-SA 3.0.

Diverse motivations drive scientific research. For many of my geoscience colleagues, curiosity seems motivation enough. However, motivations beyond mere curiosity often spur the funding of science. Geoscience offers many questions of such immediate and practical value that much funding is supplied by private industry, although work of a more basic nature may also be funded publicly by governments at many levels. In the political world, it’s often argued that responsibility for advancing science should be weighted more heavily on one sector or another. But it need not be imbalanced: What often gets lost in such conversations is that science truly needs the fullest possible portfolio of motivations — and funders — to be most effective.

One of the latest rounds in this debate played out last year when the House of Representatives' version of the America Creating Opportunities to Meaningfully Promote Excellence in Technology, Education, and Science Act of 2007, or America COMPETES Act, proposed to slash funding across a broad swath of geoscience. A campaign by geoscience organizations and individuals to remind lawmakers of the value of geoscience helped the funding to be restored in the final version of this bill passed by the Senate. One of the points taken to various congressional staffers was that early and esoteric work, funded by federal programs, emerged nearly 30 years later to boost our understanding of oil and gas shales, which are, of course, funding our current oil and gas boom.

The geoscience funding debate still simmers, however, and the conversation needs to continue. The transfer of scientific knowledge can flow the other way too: Observations made by industry scientists and engineers while trying to understand and predict rock response to hydraulic fracturing, used in oil and gas exploration, now support endeavors to understand major earthquakes.

Connecting oil and gas shales to subduction earthquakes starts with the basic science of shales, which are, more simply, lithified mud, or mudrocks. What happens if you conduct a little engineering experiment and smack chunks of mudrock with a hammer? It turns out that some mudrocks squish and others crack; and there are, in fact, mudrocks that display a full spectrum of behaviors in between. The question for science here is, “Why?” (I’ll come back to the connection between mudrocks and earthquakes later.)

The answer, developed in the latter part of the 20th century, has to do with compaction (mud particles getting pushed closer together during burial) and cementation (precipitation of rigid, crystalline glue between the mud particles). Compaction can be studied indirectly by looking at rock properties such as porosity or density, although this is only a reliable approach if you are certain the rock contains no cement, which also affects these properties. However, this is not an easy thing to be sure of given that the tiny scale of such cement particles makes them difficult to see.

If something is beyond our means of observation, then it’s not possible to collect relevant data to answer questions about it. For a long time, this was very nearly the case for mudrock cement crystals, which necessarily must be small enough to fit between the mudrock grains, themselves mostly clay-sized (less than 4 micrometers wide). Spaces available for mudrock cements between the grains are mostly smaller than 1 micrometer wide. Sporadic attempts through the 1980s and 1990s managed to image cement crystals in mudrocks, but detailed quantification and characterization of mudrock cements remained out of reach until field-emission electron microbeam instruments became widely available to geoscientists beginning about 2005.

Field-emission scanning electron microscopy (SEM) imaging routinely achieves about an order of magnitude resolution improvement over conventional SEM imaging, and this technological advance seemed tailor-made to attack questions that began vexing the petroleum industry as exploration in unconventional reservoirs expanded. Some mudrocks crack nicely under hydraulic fracture treatments and release economic volumes of oil or gas; others respond with more of a squish and fail to open up in a way that releases stored hydrocarbons.

With new tools for looking at very small things, mudrock cements could be studied in detail, and companies funded efforts to look at them. It turns out that cements are indeed present in some mudrocks, and they play a role in transforming mud into something that goes “crack.” Research to predict which oil and gas shales are more likely to contain cements is progressing.

Meanwhile, the seismological community, funded primarily by the public sector, is keenly interested in the same squish-versus-crack problem in mudrocks. Scientists who study sediments in earthquake-prone active margin settings want to know how mudrocks behave as they are consumed in subduction zones. It’s clear from research that in the shallow regions in front of and above subducting plates, mudrocks are mostly very soft (squishy), but locally, for reasons not fully understood, harder mudrocks prone to cracking during deformation occur. Such a patch of shallow, brittle mudrock is indicated by high seismic velocities off the coast of Sumatra, for example, in the region of the 2004 magnitude-9.2 earthquake and tsunami.

This year, Expedition 362 of the International Ocean Discovery Program is taking an interdisciplinary, multinational team to drill cores on the Sumatra margin to investigate the controls on rock-property evolution in this seismically hazardous region. The team will benefit from recent industry knowledge about mudrock heterogeneity — how to sample it, how to prepare specimens for high-resolution imaging and which imaging protocols work best, and what mudrock cements look like. It’s very possible Expedition 362 will add to this basic knowledge, but it’s certain they won’t have to create it from scratch.

Differing motivations for science funding serve to drive science in more diverse directions. Work on mudrock cementation would not likely have been funded by the National Science Foundation’s Geoscience Division, which has only recently accepted proposals for sedimentary geoscience, primarily under the rubric of “Surface Earth Processes.” But companies with a compelling need to understand the evolution of pore systems and mechanical rock properties in the subsurface were eager to see such work progress, despite its rather basic nature.

In reality, over time, the funding of basic and applied research is handed off, back and forth, between private and public entities, depending on the benefits imagined by different communities. Imagination, even more than curiosity and funding, is what drives science, and combining the imaginations of diverse communities gives a more powerful boost to the enterprise. One hopes that the leadership, in government and industry, will continue to recognize this vital symbiosis. If motivations for both basic and practical understanding, for both public and private needs, are represented in science funding, then progress in science will accelerate, to the benefit of all.

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