One of the most exciting new breakthroughs in discovering the connectivity in reservoirs is the use of detrital zircon. Welcome to an interview with Glenn Sharman, University of Texas at Austin, who is developing effective new techniques and approaches that will lead to accurate characterization of reservoir connectivity.
What is your name and your relationship to analytics of detrital geo- and thermochronology?
I am Glenn Sharman — a postdoctoral researcher at the Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin. I received my PhD from Stanford University and subsequently worked for ConocoPhillips in Houston in new ventures exploration. My exposure to detrital geochronology began during graduate school. My first project used this technique to debunk a long-held correlation of two Eocene sandstone formations thought to have been displaced laterally by the San Andreas fault in California. Since then I have continued to use detrital geochronology and thermochronology to study the relationship between sedimentation and tectonics in a number of basins across the globe. One of the greatest challenges in this field is the ever-increasing quantity of data that is being generated. As a researcher at the Quantitative Clastics Lab (QCL) at UT Austin's Bureau of Economic Geology, I am working on developing approaches to manage, visualize, and analyze these data efficiently to support industry adoption of a technique that has already become standard practice in academic studies. At the QCL, we work closely with Prof. Daniel Stockli who runs the (U-Th)/He and U-Pb Geo-Thermochronometry Lab at UT Austin.
What is zircon and how does it relate to geo- and thermochronology?
Detrital zircon grains. Photo was taken using transmitted light and shows the internal character of these zircons.
Zircon is an accessory mineral that forms in certain types of igneous and metamorphic rocks. In the study of dating earth materials (geochronology), zircon is particularly useful because it contains a significant amount of the radioactive element uranium that decays to lead over time. The age of individual zircon crystals can be determined by measuring the ratio of uranium and lead isotopes. Zircon also retains byproducts of radioactive decay, namely helium and evidence of radiation damage (fission tracks), after cooling below a certain temperature threshold. Thus helium and fission tracks in zircon and other minerals can be used to date the time of cooling (thermochronology).
How did the concept of using detrital zircon come about? What were the needs it addressed?
A zircon becomes "detrital" after it has been eroded and incorporated into sediment. Although small (most detrital zircon grains are fine sand to silt sized, smaller than about 0.25mm in diameter), each zircon contains a wealth of information about its geologic history — including when it crystallized and when it cooled. And because zircon tends to co-occur with more abundant minerals, like quartz and feldspar, zircon tells us a lot about the origin, or provenance, of sand and sandstone.
The number of academic articles that contain the phrase “detrital zircon” by publication year. Results were obtained from a GeoRef search for “detrital zircon” in all fields and filtering by publication year.
Because a statistical representation of zircon ages is required for each sample, multiple zircon grains (commonly between 60 and 300) must be dated. It was not feasible to collect such a large amount of geochronologic data until the 1990s and early 2000s, when a number of technologic advances increased the speed and efficiency of detrital zircon geochronology. Today, individual zircon grains can be analyzed in 90 seconds or less, allowing 100s of individual grain ages to be collected in a single day. Correspondingly, the usage of detrital zircon geochronology in academic studies has skyrocketed in in the last 15 years. Termed the "Detrital Zircon Revolution", this approach has now become standard practice in stratigraphic studies around the world.
What were some of the primary applications of detrital zircon geochronology?
Detrital zircon geochronology has a number of important applications in the field of sedimentary geology. Perhaps the most common use is in reconstructing the source areas, or provenance, of ancient sedimentary rocks. This approach is particularly powerful when combined with other, complementary methods of provenance analysis (e.g., sandstone petrography). In addition to yielding important insights about the paleogeographic and tectonic setting, this approach can provide information about the catchment size and lithology types that are up-stream of petroleum-bearing sedimentary basins. Knowledge of these parameters can help the petroleum geoscientist predict the quantity (how much?) and quality (how good?) of clastic petroleum reservoirs in frontier basins.
Example of a paleogeographic reconstruction based in part on a ‘big data’ application of detrital zircon geochronology (modified from Sharman et al., 2016, Geology).
Detrital zircon geochronology can also yield insight into the age and correlation of stratigraphic units. Because a sample cannot be older than the youngest zircons it contains, detrital zircons provide an estimate of maximum depositional age. In cases where there was active volcanism in the source area, the youngest zircon grains can provide a close approximation of when the sample was deposited. This approach is particularly useful in sedimentary sequences that are poorly dated or that are associated with active volcanism. For example, detrital zircon studies of the Paleogene Wilcox Group of the Gulf of Mexico have shown a good correspondence with the age of the youngest zircon and the depositional age of the sample.
What are some of the most exciting applications for big data and detrital zircon studies that you see today?
The proliferation of academic detrital zircon studies presents both a challenge and an opportunity. The challenge is to efficiently manage, visualize, and analyze large quantities of data, and the opportunity is to leverage that same data in robust analysis leading to the best possible interpretation. I see great potential for 'big data' in detrital studies to help geoscientists see the forest for the trees, providing a big picture view that places one's own dataset in context of other published datasets. To accomplish this, data must be mined from published sources and tools and workflows are needed for efficiency in visualization and data analysis.
How is the use of big data with detrital zircon geo- and thermochronology relevant for oil and gas exploration and development?
Example of how detrital zircon geochronologic data is typically presented. The upper plot displays a cumulative distribution and the lower plots show relative probability distributions and histograms. These plots also illustrate the power of ‘big data’ in detrital geochronology; in this case, Paleocene-Eocene sandstone samples from the Rocky Mountain region display very similar U-Pb age distributions with age-equivalent deltaic deposits in east Texas. See Sharman et al. (2016, Geology) for additional information and an explanation of data sources.
Detrital studies are not a magic bullet, but are rather one of a number of tools available to the petroleum geoscientist in developing stratigraphic models and predictions. Detrital zircon geochronology can provide insights into paleogeography, subsurface correlations, and the age of sedimentary sequences. These are all important components of any petroleum geologists' workflow in defining an exploration prospect or developing a field. For sandstone reservoirs or prospects, detrital zircon studies may be particularly relevant to predicting sand presence and quality. For unconventional mudrock plays, detrital thermochronology can provide powerful insights into the thermal and exhumation history of sedimentary basins with relevance to the timing of source rock maturation and uplift. These approaches are well-suited to subsurface evaluation because samples can be taken from either core or cuttings, provided sufficient material is available for mineral separation (typically at least 1 pound, depending on lithology type). Integration of these approaches in a 'big data' context will help the petroleum geologist be efficient in data analysis and interpretation.
What is the future of detrital zircon studies?
My opinion is that detrital studies, including zircon geochronology and thermochronology, have been largely under-utilized in the petroleum industry. There is a bright future ahead that includes increased application of these techniques to the subsurface, as industry catches up with the "Detrital Zircon Revolution" that has swept academia. There are abundant opportunities for application of big data analysis and also industry-academic partnerships (e.g., the QCL).
The future of detrital studies may also lie in combining multiple grain-based analytical approaches to yield even more insights from grains of sand. For example, collecting both the crystallization and cooling ages of detrital zircons ("double dating") provides extra insight into the geologic history of the sample. Zircon can also be analyzed for its chemical composition and hafnium isotopes that provide even more resolution on its geologic history. Finally, other detrital minerals can also be used in tandem with zircon to provide insight into source area exhumation and the thermal history of sedimentary basins. These approaches include detrital thermochronology from apatite and potassium feldspar and detrital barometry (pressure) from hornblende.
Can you recommend a few articles and books?
George Gehrels (University of Arizona) has written an excellent introduction on detrital zircon geochronology that can be found in the book Tectonics of Sedimentary Basins: Recent Advances (published in 2012 by Blackwell Publishing Ltd.). There is also a useful overview article by Christopher Fedo and others (2003) on "Detrital Zircon Analysis of the Sedimentary Record" published in Reviews in Mineralogy and Geochemistry (v. 53, p. 277-303). Additional information can be found on the websites of University of Texas at Austin's Geo-Thermochronometry Lab the Arizona LaserChron Center.
Sharman, G.R., Covault, J.A., Stockli, D.F., Wroblewski, A., F.-J., and Bush, M.A., 2016, Early Cenozoic drainage reorganization of the U.S. Western Interior-Gulf of Mexico Sediment Routing System: Geology, doi:10.1130/G38765.1.