Shale vs tight

A couple of weeks ago, we looked at definitions of unconventional resources. Two of the most important play types are shale gas and tight gas. They are volumetrically important, technologically important, and therefore economically important. Just last week, for example, Chevron bought an unconventional gas company for over $4B.

The best-known examples of shale gas plays might be the Barnett in Texas, the Marcellus in eastern US, and the Duvernay in Alberta. Tight gas plays arguably had their hyper-popular exploration boom five or so years ago, but are still experiencing huge investment in areas where they are well-understood (and have nice reservoir properties!). Prolific examples include the Bakken of northern US and the Montney of Alberta.

So if we were to generalize, perhaps over-generalize: what's the difference between shale gas plays and tight gas plays?

	Shale gas	Tight gas
Grain-size	Mostly mud	Substantially silt or fine sand
Porosity	up to 6%	up to 8%
TOC	up to 10%	up to 7%
Permeability	up to 0.001 mD	up to 1 mD
Source	Mostly self-sourced	Mostly extra-formation
Trap	None	Facies and hydrodynamic
Gas	Substantially adsorbed	Almost all in pore space
Silica	Biogenic, crypto-crystalline	Detrital quartz
Brittleness	From silica	From carbonate cement

 

Over-generalization is a problem. Have I gone too far? I have tried to indicate where the average is, but there is a space in the middle which is distinctly grey: a muddy siltstone with high TOC might have many of the characteristics in both columns; the most distal facies in the Montney are like this.

Why does this matter? Broadly speaking, the plays are developed in the same way: horizontal wells and fracture stimulation. The difference is really in how you explore for them.

Accretionary Wedge #31

This is my first contribution to the Accretionary Wedge; the theme this time is 'What geological concept or idea did you hear about that you had no notion of before (and likely surprised you in some way)?' Like most of the entries I've read so far, I could think of quite a few things fitting this description. I find lots of geological concepts surprising or counterintuitive. But in the end, I chose to write about the thing that obsessed me as an undergraduate, right at the beginning of my career:

The Devonian day was 22 hours long

In November I moved to the Atlantic coast of Canada. It's the first time I've lived right at the seaside, but I am originally from the tiny island of Great Britain so never lived too far from the edge. There is a deeply maritime feel to this part of the continent, even in the sheltered Bay of Fundy. The famously macrotidal regime there permeates the culture: artists paint the tidal landscapes; musicians sing about the eerie currents; geologists crawl around on the mud-flats and cliffs. The profound consequences of a 17-metre tidal range and its heartbeat, regular as clockwork.

← Tidal forces shape a bar-built estuary, Pamlico Sound, USA.

It's easy to see the effects of the tide in the geological record. Tidal successions are recognizable from some combination of pin-stripe lamination, mud-drapes, bi-directional ripples, proximity to shore, diagnostic fossils, brackish trace fossil assemblages, and other marvellous sedimentological tools. Less intuitively perhaps, at least for a non-biologist like me, marine animals also express these tidal frequencies in their growth patterns. So a coral, for example, might have a lunar breeding cycle. This periodicity results in growth rings just like a tree, only they record not the seasons but the bi-monthly beat of spring and neap tides. The tides are driven by the relative positions of the sun and moon relative to earth. Celestial bodies created banded coral.

From Scutton (1963): diurnal rings and and monthly bandsColin Scrutton, one of my professors at the University of Durham in the northeast of England, measured the growth ridges of rugose corals of Middle Devonian successions in Michigan, Ontario and Belgium (Scrutton 1964). He was testing the result of a similar experiment by John Wells (1963). The conclusion: the Devonian year contained 13 lunar months, each lunar month contained 30.6 days, so the year was 399 days long. According to what we know about planetary dynamics in the solar system, the year was approximately the same length so Devonian days were shorter by a couple of hours. The reason: the tides themselves, as they move westward around the eastward-spinning earth, are a simple frictional brake. The earth's rotation slows over time as the earth-moon system loses energy to heat, the ultimate entropy. Even more fascinatingly, the torque exerted by the sun is counteractive, introducing further cyclicities as these signals interfere. Day length, therefore, has probably not slowed monotonically though time.

For me, this realization was bound up with an obsession with cyclicity. I could not read enough about Milankovitch cycles: wobbles and ellipticity in the earth's dance through space scratching their pulse into the groove of the stratigraphic record and even influencing sea-floor spreading rates, perhaps even mass extinctions. The implications are profound: terametre-scale mechanics of the universe control the timing of cellular neurochemical functions.

Why anyone needs astrology to connect with this awesome fact is beyond me. 

References

Panella, G, et al (1968). Palaeontological evidence for variation in length of synodic month since late Cambrian. Science 15 (3855), p 792–796, doi: 10.1126/science.162.3855.792.
Scrutton, C (1964). Periodicity in Devonian coral growth. Palaeontology 7 (4), p 552–558, pl 86–87.
Wells, J (1963). Coral growth and geochronometry. Nature 197, p 948–950. doi: 10.1038/197948a0.

Geophysical prospecting's roots

As a schoolboy, I used to frequent the second-hand bookshops of Reading, Cambridge, and all over the south east of England. Though not much of a collector, I was taken with the challenge once: Penguin's quarterly science magazine of the late 40s and early 50s: Science News. I completed the set only a few years ago. I'll be honest, while the articles were often very interesting, I was mainly interested in the beautiful cover design. Classic mid-20th Century Penguin.

Most of the articles are very dated of course, but I find them interesting to read nonetheless. Today, I thought I'd excerpt a 1948 article by one A Harford: Advances in geophysical prospecting. It's interesting because this post-war period was really the dawn of the golden age of the oil and gas industry. Naturally, this meant rapid advances in exploration geoscience, especially well logging, reflection seismology, and gravity-magnetics. No doubt wartime technology had its effects; certainly the development of seismic and signal processing technology was accelerated by the Great War and World War II. This article is mostly about magnetic surveys, but he touches on all of these technologies.

...geophysics hardly began until the 1920’s, since when it has expanded at a furious pace. Big business found that geophysics would detect new oil-fields with greater certainty than any other means and, as they found this new technique increased their profits, they lavished money upon it for many years. As more money was spent on better instruments and interpreters the successes increased until, in fifteen years, the gravity meter for instance reached ultimate sensitivity. Between them the physicists and geologists discovered numerous oilfields with relative ease and seemed to find the pace invigorating. Certainly the oil industry has created geophysics, which even now is little used outside problems connected with oil.
DOWNLOAD THE REST OF THE ARTICLE

The blustery language ('the oil industry created geophysics'!) and fearless modernism seems quaint now, but the rate of new oil and gas discoveries at the time was several times what it is today (see chart). I sometimes wonder if the thought of technology leading us has left us jaded; one often hears people react negatively to new tools or software: "We didn't need that in my day".

← Image from Wikipedia article on peak oil

To be sure, even as a committed technologist, I love the idea of spending more money on better interpreters! Like these gentlemen geophysicists, casually examining a seismic record at Lake Arthur, Louisiana, from Harford's article:

Part of me thinks the world has changed so much—hydrocarbons are much, much harder to find today—that this is all really just ancient history. But I also recognize that the tools we have are far more powerful, and our knowledge so much more profound: plate tectonics was still a hotly-debated concept in 1948, for example. So who really has the advantage?

Disclaimer To the best of my knowledge, the original article first appeared in the October 1948 issue of Science News, published by Penguin Books of Harmondsworth, England. It is excerpted here, and made available for download, with their advice but not their explicit permission; Penguin is not involved in this website. To the best of my knowledge, the material is copyright free today; if you believe otherwise, get in touch.

Great geophysicists #2: Snellius

Willebrordus Snellius (1580–1626) was the Latin name that Dutchman Willebrord Snel van Royen, or plain Snell, used for all of his publications as a mathematician. He made great advances over his predessors by introducing trigonometric methods for measuring large distances across landcapes. He's regarded as the father of triangulation, which is widely applied in surveying, imaging, and event location. And he even tried to measure the radius of the earth.

His most famous contribution, Snell's law of refraction, has formed the basis of geometrical optics and is inherently ingrained in seismology. Snell's law is used to determine the direction of wave propagation through a refractive interface:  

How much waves bend depends on the ratio of velocities between the two media v₂/ v₁. You will notice that the right hand side of this equation is where Ibn Sahl left off.

Other mathematicians before him, Ibn Sahl for instance, were aware that light rays refracted when they entered media of different velocities, but Snellius was the first to describe this problem using trigonometry. He made his discovery in 1621, when he was 41 years old, but it was never published in his lifetime. René Descartes, the inventor of the cartesian coordinate and analytical geometry, published this law of refraction 16 years after Snell's death, as Descarte's law of refraction. But Snell was eventually widely attributed with the discovery in 1703 when Christiaan Huygens published Snell's results in his Dioptrica to explain, among other things why successive wavefronts travel in parallel.

Oleg Alexandrov via Wikipedia

In a classic analogy, a 'fast' region is the beach, a ' slow' region is the water, and the fastest way for a rescuer on the beach to get to a drowning person in the water is to run, then swim, along a path that follows Snell's law. The path a ray will take upon entering a media is the one that minimzes the travel time through that media (see Fermat's principle). Notice too that there are no arrows indicating the direction of ray propagation: whether the ray enters from above or below, the refraction behaviour is the same.

In seismology, Snell's law is used to describe how seismic waves bend and turn in accordance with contrasting velocities in the subsurface which is the foundation of surveying, image focusing, and event detection. It appears in ray-tracing, ray-parameterization, offset to angle estimations (used in AVO), anisotropy problems, velocity modeling, and traveltime tomography. Snellius, we salute you!

Aerial geology

A few recent posts by others on aerial geology prompted me to gather them for future reference. Please add any I've missed to the comments!

Large collections

Both of these pages features lots of pictures from all over the United States, plus a few from other parts of the world. I was a bit surprised not to find collections of geological pictures taken from helicopters or hot air balloons. 

•  John Louie's Aerial Geology with lots of images taken from commercial aircraft (like the one shown at right, of the Clayton Valley dunes in Nevada; from John's site).
• Geology by Lightplane by flying geologist Louis Maher and photographer Charles Mansfield has dozens of pictures taken from a Cessna mostly during the late 1950's and early 1960's. Doc Searl's blog post (below) is about this wonderful collection.

Blog posts

• Geology by plane on Doc Searls Weblog by Doc Searls (@dsearls)
• Aerial pictures on Got The Time by Aaron Barth
• Sight-seeing, airplane style on Liberty, Equality, and Geology by Helena Heliotrope (@helenasrox)
• Flying geologists and Flying with the geologists, a pair of posts on Scientific Explorations and Adventures by Paul Doherty. (If you're involved in science education, you should really check out this blog!)
• Dunedin geology from the air, from the Otago Rock and Mineral Club, Dunedin, New Zealand
• Airliner Chronicles, a series on Geotripper by Garry Hayes (@geotripper)

Publications

• Renick, B (1925), Airplanes for geologic explorations in inaccessible regions, AAPG Bulletin 9, doi: 10.1306/3D9326E2-16B1-11D7-8645000102C1865D
• Wade, A (1933), Modern developments in geological exploration, Parts 1 and 2, 1st World Petroleum Congress, London UK, July 1933. 
• Ray, R, and Fisher, W (1958), Geology from the air, Science 126 (3227), doi 10.1126/science.126.3277.725 (shown very small at right)
• Fillizola, C, et al (2002), Aerial remote sensing hyperspectral techniques for rocky outcrops mapping, Annals of Geophysics 45 (2), 233–245. Never mind just looking at rocks, you can even tell what they are! Maybe.
• Various books by Michael Collier, featuring volumes on mountains, rivers, and the San Andreas fault.

Great geophysicists #1: Ibn Sahl

Abu Sa’d al-’Ala’ ibn Sahl was a mathematician in late 10th Century Persia, working for the hugely powerful Abbasid caliphate and probably based in Baghdad. He understood the mathematics of refraction, at least six hundred years before Dutchman Willebrord Snellius wrote it all down and eventually gave the sine law of refraction his name. Snellius’ ancestors in Europe, in Ibn Sahl’s time, were in the middle of the Dark Ages, waiting patiently for the Renaissance just three or four hundred years away. I'll tell more about Snellius in a future post.

Refraction—the bending of a wave's ray-path due to a change in velocity—had been studied by the Egyptian astronomer Ptolemy in the second century, but he was unable to figure out the mathematics fully. It’s not known whether Ibn Sahl knew of this work, but Ptolemy’s Optics was certainly translated into Arabic, so it seems likely that he did. There wasn’t, after all, that much to read in those days.

The key figure from his work On Burning Mirrors and Lenses is shown here. I have cropped out part of the figure; see the whole thing on Wikipedia. I redrew the diagram, and added some annotation to show how it relates to the usual formulation of Snell's Law. The ratio of the lengths of the hypotenuses AB to DB is equal to the reciprocal of the ratio of the refractive indices of the materials on either side of the vertical line. Phew!

Ibn Sahl’s work was brought much further along by one of his successors, Ibn Haitham, widely considered to be the founding father of optics. He was the first person to make a pinhole camera, and thereby prove that light travelled in straight lines.