Episode 396 Turbidity currents

As near as I can tell in the original daily series in 2014, I never addressed the topic of turbidity currents and their sedimentary product, turbidites. But they account for the distribution of vast quantities of sediment on continental shelves and slopes and elsewhere.

You know what turbid water is: water with a lot of suspended sediment, usually fine mud particles. In natural submarine environments, unconsolidated sediment contains a lot of water, and when a slurry-like package of sediment liquifies, it can flow down slopes under gravity, sometimes for hundreds of kilometers.

It isn’t correct to think of these streams of water and sediment as like rivers on the sea floor. Rivers transport sediment, whether boulders or sand or silt or mud, through the traction, the friction of the moving water. Turbidity flows are density flows, moving because the density of the water-sediment package is greater than the surrounding water. That means they can carry larger particles than usual.

Turbidite formation. Image by Oggmus, used under Creative Commons license - source

Sometimes a turbidity flow is triggered by something like an earthquake, but they can also start simply because the material reaches a threshold above which gravity takes over and the material flows down slope. The amount and size of sediment the flow can carry depends on its speed, so as the flow diminishes and wanes, first the coarse, heavier particles settle out, followed by finer and finer sediments. This results in a sediment package characterized by graded bedding – the grain size grades from coarse, with grains measuring several centimeters or more, to sand, 2 millimeters and smaller, to silt and finally to mud in the upper part of the package. Repeated turbidity flows create repeated sequences of graded bedding, and they can add up to many thousands of meters of total sedimentary rock, called turbidites.

Other sedimentary structures in turbidites can include ripple marks, the result of the flow over an earlier sediment surface, as well as sole marks, which are essentially gouges in the older finer-grained top of a turbidite package by the newest, coarser grains and pebbles moving across it.

There are variations, of course, but the standard package of sediment sizes and structures, dominated by the graded bedding, is called a Bauma Sequence for Arnold Bouma, the sedimentologist who described them in the 1960s.

Turbidity currents are pretty common on the edges of continental shelves where the sea floor begins to steepen into the continental slope, and repeated turbidity flows can carve steep canyons in the shelf and slope. Where the flow bursts out onto the flatter abyssal sea floor, huge volumes of sediment can accumulate, especially beyond the mouths of the great rivers of the world which carry lots of sediment.

When the flow is no longer constrained by a canyon or even a more gentle flow surface, the slurry tends to fan out – and the deposits are called deep abyssal ocean fans. They are often even shaped like a wide fan, with various branching channels distributing the sediment around the arms of the fan. The largest on earth today is the Bengal Fan, offshore from the mouths of the Ganges and Brahmaputra Rivers in India and Bangladesh. It’s about 3,000 km long, 1400 km wide, and more than 16 km, more than 10 miles, thick at its thickest. It’s the consequence of the collision between India and Eurasia and the uplift and erosion of the Himalaya.

The scientific value of turbidites includes a record of tectonic uplift, and even seismicity given that often turbidity currents are triggered by earthquakes. They also have economic value. Within the sequence of fining-upward sediments, some portions are typically very well-sorted, clean sandstones. That means they have grains of uniform size and shape and not much other stuff to gum up the pores between the sand grains – so that makes them potentially very good reservoirs for oil and natural gas. You need the proper arrangements of source rocks, trapping mechanisms, and burial history too, but deep-water turbidites are explored for specifically, and with success, in the Gulf of Mexico, North Sea, offshore Brazil and West Africa, and elsewhere. The Marlim fields offshore Brazil contained more than 4 billion barrels of producible oil reserves when they were discovered in the 1980s.

Ancient turbidites sometimes serve as the host rocks for major gold deposits, such as those at Bendigo and Ballarat Australia, which are among the top ten gold producers on earth.

—Richard I. Gibson

Episode 372 Satellite-derived gravity

Welcome to the History of the Earth, which has now evolved into a general podcast covering all things geological. I’m your host, geologist Dick Gibson.

Today I’m going to talk a bit about one of my specialties, interpretation of gravity data. Specifically, gravity data derived from a satellite. Measurements of the earth’s gravity field are essentially measurements of the attraction of the earth on a spring – the more the spring extends, the stronger the pull of gravity, and the stronger pull of gravity occurs where denser materials are present beneath that spring. We can actually measure those attractions with such precision that we can identify areas where there are varying distributions of rocks of different density – or more correctly, we can identify locations of density contrast, where there is a change from one density to another. A classic example is a salt dome. Salt, the mineral halite, has a density of around 2.15 grams per cubic centimeter, while common rocks like shale and sandstone have densities of anywhere from 2.4 to 2.6 grams per cubic centimeter, within an even larger range. So when a low-density, buoyant salt dome rises up through shales and sandstones, it creates a pretty significant density contrast, and a salt dome often produces a strikingly intense, circular gravity low, representing the low-density salt versus the surrounding denser rocks.

Satellite gravity map of western India, from Technical University of Delft.
The gravity low discussed in the podcast is circled.

Since the 1920s we’ve had gravity meters that can measure the earth’s gravity field, and maps of the distribution of gravity data have guided oil exploration as well as our understanding of regional geology and tectonics ever since. Most of those gravity data were acquired by people driving or hiking across country, sitting a gravity meter down, and making a measurement. Time intensive and expensive. Eventually we developed technologies to allow the gravity field to be measured from a moving aircraft or from a moving boat – such measurements are lower quality, but they’re a lot cheaper.

In the middle 1990s incredibly accurate radar altimeters were developed and deployed on satellites. A radar altimeter is basically a range-finder, an extremely accurate tool for measuring distance. The radar signal goes out and bounces back, and the time it takes for the trip is proportional to the distance the radar beam traveled. So you can visualize a sensitive radar altimeter on a satellite as something that can give incredibly accurate measurements of the height of the land – topography. The satellite-borne radar altimeters had centimeter-scale accuracy.

But it can do more. Over the oceans, the radar altimeter measures the distance from the satellite to the surface of the ocean. That’s cool, but so what? Ocean surfaces are really very irregular, with waves, currents, and so on to make any measurements at the level of centimeters irrelevant, right? Right. But if you make the measurements dozens, hundreds, thousands of times, you effectively average out things like waves and currents. You get an average measurement of the height of the surface of the ocean. OK, really it’s the distance from the satellite – whose elevation is precisely known – to the ocean surface, but it’s OK to think of that as the sea’s height.

Again, so what? Well, the average height of the sea surface on a perfectly uniform sphere, the earth, would be a uniform surface, and it actually has a name, the geoid. But the earth is anything but uniform. And in fact we can use the satellite radar altimeter measurements to make maps that are essentially representations of the attraction of gravity – and the accuracy is high enough that we can actually see geological features.

Imagine the sharp slope on the sub-sea edge of a continent – the position where the water gets abruptly deeper. This happens around all the continents. The dramatic contrast in density between water and any rock, any rock at all, it huge, from 1.0 to more than 2 grams per cubic centimeter, so the under-water topography, called bathymetry, is by far the strongest component of the gravity maps that are made from satellite radar measurements. But where the water bottom is relatively flat, the variations in the radar measurements, which translate to gravity values, really do represent geology.

Yes, these data are lower quality than the gravity data that come from gravity meters sitting out there on the land surface. But they are essentially free – all this comes from data acquired from satellites paid for by tax money, so they are in the public domain. And they are often much better data sets than anything that might have been acquired from a ship or aircraft, just because there are not many such data sets.

These satellite-derived gravity maps are really useful for strategic planning for oil companies. Most of the consulting work I did from 1997 to 2002 was interpretation of such data. I did projects that covered all the coastal areas of Africa and India, the east coast of South America, the Maritime Provinces of Canada, the Gulf of Mexico, the South China Sea, and all the waters offshore Indonesia. The information is pretty amazing, if you can figure out how to read the data, and that was my job.

Some of it is pretty unsurprising. For example, off the coast of Bangladesh, in the Bay of Bengal arm of the Indian Ocean, there’s a really wide, flat continental shelf. The Ganges River has carved a submarine channel across that submarine shelf, and no surprise – the channel, which in terms of density is a narrow canyon of water surrounded by rock and sediment that are much denser, shows up in the satellite gravity map as a long, sinuous gravity low. Offshore southwest Africa, there are features in the gravity map that represent huge igneous intrusives that might be wonderful places to explore for minerals, if they were not 150 kilometers offshore and under 200 meters of water. But figuring out where they are located can guide our understanding of the structural geology and tectonics that may help with exploration onshore.

On the west coast of India there is a nearly circular gravity low. You should think of that as low-density rock, but compared to what? It turns out it is really a block of granitic crust, rifted away from the Indian Subcontinent, but it shows up as a low because even at a density of about 2.7 grams per cc, it’s really low compared to the oceanic crust, basalt at maybe 3.3 grams per cc, that surrounds it. This block is actually a high-standing bit of continent that tried to rift away from India – but never really managed to separate. And guess what – today, the largest oil field in India, the Bombay Field, sits right on top of that gravity low. So knowing that, we can look for similar, perhaps less obvious places where there might be additional oil or gas fields. That was the nature of the work I did in the late 1990s for oil companies interested in evaluating the strategic potential of offshore India. It wasn’t really a question of “where do we drill,” – it was more a question of “should we be interested in this region or not.”

Because the satellite data are essentially free, and a geological interpreter like me is relatively cheap in oil company terms, there was a lot of analysis of these data sets back about 15 years ago. The data are still useful and oil companies routinely use them as they plan their exploration programs.

Thanks for listening. I appreciate your interest and support.

—Richard I. Gibson

The Oily Episodes from 2014

My friend Larry Smith, a geology professor at Montana Tech here in Butte, Montana, suggested that I assemble the podcasts from 2014 into thematic packages as well as the month by month packages, which is ongoing. I thought that was a good enough idea to buy Larry a beer, and here’s the first of these packages.

This group contains all the 2014 episodes tagged with oil or oil shale keywords. There are 15 of them, including two that are mostly about oil shale deposits. Running time is about an hour and twenty minutes.

Thanks very much for your interest.

—Richard I. Gibson

December 18. Oil at Baku

The Caucasus Mountains, between the Black and Caspian Seas, hold one of the most important and early-produced oil provinces in the world. This area is part of the Alpine-Himalaya collision between pieces of Gondwana and the southern margin of Eurasia. Specifically, it’s the northern prong of Arabia that’s squeezing a small bit of continent, more or less part of the main Iran block, which itself was part of the Cimmeride continent, all that is being pushed into the south side of Eurasia.

Geographically, the Caucasus is taken as the boundary between Europe and Asia, and it contains some high mountain peaks, including Mt. Elbrus, a dormant volcano that reaches more than 5,600 meters above sea level, more than 18,500 feet. It last erupted about 2,000 years ago, showing that this area is still tectonically active.

Photo: Baku oil wells, Asbrink Collection.

One of the effects of the ongoing Alpine-Himalayan collisions was the development of fold belts along and within the Caucasus Mountains complex. Rocks of Miocene age were pushed into large asymmetrical folds, anticlines and synclines with strata arched upward and downward, respectively. This shows certainly that the tectonic action was going on after the Miocene rocks were laid down, since they are involved in the folding. This isn’t a surprise, since we know the collision is still going on today. The early Miocene rocks were probably folded in Miocene time, 5 to 20 million years ago, and in the Pliocene, 2 to 5 million years ago.


These anticlines trap lots and lots of oil. Oil was known in the area around Baku from the time of Marco Polo, and was supposedly used by locals for lubricants and fuel in the time of Alexander the Great. Baku oil was produced in quantity from hand-dug wells in the 1830s, and the world’s first paraffin factory began there in 1823. The first mechanically-drilled well in the world was drilled at Baku in 1846, 13 years before America’s first oil well in Pennsylvania, in 1859. By the 1870s, oil demand was surging worldwide, and outside investors came in to develop the oil fields around Baku. Two of the many fortunes that came from Baku oil were those of the Nobels, of Nobel Prize fame, and the Rothschilds. In 1900, half the world’s oil was coming from Baku, much of it from rocks of Miocene and Pliocene age.

Further west along the northern front of the Caucasus Range, additional fields were discovered. Grozny, in Chechnya, became Russia’s #2 source of oil until after the Revolution in 1917, and the Grozny area still produced about 7% of the Soviet Union’s oil as late as 1971. The Grozny field is in an anticline in Miocene rocks, with multiple sandstone reservoirs with impermeable shale seals. The Caucasus oil was a major target of Hitler’s forces in World War II, and it still plays a significant role in the geopolitics of the region.

Pliocene deltas (that form oil reservoirs)
coming into the South Caspian Basin.
From USGS Bulletin 2201-I

It’s no surprise that this oil was found so early, because it is practically at the surface in many cases, or just a few feet beneath the surface in the relatively young Miocene and Pliocene rocks. Marco Polo reportedly saw a natural gusher of oil. The organic rich source rocks are largely of Miocene age, called the Maykop Suite. There was a restricted seaway extending through this region, on the north side of the approaching continental blocks before they collided to raise up the Caucasus, and the marine carbonates of the Maykop Suite were deposited there. By Pliocene time, just four or five million years ago, the region became isolated from the sea, and rivers brought sandy sediment into the basin. Some of the most productive reservoirs around Baku are from Pliocene rocks deposited in deltas around the margins of the South Caspian Basin, which is an entrapped bit of old Tethys Ocean floor. The ongoing tectonic activity has created plenty of traps for the oil. 

Azerbaijan, where Baku is located, still produces about 900,000 barrels of oil per day, about 10% of what the US produces. But it’s only about the size of the state of Maine.

—Richard I. Gibson

Cenozoic oil – Azerbaijan 
Photo: Asbrink Collection.

Pliocene deltas (that form oil reservoirs) coming into the South Caspian Basin. From USGS Bulletin 2201-I, by Linda Smith-Rouch, 2006.

November 27. The Athabasca Tar Sands

The McMurray Formation is a package of Cretaceous rocks in northern Alberta and adjacent areas of Canada that was laid down in early Cretaceous time, so I should have talked about it earlier this month, but I didn’t. 

The basin that the McMurray sediments accumulated in was an early aspect of the Cretaceous Interior Seaway that reached its height in Late Cretaceous time. The sea invaded from the north, the Arctic, so the transgression of the sea affected Canada earlier than it did the western United States. The McMurray environment was similar to that of the shores of the later Cretaceous Interior Seaway – rivers, deltas, estuaries, and coastal plains. Settings like that typically give rise to interbedded and alternating sands in river channels and beaches, silts in the more distant parts of deltas and estuaries, and muds in overbank deposits, flood plains, and in the parts of the coastal sea even more distant from shore. 

What makes the McMurray special is that it holds one of the largest accumulations of hydrocarbons in the world – the Athabasca Tar Sands. You sometimes hear this called oil sands, but it really isn’t oil in the sense of liquid oil. It’s solid bitumen, tar, oil that has lost the volatile components that help make oil liquid. It’s called the Athabasca Tar Sands because the McMurray formation crops out in the drainage basin of the Athabasca River.

Map by Norman Einstein, released to public domain.

There’s an estimated 1.7 trillion barrels of hydrocarbon in place in these deposits, similar to or more than all the rest of the known conventional oil in the world. Only about 179 billion barrels is listed as economically producible – but that’s “only” about the same as all the oil in Saudi Arabia.

This stuff, because it is solid, is a lot harder to produce. About half of production is mined, dug out of the outcrop, and the rest is produced using steam injection and other methods to essentially melt the material in the subsurface to liquefy it. All methods combined yield about 2.3 million barrels of oil per day – more than half of Canada’s total oil output. Canada is the #5 oil producer in the world, after the U.S., Russia, Saudi Arabia, and China, and Canada is the leading source of U.S. imports, at more than 3.5 million barrels per day – more than all OPEC nations combined. Canadian imports amount to more than one-third of all US imports, which totaled about 9.2 million barrels a day in September 2014, and Canadian imports made for about 19% of total US consumption, which is just over 19 million barrels a day.

Canada projects that it may be able to double its production of oil from tar sands by 2020.

Leaving apart the controversies over this production, I’m going to focus on the origin of the deposit. I used to think it was more or less just an oil reservoir at the surface with the volatiles gone, but it’s more complicated than that.

First, there’s the question of the source rocks for the bitumen. The most common speculations for source rocks include marine Devonian carbonates or Triassic-Jurassic shales, or both. Those rocks are known to be the sources of conventional oil in the Western Canada Sedimentary Basin, but they may be inadequate to generate the volumes we see in the tar sands. It’s been suggested that they formed in place, from organic matter washed into the McMurray formation along with the sands and shales, or that it was derived from non-marine Jurassic and Lower Cretaceous coals.

There’s also argument about the migration history – how the bitumen got there – ranging from in-place formation to migration from deep sources. The process was complicated by dissolution and collapse of salt-bearing horizons in the Devonian. Their collapse changed the topography of the surface the McMurray sands were deposited upon, so that some places had thicker sands than others, and were more favorable for the oil and bitumen to accumulate.

Agreement is not unanimous, but I think there’s at least a slight preference for the Jurassic Fernie formation, a black shale, as the ultimate source of the organic matter. It would have migrated in a fairly standard way, and then it would have to be degraded, driving off the volatiles to leave the heavy, viscous tarry stuff behind in the sand reservoir. The standard view of degradation of oil to make bitumen is that it’s driven mostly by bacterial action – microbes consume the lighter, more volatile compounds in the oil and leave the tar behind. That certainly happens, but it begs the question of why did it happen on such a massive scale here in Canada. Even the weight of glacial ice on the reservoir may have been a factor in the degradation of the oil from a fluid to the bitumen, or from solid coal to tarry stuff.

There are lots of other tar sands and accumulations of heavy oil around the world, but the one in Canada and the one in Venezuela – which may be even slightly more voluminous than the Canadian deposits – are by far the largest.

—Richard I. Gibson

LINKS:
Geologic features of tar sands 
Geology of the Oil Sands (PDF)
Origin of the tar sands 

Western Canada Basin oil – Jurassic sources

Canadian oil 

Map by Norman Einstein, released to public domain.

November 13. The South Atlantic opens

Base from NOAA (annotated by Gibson)

It was the obvious good fit of the coastlines of Africa and South America that helped lead Alfred Wegener to his theory of continental drift back in 1915. The South Atlantic has a different history from the North Atlantic, not least in being rather younger than the northern portion.

The geometry of the coasts reflects differences in the way parts of the South Atlantic opened. The near east-west margin of West Africa turns to north-south at the Gulf of Guinea, the corner where the northeastern tip of South America used to be attached. In fact, that corner is probably really a triple junction, a tectonic location where three relatively distinct rifts began, radiating away from the location of the Niger Delta today.

The combined South America-Africa continent, which we’ve referred to previously as West Gondwana, began to separate in several places during the Jurassic. In the north, the irregular boundary between West Gondwana and North America left a piece of West Gondwana attached to North America – Florida. And the zone between West Africa and northern South America began to crack, too. In the south, also during the Jurassic, we talked last month about the separation of East Gondwana, and that also put the beginnings of a rift between southern Africa and southern South America. But the middles of what are now Africa and South America were still attached to each other until the Cretaceous.

At the corner, today’s Gulf of Guinea, two pull-apart rifts started. One ultimately became the rift that makes up most of the South Atlantic, with the north-south margin of Africa from Gabon down to South Africa on one side and Brazil on the other. The other rift was within the African continent, extending northeast from today’s Niger Delta, practically all the way across Africa, through Chad and Sudan to the Red Sea. In places this was a pull-apart zone, especially in Nigeria, where it is called the Benue Rift, but further into Africa it is a tear or shear zone, with the northern and southern parts of Africa moving alongside each other. If this rift had not failed, we would have two continents today instead of one single African continent.

Another shear zone developed in the third branch of the triple junction. This branch ran west from the Niger Delta today, along the east-west trending coast of West Africa, from Nigeria to Ghana to the Cote D’Ivoire to Liberia. On the southern side of this zone, the northern coast of Brazil, from the tip at Cabo San Roque up to the mouth of the Amazon, that section was sliding to the west. So the tectonic boundary between South America and Africa was mostly strike-slip faults, also called transform faults, where the two big blocks slid alongside each other and ultimately parted, leaving the Middle Atlantic Ocean in their wake. The southern Atlantic Ocean was formed in a more straightforward way, with the two sides pulling apart more or less perpendicularly from the spreading center at the Mid-Atlantic Ridge.

But as usual, there was plenty of variety within those overall general parameters. In the southern, relatively simple rifted system, things were complicated by two hotspots that poured basalt and other volcanics into the widening ocean and on the adjacent continents as well. We talked about one of them, the Tristan Hotspot that produced the Parana Basalts and the Rio Grande-Walvis Seamount Chain, the other day. (November 11) The second one was further north, and today it is reflected in active volcanoes like Mt. Cameroon in Africa and St. Helena, the volcanic island near the Mid-Atlantic Ridge. Between them under the ocean, the St. Helena seamount chain represents the movement of the African Plate over the hotspot as the South Atlantic opened. And there is a conjugate submarine ridge on the South American side too.

The big deal about these two volcanic centers, the Tristan and St. Helena hotspots, is that as the South Atlantic was opening, the volcanics erupted from these centers blocked off, segmented, the widening ocean basin. Instead of a long, narrow oceanic seaway, we ended up with long, narrow segments with differing history. In particular, the central section between the hotspots was periodically cut off from the open sea. You won’t be surprised that in that low-lying part of the rift thick evaporites developed as salty marine waters came in and evaporated. Those evaporite beds are hugely important to the economics of Brazil, and across the ocean, to Angola, Congo, and Gabon. Not for the salt, but for the oil that the salt helps trap beneath it.

Early in the formation of the South Atlantic rift, when it was still part of the combined continent of West Gondwana, the region that would ultimately break the continent apart was a low-lying, down-faulted zone. Think of it like the Triassic grabens that we talked about a lot in Eastern North America as the North Atlantic was beginning to form. In West Gondwana, the low-lying, tropical basins held extensive lakes, and the organic rich material that was deposited in those lakes became an excellent oil source rock. As the region pulled apart more and more, the volcanic centers to north and south restricted the basin allowing for the salt deposits to form and serve as the seal to keep the oil from escaping to the surface.

There is some argument about the origin of the salt, and it may be more complicated than simple incursions of marine waters and evaporation; some of the mineralization may have come from hydrothermal sources. The salt was deposited mostly during the Aptian Stage of the Early Cretaceous, about 125 to 115 million years ago. Eventually, about 112 million years ago, the South Atlantic became wide enough that we had an open ocean between the two continents, and the salt basins were split into two – one now offshore Brazil, and the other offshore (and in a few places, onshore) Angola to Gabon. These two areas are among the most prolific oil provinces in the world. Almost all of Brazil’s oil production comes from these sources, more than 2 million barrels a day, from fields that probably contain at least 13 billion barrels of oil. Brazil’s oil also comes from some of the deepest water depths ever drilled, with the sea floor around 7,000 feet below the sea surface. On the opposite side, the African coastal offshore from Nigeria down to Angola produces around 4 million barrels a day.

There’s plenty more to explore about the opening of the South Atlantic, and I have a handful of links and references below if you are interested in more.

* * *

The volcano Nevado del Ruiz erupted in Colombia on November 13, 1985. The eruption melted glaciers on the mountain summit, which is more then 5,300 meters, or 17,400 feet above sea level. The eruption was really quite small, only about 3% of the volume erupted from Mt. St. Helens in 1980, but the glacial melting proved catastrophic. The volcanic debris mixed with the glacial meltwater, loose rock, and surface soil on the mountain flank to produce a pyroclastic flow, also called a lahar, which increased in volume as it came down the slopes. The flow wiped out the town of Armero and several villages, with a total death toll of about 23,000, the fourth-deadliest volcanic eruption in recorded history. Tectonically, Nevado del Ruiz is a typical volcano in the Andes, the result of the subduction of the Pacific Oceanic Plate beneath the South American Continental Plate.

—Richard I. Gibson

Links and References:
A new scheme for opening of the South Atlantic

Aptian evaporites (2012)

Tectonic evolution of South Atlantic (2000)

South Atlantic 

Brazil’s offshore oil 

Evaporites Through Space and Time, edited by B. Charlotte Schreiber, S. Lugli, M. Ba̜bel (The Geological Society, 2007)

October 27. Late Jurassic oil source rocks

On October 22, we talked about the lush vegetation of the Jurassic that thrived in greenhouse conditions. The volume of that plant matter is likely a factor in the astonishing hydrocarbon source rocks that are found in the upper Jurassic. By some estimates, a quarter of all the discovered oil and gas on earth was sourced by Jurassic rocks.  

We’ve mentioned Saudi Arabian oil already, back in the Silurian (April 18). But a much larger source of Arabian oil lies in the Jurassic section. During the late Jurassic, as Pangaea broke up, new mid-ocean ridges were displacing more and more water, so the sea transgressed over the land in many places. The northeastern part of Africa – the Arabian Peninsula today – was just about on the equator and was along the southern margin of the Tethys Ocean. As sea level rose, a wide, shallow, tropical marine shelf formed – a perfect place for life. Meanwhile the land contained all that plant matter, and as it washed into the oceans, it would typically be dispersed and added to the ocean’s store of nutrients to support marine life. But that shallow marine shelf in what is now Saudi Arabia wasn’t flat.

The shelf had deeper basins in the sea floor where oceanic circulation was restricted, so when plant matter washed into them, they tended to accumulate without being dispersed. It was probably something like the modern Bahamas, where shallow shelves give way to deep troughs. Another analog would be the basin where the Solnhofen Limestone formed, but in this case, with a lot more organic matter. You know what’s coming – the organic matter, most of which was actually algae, which piled up in those troughs didn’t decompose as it would in the open ocean, so it became part of a limy, organic-rich mudrock. The Tuwaiq Mountain and Hanifa Formations contain as much as 5% total organic carbon, a huge value, making for a world-class oil source rock.

Don’t visualize instant oil formation, though – it took at least 50 million years, and probably closer to 100 million years, for the source rocks to be buried sufficiently for the oil to be cooked out of the source rocks and to begin to migrate into overlying reservoir rocks. By that time, the ocean was closing, and Arabia was beginning to impinge on what is now Iran and adjacent areas that were part of the Cimmerian Continent that took off from the margin of Gondwana back in the Permian. The collision produced folds, like a carpet caught between two pieces of furniture being pushed toward each other. Those folds made excellent traps, and the oil migrated up into porous rocks that formed in the Tethys Sea before the collision began. As the collision proceeded, the seaway between what is now Arabia and Iran became restricted – just as it is today, in the Persian Gulf, but more so – and thick evaporites formed. So you had everything needed for a great oil province – world-class Jurassic source rocks, magnificent reservoirs crunched into big, broad upwarps, and covered by evaporites to seal the oil into the reservoir. There are variations, of course, but this is pretty much the scenario for most of the oil in the Middle East.

Oil & Gas Fields of West Siberian Basin

Similar situations developed in other areas, which were less tropical and more temperate, but recall that the temperate zones during the Jurassic were wider and warmer than they are today. One is the present-day northwest shelf of Australia, another margin of the Tethys Ocean. And another was in the West Siberian Basin, where a restricted, narrow sea developed between the remnants of the Ural Mountains, formed in the collision between Europe and Siberia, and the high-standing block of the Siberian craton itself.

You may recall from the episode on September 27 that the West Siberian Basin was initiated by Triassic rifting, rifting that never went to completion to make an ocean basin like the rifting in the Atlantic did. But the region sagged, so that during the late Jurassic a deep, restricted basin formed in which organic-rich shale was deposited. The Bazhenov Suite of rocks contains an estimated oil-in-place of around two trillion barrels, of which anywhere from 75 to 360 billion barrels may be recoverable. For comparison, Prudhoe Bay, the largest oil field in North America, has produced about 13 billion barrels, and it is largely depleted.

Most of the oil and gas fields of the West Siberian Basin are sourced in the Bazhenov Suite, a package of rocks that covers something like a million square kilometers, but it’s typically only about 150 meters thick. Some estimates suggest that in the long run, the Bazhenov may be a more prolific source of oil and gas than the rocks in the Arabian-Iranian basin. Much of the Bazhenov oil is probably tightly locked in the rock, so it will have to be produced using horizontal drilling and hydrofracturing techniques, like the Bakken formation in North Dakota.

As I mentioned, virtually all of this oil and gas, and in fact almost all oil and gas, of every age, everywhere on earth, comes from the organic matter in plants, especially marine plants like algae. For all the dinosaurs we’re talking about this month, their contribution to oil and natural gas accumulations is practically zero.

—Richard I. Gibson

Why so much oil in the Middle East?

Petroleum geology of the West Siberian Basin  (source of map)

August 22. New Red Sandstone

If you’ve been with us since May 3, maybe you remember the Old Red Sandstone. Those were the sands laid down in largely terrestrial environments, in basins scattered through the Caledonian Mountain Range that resulted from Baltica – Europe, especially the British Isles and Scandinavia – colliding with North America, specifically Greenland, Labrador, and Newfoundland. The early European geologists recognized two red sandstones, and applying the law of superposition, also recognized that one was very definitely older than the other. Hence Old Red and New Red Sandstones.  

The Old Red became assigned to the Devonian Period as the early and middle Paleozoic strata were sorted out. The New Red was above, and therefore younger than the distinctive coal measures of the Carboniferous. Ultimately the rocks of the New Red Sandstone were recognized as being of Permian and Triassic age. In Britain, much of the package is quartz sandstone cemented by hematite, iron oxide, which gives it the red color. As we’ve indicated several times, oxidized iron usually indicates that the rock was alternately under water and exposed to the atmosphere for the oxidation to happen. This gives us some good clues about the environment, and like the Old Red, the New Red was laid down in largely terrestrial environments, including rivers, deltas, and ephemeral seas and lakes.

East of Britain, beneath the waters of today’s North Sea, and extending into the Netherlands, Germany, and Poland, the later New Red equivalents include the Zechstein salt that we talked about a few days ago. Salt and other evaporites, including gypsum and sulfates, indicate a restricted sea or large lake in a relatively hot, arid environment. Sea level changes would alternately flood the land or recede leaving mud flats and sandy delta plains exposed to the atmosphere so the iron in the sediment could oxidize and the salts could precipitate out. Today this environment is called a sabkha, an Arabic word for salt flat.

The Zechstein formed in Late Permian time, and the older equivalents of Britain’s New Red are called the Rotliegend Group in Germany, the Netherlands, and in the subsurface of the North Sea. Rotliegend means “the underlying red,” for their presence beneath the distinctive Zechstein evaporites. The basin containing the New Red Sandstone and the Rotliegend Group was surrounded by the mountains uplifted when Gondwana collided with central Europe, so the setting was very similar to that of the Old Red Sandstone – just with a somewhat different geography of mountain ranges.

Carboniferous-Rotliegend oil and gas fields (from USGS Bulletin 2211)

There are quite a few oil and gas fields in the North Sea whose reservoirs are in the Permian Rotliegend rocks. The complex changing environments during the deposition of the Zechstein also make for abundant source and reservoir rocks, and the salt can flow and move in ways that help create significant traps for oil and gas. The Rotliegend strata probably spanned a considerable time during the early to middle Permian, perhaps as much as 30 million years. Zechstein deposition was more constrained, maybe only 5 to 7 million years, very near the end of the Permian Period. It’s possible that the incursion of salt water into the Rotliegend basin represents the very first pulse of the break-up of Pangaea. Some reconstructions show a narrow rift beginning between Greenland and Scandinavia by Late Permian Zechstein time, but alternatively, the sea could have come in through a trough extending southeast from Poland to the Tethys Ocean.

* * *

Laurence McKinley Gould was born August 22, 1896, at Lacota, Michigan. He was a geological explorer of the Arctic and Antarctic, working as Richard Byrd’s chief scientist on Byrd’s first trip to the Antarctic. Gould was on the faculty at Carelton College in Minnesota for 30 years.

—Richard I. Gibson

References:

• Permian and Triassic red beds   p.11, 17
• Petroleum Geology of the North Sea, K.W. Glennie, ed. (1998, Blackwell Science)
• Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter Ziegler (1988, AAPG Memoir 43)
• Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary by Donald Gautier, USGS Bulletin 2211

July 27. Lansing Formation

Throughout the Pennsylvanian Period, I’ve focused a lot on the coal swamps that characterized the time, as well as mountain uplifts. But there were plenty of regions still covered by shallow seas like those that were typical of so much of the earlier Paleozoic Era.  

Take Kansas, for example. Many of the Pennsylvanian rocks there are shallow-water limestones similar to those that covered much of the interior United States for much of the previous 200 million years. One difference is that those Kansas rocks do also reflect the alternations in sea level that produced the coal cyclothems further east. Cyclothems are alternating layers of coal and sandy sediment, reflecting high stands and low stands of the sea. In the Lansing Limestone of Kansas, the same cycles are shown by alternations between limestone and shale in the Pennsylvanian strata.

It’s pretty clear that the limestones were formed in very shallow water. A lot of the rock is made up of broken shells, which must have been fragmented by wave action. They were piled up into shoals, just like sand banks at or near the water line or at least above wave base. When sea level rose just a bit, fine-grained deeper-water shaly sediments could wash over those carbonate banks. The skeletal shell fragments in the limestone made for some excellent porosity in the limestone – commonly as much as 15% of the rock, and the tight, fine-grained shale made for a nice impermeable barrier – both necessary conditions for oil and gas accumulations. All that we need is a source rock with time to mature – and we have that too, in the same shales that washed organic material into the sea along with the fine sediment.

Drawing of shoal environments of Lansing Limestone (from Harbaugh, 1960)

The Lansing and Kansas City Group of limestones has produced something like a billion barrels of oil over many decades, and that adds up to at least a fifth of all the oil Kansas has produced, and Kansas is one of the leading states in terms of total cumulative production. About 25% of all U.S. oil production comes from rocks of Pennsylvanian age. The Lansing Formation and its equivalents extend into Colorado and Oklahoma too. They were laid down about 210 million years ago, contemporaneous with the coal swamps in Pennsylvania and West Virginia and elsewhere.

—Richard I. Gibson

Links:
Petrology of Marine Bank Limestones of Lansing Group (Pennsylvanian), Southeast Kansas, by John W. Harbaugh - Originally published in 1960 as Kansas Geological Survey Bulletin 142, part 5

Oil & Gas

Oil Potential  

July 8. Rangely Oil & Gas Field

Each dot is a well in Rangely Oil Field, averaging about 6000 feet to the Weber Sandstone.
The squares are one mile on a side. After Dobbin, 1956 (USGS)

Rangely Field in northwestern Colorado is an elliptical dome about 11 miles long that contains oil and natural gas in the Upper Pennsylvanian Weber Sandstone. The Weber is mostly a river sand deposit, but some eolian, wind-borne, sand dunes are present as well, and they form some of the best oil reservoirs. Rangely is one of the largest oil fields in the United States, with cumulative production of about 900 million barrels of oil and 700 billion cubic feet of natural gas. That makes it about the 18th or 19th largest oil field in the U.S. in terms of total production. The dome, a structure like an inverted bowl, is caused by a large deep-seated fault on the southwestern flank of the structure. That fault which produced the fold or dome in the Weber Sandstone didn’t form until toward the end of the Cretaceous Period, 200 million years or more after the Weber was laid down. The fault was part of the Laramide Orogeny, and the anticlines and domes that Rangely is part of are essentially a buried extension of the Uinta Mountains of northeastern Utah. 

The rivers whose sand became the Weber Sandstone were flowing off the Uncompahgre Uplift, one of the high mountain ranges formed by the Ancestral Rockies uplifts. The dome makes a nice anticline that’s quite evident on the surface, so it was an early target for oil exploration, with the first discovery in 1933 by the California Company, which we know today as Chevron. It’s a pretty remote area, however, and production didn’t begin until after World War II, and the depth to the Weber is around 6,000 feet or more, which would be a pretty deep well in those days. Because it has been produced for so long, the easy-to-get oil has all been pumped out. In the late 1980s producers were working to get the last bits of oil out of the field by pumping carbon dioxide into the reservoir to force the oil out. During earlier water injection, in the 1960s, it was shown that the deep injection was causing small earthquakes in the Rangely area, some with magnitudes of 4, but mostly smaller.

With the ongoing CO2 injection, Rangely in 2011 was producing about 11,000 barrels per day from almost 1000 wells, which works out to about 11 barrels per day per well, just a bit above the US average oil well production. The CO2 injection has significantly increased the projected production of the field, which otherwise would probably have been half or less than the 11,000 barrels a day. And the CO2 injection does not appear to be causing any earthquakes. See below for a link to a report on the CO2 project.

—Richard I. Gibson

References:
Weber Sandstone 

Earthquakes 

Rangely today – CO2 injection project

Top oil Fields (US)

Drawing after Dobbin, 1956 (USGS)

May 22. Grant Canyon Oil Field

For many years in the 1980s, the most prolifically producing oil wells in the onshore 48 states were in Nevada. Nevada? Yep – not the first place you think of for oil, but there’s oil there, in some pretty unusual traps.

Oil was first discovered in Railroad Valley, in desolate central Nevada, back in 1954. It was kind of a fluke – the seismic data they had were pretty poor, and they drilled one thing but found another. The oil reservoir at Eagle Springs Field is mostly fractured volcanic rocks called welded tuffs – essentially, the result of hot ash erupted from a volcano perhaps 10 million years ago – just yesterday, geologically speaking, during the Cenozoic era. The ash fell and landed while still pretty hot, hot enough to weld itself together into a hard, almost glassy rock. Such rock is pretty easy to fracture naturally, and the fractures trap the oil.

Oil Fields of Railroad Valley (data from Nevada BuMines;
interpretation by Gibson)

The oil comes from a rich source, organic-rich black shale in the Mississippian-age Chainman Shale which is buried beep beneath the basins of Nevada. Some of the Chainman has as much as 8% total organic carbon in it, and if you recall some of our previous episodes on oil source rocks, you know that’s fantastic. Even 1% or 2% total organic carbon can make an excellent source rock.

OK, so Mississippian source rocks and Cenozoic volcanics as reservoirs. Aren’t we in the Devonian this month? Yes. Hang on, we’ll get there.

Fast forward to 1976. Another oil field was discovered in Railroad Valley. Trap Spring Field was also in fractured volcanic rocks, but it was across the valley from Eagle Springs. Eagle Springs was a small but steady producer, with today something like 5 million barrels total produced in 60 years. Trap Spring was better, and it has yielded around 15 million barrels in less than 40 years. For perspective, the United States today consumes close to 20 million barrels of oil every day.

The discovery of Trap Spring stimulated a renewed interest in Nevada. At the time, even major oil companies, like Gulf Oil where I worked, were interested. My first work on trying to understand the geology and to use geophysical data to predict where analogs to the existing production might be found began in 1978. And my most recent work on Nevada was this year.

In 1983 another oil field was discovered, in another corner of Railroad Valley. This one was entirely different from the others in terms of the reservoir. Instead of fractured volcanics, the reservoir was extremely porous dolomite – Devonian dolomite, buried within the Cenozoic sands and gravels that fill the basins of Nevada. Nevada’s basins and ranges are formed by long normal faults – the kind formed by pulling apart, extension of the earth. Think of the basins as the parts that dropped down, and the mountain ranges as the high-standing parts that were left back, that did not subside. As the faulting continues, and one side goes down and the other side goes up, relatively, you get these alternating high ranges and low basins. And of course you get erosion of the mountains, dumping sediment into the adjacent basins. Some of the basins in Nevada have more than 10,000 feet of sediment that was eroded off the mountain ranges, and most of that has happened in the past 10 to 15 million years. All of the known oil in Nevada is trapped in various kinds of rock that’s been dumped into the basins.

So back to the new oil field discovered in 1983, named Grant Canyon. If all the oil is in the Cenozoic fill in the basin, how can I say it’s in a Devonian dolomite?  Think of a fairly rapidly downdropping basin. Fairly rapidly means just a few million years. That can make a pretty steep scarp, the face of the mountain range. Steep scarps lend themselves to massive landslides on occasion – and that appears to be what happened here. A huge slice of the mountain range – composed of those Devonian dolomites and other rocks – slumped off the mountain and into the basin, maybe 6 or 8 million years ago. And then it was buried by more and more sediment coming off the mountain front, until that huge landslide was buried under around 3500 to 5500 feet of later sediment. You can think of it as a landslide, as I described it above, but it’s probably a little more accurate to think of it as another fault that dropped part of the mountain front down into the basin. Either a large landslide or a small fault block. The entire area of material is less than a square mile.

What’s the big deal? Well, in those highly porous Devonian dolomites, oil migrating up from the Chainman shale accumulated. Most of the time you should think of oil in rocks as simply filling the tiny pore spaces between grains of rock, but in this case it’s actually fair to visualize a real pool of liquid oil down there. Some of the porosity in these rocks is called cavernous porosity – essentially, little caves eroded out of the carbonate. With a really good seal, an impermeable layer of rock sitting above it, the Devonian dolomite became a small, but excellent oil reservoir.

How excellent? For about 9 years, from 1983 through 1992, the two wells in Grant Canyon Field yielded close to 6,000 barrels per day – the most of any wells in the onshore 48 states. I’ve said it before, but as a reminder and for perspective, the average US oil well produces 10 barrels per day. 6000 is Saudi Arabian levels. The total volume was nothing like a Saudi Arabian field, but Grant Canyon and the associated Bacon Flat Field produced about 25 million barrels over about 30 years.

There have been several other important oil discoveries in Railroad Valley and some in Pine Valley, further north. The last large discovery came in 1986.

In 2009 and 2010 I did some work for an Irish oil company in Hot Creek Valley, across one mountain range to the west of Railroad Valley. I used a predictive model based on analysis of gravity, magnetic, and geologic data to point to possible analogs to the existing production in Railroad Valley. The company used my recommendations to do a lot of additional work, including geochemical surveys and other approaches, and in 2012 they drilled the second exploratory well ever located in Hot Creek Valley. The 400 barrels per day that they tested was deemed non-commercial, but I can tell you that as far as I am concerned, I felt like I had found oil. It was for me a proof of the concept used to identify analogs to existing production, and I was really happy!  The last I’ve heard, the company is using the information it gained in the first well to plan a second well. Stay tuned.

—Richard I. Gibson

Link:
My Nevada oil exploration page

May 17. Bakken formation

Today’s topic, the Bakken formation of North Dakota and Montana, is probably familiar to most listeners, and there’s a vast amount of easy-to-find information available, so this will just be a summary. I actually started the Wikipedia page for the Bakken, back in 2007 when it was just taking off – if you go into the history of the article, and go back to the start, you’ll see it was started by Geologyguy – that’s me. The announcement a couple weeks ago that the Bakken had produced its one billionth barrel of oil also indicated that most of that production had come since 2008.

First, let’s talk a little about terminology. The Bakken is not an oil shale. I talked about oil shale in Estonia on March 27. Oil shale is a solid rock, no liquid. It contains a lot of organic material in it, which can be cooked to covert the organic stuff to liquid oil. The confusing, but somewhat better term for the Bakken is shale oil – liquid oil that is tightly trapped within very fine grained shale. The Bakken is the oil version of the Marcellus Shale that we talked about a few days ago – the Marcellus has natural gas, and the Bakken has oil. In both cases, the hydrocarbons are trapped in tiny, tiny pore spaces that are poorly interconnected, if at all. Consequently, techniques like horizontal drilling and hydraulic fracturing, which I described for the Marcellus on May 11, are used to extract the oil or gas.

In the Bakken formation, the oil bearing horizons are about 9,000 feet down, a little short of two miles. So the well is drilled that far, pretty much straight down, then the drill bit is turned to nearly horizontal and navigated through the oil-rich part of the Bakken, only about 140 feet thick. The horizontal portion of a typical Bakken well may extend for two or three miles, 10,000 to 15,000 feet. The kinds of production that such wells yield range from around 100 barrels a day to more than 1000 barrels a day – obviously 1000 is better than 100, but 100 is pretty good considering the average production of all oil wells in the United States is around 10 barrels per day per well. These wells in the Bakken are also a lot more expensive than conventional oil wells, costing $2 million to $5 million or more, when a standard conventional well might cost $1 million or even less.

During much of the early Paleozoic Era, the Williston Basin in western North Dakota and eastern Montana was a deep depression, much like the Michigan Basin that we’ve talked about several times. The water was deeper, and because it was kind of like a deep bowl in the sea floor, the water was also restricted in terms of circulation, so it became anoxic, at least at times. That happened on multiple occasions during the Devonian.

The Bakken lies above, and is therefore younger than the Devonian Jefferson Formation that we talked about the other day, and it’s also younger than another formation called the Three Forks. Both the Three Forks and the Bakken contain black shales, similar to those of the Marcellus, that accumulated in the deep, quiet, anoxic waters of the Williston Basin. But the Bakken is multiple layers, including typically a lower black shale, a middle dolomite, and an upper black shale. The dolomite, calcium magnesium carbonate, probably represents a change to a shallow, more well-oxygenated environment.


A few minutes ago I talked about the Bakken shale being tight – low porosity and low permeability, or interconnectedness of the pores, and that’s true, but it’s not 100% tight. Some of the oil has been squeezed from the two black shales into the dolomite, and the dolomite is in fact the main oil reservoir for the Bakken, so even calling it shale oil is misleading. The shale has some oil in it, and it most definitely served as the source for the oil that’s in the dolomite. Even in the dolomite, the porosity is quite low – maybe 5% of the rock – and the permeability is also very low. And it’s not all uniform – there are definitely “sweet spots,” places where there’s significantly more oil in the reservoir than elsewhere. It’s not a case of drill anywhere.

How much oil is there in the Bakken? Lots of guesses, and some of them are actually intelligent guesses. But you should ignore the hype that says there’s a trillion barrels of oil there. Or, more accurately, you should dig a little deeper. There might be a trillion barrels of oil in place – but by no means can all that oil be produced, and certainly all of it cannot be produced economically. The U.S. Geological Survey and the North Dakota Department of Mineral Resources give pretty reasonable estimates of 150 to 400 billion barrels of oil in place.

How much is producible? That’s the more useful question, and the answer is a moving target based on changes in technology and increasing understanding of both the volumes present, how they are distributed geologically, and how the production declines over time. Again, take various estimates with a few grains of salt. If a company producing oil estimates that there might be 20 billion barrels to produce, maybe that’s correct, but remember that it would certainly be to the company’s advantage to have such a high value for its reserves.

The U.S. Geological Survey, a reliable if somewhat conservative organization, estimated about 3.6 billion barrels producible in 2008, and they’ve more than doubled that estimate now, to 7.5 billion. Proved reserves – that’s a more reliable number, based on actual drilling rather than projections – amount to about two billion barrels or so.

All this is very good. The Bakken has propelled North Dakota from about the #10 state in terms of oil production to #2, after Texas. And its production continues to grow. In November 2013, according to the journal World Oil, North Dakota was producing an average of 972,000 barrels per day. That’s out of about 8 million barrels a day for the entire US, so North Dakota is producing more than 12% of all U.S. oil. North Dakota, combined with significant production increases in the Gulf of Mexico, has dramatically increased U.S. total oil production, from less than 5 million barrels a day 5 or 6 years ago to 8 million today. That’s still well short of the U.S. peak of oil production, at more than 10 million barrels a day back in 1970. Will the surge continue, and bring the U.S. to a new peak? Time will tell. You can find many headlines that say U.S. oil production will exceed that of Saudi Arabia within a year, or a few years. That’s not impossible, of course – Saudi Arabia produces about 9½ million barrels per day, only 1½ million more than the U.S. today. Personally, though, I wouldn’t bet the farm on it. Your mileage may vary, of course.

 * * *

On May 17, 1776, Amos Eaton was born at Chatham, New York. He was a geologist and botanist, and he significantly influenced education in the United States through a philosophy of applying science to daily life. In 1824 he co-founded the school that became the Rensselaer Polytechnic Institute in Troy, New York. Among his students was Mary Mason Lyon, founder of Mount Holyoke College, and James Hall, first state geologist of New York.

—Richard I. Gibson

Cross section from USGS

Map from Energy Information Administration

April 18. Saudi Arabian Oil

April 18, 1906. At 5:12 in the morning San Francisco was rocked by a powerful earthquake. The magnitude was 7.8 or 7.9 and the death toll is estimated today at about 3,000 people. 

* * *

Now back to the Silurian. Oil is another resource associated with Silurian rocks.  Worldwide, something like 9% or so of all the oil comes from Silurian source rocks – and most of that is in Saudi Arabia. The Silurian Qalibah Formation is a thick shale and sandstone package. The lowest part is a black shale rich in organic material. It seems to have been deposited relatively early in the Silurian, when the land was being flooded as the glacial period at the end of the Ordovician Period ended. It appears that this shale was deposited during a sea-level drop, an anomaly during a time of generally rising sea levels as the ice melted. The receding sea might have stranded piles of plants and other shallow-sea life in shallow pools on land, where enough organic material was caught in the mud to make these black shales. It may be that there was a proliferation of life associated with the end of the glaciation – there was such a proliferation, but how fast and extensive it was can be debated. But with the glaciers’ withdrawal, there was more land to erode and to provide nutrients into the sea for life to take advantage of in the relatively suddenly warmer waters. The exact origin of these rocks remains somewhat contentious.

They call these source beds “hot shales” because they also tend to have concentrations of radioactive elements, which make them easy to recognize on well logs, which are basically measurements of information taken down a drill hole. One tool measures gamma-ray intensity, which is greater in radioactive materials. Similar rocks of similar age are found in North Africa as well – Algeria, Libya, and Egypt have such shales and oil explorationists are really only just beginning to understand them.

The known oil from Silurian source rocks in Arabia and Algeria amounts to probably about 95% of the Silurian-sourced oil in the world, but there’s some oil in Silurian rocks in the U.S. as well. We’ll get to that in a few days. 

—Richard I. Gibson

References:
Hot Shale 
Role of glaciation
North Africa

March 25. Trenton oil and gas field

In 1883 pioneers of oil and gas exploration drilled more than 1,000 feet – a deep well in those days – to reach the Middle Ordovician Trenton limestone in northeastern Ohio, encountering a significant flow of natural gas. Within a few years, an extensive area of Trenton oil and gas production had been established in northwestern Ohio and eastern Indiana. It was one of the largest accumulations discovered in the United States before 1900.

Most of the Ordovician production in this area has now been depleted, but in 1887 a group of small companies, threatened by John D. Rockefeller’s Standard Oil, based in Cleveland, joined forces in Findlay, Ohio, to form the Ohio Oil Company, known today as Marathon.

Just two years after its founding, the Ohio Oil Company was gobbled up by Standard, in 1889, and remained part of the Standard Oil Trust until the breakup of Standard Oil in 1911. In 1930, the Ohio Oil Company acquired a small marketer, Transcontinental Oil Company, and also acquired Transcontinental’s brand, Marathon, which ultimately became the trademark of the original company. US Steel bought Marathon in 1982, but the combined company spun off the steel business in 2001 and now it’s Marathon Oil Company again, an important multinational oil company.

The middle Ordovician limestones of Ohio and Indiana that held the hydrocarbons, oil and natural gas, that made Marathon, are part of the quiescent, warm, shallow-water deposition that dominated Ordovician times in what is now North America – at least until the action started in late Ordovician time, over in what is now the Appalachian Mountains.

Production from the Trenton Oil and Gas Field peaked long ago, about 1902 to 1905. The estimated total production over time is a trillion cubic feet of natural gas and 105 million barrels of oil. For context, while that was a huge boom back in its day, 105 million barrels of oil is just five and a half days worth of US oil consumption today. Winter natural gas consumption in the United States is about 3 trillion cubic feet per month, so the Trenton Field would have provided three months’ supply at today’s rates.

By no means was all of the oil and gas removed – it never is – and newer technology and higher prices have driven some rejuvenation of production in this area today. But the production is small, and expensive compared to that of the late 19th century.

An interesting side light to the natural gas boom in Indiana in the 1880s was the development of a flourishing stained glass industry around Kokomo. The Opalescent Glass Company was established there in 1888 because of the abundant natural gas in the area, which served as a ready fuel for the glass kilns. They’re still in business today.

—Richard I. Gibson

Photo of gas flares from Trenton Field, Jan, 18, 1889, Leslie’s Illustrated Magazine (public domain)

Oil & gas statistics from EIA  

February 21. Ohio oil fields

You don’t expect much oil and gas in the Cambrian. Partly that’s because it’s so old and deep, the rocks that might hold oil or gas may have been buried so deeply that the hydrocarbons, the oil and gas, may have volatilized, turned to gas and seeped out. Or the pressure could have reduced the porosity to not much. And since oil and gas come mostly from decaying plants, you have to wonder if there was enough life around to accumulate to be cooked into oil. But we do find some oil in Cambrian rocks.

Oil wells in back yards in Cardington, Ohio, 1964. Photo from Ohio Geological Survey.

In Morrow County, Ohio, just north of Columbus, in the 1960s there was quite an oil boom for a while. It discovered a bunch of little fields in a special kind of oil trap. The upper part of the Cambrian in Ohio includes carbonates – limestones and dolomites, the kinds of rocks that flowing water can dissolve to make caves. At some point not too long after they were deposited, parts of those layers were eroded into hills, and the hills, standing above water line were dissolved – not really into caves, but little dissolution cavities developed in the rocks, excellent spaces to have an oil reservoir.

Ordovician rocks – we’ll talk about the Ordovician next month – were deposited over those rocky hills and served as a tight, impermeable seal to keep fluids from escaping. Oil migrated into those little hills, and stayed trapped until the 1960s, when thousands of wells tapped hundreds of fields. They have produced about 38 million barrels of oil and 35 billion cubic feet of natural gas over time – not that much in the grand scheme of things, but not too shabby, either. To put it in perspective, the United States today consumes almost 20 million barrels of oil every day, so all of the 38 million barrels produced by those wells in the 50 years since the 1960s amounts to about 2 days’ consumption.

Where did the oil come from? Good question. It’s in Cambrian rocks now, but did it start there? Oil reservoirs are not usually where the oil originates. It starts in a rock with lots of organic material, a source rock. Heat, from burial, cooks that solid organic matter over sometimes millions of years, and oil is generated. Then it migrates until it reaches a suitable place to accumulate, a reservoir. The oil in Cambrian reservoirs in Ohio is probably from organic-rich black shales of Ordovician age – younger than the reservoir. How do you push light oil DOWN into older formations? Well, you don’t, with some unusual exceptions. The deep Appalachian basin where the Ordovician shales were heated up – oil people call it the oil kitchen – was deeper than the reservoirs up in central Ohio. The oil did migrate up, as it pretty much must – but it got into older rocks that were physically above the younger source beds. Seems counterintuitive, but it happens more often than you might think.

—Richard I. Gibson

Photo: Oil wells in back yards in Cardington, Ohio, 1964. Photo from Ohio Geological Survey.
http://pubs.er.usgs.gov/publication/70020638