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)