Plate Tectonics
Plate tectonics is the unifying geological theory that Earth's outer shell (the lithosphere) is divided into a dozen major and several minor rigid plates that move relative to each other over the underlying, partially molten asthenosphere. The plates interact at their boundaries in three ways: they spread apart at mid-ocean ridges (divergent boundaries), collide at subduction zones and continental collision belts (convergent boundaries), or slide past each other along strike-slip faults (transform boundaries). These interactions control where mountain ranges, ocean basins, volcanoes, and earthquakes occur. For petroleum geology, plate tectonics determines where and when sedimentary basins form (as plates rift apart or collide), where source rocks are deposited (in the deep, organic-rich environments that often form in rifts and passive margins), and where structural traps develop (along convergent belts).
Key Takeaways
- Rift basins form where plates pull apart. As the crust stretches and thins, it subsides and creates a topographic low that fills with sediment. The heat from the underlying mantle provides organic matter maturation. Major petroleum provinces formed in ancient rifts include the North Sea (Viking Graben and Central Graben), the Gulf of Mexico, the West African and Brazilian margins (South Atlantic rift systems), and the Red Sea basin. The rifting phase is when source rocks are often deposited in deep, restricted, oxygen-poor rift lakes or marine conditions.
- Passive margins (continental margins that form after rifting and spreading, no longer tectonically active) are the world's most prolific petroleum provinces. The post-rift thermal subsidence creates a broad, gently tilted shelf where thick sediment prisms accumulate over tens of millions of years. The great weight of sediment buries older source rocks to maturity depth, generating oil and gas that migrates up into structural and stratigraphic traps in the overlying sedimentary wedge. The Norwegian, UK, and Nigerian offshore provinces are all passive margin settings.
- Foreland basins develop on the cratonic side of a continent-continent collision belt, as the lithosphere flexes downward under the weight of the mountains being thrust up on one side. The Western Canada Sedimentary Basin east of the Foothills is a Laramide-age foreland basin, where the weight of the Rocky Mountain fold-and-thrust belt depressed the crust of the Alberta platform and allowed thick Cretaceous marine sediments to accumulate. The oil sands of the Athabasca region and the conventional oil fields of central Alberta all owe their existence to the foreland basin created by the Laramide orogeny.
- Fold-and-thrust belts (convergent zones where one plate overrides another) generate compressional structural traps: anticlines above thrust ramps and fault-bend folds are the targets of conventional oil exploration in the Foothills of Alberta, the Zagros fold belt of Iran and Iraq, the Andean foothills of Colombia and Peru, and the sub-Himalayan thrust belt. Some of the world's largest oil fields (Kirkuk in Iraq, Ghawar in Saudi Arabia, at least in part) are in compressional structures related to tectonic convergence.
- Transform margins (formed where plates slide past each other, like offshore California) can develop pull-apart basins in restraining and releasing bends along the transform. These small, deep basins rapidly subside and accumulate thick organic-rich sediments that mature quickly. The Los Angeles Basin and Ventura Basin of California, small but prolific oil producers, are pull-apart basins associated with the San Andreas transform system.
How Plate Tectonics Controls Petroleum Provinces
Most of the world's oil and gas is found along the edges of the major continents, in basins that formed when continents rifted apart to create new ocean basins and passive margins, or where ancient oceans closed and continents collided to create mountain belts and foreland basins. This is not coincidence. Plate tectonics controls the location, timing, and character of sedimentary basins, and sedimentary basins are where petroleum forms and accumulates.
The Atlantic Ocean opened as North America, South America, Europe, and Africa pulled apart over the past 200 million years. As the continents drifted away from each other, the rift margins accumulated thick wedges of sediment. Organic matter was buried in the deep, warm, oxygen-limited rift lakes and early marine environments. As subsidence and burial continued, these organic-rich intervals were heated to oil and gas generation temperatures. The mature source rock fluids migrated upward and laterally, filling structural and stratigraphic traps in the overlying sedimentary section. Today, the passive margins of the Atlantic (the North Sea, the Gulf of Mexico, the West African and Brazilian margins, the Norwegian and UK shelves) host the major oil and gas provinces of the Western world.
The Middle East oil province is a product of the closure of the ancient Tethys Ocean and the collision of the Arabian plate with the Eurasian plate. The Tethys was a warm, shallow sea throughout much of the Mesozoic, ideal for carbonate platform deposition and the accumulation of thick organic-rich source rocks (the Jurassic Hanifa, Tuwaiq, and Silurian Qusaiba source rocks). When the Arabian plate began converging with Eurasia, the Zagros fold belt developed along the collision zone, creating an enormous series of compressional anticlines (Kirkuk, Ghawar, Rumalia, Safaniya) that trapped the oil generated by the Tethyan source rocks.
Fast Facts
Alfred Wegener proposed the theory of continental drift in 1912, based on the fit of the continents across the Atlantic and the matching of fossil assemblages on separated continents. His theory was rejected by most geologists for decades because no mechanism for moving continents was known. The discovery of seafloor spreading at mid-ocean ridges in the 1950s and early 1960s (Harry Hess, Robert Dietz) and the measurement of magnetic anomaly stripes on the ocean floor (Vine and Matthews, 1963) provided the physical mechanism and the kinematic record that Wegener lacked. By the late 1960s, plate tectonics was accepted as the unifying framework of geology. The impact on petroleum exploration was immediate: basin types could now be predicted by their plate tectonic setting, and petroleum systems analysis became a global discipline.
The Western Canada Sedimentary Basin as a Plate Tectonic Product
The Western Canada Sedimentary Basin (WCSB) is one of the world's great petroleum provinces. It owes its existence to two plate tectonic events: the passive margin phase (when the western edge of North America was a rifted, sediment-receiving margin from the Cambrian through early Jurassic) and the foreland basin phase (when the Laramide orogeny created the Rocky Mountains and the adjacent foredeep basin from roughly 80 to 50 million years ago).
During the passive margin phase, thick Paleozoic carbonate platforms accumulated on the western margin of the craton, including the Devonian reef complexes (Leduc, Swan Hills, Nisku, Wabamun formations) that are now prolific oil reservoirs in Alberta. These carbonates were buried, tilted, and faulted during the Laramide orogeny, creating the structural configuration of the Alberta Basin today.
The Laramide orogeny generated the thrust faults of the Foothills fold-and-thrust belt and loaded the crust with the weight of the mountain belt, causing the foreland basin to subside eastward into central Alberta and Saskatchewan. The subsiding foreland basin was flooded by the Western Interior Seaway, which deposited the Cretaceous marine and paralic sequences (Colorado Group, Belly River, Edmonton) that host the Viking, Cardium, and Peace River sandstone reservoirs. The foreland loading and subsidence also buried Devonian carbonate source rocks to oil generation temperatures across the southern Alberta Deep Basin.
Synonyms and Related Terminology
Plate tectonics is also called continental drift theory (an older term, now largely superseded) or lithospheric plate theory. Related terms include rift basin (a sedimentary basin that forms where the crust is pulled apart at a divergent plate boundary or continental rift; rifts create the subsidence and warm conditions favorable for source rock deposition), passive margin (a continental margin that is no longer tectonically active; formed after rifting, characterized by a thick sedimentary wedge and prolific petroleum systems; examples include the Norwegian margin, the US Gulf Coast, and the Brazilian margin), foreland basin (a sedimentary basin that forms on the craton side of a mountain belt due to flexural subsidence under the weight of the mountains; the Western Canada Sedimentary Basin east of the Foothills is a classic foreland basin), fold-and-thrust belt (a compressional tectonic setting where rock is folded and faulted by lateral stress during plate convergence; structural traps in thrust-related anticlines are important petroleum targets worldwide), and petroleum system (the set of geological elements (source rock, reservoir, seal, trap) and processes (generation, migration, accumulation) that must work together to create an oil or gas accumulation; petroleum systems are controlled at the largest scale by plate tectonic setting).
How Plate Tectonics Predicted a New Petroleum Province Offshore West Africa
In the early 1970s, a geologist at Gulf Oil Corporation (now Chevron) was studying the plate tectonic history of the South Atlantic. The Atlantic Ocean had opened beginning approximately 130 million years ago, as South America and Africa rifted apart. Before the opening, the area that is now the Niger Delta and the deep Gulf of Guinea was an active rift system, with rapidly subsiding basins accumulating organic-rich lacustrine sediments in an isolated, restricted environment.
The geologist mapped the kinematic history of the South Atlantic opening using magnetic anomaly data and calculated when the rift transition to passive margin occurred along the West African margin. He predicted that the synrift sediments (deposited before the transition) would be buried to oil generation depth along a specific band of the offshore margin, and that the post-rift (passive margin) sediments would provide the reservoir and trap framework above the synrift source rocks.
The prediction pointed to the deep water offshore Nigeria and Angola as having a major petroleum system that had not yet been explored, because at the time, offshore drilling was limited to water depths below 200 metres. As deepwater drilling technology advanced through the 1980s and 1990s, exploration confirmed the prediction: the Bonga, Girassol, Dalia, and Agbami fields, all in 300 to 1,500 metres of water, confirmed one of the world's most productive petroleum provinces. Total deepwater West Africa resources now exceed 30 billion barrels of oil equivalent. The plate tectonic prediction, made decades before drilling was possible, guided exploration toward one of the 20th century's major petroleum discoveries.