For geoscientists, the Gulf of Mexico is particularly interesting, as it is the main offshore oil and gas province for the United States and Mexico, and much of what we know about the GOM’s basin and seafloor comes mainly from oil and gas exploration over the past century. Let’s look at some recent studies surrounding the GOM.
Rasoul Sorkhabi
Editor, Core Elements
The Age of the GOM Basement
Joao Cachapa/ Shutterstock.com
One of the lesser-known aspects of the GOM is the nature and age of its basement rocks. A recent paper in Geosphere sheds light on this topic.
Deepwater Raptor Well: In 2013, Anadarko Petroleum (later acquired by Occidental) and BHP Billiton (now merged with Woodside) drilled in the Raptor Prospect in Desoto Canyon Block 535 at a water depth of 8,200 feet. Well 535 No. 1 Raptor was drilled into a structural high (Middle Ground Arch) targeting the Upper Jurassic Norphlet Formation. The well was dry, but drilling continued into the Louann Salt, below which the borehole encountered igneous rocks.
Why it matters: Igneous rocks are not oil and gas prospects, but they provide a lot of information about rocks underlying petroleum basins and are particularly valuable for offshore basins.
New research: Jillian Mallis of Occidental and her academic colleagues in Houston have utilized this opportunity to study the lithology and age of the basement rock at Well 535 No. 1 Raptor at subsea depth of 22,004 feet.
Major findings:
The basement rock is a reddish crystal-rich volcanic tuff. Geochemical analysis shows it to be calc-alkaline trachyte, including monzonite fragments.
The researchers also used zircon U-Pb geochronology to determine the age of the rock to be 509 Ma.
Tectonic setting:
The reported age resembles 50 other radiometric ages previously reported from wells that encountered basement rocks in onshore Florida and the West Florida Shelf of the GOM.
Unlike previous reports, this new age comes from a deepwater well.
All of the reported 412–709 Ma (with a cluster at 500 Ma) from igneous rock samples belong to the Suwannee terrane of Gondwana supercontinent.
The Suwannee terrane formed by subduction-related magmatism during the Cambrian before this terrane became a passive continental margin.
Above the GOM basement lies a distinct layer of Jurassic-age salt and anhydrite deposits called the Louann Formation in the U.S. sector and Campeche Salt in the Mexico sector. The salt has obscured the nature and structure of deep GOM crust. Two recent studies have utilized seismic images to shed light on the crustal architecture of the GOM.
GOM Basin formation: The GOM basin formed through three major phases:
Phase 1 (Late Triassic to Middle Jurassic) involved NW-SE oriented continental rifting related to Pangea fragmentation. The rift basins were filled with evaporitic, fluvial, lacustrine sediments.
Phase 2 (Middle Jurassic to Late Cretaceous) created oceanic crust of 162–140 Ma as Yucatan terrane separated from North America and moved southward in a counterclockwise rotation.
In phase 3, after the ocean floor spreading ceased, the GOM became a passive margin that is filled with sediments shed from the coasts of the United States and Mexico.
Data sets for new studies:
Curry and colleagues, published in Tectonics, used two seismic lines in offshore Florida and two lines on the Mexico side. They supplemented these 2D lines with 3D images, providing detailed information on supra-salt sediments and structures.
Vasileiou and colleagues, published in Basin Research, used nine 2D seismic reflection lines from the eastern region of the GOM. Their use of gravity and magnetic data enhance their geophysical interpretation.
Major findings:
Curry and colleagues’ U.S. seismic aligned with those on Mexico side. Their structural construction includes continental crust separated from the thin oceanic crust by a wedge of exhumed mantle or lower continental crust. Rift-basin and passive-margin-filled sediments cover the basement variably depending on local structural style.
Vasileiou and colleagues also construct hyper-extended continental crust in offshore Florida filled with rift-basin and passive-margin sediments. In one of their seismic sections, they also report a patch of seaward dipping reflection sequence (SDRs)—probably volcanic flows—similar to those reported from ultradeep waters in offshore Namibia. The interpreted GOM SDRs are below the Jurassic salt but sit atop the continental crust.
Based on stratigraphic position, both articles consider a Bajocian-Bathonian age (170–165 Ma) for the Louann salt.
Curry and colleagues suggest a relatively deeper water environment for Louann salt and report that extension-shortening of supra-salt sediments is not balanced. The extension exceeds shortening by 18–30 kilometers.
Why it matters: As the industry moves to high-cost, high-reward ultra deepwater ventures, knowledge of basement-basin architecture close to the ocean floor and rift-to-drift basin evolution become critical for successful oil and gas exploration.
Last week’s quiz question was: What are the second and third largest conventional oil fields in the world?
The second largest conventional oil field is the Burgan Field in Kuwait, which was discovered in 1937 with recoverable reserves of 70 billion barrels. The third position likely goes to the Gachsaran Field in southwest Iran, with recoverable reserves of 67 billion barrels. It was discovered in 1927 not far from Burgan.
Core Elements and the weekly quiz will take a break next week in the spirit of the season. Happy holidays!
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