WATCHING THE WORLD FLESHING OUT GEOLOGIC AGES

With David Knott from London Parents and grandparents throughout the world have been subjected recently to intense questioning about dinosaurs, thanks to the film "Jurassic Park." Trying to answer those questions helps bring to life some of the descriptions used by geologists because many oil bearing formations were being laid down when dinosaurs flourished. Many oil reservoirs probably started to take shape in Triassic, Jurassic, and Cretaceous periods, which coincided to a great extent with

Aug. 9, 1993

3 min read

Parents and grandparents throughout the world have been subjected recently to intense questioning about dinosaurs, thanks to the film "Jurassic Park."

Trying to answer those questions helps bring to life some of the descriptions used by geologists because many oil bearing formations were being laid down when dinosaurs flourished.

Many oil reservoirs probably started to take shape in Triassic, Jurassic, and Cretaceous periods, which coincided to a great extent with the age of the dinosaurs.

Dinosaurs lived in the Mesozoic era 245-63 million years ago, according to London's Natural History Museum. The first part of the Mesozoic age, the Triassic, lasted from 245 million to 210 million years ago. The climate was temperate. Much of the swampier land was covered with giant horsetail ferns, while tree ferns and tall conifers grew in dry areas.

TRIASSIC

Reptiles rather than dinosaurs dominated the Triassic, There was a great variety of marine reptiles. Onshore, Dimetrodon, a huge lizard with a sail-like fin down its back, is perhaps the best known.

True dinosaurs took over in the Jurassic era, from 210 million to 145 million years ago. Then the land was still,covered by ferns, but tall conifers such as sequoia and monkey puzzle had appeared. The weather was hotter and steamier than in the Triassic, like today's tropical climate.

Apatosaurus, which used to be known as Brontosaurus, is perhaps the archetypal dinosaur, with its huge body, long neck and tail, and relatively tiny head. It lived 154-145 million years ago. Biggest predator of the Jurassic age was Allosaurus, which looked like a smaller version of Tyrannosaurus.

Tyrannosaurus Rex arrived on the scene after Apatosaurus had died out, living 67-65 million years ago. A favorite meal for Tyrannosaurus probably was Triceratops, the three-horned dinosaur that looked like a giant rhinoceros with a stiff collar.

RESERVOIRS

During the Cretaceous period, 145-65 million years ago, the first flowering trees and shrubs appeared, including oak and beech. About 65 million years ago, dinosaurs died out.

For most of the Mesozoic period the North Sea was landlocked, containing many shallow, stagnant lakes. Toward the end of the Cretaceous, about 70 million years ago, the area was flooded by the ocean.

During the Tertiary period that followed, sandstones and shales were deposited above the remains of the. plants and animals that eventually would decompose into oil.

Those porous rocks became reservoirs as oil migrated upward. Among the more famous North Sea formations, Brent sands are from the middle Jurassic, Statfjord sands are lower Jurassic, while Ekofisk is upper Cretaceous.