of methane gas, the majority of geoscientists consider oil and gas found in sedimentary basins to originate from source rocks. Without these, all the other components and processes needed to form hydrocarbons become irrelevant. Te source rock’s hydrocarbon-generating potential is directly related to its volume (thickness times areal extent), organic richness and thermal maturity (exposure to heat over time).


In the Earth’s history, most rich source rocks were deposited


during six time periods that favoured their deposition and preservation, starting with the Silurian (444–416 million years ago, or Ma), including the above mentioned Late Jurassic (165–145 Ma) up to the Oligocene–Miocene period (34–5 Ma) (see Figure 1.4). Since oil and gas are relatively low density compared


Figure 1.4: The geological time scale is used by earth scientists to describe the timing and relationships between events that have occurred throughout the 4.6 billion years of Earth’s history. It is organised into intervals based on important changes that have been seen in the geologic record. The largest defined unit of time is the eon. Eons then are divided into eras, which in turn are divided into periods, epochs and ages. Eras, periods and epochs are usually named after places on Earth where the rocks of those times were first discovered. More specifically, eras or ‘chapters’ in Earth’s history are separated according to the nature of life they contained. Eras begin and end with dramatic changes in the types of animals and plants living on Earth. Periods are based upon the nature of the rocks and fossils found there.


Life on Earth started 3.5 billion years ago. During the Cambrian period of the Palaeozoic era, shellfishes and corals developed. In the Silurian period, around 435 million years ago, the first land plants, swamp trees and primitive reptiles evolved, and around 225 million years ago during the Triassic period the dinosaurs came on the scene. The Jurassic saw the first mammals and birds. Around 2 million years ago, in the Pliocene, Homo habilis appeared – the first human species to be given the genus name Homo, meaning ‘man’. Because of the geological record we know that continental drift – the movement of the plates on the earth’s crust – resulted in the current position of the continents. Further, we know that the geological record contains evidence of several extinctions, including the Cretaceous–Paleogene (K–Pg) extinction event that marks the end of the Cretaceous and the beginning of the Paleogene 65 million years ago, when more than half of the earth’s species were obliterated, including the dinosaurs. Scientists have two main hypotheses to possibly explain this extinction: an extra-terrestrial impact, such as a huge meteorite or an asteroid, or a massive bout of volcanism. A layer of rock rich in the metal iridium dated to the extinction event is found all over the world, on land and in the oceans. Iridium is rare on Earth but is commonly found in meteorites. Later in this chapter you will read the story of the Chicxulub crater, found during geophysical surveying over Mexico’s Yucatán Peninsula, and dated to 65 million years ago.


First horses


First whales


First monkeys


First apes


First hominids


modern humans


First


Palaeocene million years First dinosaurs


First land plants First insects


First shellfish & corals


Cambrian million years 542


First tetrapods First reptiles


First fish


First mammal- like reptiles


Ordo- vician SilurianDevonian Carboni-


488 444 416 359 First mammals


First birds


dinosaurs


First flowering plants


ferous Permian Triassic Jurassic Cretaceous 145.5


299 251 200


Paleo- gene


65.5 Last 65.5 56


Eocene 34


Oligocene 23


Miocene


Pliocene 5


1.8


Neo- gene


0


2,000 4


1,000 million years


Paleo- zoic


Meso- zoic


0


Pleistocene / Holocene


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