Sedimentary Basins and Geological Time
How basins form and fill, how rock layers record time, and why basin history controls where petroleum is found.
11 min
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Where petroleum lives
Almost all oil and gas is found in sedimentary basins: large areas of the earth's crust that sank over millions of years and filled with layers of sediment. Sand, mud and the remains of organisms were buried, compacted and turned into rock. The same burial that created reservoir rocks also heated source rocks and generated hydrocarbons. To understand a field, you start by understanding the basin it sits in.
Main basin types
Reading time in the rocks
Layers of sedimentary rock are generally younger towards the top, unless later folding or faulting has disturbed them. Geologists divide time into named periods, such as the Jurassic and Cretaceous, using fossils and other methods to date rocks. An unconformity is a surface where time is missing because rock was eroded or never deposited. Unconformities matter because they can cut across reservoirs and seal them, or open migration paths.
Burial history describes how deep each layer has been through time. It controls when a source rock entered the oil window, which you met earlier in the course, and whether traps already existed then.
Two basin examples
A basin formed by rifting has a rich source shale deposited in a deep lake during the active rifting stage. Tilted fault blocks hold sandstone reservoirs, sealed by later marine shale. Most fields are fault-block traps along the rift edges. Explorers in this basin focus on fault-block crests and on whether faults seal.
Match the basin type to its typical traps
- Rift basin
- Foreland basin
- Passive margin
- Intracratonic basin
Why does burial history matter for exploration?