Geologic time and sedimentary basins

The timescale, in broad strokes

Earth is about 4.54 billion years old. Geologists divide time into eons (billions of years), eras (hundreds of millions), periods (tens of millions), and finer subdivisions. For most exploration-scale work, three eras cover almost everything:

• Paleozoic — roughly 541 to 252 Ma (million years ago). Includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Many of the world’s great carbonate reservoirs (Middle East, parts of North America) are Paleozoic.
• Mesozoic — 252 to 66 Ma. Triassic, Jurassic, Cretaceous. The North Sea, Gulf of Mexico deep plays, and West African margin have important Mesozoic sections. The Cretaceous in particular was a time of warm climate and widespread carbonate and chalk deposition.
• Cenozoic — 66 Ma to present. Paleogene, Neogene, Quaternary. Much of the producing Gulf of Mexico, the Niger Delta, offshore Brazil, and the California basins are Cenozoic clastic systems.

You do not need to memorize the full chart, but you do need to know: an "Upper Cretaceous sand" is roughly 85–66 Ma old; a "Paleocene shale" is roughly 66–56 Ma old. The boundaries often correspond to real, mappable surfaces in the subsurface.

The principle of superposition

Sediments deposit from the bottom up. Within an undeformed sequence, the deepest layer is the oldest and the shallowest is the youngest. This sounds obvious but it is the fundamental tool for ordering events on a seismic section. A fault that cuts the blue reflector and stops below the green reflector must be younger than the blue and older than the green.

Unconformities: surfaces where time is missing

When deposition stops — either because the sediment supply runs out, or because the basin gets uplifted and erosion starts removing the top layers — a gap opens in the rock record. When deposition resumes later, the new sediment sits on top of older sediment with a gap in age between them. The surface that separates them is an unconformity. Three kinds are worth knowing:

• Angular unconformity — the older layers were tilted or folded, then eroded flat, then overlain by younger flat-lying beds. The unconformity surface truncates the older dipping beds and the younger flat ones onlap it. This is the easiest kind to recognize on seismic.
• Disconformity — both sets of beds are parallel, but a time gap exists in the middle. Often harder to see on seismic; often inferred from biostratigraphy in a well.
• Nonconformity — sedimentary beds lying on top of an older eroded crystalline basement (granite, metamorphic rock). The basement appears as chaotic, non-reflective seismic because it has no layering.

Unconformities matter for interpretation for two reasons. First, they are often the most prominent reflectors in a section and make excellent regional markers. Second, they can be reservoir traps themselves — sediment that onlaps an unconformity may create a stratigraphic trap, a class of exploration target we will discuss in Part 4.