Petroleum Play

The petroleum play is a perception (or model) of how a specific region of the Earth’s subsurface may be an appropriate target for exploration drilling. More specifically, how a:

• producible reservoir (the rock with its connected pore or fracture system),

• petroleum charge system (the source rock for the hydrocarbons and its migration path to the subject reservoir),

• regional topseal (the capping rock preventing migration out of the reservoir), and

• trap (the geological features defining the physical limits to the reservoir rock in the subsurface)

may combine to produce significant petroleum accumulations at a specific stratigraphic level.

Depositional Environments

The grain size and sorting will depend on the physical conditions where the sands ceased to be transported and so were deposited, prior to being lithified (i.e., the depositional environment), because of:

• nearness or proximity to sediment source (coarser material is usually transported over less distance).

• nature of sediment source which is known as provenance (an eroded sandstone will generate a different sediment than a granite, both in grain size and composition).

• the energy of the depositing currents

• fluctuations in depositing current strength and direction

Key reservoir points

• In clastic reservoirs the reservoir quality is a function of grain size and sorting.

• Grain size and sorting are a function of depositional environment.

• Primary porosity generally (but not always) reduces with depth of burial.

• Porosity and permeability development in carbonates is dominated by secondary processes.

Seal

A seal is a fine-grained rock that prevents the oil migrating to the surface (which happens in many parts of the world - leading to natural oil seeps). In some situations, salt provides an effective seal but muddy, clay-rich rocks represent most seals.