Geology of Source and Reservoir Rocks

Study Notes

Marine origin rocks are typically oil-prone, while terrestrial source rocks like coal are gas-prone.
Preservation of organic matter without degradation is essential for quality source rock.
Under certain conditions, source rocks can also function as reservoir rocks, exemplified by shale gas reservoirs.

Reservoir rocks, like sandstones, have the capacity to hold oil, similar to a sponge soaking up spills.
Sandstones are the most common reservoir rocks due to their high porosity.
Reservoir rocks can also include limestones and dolostones, often derived from ancient coral reefs.
Pore spaces within reservoir rocks allow for the movement of oil, facilitating storage and flow.

Also referred to as cap rocks, seal rocks prevent fluid migration, forming a protective layer above reservoir rocks.
Common types of seal rocks include shale, anhydrite, or salt, which are relatively impermeable.
A seal rock's permeability is generally between 10-6 to 10-8 darcies, critical for retaining fluids over geological time.

Conventional reservoirs involve traditional drilling methods for extracting oil and gas, relying on natural pressure.
Production may decline over time, prompting the use of artificial lifts or water and gas injections.
Unconventional reservoirs arise due to the increasing difficulty of extracting oil and gas with standard methods.
Unconventional drilling may include horizontal drilling and hydraulic fracturing (fracking) to access tight oil sands.

The rock cycle describes the transformation of rock types: igneous, sedimentary, and metamorphic.
Igneous rocks form from cooled magma, which can solidify slowly underground or quickly above ground as lava.
Sedimentary rocks originate from layered sediments that are compressed and cemented over time.
Metamorphic rocks result from heat and pressure transforming existing rocks into new forms.

Source Rock: Organic-rich rock, primarily shale or limestone, containing 1% to 10% organic matter that can generate oil and gas.
Reservoir Rock: Rocks, often sandstones, capable of trapping and holding hydrocarbons due to their pore spaces.
Seal Rock: Impermeable rock acting as a barrier to prevent fluids from escaping the reservoir.
Overburden Rock: The layers of rock situated above reservoir rocks, influencing pressure and temperature conditions.

Magnetic techniques are utilized to detect subsurface features.
Gravimetric surveys measure variations in Earth's gravitational field to locate resources.
Seismic methods involve generating waves to map geological structures beneath the surface.