Category:Petroleum Geology

From Eurêka

Petroleum geology Study of the way in which petroleum forms, migrates, and may become trapped in the crust of the Earth. It also encompasses the study of the generation of organic matter on the surface of the earth, the way in which this may be buried and preserved in sediments, and the physical and chemical processes that lead to the formation of petroleum and its expulsion from sedimentary source beds.

Oil geology Occurrence of oil in sedimentary rocks of marine origin in suitable reservoirs. Requisites for recoverable petroleum are:

1. Source rock Sedimentary rock such as shale or limestone which was formed when layers of mud containing microscopic animals and plants sank to the bottom of a sea or lake. These muds were subjected to heat and pressure, causing them to solidify and form shales. This process also led to the decomposition of the organisms into compounds of hydrogen and carbon to form petroleum.
2. Reservoir rock Porous and permeable rock such as a sandstone or limestone layer, which was in contact with source rocks so it could receive petroleum that was squeezed out of the source rocks as they were compressed by overlying sediments. The reservoir rock had to be porous to provide space to store the petroleum. It also had to be permeable so that the oil and gas could move through it.
3. Trap Any barrier to the upward movement of oil or gas that allows either or both to accumulate. There are two types of traps:
   1. Structural traps Traps formed by folding and faulting or the reservoir rock layers, e.g. an anticline.
   2. Fault traps Oil trapped by an impermeable fault plane.
   3. Stratigraphic traps Traps created when layers of porous, permeable reservoir rocks are sealed off by superimposed impermeable beds.
   4. Barren traps Water-filled traps.
   5. Dry syncline trap Water-free synclines.
   6. Salt-cored traps Oil structures associated with salt intrusions which transect the stratification of the intruded sediments.
   7. Piercement salt domes

Hydrodynamic trap Inclined oil-water contact surface in which the potentiometric defines the heights to which artesian water would rice if wells are drilled into the carrier bed.

1. Reservoir seal (Seal) Layer of impermeable strata that overlies the trap and prevents the petroleum liquids from escaping.
   1. Parallel seal Where the sealing surface is parallel to the bedding planes of the reservoir rock.
   2. Transverse seal Where the sealing surface crosses the stratification of the reservoir rock.

Anticlinal theory Theory of petroleum and gas accumulation in anticlinal structures, first observed by Canadian William Logan in 1842. The first clear statement of the theory was made by Canadian T. S. Hunt 1861.

Cap rock Layer of impermeable strata sealing petroleum within a reservoir.

Sedimentary basin Part of the Earth’s crust containing a thick sequence of sedimentary rocks.

Play Rock formation or group of formations within a sedimentary basin with geological characteristics similar to those that have been proven productive. A play serves as a planning unit around which an exploration program can be constructed. A play is composed of one or two prospects.

Prospect Area that is a potential site of economically recoverable petroleum accumulation based on preliminary exploration.

Oil field Geographical area of oil-bearing rocks composed of a single pool or multiple pools that are grouped on or related to a single structure and/or stratigraphic feature.

1. Elephant field (trade term) Oil field that contains hundreds of millions of barrels of oil.

Pool (Reservoir) Formation of reservoir rock containing recoverable oil and/or gas. A petroleum reservoir usually contains gas in the upper portion, water in the lower part, and oil in the intermediate zone.

Zone Interval of a subsurface formation containing one or more reservoirs.

1. Zone isolation Method of sealing off temporarily, a producing formation while the hole is being deepened. A special substance is forced into the formation where it hardens allowing time for the well bore to be taken down. After a certain time, the substance breaks down unblocking the producing formation.

Unitization Joint development by a number of companies of an oil field which extends into territory owned by each of them.

Black elephant Oil industry jargon for a massive oil find in a new oil field, enough to encourage the oil industry to make the large investment for the production infrastructure.

Creaming theory Statistical technique which recognizes that in any exploration province after an initial period in which the largest fields are found, success rates and average field sizes decline as more exploration wells are drilled and knowledge of the area matures.