petroleum geology - origin of petroleum

JAMES A. CRAIG

Formation

Migration

Accumulation

ORIGIN OF PETROLEUM

To be considered are these 3 stages:

Formation of Petroleum Migration of Petroleum Accumulation of Petroleum

Formation of Petroleum

There are 2 theories concerning the formation of petroleum:

The Inorganic theory The Organic theory

Inorganic Theory

Berthelot (1860) & Dmitri Mendeleev (1902): Iron carbide in the earth’s mantle would react with

percolating water to form methane. FeC2 + 2H2O → CH4 + FeO2

This theory is called Deep-Seated Terrestrial Hypothesis.

Sokoloff (1890): Hydrocarbons precipitated as rain from the

original nebular matter from which solar system was formed.

The hydrocarbons were the ejected from earth’s interior onto surface rocks.

This theory is called Extraterrestrial Hypothesis

Problems with Inorganic Theories : No field evidence that inorganic processes have

occurred in nature. Commercial accumulations are restricted to

mainly sedimentary basins. Accumulations are absent from igneous and

metamorphic rocks.

Organic Theory Early marine life forms living on earth were

primarily plankton (rich in hydrogen & carbon) Over 95% of living matter in the ocean is

plankton.

As these plankton died, their remains were captured by the process of erosion and sedimentation.

Successive layers of organic-rich mud & silt covered preceding layers of organic-rich sediments & overtime created layers on the sea floor rich in the fossil remains of previous life.

Thermal maturation processes (decay, heat, pressure) slowly converted the organic matter into oil & gas over periods of millions of geologic years.

Conversion of the organic material is called Catagenesis.

It usually occurs under anaerobic conditions.

Clay & silt are carried together with the dead organic remains & deposited under deltaic, lacustrine & marine conditions to form Source rocks.

Black-coloured, organically-rich shales deposited in a non-oxidizing, quiet marine environment are considered the best source rocks.

Shales

Shale rock = 99% clay mineral + 1% organic material.

Kerogen Insoluble High molecular

weight Polymeric

compound

Thermal alteration of kerogen forms crude oil by increasing the carbon contents.

At shallow depths (< 3,000 ft), bacteria actions on organic materials form Biogenic Gas (natural gas).

At great depths (high temperature & overburden), Thermogenic Gas is formed.

Later stages of thermogenesis will form wet gas and condensate.

Hydrocarbon maturation

Supports for organic hypothesis: Carbon & hydrogen are the primary constituents

of organic material, both plant & animal. Nitrogen & Porphyrins are found in organic

matter & in many petroleum. Porphyrins are chlorophyll derivatives in plants &

blood derivatives in animals.

Migration of Petroleum

Produced hydrocarbons migrates upward from the deeper, hotter parts of the basin through permeable strata into suitable structures.

There are 2 stages of migration: Primary Migration – Kerogen transformation

causes micro-fracturing of the impermeable & low porosity source rock, allowing hydrocarbons to move into more permeable strata.

Secondary Migration – The generated fluids move more freely along bedding planes and faults into a suitable reservoir structure.

Migration can occur over several tens of kilometers in lateral directions.

Accumulation of Petroleum

Accumulation & storage occur when the migrating fluids encounter an impermeable shale or dense layer of rock.

This is called a Trap. After accumulation, the fluids tend to stratify

according to their relative densities: Gas Oil Water

If the migrating fluids do not encounter a trap, they tend to flow to the surface or deposited on the ocean floor.

Examples are: Seepages Escaping natural gas Bituminous lakes

Reservoirs are composed of either: Clastic formation – sandstone reservoirs made

from silicates (quartz, SiO2). Carbonate formation – carbonate reservoirs

made from detritus (coral or shell fragments). Reservoirs must be:

Porous Permeable Trapped.

Types of Traps

Anticlinal Traps Result of ductile crustal deformations.

HCWC

Fault Traps Result of brittle crustal deformations.

HCWC

Stratigraphic Traps Impermeable strata seal the reservoir.

HCWC

Salt Dome Traps

Combination Traps Fault bound anticlinal structures, i.e. combination of anticlinal & fault traps. Many global reservoirs have this type.