Lecture 4 ERSC 5051 Petroleum Geology Source Rocks Spring 2012

SPRING 2012

ERSC 4 5051 PETROLEUM GEOLOGY

SOURCE ROCKS - A

Definition Of Source Rock Carbon Cycle

Types Of Organic Matters Chemistry Of Organic Matter Deposition Of Organic Matter Maturation Of Organic Matter

Source rocks are any rocks in which sufficient organic matter to form petroleum has been accumulated,

preserved, and thermally matured. Organic particles are usually fine-grained, and will settle out most easily in

quiet-water environments. Therefore, source rocks are most commonly fine-grained rocks, particularly shales.

Other potential sources are fine-grained carbonates (lime mud), mud-carbonate mixtures (marl), or coal.

Definition Of Source Rock

Carbon Cycle and Organic Matter Production

The carbon cycle is the cycling of carbon from the inorganic

carbon dioxide reservoir of the atmosphere and hydrosphere,

through life processes (reduction) into organic

materials, and then back again through degradation processes

(oxidation) into the carbon dioxide reservoir organic matter distribution and productivity in

the organic carbon cycle.

ORGANIC MATTER Carbonaceous material generated by life process.

For geochemical issues, total organic matter (TOM) content is measured by determining the biotic carbon present in a sample and then multiplying that

weight percent by 1.1 to 1.2 to adjust for other elements (e.g., H, N, S, 0), characteristically incorporated in organic compounds.

Origin Of Organic Matter

Organic matter may be ALLOCHTHONOUS (derived, detrital, washed in) or AUTOCHTHONOUS (produced in the

depositional environment).

Allochthonous Organic Matter • Terrestrial plant and animal debris

• Spores and pollen (aeolian or waterborne) • Recycled (old) kerogen from sedimentary rocks

Autochthonous Organic Matter • Phytoplankton (algae, diatoms, etc.) – producers by

photosynthesis • Zooplankton (foraminifera, etc.)

• Fish (nekton) • Benthos (corals, sponges, etc.)

• Bacteria

Production and Accumulation of Organic Matter

Most oil is biological in origin and derived from organic matter in sediments.

Marine organic matter is formed in the photic zone by phytoplankton (primary producers) that fix carbon through photosynthesis.

The highest productivity occurs in the uppermost 50 m of the ocean, declining with depth as sunlight penetration decreases.

Most organic matter [C] fixed by photosynthesis in upper 100–150 m is recycled in the water column by passing through the food chain.

Phytoplankton (diatoms, algae: primary producers of OM) are oxidised or eaten by zooplankton.

Both types of plankton are then consumed by other higher organisms. They defecate, producing pellets that contain the indigestible part of

the organic matter. The pellets sink relatively quickly to the bottom, whereas plankton are

commonly degraded in the water column. The organic matter that arrives on the ocean (or lake) floor can then be

consumed by benthic organisms. Only a few percent of the organic matter produced is buried in

sediments, especially in the deepest parts of the oceans. High organic productivity in the oceans depends mainly on adequate

sunlight (for photosynthesis) and availability of nutrients. In surface waters, sunlight generally is not a limiting factor except

seasonally (winter) at high latitudes.

Production And Accumulation Of Organic Matter

Nutrients (mainly N and P) have a very heterogeneous distribution in marine waters.

The highest concentrations are commonly found in coastal regions, where they are land-derived (e.g., soil erosion with leaching to rivers),

and in zones of upwelling. Upwellings are present mainly on the western margins of the continents

(e.g., offshore Peru, Chile, Namibia, etc.), and in areas of oceanic divergence, as for example in the equatorial Pacific.

In polar regions, cold oxygen and nutrient-rich water sinks to great depths and flow slowly toward low latitudes.

In areas with strong prevailing land winds, that cold water may well up to the surface.

The nutrients stimulate phytoplankton growth that, in turn, sustains an abundance of zooplankton, fish, etc. At such locations, above average

quantities of organic matter may reach the ocean floor. On the ocean floor, organic matter will be degraded by microorganisms

(mainly bacteria) and consumed by burrowing organisms.

Production And Accumulation Of Organic Matter

The organisms reduce the organic content of the sediments because most of the organic matter is digested.

Bioturbation may stir up the sediments and allow exposure to oxygen-bearing bottom water.

If the water is stagnant, with little (dysaerobic or suboxic) or no (anaerobic) oxygen, more organic matter can be preserved.

Production And Accumulation Of Organic Matter

Production and Accumulation of Organic Matter

Production and Accumulation of Organic Matter

Production and Accumulation of Organic Matter

Production and Accumulation of Organic Matter

Production and Accumulation of Organic Matter

Chemistry of Organic Matter

The main components of living organisms:

Carbohydrates • Sugar chains, • Rapidly break down; unstable, • Include cellulose

Lignin • Walls of higher plants, • Very resistant to decay, • Contains

aromatic rings Tannin,

• Contains aromatic rings Proteins

• Amino-acid polymers, • Contain most N-compounds in organic matter, • Rapidly decomposed, • Include enzymes, hemoglobin, structural

components of shells, corals, sponges, etc. Lipids

• Animal fats and vegetable oils, • Insoluble in water, • Include spores, fruit, muscle, waxes, etc., • Some contain paraffin chains.

Plant and animal pigments Essential oils

DEPOSITION OF ORGANIC MATTER IN DIFFERENT ENVIRONMENTS

DESERTS (< 0.05% OM) • Waxy organic matter

• Almost all converted to CO2 and H2O • Almost no source-rock potential (but sandstones in deserts may have

high reservoir potential)

ABYSSAL OCEAN PLAINS (< 0.1% OM) • Pelagic muds and oozes

• Oozes may be calcareous (e.g., from coccoliths, foraminifera) or siliceous (e.g., from diatoms, radiolaria)

• In the deepest, central parts of the oceans, bottom waters are undersaturated with respect to CaCO3 and amorphous silica: oozes

cannot form (shells dissolve); only detrital clays can accumulate • Most OM produced is consumed in water column and recycled

OM that sinks through the water column to reach the ocean floor may then be consumed by benthic organisms

• Fecal pellets allow rapid delivery of OM to the seabed • Nutrients are not abundant in the central part of the oceans, so

primary productivity is often low

HIGH ENERGY COASTS (0.2–0.5% OM) • Adequate productivity – nutrients often supplied from the land;

abundant oxygen • Waves and currents may produce coarse sediments

• High oxygenation of the permeable sediment can lead to early biodegradation (biological breakdown of organic matter to CO2 and

water

LOW ENERGY COASTS (0.5–5% OM) • High productivity

• Muds or carbonate muds deposited • Can produce good source material if rate of biogenic decay of OM is

not too high

DEPOSITION OF ORGANIC MATTER IN DIFFERENT ENVIRONMENTS

DISTAL FLOODPLAINS AND DELTAS (0.5 – > 10% OM) • Mainly clay sedimentation

• Organic matter is mainly terrestrial • Yields much coal and gas, but little oil

SILLED BASINS, ENCLOSED SEAS (< 2 – > 10% OM) • High productivity

• Clays • Often anoxic

• Can produce highly favorable source rocks

EPEIRIC (EPICONTINENTAL) SEAS (< 1 - > 10%) • Muddy sediments

• Can be very favourable if circulation is restricted

DEPOSITION OF ORGANIC MATTER IN DIFFERENT ENVIRONMENTS

LAKES, COASTAL LAGOONS (< 1 - > 10%) Favourable if:

• Low clastic input • Clay sedimentation

• Stratified waters • Most are not stratified

• May be eutrophic (algal blooms)

COASTAL SWAMPS (10 – 100%) • High vegetation; stagnant

• Peat produced (coal + methane)

DEPOSITION OF ORGANIC MATTER IN DIFFERENT ENVIRONMENTS

MATURATION Evolution Of Organic Matter To Kerogen

DIAGENESIS of organic matter leads from BIOPOLYMERS synthesized by

organisms through “humin” to KEROGEN, a GEOPOLYMER, by partial destruction and rearrangement of the

main organic building blocks as:

Between modern organisms and

recent sediments, the main changes

are:

• A large decrease in carbohydrates

• An increase in lignin-humus, and

N-compounds

Early changes caused by chemical

and microbial reactions hydrolyze

some OM to sugars and simple

molecules that polymerize to form

lignin-humus and nitrogenous

compounds that are the precursors

of KEROGEN.

MATURATION Evolution Of Organic Matter To Kerogen

MATURATION Evolution Of Organic Matter To Kerogen

MATURATION Evolution Of Organic Matter To Kerogen

MATURATION Evolution Of Organic Matter To Kerogen

MATURATION Evolution Of Organic Matter To Kerogen

What happens when we subject kerogen to subsurface conditions?

KEROGEN

Diagenesis

Catagenesis

Metagenesis

Shallow subsurface Normal pressure and temperature

Released: CH4, CO2, H2O • Overall decrease in O

• Overall increase in H and C

Deeper subsurface Increased pressure and temperature

Released: oil & gas • Overall decrease in H and C

Metamorphism High temperature and pressure

Only C remains: becomes graphite