7.3: Organic-Rich Sedimentary Rocks
Organic-rich rocks (we can arbitrarily define them as having >5% organics) have 3 main components:
1. Clastic particles - sand, silt, clay and other types of material that were transported by wind or water.
2. Humus – dark fibrous plant material found in terrestrial settings.
3. Sapropel – jelly-like ooze of plant remains (usually algae) putrefying in an anaerobic environment
Petroleum-bearing rocks
For oil and natural gas to accumulate in significant amounts, all components, conditions, and processes of a " petroleum system " must align.
Petroleum is derived from a sapropel-rich source rock that typically formed in aquatic environments where the remains of algae and plankton were able to escape decay through burial in oxygen-poor settings. As sediment continues to accumulate, the organic material is eventually transformed into kerogen , which is a waxy, insoluable organic residue. Given the microscopic nature of kerogen, geochemical analysis is necessary to know what the source was and to predict how gas or oil prone the kerogen is. The three types of kerogen include:
- Type 1 - made mostly of algal remains; likely to produce oil
- Type 2 - a mixture of terrestrial and marine organics; may produce oil
- Type 3 - mixture of woody and algal material; likely to produce natural gas
As pressures and temperatures increase with burial of the source rock , the kerogen is eventually converted to oil and/or natural gas. Peak oil generation (the "oil window") typically occurs between 75 and 150 ⁰C and peak gas production occurs at temperatures between 125 and 200 ⁰C.
Accumulation of significant volumes of oil and/or natural gas require numerous other conditions to be met. In the case of a conventional petroleum system, the hydrocarbons must migrate into a reservoir rock that is capped by a seal rock that forms part of a structural or stratigraphic trap . Unconventional systems occur where the hydrocarbons cannot be recovered through traditional vertical drilling techniques; they include tar sands, biodegraded heavy oil, tight reservoirs that have to be horizontally drilled and hydraulically fractured, and others.
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Figure \(\PageIndex{1}\): Diagram showing the approximate relationship between hydrocarbon generations, source rock type, burial depth and temperature for an area with a geothermal gradient of 25 ⁰C/km. These systems are complex and time dependent; all values and relationships are rough approximations ( Michael C. Rygel via Wikimedia Commons ; CC BY SA 4.0 ).
Coal-bearing rocks
The organic material that forms most coal comes from humus (terrestrial plant material) which, under swampy reducing conditions can be preserved as peat . By definition, coal has to be at least 50% weight percent organic material. Economic coal deposits occur in discrete seams (beds) but small fragments of plants can be coalified and preserved within other sedimentary rocks. Although we think of coal as a massive and homogeneous material, many coal seams contain layers of impurities (which turn into ash when burned) and a variety of different types of plant material ( macerals ) that can be identified with microscopic analysis and sometimes in hand sample.
In certain swampy, clastic-poor environments significant accumulations of plant material can escape decay and accumulate to form relatively thick layers of peat. With increased burial and compaction, volatiles (water and gasses) are driven out and the percentage of carbon increases and the peat can progressively transitions to lignite, bituminous coal, and eventually anthracite coal.
Given its composition, coal occurs only in Devonian or younger rocks; the majority of it accumulated in the Late Paleozoic when vast areas of continental crust were subsiding in response to mountain building and within the humid tropics.
Readings and Resources
- Petroleum System on the AAPG Wiki: https://wiki.aapg.org/Petroleum_system
- Kentucky Geologic Survey's webpage on coal: https://www.uky.edu/KGS/coal/