[Notes] Petroleum Geology Part 6

Oil & Gas

- comes from organic matter (OM) through transformation involving heat and geologic time

Source Rocks

- capable of generating oil & gas

- must be:

        * rich in organic content

        * mature enough to expel the oil or gas

Organic Matter

- material composed of organic molecules

- all organic matter was originally atmospheric CO2

3.1 -3.3 BYA - oldest form of organic life

3.7 - 4 BYA - oldest know rocks

Pre-Cambrian to Devonian - marine phytoplankton

Devonian - increasing amount from terrestrial sources

4 Contributors of OM

1. Phytoplankton

2. Zooplanktons

3. Higher plants

4. Bacteria

Sequence of OM

- Production

- Accumulation

- Preservation

Production of OM

- controlled by LIGHT, TEMPERATURE & CHEMICAL COMPOSITION of sea water

- 60 to 80 meters (large part of biological production)

- coastal waters 2x as that of open seas productivity

Accumulation & Preservation of OM

- restricted to an aquatic environment

- to high energy level causes erosion & high sed rate

- to low, little sed supplied to bury the OM

- anoxic environment

- fine-grained more favorable

- continental shelf: lagoons, estuaries and deep basins