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Source Rocks
Source rocks are subsurface sedimentary rock usually shale or limestone, they have the ability of generating or already generated movable quantities of hydrocarbons.
A rock should have following features to be a source rock:
1.Organic matter availability in sufficient amount
2.Should be able to produce movable hydrocarbons
3.Thermal maturity.
Source rocks are mainly categorized into three basic categories on the basis of kerogen they contain. They are described in the next table
- Type1 source rocks:-Formed from algal remains deposited in oxygen depleted conditions in deep lakes.These source rocks when subjected to high thermal stress during deep burial produce waxy crude oil.
- Type 2 source rocks:-These type of source rock are produced by marine planktonic and bacterial remains preserved under oxygen depleted conditions. Type 2 source rock tend to produce both oil and gas when subjected to high pressure and temperature.
- Type 3 source rocks:-These type of source rocks are mainly formed by land plants that got decomposed by bacteria and fungus .They tend to generate mostly gas and some associated light oil.
Source rock potential is determined using below table:-
Measuring the “Total organic carbon “(TOC) present in weight percent (wt %) is the most common method for determining source rock potential. Hydrocarbons are 75-95 wt % carbon by molecular weight and average 83 wt %. The organic carbon quantity present in rock determines rock’s ability to generate hydrocarbons. In below table (from Chinn,E.W ,1991) we are presenting average total organic carbon (TOC) values for various sedimentary rock .
| Rock type | TOC ( %) |
| “Shale averages” | 0.8 |
| “Shale source rock averages” | 2.2 |
| “Calcareous shale source rock averages” | 1.8 |
| “Carbonate source rock averages” | 0.7 |
| “All source rock averages” | 1.8 |
In the below table assessment guidelines to estimate source rock richness is presented
| Generation Potential | Shales TOC in Wt % | Carbonates TOC in Wt % |
| Poor | 0.0 to0.5 | 0.0 to0.2 |
| Fair | 0.5 to1.0 | 0.2 to0.5 |
| Good | 1.0 to2.0 | 0.5 to1.0 |
| Very Good | 2.0 to5.0 | 1.0 to2.0 |
| Excellent | Greater than 5.0 | Greater than 2.0 |
If the analyzed sample to find out sample richness is mature , then the current maturity is determined to find the initial (pre-maturation) value of organic carbon for the given interval. The TOC value found does not help in identifying source potential of sample