We all know, that petroleum system element are consist of source rocks, reservoir rocks, seal, trap, and proper timing of migration. So, which one is the most important element of petroleum system? Actually every single part of it has their own role, but source rock is the most important element that we have concern to. As the definition, source rock is rock that generated and expelled hydrocarbons. Rocks that probably could be Source Rocks are: Shale (rich organic matter), marl, carbonate, etc.
Source Rock Maturity and Capacity
After we found souce rock, we have to know the maturity of it and what kind of fluid that source rock generate, is it oil or gas. Thermal maturation of source rock is around 125o-170oC. But, the oil window formed at around 60o-120oC. The source rock maturity can be looked by amount of it’s vitrinty reflectance. Diagenesis (0,5-0,7), katagenesis of oil window (0,7-1,3), katagenesis of gas (1,3-2,0), and metagenesis (>2,0). The other way to see the maturity of source rock are using thermal alteration index and fixed carbon ratio.
When we found source rock, we have to calculate its total organic carbon with Leco. But, we still have to be careful when the total organic carbon that we found is more than 10%, why? Because maximum amount of hydrocarbon is around 10%, so if we found its more than 10% it might be become coal or has been on coaling process. After we calculate the total organic carbon we do pyrolisis. Pyrolisis is heating rocks until the oil expelled.
So, we have three main things that we have to do after founding source rock. There are calculating or counting the total organic carbon (TOC), if less than 0,5% TOC just throw it, stop the exploration. After that we have to do pyrolisis to expel the oil. And the last is counting the Tmax of the source rock, then we can see from gamma ray table that exploration profit enough to continue.
We can conclude that after we found the source rock, the first thing we’ve to know is “Is the source rock mature?”