In the first part of this series, I’ve tried to explain the processes that make up the oil and gas underground. In this part, I will be trying to give information about how we determine where this oil and gas is accumulated.
The so-called magic 5 of exploration are;
1)Source Rock (OM%, kerogen type (origin of that organic matter), etc.)
2)Reservoir (porosity (Φ), permeability (k), thickness (h), etc.)
3)Heat (Oil Window temperatures)
4)CapRock (Shales, evaporites, etc.)
5)Trap (has to be ready before the accumulation)
The plate tectonics theory built on the continental drift (2-10 cm/year) theory by Alfred Wegener shows that nine major plates with many other smaller ones that make up the Earth’s surface are in constant relative motion caused by the convection in the mantle.
This heat convection causes uplifting, and subduction, basically hot material near the core rises while cold mantle rock sinks.
By paleolatitude (the north-south coordinates in the past), the zones which are thought support life and, therefore, with the potential of having today’s petroleum geologists know source rocks. Furthermore, the visible signs on the surface (seepages, stratigraphy, structures) hint the place to search for based on the principle of uniformitarianism (today’s occurrences result from the same dynamics of the past).
For example, you can see type 1 kerogen algae as green debris on the water surface or type 2 kerogen might be observed as moonlight reflecting on murky water behind a ship. Seepages are especially crucial for marine exploration activities since the sea bottom can not be seen directly with the eye.
Before the seismic tools were developed, the explorers just looked for the anticlinal traps on the surface, thinking there might also be one underground, drilled, and just hoped for the best (creekology).
Since today’s technology is far more advanced, modern tools are used to analyze and interpret the petroleum systems. Many logging devices, gravimeters, magnetometers, and seismic instruments have been developed. However, the human factor is still the essential aspect of the whole process is open to different interpretations.
An experienced, sensible petroleum geologist is priceless, and when he demands that you drill, you better be drilling. In the third part of The Tale of Oil&Gas, I will try to explain how reserves are classified and what is the current situation right now.