I believe that the 67% heavy metal elements reduce the amount of metal deposits.
As for oil, I find there is always too much in planets with life. It's almost as if civilization couldn't burn all fuel reserves in a century or two and the old empire made a point to keep all the deposits for future generations.
I try to pick planets with as much heavy metal as possible without having to roll too many times to balance it with making it suitable for rain/forests
Although I also find there is a massive amount of oil on the planets I pick (i always go for forests, which i believe offers the most oil deposits on maps), vehicles use such a massive amount of fuel I find you need an equally massive amount of oil wells to operate a vehicle-heavy force (a 'fun' army)
I just find it's a case of feast or famine situation. It's nice that there are feast planets, as far as I'm concerned there could even be planets with surface hydrocarbon (like on Titan) but I feel like the game is missing planets where there would be some oil, enough to not rush oil producing assets like biofuel, but at the same little enough to try and get the best out of every drop.
I am a petroleum engineer, retired. When I was in school, the prevailing theory was that petroleum was a product of oceanic biomass, primarily plankton type organisms, and algae. Terrestrial plants, i.e. trees, are not necessary unless one is modelling coal creation.
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