For reference, this can be fixed on your local copy.
In your mod folder ( %APPDATA%\Factorio\mods ), unarchive the RSO mod (rso-mod_6.1.2.zip) in the same directory ; edit the file Factorio\mods\rso-mod_6.1.2\resourceconfigs\pypetroleumhandling.lua to add the following block beneath the first line ( function fillPyPetroleumHandlingConfig(config) )
config["bitumen-seep"] = {
type="resource-liquid",
minimum_amount=500000,
allotment=60,
spawns_per_region={min=1, max=1},
richness={min=3000000, max=5000000},
size={min=1, max=1},
useOreScaling = true,
starting={richness=250000, size=2, probability=1}
}