I was wondering why are coal and petroleum being both fossil fuels are found at different places i.e. coal at land and petroleum at sea.
Coal, oil and gas can be found anywhere on Earth: on land and on sea.
The Japanese once mined coal from beneath the sea floor, using Hashima Island as the base of operations. The island is only 6.3 ha in size.
As for oil, the following picture from USGS shows the current oil-prone petroleum basins in the world, outside of the USA.
The following oil basins are all on land:
- Arabia, Kuwait, Iraq and north Africa
- Eastern Europe
- The Canadian oil sands and other plays in the province of Alberta, Canada
- The oil fields of east Texas, USA
- The former oil fields in Pennsylvania
When oil & coal deposits formed they each had different mechanisms of formation. Coal is derived of plant material that once lived on land, died, formed peat, which then got buried.
Oil however, owes its origins to algae & plankton that lived in the oceans. When they died they sank to the bottom of the ocean and deposited in waters with very low, if any oxygen. Eventually, these deposits were covered by sediments. If the deposit was subject to a significant amount of geothermal heating gas deposits formed, otherwise oil deposits formed. Movements of the Earth's crust then repositioned some of the deposits which is why some coal is now under the sea and some oil is now on land.
There's no significant difference in distribution beneath land and sea. Recovery is all about accessibility. There's plenty of coal under the sea, but we just can't get at it - fortunately! To avoid carbon-driven disaster we have to leave most of the coal and oil where it is, under land or sea.
A more fair comparison would be between the presence of coal and that of hydrocarbon-generating source rocks. Although hydrocarbons can be generated by cooking any rock with a high enough concentration of hydrocarbon-bearing organic matter (i.e. kerogen), the maps you are showing are for the accumulations of trapped hydrocarbons after they have migrated away from their source rock. The vast majority of hydrocarbons ever generated by cooking a source rock in depth have migrated to the surface and been degraded by bacteria.
The places where you will find hydrocarbons not only require a buried source rock, but also a migration pathway, a reservoir, a trap, a seal, and all of it happening at the right time; you need a whole petroleum system.
I am sure that there are other constraints about where to find coal seams as well. For example, There are probably many coal seams that have been buried in a way that they are not accessible and are therefore not counted on those maps. But I'm not that familiar with the geology of coals.