It depends on the source material , and the temperature and pressures the source rock is cooked at .
Organic material is laid down under anoxic conditions. It gets buried and compressed with sediments and eventually forms into a shale. The organic material, as it is buried and heated turns to kerogen. This shale is often referred to as the source rock , as it is where the hydrocarbons come from.
The Kerogen is heated down in the earth, and eventually forms oil or gas. Generally speaking the higher the temperature , the more likely gas is produced. That said , the organic material laid down can have an impact, algae, plankton etc often form a kerogen that forms oil, plant matter tends to form gas.
The oil and gas then tends to migrate up through the rock, and eventually gets to surface and evaporates off into the atmosphere. If the migrating oil and gas hits a seal, with a porous formation under the seal, we end up with an oil or gas (or often both) reservoir. The gas and oils can all migrate at different rates through the rock to get to the reservoir as well.
So really what determines the reservoir is
- the type of organic material laid down,
- the temperature it was heated up to , and to a certain extent, the rate at which it was heated.
On one further note, the shale oil drilling you hear about is basically going straight for the source rock.
So that is the - why oil or gas or both
The question to there being any hydrocarbon is basically,
- was there any organic material being deposited in a sedimentary basin under anoxic conditions several million years ago.
- And if so, was there sufficient burial and heating to form kerogen and then oil and gas,
- Did subsequent sediment deposits also form a porus reservoir rock
- did a seal form over that reservoir.