How is petroleum source rock different from reservoir rock?

• A. The source rock is fine-grained, and the reservoir rock is coarser and more porous.
• B. The reservoir rock is fine-grained, and the source rock is coarse.
• C. Both rocks have the same porosity.
• D. The reservoir rock is impermeable.

Answer: (A)

In petroleum geology, source rocks and reservoir rocks serve different roles in a hydrocarbon system. Source rocks are the organic-rich rocks where oil and gas are generated under heat and pressure; they are typically fine-grained (like shale or mudstone) with low permeability, which keeps generated hydrocarbons in place until they begin migrating. Reservoir rocks, by contrast, are the rocks that have enough porosity and permeability to store and transmit hydrocarbons; they’re usually coarser-grained (such as sandstone) and have interconnected pore spaces that allow fluids to move and accumulate. So the best description is that the source rock is fine-grained, while the reservoir rock is coarser and more porous.