Petroleum is usually found in

Step 1: Analyze the question The question asks which type of rock petroleum (crude oil and natural gas) is usually found in.

Step 2: Explain the concept Petroleum forms from the remains of tiny marine plants and animals (plankton, algae) that died and settled on the sea floor millions of years ago. Over time, they were buried under layers of sediment (sand, silt, clay). Heat and pressure slowly turned the organic matter into oil and gas.

For this to happen, you need:

• Source rock: sedimentary rock (like shale) rich in organic material.
• Reservoir rock: porous and permeable sedimentary rock (like sandstone or limestone) that can hold the oil.
• Cap rock: an impermeable layer (like clay or shale) that traps the oil.

Step 3: Eliminate the options

• A. Resistant rocks: This is not a rock class. It describes how well a rock withstands erosion, so it is not correct.
• B. Metamorphic rocks: These formed under intense heat and pressure, which would destroy the organic matter and the pore spaces needed to hold oil.
• C. Igneous rocks: These formed from cooled magma or lava. The high temperatures involved leave no organic remains, and the rocks are generally non-porous.
• D. Sedimentary rocks: These are formed from layered deposits, contain fossils and organic matter, and have pore spaces that store oil. This is correct.

Step 4: Answer Petroleum is usually found in sedimentary rocks, which is option D.

Nigerian context: The Niger Delta, where most of Nigeria's oil is produced, is a large sedimentary basin. The same is true of the Anambra and Benue basins.

Exam tip: Remember "Fossils, Fuels, and Folded layers = Sedimentary." Coal, petroleum, and natural gas (fossil fuels) are all linked to sedimentary rocks.