A hydrocarbon reservoir is a geological formation that contains accumulations of hydrocarbons, which are organic compounds made primarily of hydrogen and carbon. These reservoirs can be found in various types of rock formations, such as sandstone or limestone, where oil and natural gas can be trapped. The hydrocarbons are formed from the remains of ancient plants and animals that have been subjected to heat and pressure over millions of years. The ability to extract hydrocarbons from a reservoir depends on several factors, including the reservoir's porosity and permeability. Porosity refers to the amount of empty space within the rock that can hold fluids, while permeability measures how easily those fluids can flow through the rock. Understanding these properties is crucial for the successful exploration and production of resources like oil and natural gas.