Oil reserves
The term refers to the total amount of petroleum (oil) discovered in any given oil field or nation. Thus it can be said that Kuwait has xxxx millions of barrels (mb) of oil in the ground. However, the exact amount can never be known, simply because of the difficulty in sensing or "seeing" beneath the surface of the Earth. The term Proven Reserve or PR refers to an amount of oil that is generally accepted by geologists to be the actual amount of petroleum in the ground.
Saudi Arabia has the largest generally accepted oil reserves in the world followed by Iraq, Iran and some gulf states. The Gawhar field in Saudi Arabia is the largest known oil field, measuring about 110 miles long by 15 wide, although its actual size is kept secret by the Saudis. Gawhar's estimates vary from as little as 150 billion barrels to as much as 500 billion but the lower number is more reliable since the Saudis protect their petroleum data with much secrecy. In addition much of this field is now increasingly sour crude and is less desireable.
Oil reserves can be of various types or qualities, light sweet being the most desireable. Heavy crude is a much thicker, and highly sulfurous variety. Oil sands and oil shale are the least desireable type owing to the expense of extracting the oil from the shale or sands. The term sweet refers to the minimal amount of sulphur in the crude, light sweet crude being the least sulfurous and heavy crude being the most.