Sedimentary rock consisting of at least 50% calcium carbonate. The term limestone is applied to any sedimentary rock consisting essentially of carbonates. The 2 most important constituents are calcite and dolomite, but small amounts of iron-bearing carbonates might also occur. The oil reservoirs of the Middle East and North Africa contain some 70% of the world’s known oil reserves and about 50% of the world’s natural gas reserves. Most of these are contained in high-energy carbonate platform sediments, or in fractured limestones of large, doubly plunging anticlines