The crust of the earth in oil- and gas-producing areas is composed ofsedimentary rock layers. Sedimentary rocks can be source and reservoirrocks for gas and oil. These rocks are called sedimentary rocks becausethey are composed of sediments that were formerly loose particles suchas sand grains, mud, and seashells or salts that precipitated out of water.Sedimentary rocks are millions of years old and were deposited when thesea level rose and covered the land many times in the past. These sedimentsare relatively simple materials such as sands deposited along beaches, muddeposited on the sea bottom, and beds of seashells. Ancient sediments,piled layer upon layer, form the sedimentary rocks that are now sandstonescomposed of sand grains, shales composed of mud particles, and limestonescomposed of seashells. These are drilled to fnd and produce oil and gas.
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