Carrizo Oil & Gas, Inc. (Carrizo) is an independent energy company. Carrizo together with its subsidiaries is engaged in the exploration, development, and production of oil and gas in the United States and the United Kingdom. Its operations are focused in proven, producing oil and gas plays primarily in the Barnett Shale in North Texas, the Marcellus Shale in Appalachia, the Eagle Ford Shale in South Texas, the Niobrara Formation in the Denver-Julesberg Basin (the Niobrara) in Colorado and the United Kingdom North Sea where its Huntington field discovery was under development during the year ended December 31, 2011. It controls acreage blocks and utilizes advanced drilling and completion technology along with three dimensional (3-D) seismic techniques to identify potential oil and gas drilling opportunities and to optimize reserve recovery. During 2011, it added, net of production, 140 billions of cubic feet equivalents to proved reserves, for a reserve replacement ratio of 311%.