Complex sentence: Oil shale resources can be either concentrated in a large confined deposit such as the Green River formations which was the result of a large inland lake and so can be many meters thick but limited by the size of the original lake.
Split into simpler sentences: Oil shale resources can be concentrated in a large confined deposit such as the Green River formations, which were formed by a large inland lake. These can be many meters thick but limited by the size of the original lake.