Even in a single horizontal drill hole, the amount recovered may vary, as may recovery within a field or even between adjacent wells. This makes evaluation of plays and decisions regarding the profitability of wells on a particular lease difficult. Production of oil from tight formations requires at least 15 to 20 percent natural gas in the reservoir pore space to drive the oil toward the borehole; tight reservoirs which contain only oil cannot be economically produced. Formations which formed under marine conditions contain less clay and are more brittle, and thus more suitable for fracking than formations formed in fresh water which may contain more clay. Formations with more quartz and carbonate are more brittle.
