Oil occurs in several different rock associations, but tends to follow particular facies subdivisions within each type. In the micaceous sand and shale association, linear trends of pools are well developed and parallel with lithologic ratio contours. Oil occurs also in similar rocks deposited in bird-foot deltas resembling the delta of the Mississippi. The zones favorable to the growth of reefs or bioherms are rigidly controlled by environment and the occurrence of reefs can be related to the nature of their enclosing rocks. The widespread quartzose sands found associated with shelf limestones are permeable over large areas and many local pools are structurally controlled. This rock association is generally poor in organic matter, and the location of some oil pools can be related to belts of oil-source facies in associated beds.
Lithologic conditions are especially important in deciding whether to lease and drill a prospect. In a large and growing number of reservoirs where structural closure is absent, they are the only geological considerations. All types of lithologic and stratigraphic information can be shown quantitatively by contours in the same manner as structural data. Quantitative expressions provide the best means of presenting lithologic data areally. Such presentations permit the definition of areas lithologically favorable for oil occurrence.