The problem of determining the time of accumulation is related closely to that of determining a sequence of geological events. It, in fact, includes the one of determining the time of unconformity. Thus far, then, the criteria of the two problems are identical, and the determination is dependent upon geological observations in the region being given consideration. Major and minor unconformities have distinct significances with respect to migration and accumulation. But the determination of the time of accumulation is also dependent upon criteria which are mechanical in nature, and which are only to be observed during drilling and production.

A diagram entitled "Restricted Sequence of Geological Events" is used as a basis of the present treatment of the subject. The axis of time is divided into nine periods, each of which has particular significance with respect to the problem. This diagram may prove to be deficient for many regions. It is designed to cover the general problem only in so far as we find it necessary for the consideration of accumulations under the special circumstances considered here.

Accumulations are classified according to two categories. The first relates to such as are found in porous strata of Cenozoic age, and the second relates to others in porous strata of Paleozoic age. Which of the two is appropriate for strata of Mesozoic age is at present unknown because of the lack of sufficient data.

Accumulations of the second category are found from north Texas to the province of Ontario, where they lie in strata ranging in age from Ordovician to Permian. Migration and accumulation took place before the truncation by erosion or the subsidence of the petroliferous strata along their upturned edges around the area in which the accumulations are situated, and any subsequent elevation or subsidence, folding, warping, or tilting of the accumulations was not accompanied by a lateral shifting of the pools to accommodate new structural conditions. Accumulations are, therefore, frequently found on a flank of an otherwise barren anticline, on a terrace, or in a syncline. Occurrences of this sort played an important part in early discussions (1865-1885) relating to the anticlinal theory of accumulation.