In considering the accumulation of oil and gas in limestone reservoirs according to the theories involving capillarity, water circulation or gravitational adjustment, one is faced by many difficulties in explaining migration through impervious rocks surrounding the reservoir. It is therefore suggested that oil-forming organisms collected within the reservoir itself and the area immediately tributary thereto, and that there is consequently no true distinct source rock. Neither is there any necessity for accounting for any migration excepting within the porous limestone itself.
The accumulation of oil in limestone may be effected in various ways, depending on the nature of the openings in the limestones. These openings may be classified as: (1) fractures; (2) primary porosity; and (3) secondary porosity, (a) with partial later cementation, and (b) without later cementation.