This article summarizes what is known of the nature of the limestone reservoir rocks in the Mexican oil fields, and discusses the probable source of the oil. Porosity in the Northern fields (Pánuco area) is due to faulting and fracturing of the compact Tamaulipas, the Agua Nueva, and the San Felipe limestones, of Cretaceous age. Some porosity in the limestones is due to jointing. Oil accumulation in the Northern fields occurs on the top, or on the flanks, of anticlinal structures. The basal Agua Nueva beds, of Eagle Ford age, are considered to have furnished the main source of the oil.
In the Southern fields (Dos Bocas-Alamo) the oil occurs in the El Abra limestone, a porous rock representing a reef facies, of Cretaceous age. Production is found at the top of asymmetric anticlines. The El Abra limestone reservoir rock of the Southern fields is considered to be the source of the oil which it contains.