GEOLOGY OF THE FIELD 255 



forces have done in creating disturbance and tilting of sedimentary beds 

 in the oil and gas fields of the United States, so as to permit the small 

 quantities of petroleum, gas, and water present in most undisturbed sedi- 

 mentaries to segregate and accumulate into reservoirs of commercial 

 quantity and value. Senor Ezequiel Ordonez, who joined our party at 

 Monterey, accompanying it as far as Casiano, and who communicated to 

 !he writer freely the results of his studies of the Mexican oil fields, be- 

 lieves that these intrusions of volcanic matter in the shape 1 of domes, 

 lidges, etcetera, along the (hilt coast are of quite recent date, geologically 

 speaking, and his views would appear to be confirmed by the topography 

 of the basaltic domes, as also by the newness and character of the prin- 

 cipal seepages, since the asphaltic residua, even in the most active vents, 

 have not accumulated to a depth of more than 20 to 25 feet. Senor 

 Ordonez also thinks that in all the shallower wells which do not penetrate 

 to the Tamasopa limestones, the parent strata of the petroleum, that the 

 principal reservoirs have been created by the intrusion of volcanic debris 

 in a porous or vesicular condition horizontally into the shales, thus frac 

 turing them and forming additional reservoir space. 



Age ok the Oil-bearing Strata 



There can be practically no doubt that the mother rock or mam reser- 

 voir of petroleum along the Gulf coast of Mexico is the Tamasopa lime 

 stone of the Mexican Cretaceous fcerranes, supposed to be several thousand 

 feet thick, and which, rising suddenly out of the phi in, makes a great 

 wall of limestone, the first rock terrace of sedimentaries, 1,500 to 2,000 

 i'vvi high, which we encounter in going from the Gulf coast westward 

 toward the region of the great- Mexican plateau. On above the Tamasopa 

 limestone there comes in a series of flaggy and shelly limestone beds 

 inlerst ratified with marly shales, the whole being known as (he San 

 Felipe series, with a. thickness of -100 to 500 feet. The limestones of 

 this series appear to hold oil at times, hut the reservoir may be from 

 fractures and dikes of igneous Intrusions. Succeeding the San Felipe 

 series come the Mendez marls and shales, with a thickness of 2,000 to 

 .'5,000 feet, which make an excellent cover to protect the underlying oil 



deposits from all escaping to the surface through the numerous faults 

 and fractures which the extrusion of basaltic domes and dikes has ef 

 feeted. 'These two formations San Felipe and Mendez \)\\ C. W. 

 Haves, the new general manager of the Tierson oil interests in Mexiro. 

 thinks are of the same age as the Laramie \)C<W of the United States. 



namely, intermediate between (he true Cretaceous and Tertiary. 



