ABSTRACT

Thirty five wells have been drilled in Northeastern Mexico which are of interest in the study of paleogeographic elements existing during the Jurassic period and also of the lithology and environment of deposition of the Upper Jurassic sediments. (Figure 1) The Upper Jurassic stratigraphic section of Northeastern Mexico, on the basis of electrical logs and lithology, may be subdivided into: (1) La Casita, equivalent to the Cotton Valley of N. E. Texas; (2) Olvido, to the Buckner; (3) Zuloaga, to the Smackover; (4) Minas Viejas and Basal Upper Jurassic, correlative with the Louann Salt and Eagle Mills.

La Casita is characterized by carbon black calcareous shales, in which ammonites of Portlandian age have been identified in some wells, and black limestones with few dolomites near its base. The top of the Olvido is placed arbitrarily below La Casita where anhydrite becomes conspicuous, but in most wells where anhydrite is not easily identified the Olvido is merged with the Zuloaga Group. The Olvido is composed of anhydrite, black and gray shales, black and gray limestones and some dolomite. The Zuloaga is a limestone section with interbeds of dolomite and oolitic limestone having incipient porosity. Minas Viejas is a thick section of anhydrite, gypsum and salt below Smackover limestones and above a reddish or pink colored section of sands, shales, conglomerates and igneous flows which is named Basal Upper Jurassic.

The presence of a Jurassic-Lower Cretaceous landmass (The Tamaulipas Peninsula) is identified in Peyotes 2-A and La Perla No. 1 where limestones, sandstones and gravels of Lower Cretaceous age overlie metasediments similar to those penetrated in the interior part of the frontal zone of the Ouachita belt in Central Texas. From Peyotes and La Perla towards the Garza, Anahuac and Escalera wells, marine and non-marine Upper Jurassic sediments become progressively thicker. This Tamaulipas landmass is also identified in the Chapa 101, Carbajal 1 and Cerralvo 2-A because of the metasediments drilled in the basal part of these wells below a reduced Upper Jurassic section.

Shows of gas have been obtained in most wells within the Zuloaga section. In Anahuac No. 1 a large volume of dry gas flowed from Zuloaga dolomites just before the well was lost because of mechanical difficulties. In Cadena No. 3 a significant amount of gas with 61 per cent Hydrogen Sulphide was encountered which corroded the connections of the blow out preventer, casing and drill pipe making it necessary to plug the well.

The environment of sedimentation, the lithology and the gas shows, found in the sediments of Zuloaga age of Northeastern Mexico, indicate more or less similar stratigraphic and sedimentary conditions to productive Smackover fields in East Texas and to the Constituciones and San Andres Upper Jurassic fields of the Tampico-Poza Rica region. Optimistically it can be said, that the large area in the Gulf Coast Basin between Poza Rica and N. E. Texas, where the Upper Jurrasic strand lines can be found at economical drilling depths, is a promising hunting ground for future oil fields.

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Figure 1. (unnumbered) Location map of Jurassic wells drilled in Northeastern Mexico.

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