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Jurassic rocks, with a stratigraphic thickness close to 400 m, have been cored in 172 wells in an area of over 8,000 sq km. These rocks are quite important because they are the only rocks in Mexico of this age that produce petroleum. A lithostratigraphic study did not explain the nature of the reservoir, the time-stratigraphic method was used to comprehend the lithofacies changes, which occur at the stage level. Stratigraphic study based on analysis of cores from the western Sierra Madre Oriental established nine zones of ammonites: Wagnericeras, Kepplerites, Reineckeia, Discosphinctes, Ataxioceras, Idoceras, Virgataxioceras, Mazapilites, and Suarites (Bathonian-Tithonian). Pelecypods also were present in the cores.
The common occurrence of the same fossils in other characteristic beds, which have been identified by radioactive well logs, permits their used as time-stratigraphic markers. These data were used to make subsurface structural and isopach maps.
In the northwestern Poza Rica area the Middle Jurassic transgression began during the Bathonian. Later it advanced to the central, east, and west parts, and the covered area is characterized by distinct transgressive lithofacies. The last phase of the transgression was in the early Tithonian in the southeast part of the trend, and calcarenites were formed which now produce hydrocarbons (San Andres). In northwestern Poza Rica, the San Andres calcarenitic member is within the uppermost part of the lower Kimeridgian stage.
The reservoirs are stratigraphic and structural traps. On the southeast the San Andres calcarenitic member can be subdivided at the stratigraphic level in the lower Tithonian; this fact is related to the occurrence of oil or salt water.
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