ABSTRACT

The Poza Rica Trend, Veracruz, Mexico, has been Mexico's largest petroleum producer since the 1930's. The productive Cretaceous Tamabra Limestone of this trend consists of rudist-fragment packstone, wackestone, and grainstone., breccia; and pelagic wackestone. The breccia and rudist limestones are interpreted as mass-flow deposits from the 1,000m-high, east-adjacent escarpment of the Golden Lane "atoll".

The major stages in porosity development which can be identified, despite local complexities, are: 1) primary, sedimentary porosity, 2) early cementation 3) matrix lithification (probably overlaps cementation), 4) skeletal mold and vug formation, 5) fracturing (overlaps 4), 6) dolomitization (limited), and 7) calcite cementation. Porosity at each stage varied with depositional texture. Primary porosity, which was accompanied by effective permeability only in low-matrix packstone, grainstone, and low-matrix breccia with porous clasts, was almost completely destroyed in early stages of diagenesis. The slightly greater porosity retention in the more matrix-free rocks was probably significant for later porosity development.

Productive porosity is almost entirely skeletal molds in the rudist limestones. Favourable porosity is developed in some non-productive wells. Production is determined by a basinward stratigraphic pinch-out of the coarser-grained limestones coincident with a broad plunging anticline; good porosity is developed over a wider area, although not basinward of the pinch-out of the coarser-grained rocks. Leaching is ascribed to meteoric waters descending westward from cavernous limestone during intermittent periods of subaerial exposure of the Golden Lane escarpment. By this model, impermeable pelagic carbonates deposited during and immediately following deposition of the Tamabra Limestone confined the water until it reached areas where the pelagic carbonates were missing and emerged as submarine springs. Dolomitization may have resulted from brines seeping downward along the same routes from evaporate lagoons within the Golden Lane "atoll" or from mixing of fresh water and sea water within the aquifer. An average of¹⁸ -2.5 in the dolomites suggests low-salinity waters.

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