--> Abstract: Diagenesis of a Giant--Poza Rica Trend, Mexico, by Paul Enos; #90967 (1977).
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Abstract: Diagenesis of a Giant--Poza Rica Trend, Mexico

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The Poza Rica trend, Veracruz, Mexico, has been Mexico's largest petroleum producer since the 1930s. The productive 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 1,000-m-high, east-adjacent escarpment of the Golden "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, which overlaps stage 4, (6) limited dolomitization, 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 is probably significant for later porosity development.

Productive porosity is almost entirely skeletal molds in the rudist limestones. Favorable porosity is developed in some nonproductive wells. Production is determined by a basinward stratigraphic pinchout of the coarser grained limestones coincident with a broad plunging anticline; good porosity is developed over a wider area, although not basinward of the pinchout of the coarser 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 carbonate material deposited during and immediately following deposition of the Tamabra Limestone confined the water until it reached areas where the pelagic carbonate sediments were missing, and there it merged as submarine springs. Dolomitization may have resulted from brines seeping downward along the same routes from evaporite lagoons within the Golden Lane "atoll" or from mixing of fresh water and seawater within the aquifer. An average ^dgrO18 of - 2.5 in the dolomites suggests low-salinity waters.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas