Evolution of pore space in the Poza Rica trend (Mid-Cretaceous), Mexico

ABSTRACT The Poza Rica trend of the Tampico embayment, Mexico, will ultimately produce more than 2.3 × 10⁹ barrels of oil from Mid-Cretaceous (Albian-Cenomanian) basin-margin deposits. Bioclastic grainstone, packstone, and wackestone are interbedded with polymictic lime breccia and dolomitized debris; all were deposited by sediment gravity flow. Indigenous sediment was pelagic lime mud. Typical reservoir porosities are about 10%; permeabilities average 2 md and rarely exceed 100 md. Porosity is largely the result of selective dissolution of rudist fragments, which were originally aragonite. Detailed petrographic study, with emphasis on the diagenetic products, allows quantitative assessment of porosity at various diagenetic stages from original sediment to reservoir rock. A relatively simple diagenetic history is reflected by about 90% of the samples studied: primary porosity was reduced through lithification of matrix mud and initial cementation by clear, equant to bladed, non-ferroan calcite. Later dissolution produced extensive skelmoldic and minor vuggy porosity. Subsequently, non-ferroan calcite cement reduced porosity before the emplacement of hydrocarbons. Reconstructed sediment porosities are comparable to, but lower than, modern counterparts. The initial phase of cementation and presumed lithification of mud greatly reduced porosity in all lithologies, but appreciably more porosity persisted in grainstone and packstone than in wackestone or mudstone. Dissolution produced a porosity resurrection, which exceeded that of the initial sediment in some grainstones. Calcite cementation and local multiphase quartz cementation and dolomitization reduced porosity to present average values of 8–12% in grain-supported rocks and 3% in mud-supported rocks. The greater persistence of primary porosity and, therefore, permeability in grain-supported rocks probably accounts for their greater secondary porosity development and ultimate reservoir quality. Geometrically averaged permeabilities range from only 0.17 md in wackestone to 3.85 md in dolomite, but differ significantly with rock type and grain size. Permeability increases with porosity in all lithologies; the rate of increase is greater at higher porosities and with coarser grain sizes. The agent for both early cementation and development of secondary porosity appears to have been meteoric water. Subaerial exposure appears to be ruled out, however, by a basin-margin depositional environment and continued burial beneath Upper Cretaceous pelagic sediments. Early exposure to meteoric water can be explained by a hydrologic head developed during penecontemporaenous exposure that produced cavernous porosity in the adjacent Golden Lane trend. Descending meteoric water likely emerged as submarine springs along the Tamabra trend. Deposition of pelagic limestone during the Turonian blanketed part of the Golden Lane escarpment to enhance development of a large freshwater lens; gaps in the blanket localized springs and influenced flow patterns within the Poza Rica field. Analogous freshwater circulation exists today in northern Florida.