Selectively oil-impregnated limestones from the Upper Cretaceous Guzmantla Formation, outcropping in the Cordoba Platform of eastern Mexico, were studied to determine the factors controlling the porosity and hydrocarbon distribution and to reconstruct the fluid-flow history.
In the two exposed upward-coarsening (i.e., upward-shoaling) sequences, three limestone lithotypes were distinguished, based on sedimentary, diagenetic, and oil-impregnation characteristics. Lithotype I is comprised of mud-dominated low-energy deposits, which have been affected strongly by compaction. These strata are oil impregnated only along stylolites. Lithotype II consists of bioclastic wackestones to packstones deposited in an open-platform lagoonal environment. This lithotype is pervasively oil impregnated. The preservation of porosity is explained by the development of framework-stabilizing, interparticular, early diagenetic (marine and meteoric) calcite cements. Furthermore, secondary porosity was created after layer-parallel shortening (LPS), when LPS-related structures were opened during subsequent folding of the strata. Lithotype III consists of bioclastic shoal grainstones that have been cemented pervasively during early-marine and later meteoric diagenesis, occluding primary porosity and thus preventing oil impregnation.
However, Lithotype III strata display an important modern macroporosity, related to a telogenetic phase of karst development that postdates oil migration. Due to the lack of driving forces, the oil did not migrate into these karst-related pores. In Lithotype II, the presence of oil reduced the effective porosity and hindered further fluid migration. Lithotype II strata thus were less affected by the telogenetic karstification. Lithotype I was less affected because of the completely compacted matrix. This late-stage (postoil migration) dissolution phase is not important in this specific history, but it may be very important in similar deposits in the subsurface, where it can enhance appreciably the reservoir capacity.
Factors controlling porosity-permeability are, first, the sedimentary environment, which influenced early and, thus also, later diagenetic evolution. Furthermore, stylolite development (compactional as well as tectonic), which exerts a negative effect on porosity-permeability because of pressure-dissolution and related matrix cementation, also is an important factor. However, because of tectonic opening of some of the stylolites and channelling of meteoric fluids, with porosity development as a result, these stylolites also may increase permeability and total porosity. Finally, fracturing of the strata, whereby tectonic opening and/or cementation can take place, exerts a major influence on reservoir characteristics.
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The Circum-Gulf of Mexico and the Caribbean: Hydrocarbon Habitats, Basin Formation and Plate Tectonics
The Circum-Gulf of Mexico and the Caribbean has long been one of the world's most important petroleum provinces, as well as one of the world's most geologically complex regions. These two characteristics have resulted in an extensive amount of ongoing research by both industry and academia. AAPG Memoir 79, The Circum-Gulf of Mexico and the Caribbean, is the first volume in more than a decade to document such a wide range of research on the geology of this vast area. Of the total 44 papers, roughly two-thirds pertain to the Gulf of Mexico, with an emphasis on the Mexican portion of the basin, and to the petroliferous areas of the southern Caribbean, including Colombia, Venezuela, Cuba, and Trinidad and Tobago. The remaining papers relate to the Antilles and Central America, as well as a series of papers that address region-wide topics such as plate tectonic evolution. A significant number of papers were contributed by authors from national oil companies and universities from within the region.