Petrography and Reservoir Quality of Tertiary Deepwater Sandstones in the Veracruz Basin, Mexico
DUTTON, SHIRLEY P., JENNETTE, DAVID C., and AMBROSE, WILLIAM A.
Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,
PEMEX Exploración y Producción, Veracruz, Mexico
Petrographic analysis of Miocene and Pliocene sandstones from the Veracruz Basin was used to evaluate diagenetic controls on reservoir quality. The sandstones are turbidites and debrites deposited in a deepwater (slope to basin floor) setting. Sandstones from nine conventional cores between depths of 1460 and 3430 m have an average composition of Q29F18R53. The proportions of the most common rock fragments—carbonate, metamorphic, and volcanic—vary with distance from the thrust fault bounding the basin on the west and volcanic centers. Average cement volume is 11 percent; calcite and dolomite are the most abundant authigenic minerals. Thin-section porosity ranges from 0 to 29 percent and averages 15 percent; most pores (80 percent) are intergranular.
The main controls on porosity are (1) volume of cement, (2) volume of ductile grains, and (3) sorting. Compaction by ductile-grain deformation and intergranular pressure solution increases with burial depth, resulting in a decrease in intergranular volume and loss of primary porosity. The volume of carbonate cement also increases with burial depth; the source of the carbonate cement is probably from pressure solution of carbonate grains. Some sandstones are extensively cemented by calcite that precipitated relatively early in the burial history. Porosity is inversely related to grain size because coarse sandstones and conglomerates contain abundant ductile grains and are poorly sorted. Reservoir quality is thus largely controlled by factors inherited from depositional environment and stratigraphic position. Middle and upper Miocene and Pliocene sandy debrites and laminated turbidites retain the best reservoir quality.