ABSTRACT

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 1,460 and 3,430 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 from volcanic centers. Average cement volume is 11%; calcite and dolomite are the most abundant authigenic minerals. Thin-section porosity ranges from 0 to 29% and averages 15%; most pores (80%) 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 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 sandy turbidites retain the best reservoir quality.