PALEO-HYDROSTRATIGRAPHY OF FORMATION FLUIDS IN FORELAND BASINS

In both modern and ancient sedimentary basins basinal fluids are commonly highly stratified by composition and temperature, and may also vary spatially in composition and temperature. The fluid stratification may be controlled by lithologic packages with specific porosity and permeability characteristics and/or by mechanical stratigraphy. Boundaries between different fluids may be either diffuse spanning 10s to 100s of meters or sharp along a single formation boundary. The establishment of a fluid hydrostratigraphy may be related to: original depositional fluids (freshwater/seawater), diagenetic reactions, catagenetic reactions, microbial activity, host rock chemistry, fluid migration, fluid mixing, meteoric influx, structural events such as pressure solution, and mineral precipitation.

A fluid hydrostratigraphy may be perturbed by breaching hydrostratigraphic boundaries by structural-tectonic events such as folding, faulting, and/or fracturing. This perturbation of the fluid system may initiate fluid migration and fluid mixing resulting in mineral precipitation and/or mineral dissolution, dolomitization, and ore deposition. In particular, the development of a fold-and-thrust belt from foreland basin rocks may significantly alter the fluid hydrostratigraphy and initiate topographic or tectonic driven fluid flow.

Modern hydrostratigraphy can be documented by sampling deep well fluids during the exploration for, and extraction of hydrocarbons. Establishing a paleo-hydrostratigraphy for an ancient sedimentary basin relies on the analysis of vein minerals using fluid inclusion microthermometry, stable isotopes, Raman spectroscopy, cathodoluminescence, trace metal analysis, and other techniques.

Published examples will be presented of modern hydrostratigraphy in the Alberta basin, Gulf Coast, Columbia, and Appalachians. In addition, examples will be shown of paleo-hydrostratigraphy in the central Appalachians, Sierra Madre Oriental, and Sawtooth Range.