Abstract
We have devised a technique for determining chloride in interstitial water of consolidated rocks. Samples of rocks ranging from 5 to 10 g are crushed and sieved under controlled conditions and then ground with distilled water to submicron size in a closed mechanical mill. After ultra-centrifugation, chloride content is determined by coulometric titration. The chloride concentrations and total pore-water concentrations, obtained earlier from the same pore-water concentrations, obtained earlier from the same samples by low-temperature vacuum desiccation, are used to arrive at the "original" pore-water chloride concentrations by a simple iteration procedure. Interstitial chlorinity results obtained from Cretaceous and Jurassic strata in the Gulf of Mexico coastal areas ranged from 20 to 100 g/kg Cl with reproducibility approaching +/- 1%. We have also applied the technique to igneous and metamorphic bedrocks as well as ocean basalts containing 1 % water or less. Chloride values ranging from 6.7 to 20 g/kg with a reproducibility of about 5% were obtained.
Introduction
This paper outlines a technique for precision analysis of interstitial chloride and water content (porosity) of shales and other consolidated rocks from deep-earth strata. Nearly all the literature on the composition of interstitial water (formation fluid) of deep-earth strata refers to fluids from reservoir rocks or permeable horizons. In many areas, shales or other nonreservoir rocks constitute the bulk of sedimentary sequences. These rocks contain interstitial fluids of generally unknown composition. The paucity of data is caused partly by the lack of access to fresh cores and partly by analytical difficulties in obtaining interstitial water from such materials. Until the late 1960's, much of the analytical literature dealing with pore fluids from deep sedimentary nonreservoir rocks was published in the Soviet Union and in references cited by those authors. Since then, interest in several hydrochemical phenomena relating to nonreservoir rocks has increased among phenomena relating to nonreservoir rocks has increased among scientists in the U.S. and other Western countries: interest in hydrocarbon resources in overpressured strata dominated by undercompacted shales that may have anomalous chloride content; need for knowledge of the proportion of bound water (electrolyte-poor) in porosity proportion of bound water (electrolyte-poor) in porosity during quantitative interpretation of electrical logs for oil and gas saturation in shaly sands; need for better understanding of nonreservoir rocks as sealing beds for deep waste disposal; and, finally, a desire to understand better the hydrochemical history of deeper sedimentary basins. However, only a relatively few field studies are available on the topics in question. Many of these are student theses or work based on them.
The basic procedure underlying the studies of interstitial water composition of shales is simply crushing and grinding a rock sample, leaching it with distilled water, and analyzing the leachate. The salt content of the solid is then related to an independent determination of total pore fluid or porosity. Techniques based on this principle were used for shallow groundwater studies, for general rocks, and for studying oilfield drill cores. Comments in the literature and our own experiments suggest that simple approaches to the leaching process may yield accuracies of 10 to 20% for chlorides in rocks with a significant PV fraction. As water contents decrease to 1%, however, an uncontrolled system may easily yield errors of several hundred percent and uncertainties associated with the bound water (see the section called Discussion). Most of the studies of interstitial chlorinity of water composition in deep oilfield strata have been performed on stored, dried, or partly dried materials and/or have used insufficiently documented or quantified techniques. The goal of this study has been to approach a reproducibility and relative accuracy of I % in the values of interstitial chloride, given our definition of mobile water discussed later.
Sampling and Handling of Drilling-Core Samples
A potential source of error in interstitial fluid analysis is the contamination of cores by drilling fluid. However, experience in the Deep Sea Drilling Project and other drilling studies 11–15 show that, if external contaminated layers are cut or chipped away from undeformed normal, non-fractured silty-clay cores soon after recovery, virtually unaffected inner sections can be obtained. Even permeable (reservoir-type) rocks sometimes may be sampled successfully for pore-fluid study. During wireline coring by the AMCOR project with the drilling vessel Glomar Conception on the Atlantic Continental Shelf, virtually identical pore-fluid chloride profiles were obtained in repeated drillings performed with seawater and freshwater drilling fluids (Fig. 1).
SPEJ
P. 704
