Abstract

Three categories of dispersed clay are described for hydrocarbon-bearing sandstones using scanning electron microscopy and correlated to rock geological-petrophysical parameters commonly used in reservoir evaluation. Dispersed clays can occur in pores as (a) discrete (not intergrown) particles, pores as (a) discrete (not intergrown) particles, (b) intergrown crystal linings on pore walls, and (c) crystals bridging across pores. These different clay morphologies significantly affect sandstone porosity/permeability, capillary pressure curve and porosity/permeability, capillary pressure curve and associated pore-size distribution parameters, "shaliness" indicators from X-ray diffraction and Qv measurements.

Introduction

The increased utilization of scanning electron microscopy (SEM) and associated elemental detection systems (e.g., energy dispersive analysis of X-rays) has greatly expanded our knowledge of dispersed clay morphology and mode of occurrence in sandstone reservoir rocks. The marginal or questionably productive nature of particular hydrocarbon-bearing productive nature of particular hydrocarbon-bearing zones can be due to silicate clay crystals developed within rock pores in a manner adversely affecting fluid flow. Variations in clay crystal morphology can change a hydrocarbon zone from economically nonproductive to productive, and vice versa. In the past, the direct observation and description of this past, the direct observation and description of this important rock property was limited to petrographic microscope techniques. Recent technological advances in SEM, however, have enabled the petroleum geologist routinely to study rock mineral, textural, and pore-space properties at magnifications, resolution, pore-space properties at magnifications, resolution, and depth of focus far exceeding petrographic thin section capabilities.

Several recent papers have described and discussed the importance of dispersed clays in sandstones. This paper defines "dispersed" clay as silicate clay minerals (e.g., kaolinite, illite, smectite, chlorite) developed within the rock pore system and generally attached to rock mineral surfaces. Dispersed clay in rocks is of diagenetic (i.e., authigenic) origin, having developed subsequent to sediment deposition by precipitation of clay crystals from pore fluids. Particular clay mineral species develop in response to changes in pore water chemistry brought about by changing pore water chemistry brought about by changing temperature, pressure and groundwater conditions during burial and compaction. Since dispersed clays generally occur as a rock pore-filling component and haze a variety of crystal sizes and shapes, they exhibit a broad spectrum of adverse effects on rock fluid flow and fluid saturation properties.

This paper describes (a) three basic types of dispersed clay in sandstones and (b) several laboratory measured geological-petrophysical properties associated with each of these three clay types that are commonly used in evaluating hydrocarbon-bearing reservoirs. Specifically, sandstone samples are classified into three general categories on the basis of dispersed clay morphology as revealed by SEM. Each category is characterized by the following rock petrophysical parameters: porosity and air petrophysical parameters: porosity and air permeability, oil and water relative permeability, permeability, oil and water relative permeability, air/mercury capillary pressure curves, pore size and sorting, cation exchange capacity, and the amount of pore-filling clay estimated from X-ray diffraction and pore-filling clay estimated from X-ray diffraction and thin section analysis.

DISPERSED CLAY CATEGORIES

The basic criteria used to define and contrast the three general types of dispersed clay in sandstones are (a) clay crystal structure and (b) location on pore walls (i.e., mineral surfaces) and/or within intergranular pores and pore throats. Since the primary objective of this paper is to set forth the relationship of dispersed clay types to sandstone petrophysical properties, detailed discussions of petrophysical properties, detailed discussions of specific clay types and morphologies are not presented. The aforementioned Refs. 1 through 3 can presented. The aforementioned Refs. 1 through 3 can be referred to for this information.

The three categories of dispersed clay are as follows.

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