Page 521 - Petrophysics

INTRODUCTION 489

on the direction of the maximum stress and the rock type. A fracture
may consist of two rock surfaces of irregular shape, being more or less in
contact with each other. The volume between the surfaces is the fracture
void.
Naturally fractured rocks can be geologically categorized into three
main types, based on their porosity systems:

Intercrystalline-intergranular, such as the Snyder field in Texas, the
Elk Basin in Wyoming, and the Umm Farud field in Libya;
Fracture-matrix, such as the Spraberry field in Texas, the Kirkuk field
in Iraq, the Dukhan field in Qatar, and Masjidi-Sulaiman and Haft-Gel
fields in Iran; and
Vugular-solution, such as the Pegasus Ellenburger field and the
Canyon Reef field in Texas [ 11.

The accumulation and migration of reservoir fluids within a naturally
fractured formation having the first type of porosity system are similar
to those found in sandstone formations. Consequently, the techniques
developed to determine the physical properties of sandstone porous
media in Chapter 3 could be directly applied to formations having
intercrystalline-intergranular porosity. Unfortunately, this is not the case
for reservoirs having the other two types of porosity system. The pores
in the matrix of a fracture-matrix formation are poorly interconnected,
yielding a pattern of fluid movement that is very different from that
of sandstone formations. Rocks with vugular-solution porosity systems
exhibit a wide range of permeability distributions varying from relatively
uniform to extremely irregular as shown in Figure 8.1.

Figure 8.1. Naturally fractured rock cores taken from wells.