TABLE OF CONTENTS The geologic characteristics of naturally fractured reservoirs found in sandstones, carbonates, shales, cherts, basement rocks, and coal seams were discussed in Chapter 1. This chapter will consider the case histories of fractured reservoirs in some of these lithologies.
The Spraberry field is located in the Midland Basin, Texas. It is cut by regional fractures and is considered to be a reservoir of Type A. Very low oil recoveries (< 10% OOIP) are attributed to low reservoir energy as the reservoir is underpressured (0.33 psi/ft) and capillary end effects.
The geologic aspects were discussed originally by Wilkinson (1953) who indicated that the main producing structure was a fracture permeability trap on a homoclinal fold made out of alternate layers of sand, siltstone, shales, and limestone. Oil was stored primarily in the sandstone matrix, and paper-thin fractures provided channels for conducting the oil to the wellbore.
More recently, Guevara (1988) and Tyler and Gholston (1988) presented a detailed geological characterization of the Permian submarine fan reservoirs of the Driver Waterflood Unit in the Spraberry Field. Guevara (1988) examined more than 350 well logs and substantial amounts of core and production data to characterize the reservoir and to determine the relationship between oil recovery and reservoir stratigraphy. From his study, Guevara (1988) concluded that it was possible to increase oil recovery by the proper selection of completion intervals and strategic infill drilling. The importance of natural fractures on oil production was stressed throughout the study.
