The giant oil fields of Saudi Arabia are similar to other fields in that there are thick obvious pay zones, and intervals of low contrast pay. Low contrast pay can be caused by a number of factors such as deep invasion, shale effect (as in dispersed clay effects) and thinly bedded intervals. Thinly bedded intervals are often the most interesting, in that even though the productivity of the interval appears poor from conventional interpretation, there are often high porosity high permeability layers to be exploited. These thinly-bedded low contrast intervals need to be identified and evaluated so that they are recognized and included in reserves and completion strategies.
A study was undertaken to better evaluate thinly bedded intervals in a large clastic reservoir. The study involved the application of different technologies to better evaluate thinly bedded intervals. Magnetic resonance was used to better define the free fluid portion of the reservoir, and to delineate permeable sections. Oil base mud imaging was utilized to characterize the bedding and resistivity anisotropy measurements were integrated using standard published techniques that led to a better evaluation of the thinly bedded intervals. These technologies allowed for the evaluation of different approaches to the reservoir evaluation. These approaches did not change the conventional evaluation in thick clean reservoir zones and intervals with dispersed clay. In thinly bedded intervals our evaluation led to a better estimate of reservoir potential.
The approaches taken are discussed and their limitations to other clastic reservoirs and future developments are reviewed.