Abstract

This study focuses on the Tertiary Wasatch Formation in Natural Buttes field, Utah which is a highly complex clastic reservoir not easily evaluated with conventional wireline logs. Similar to many other Rocky Mountain clastic Tertiary reservoirs, it is characterized by authigenic clay minerals which present major problems in evaluation and production. Standard clay analysis from logs is not adequate in the Wasatch due to a high percentage of potassium in the matrix. Owing to the complex nature of the Wasatch, the formation was selectively sampled in a representative well by obtaining drilled-sidewall cores to provide additional information for more complete formation evaluation. Basic and advanced core analysis, including X-ray diffraction, thin-section petrography, scanning electron microscopy and mercury injection greatly facilitated analysis of this variable and complex reservoir.

The results of X-ray diffraction analysis, thin-section petrography and especially scanning electron microscopy identified the relative amount, type and overall distribution of the clay minerals in these samples. Authigenic chlorite and kaolinite occur as pore lining and filling materials and may cause extensive damage to the formation if migration takes place. Optimized production may be achieved for these types of reservoirs by being aware of the type, distribution and relative amount of clay minerals present, and by conducting drilling, stimulation, testing and production operations accordingly. Clay mineral mobilization, fluid sensitivity and iron/acid susceptibility can be significantly reduced with proper planning. Without cores, either conventional or drilled-sidewall, it is difficult to adequately evaluate formations like the Wasatch for optimum completion. Utilizing core data as an input to “inverse” log evaluation programs such as ULTRA* (Universal Logging Tool Response Analysis), provides a powerful method for understanding and making best use of conventional log data.