Jon R. Schwalbach1, William C. Benmore2, S. Anthony Reid2, Roger D. Tucker2, Edward J. Hanley2
(1) Occidental of Elk Hills, Tupman, CA
(2) Occidental of Elk Hills
ABSTRACT: Silica Phase and Compositional Controls on Pore Throat Distributions--Porcelanite and Siliceous Shale of the Elk Hills Field, California
Porcelanite and siliceous shale make up the majority of the rock volume of the Miocene reservoirs at Elk Hills Field. Both rocks have high matrix porosity and have the potential to store vast quantities of hydrocarbon. Hydrocarbon saturation, however, varies widely and is partly controlled by pore throat distribution. Pore throat distribution as measured by mercury injection techniques is a function of silica phase and rock composition. Silica phase is a primary control - rocks that are predominantly opal CT have very small pore throats (0.1-0.01's of microns). This limits the hydrocarbon saturation these rocks can achieve given the column heights encountered in the reservoir. Quartz-phase siliceous shale and porcelanite have pore throats that are generally an order of magnitude larger than opal CT rocks of similar composition. Although the porosity of the quartz-phase rocks is lower, the hydrocarbon saturation and oil contained in a given rock volume is significantly higher.
Chemical analyses, photomicrographs, and SEM photographs illustrate the geometry of the pores and relate the rock composition and fabric to the pore throat distribution. For example, rocks with high clay content have small pore throats regardless of silica phase. In some instances the mercury data indicate bimodal pore throat distributions. Often these rocks have silt or fine sand laminations within a finer-grained matrix, or are in a transitional diagenetic state and contain both opal CT and quartz silica.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado