PARASEQUENCE-SCALE DEPOSITIONAL CYCLES AND SILICA DIAGENESIS CONTROL RESERVOIR PROPERTIES FOR MIOCENE BIOSILICEOUS RESERVOIRS OF THE ELK HILLS FIELD, CALIFORNIA

Silica phase and composition are the primary controls on matrix reservoir properties of the biogenic Monterey Formation reservoirs of the Elk Hills Field, California. Porcelanite, siliceous shale, and chert are the rocks that contain significant biosiliceous precursors. Silica phase is a function of burial diagenesis and can be predicted with a knowledge of burial history. Composition varies in depositional cycles ranging from millimeter-thick seasonal lamina to several-meter-thick packages that probably represent parasequences. These parasequences are characterized using cores and open hole well logs, and stack predictably in certain stratigraphic intervals.

We use a rock’s pore throat distribution measured by mercury injection techniques as a proxy for reservoir quality. Rocks with abundant larger pore throats achieve greater hydrocarbon saturation and generally have higher matrix permeability. Silica phase is a primary control – rocks that are predominantly opal CT have very small pore throats (0.1-0.01’s of microns) with significant internal rugosity. Quartz-phase rocks have pore throats that are generally an order of magnitude larger than opal CT rocks of similar composition, and tend to be of simpler geometry. Although the porosity of the quartz-phase rocks is lower than compositionally similar opal CT rocks, the hydrocarbon saturation and oil contained in a given volume of quartz-phase rock is significantly higher. Chemical analyses, photomicrographs, and SEM photographs enable us to compare the geometry of the pores and relate the rock composition and fabric to the pore throat distribution. Our data document that rocks with high clay content have small pore throats regardless of silica phase. The mercury data for some samples 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. These observations combining rock data, stratal stacking, and burial history, enable us to predict reservoir quality within a specific stratigraphic framework.