A PRACTICAL APPROACH TO PETROPHYSICAL MODEL CONSTRUCTION IN ARKOSIC AND SUB-ARKOSIC SEDIMENTS OF THE SESPE FORMATION USING LOG BASED GEOCHEMICAL SPECTROSCOPY DATA

The Oligocene Sespe formation has produced more than 400 MMBO in Ventura County, California. The arkosic reservoir sands contain highly variable mineralogy with significant quantities of feldspars, lithic fragments and clay minerals. The petrophysical response of the Sespe has been very difficult to interpret since there is often little resistivity contrast between reservoir and non-reservoir rocks. The practical goal is to infer the relative quantities of oil and brine in the pore space of the rocks from knowledge of the complex mineralogy and the porosity of the rocks. This task is greatly facilitated by knowledge of the elemental composition of the rock matrix and its relationship to detailed mineralogy of the composite reservoir material. A geochemical instrument capable of measuring the elemental abundance of the major rock forming minerals provides a means to quantify the complex mineralogy of the Sespe formation and provide an accurate petrophysical interpretation.

A variety of models have been proposed by various authors for brine saturated rocks and other synthetic composite materials. While it is unrealistic to assume the existence of a universal mixing law appropriate for all rocks (irrespective to their depositional source), it seems reasonable that specific families of rocks, each characterized by a suite of common minerals, could be described by a specific geochemical model. Thus for example an arenitic sandstone might be described by a different geochemical model than that which is appropriate to describe an arkosic sandstone.

In this paper we propose an analytical methodology to construct a mineralogy model to describe the arkosic and subarkosic rocks of the Sespe formation. This geochemical based model is shown to be in good agreement with laboratory mineralogy and chemistry measurements of formation samples collected over a limited range of sedimentary rocks contained within the Sespe formation. The core based model can then be applied to the much larger Sespe formation using downhole geochemical logs. This scenario has been successfully applied in the West Montalvo field, with good agreement between log based petrophysical interpretation and production results.