Abstract

Quantitative information about the reservoir rock minerals is important for making technical and business decisions in hydro-carbon exploration and exploitation. Minerals are usually quantified using mineral properties available from published data and rock properties measured in the laboratory used cored samples or in the field using geochemical well logs. Despite considerable efforts by many researchers, the rapid quantification of minerals. with error estimates remains a challenge. The most widely used, method for rapid mineral quantification is the matrix algebra method that uses the least-squares principle. Although fast and easy to implement, the conventional matrix algebra method is computationally unstable leading to unrealistic values (negative or greater than one) for mineral fractions.

In this paper, we present a computationally stable method that retains the speed of the conventional matrix algebra method while overcoming its limitation. The present method can be applied to both laboratory (core samples) and downhole (geo-chemical well logs) analyses. It is effective in handling over determined, determined, and under-determined systems. It also handles both fixed and variable mineral properties.

Unlike the conventional matrix algebra method, the present method supplements the rock and mineral properties with several constraining equations that incorporate prior information about the mineral fractions. The prior information about the mineral fractions, and the measured rock properties are weighted by the reciprocals of their respective error variances. Involving only matrix operations, the resulting equation to obtain mineral fractions is easy to implement and fast to compute.

Programmed as an Excel macro or in Visual Basic, the method has been successfully implemented in our laboratory since 1993 for quantifying minerals in core samples from diverse rock formations.

Introduction

Reservoir rocks are an assemblage of minerals such as quartz, carbonates, feldspars, kaolinite, illite, and smectite. Quantitative analysis of these minerals is used by exploration and production geologists as well as by reservoir and production engineers.

Exploration geologists use the rock mineralogy to reduce the risk in discovering oil by determining the thermal and diagenetic history of a basin, defining the provenance (source area) and the depositional environment of the sediments, and correlating certain minerals with well logs. Exploitation geologists and reservoir engineers use the rock mineralogy to assess reservoir quality develop effective depletion strategies, and predict the effect of rock-fluid interactions. Production engineers use the rock mineralogy to design work-over and completion strategies such as selection of drilling fluids and proper stimulation methods (e.g., acidizing).

Traditionally, rock mineralogy has been determined in the laboratory using cored rock samples(1–4). Over the past few years, there have been efforts to develop mineral logs by downhole geochemical welllogging(5–7).

Laboratory and Downhole Mineral Quantification

The methodologies for routine laboratory and downhole mineral quantification are similar. In both cases, the rock properties and mineral properties are used to estimate mineral fractions. In the laboratory, X-ray fluorescence is used to measure rock properties (elemental composition expressed as oxides). These properties can be supplemented by other rock properties such as loss on ignition (LOI) and cation-exchange capacity (CEC).

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