Heterogeneity in Carbonate Reservoirs

Understanding carbonate reservoirs can be challenging due to the intrinsic heterogeneities that occur at all scales of observation and measurement. Heterogeneity in carbonates can be attributed to variable lithology, chemistry/mineralogy, pore types, pore connectivity, and sedimentary facies. These inherent complexities can be related to processes controlling original deposition and their subsequent diagenesis. Although it is widely stated that carbonate heterogeneities are poorly understood, the term ‘heterogeneity’ is rarely defined or numerically quantified.

This work investigates how heterogeneity can be defined and how we can quantify this term by describing a range of statistical heterogeneity (e.g. coefficient of variation and the Lorenz coefficient). These measures can be used to interpret variability in wireline log data; enabling a comparison of heterogeneities between different measurements and tools, and within individual or between multiple reservoir units. A Heterogeneity Log has been developed as a result of applying these techniques to wireline log data through a carbonate reservoir, over set intervals of 10, 5, 2 and 1m. Strong heterogeneity contrasts are identified across a suite of logs, indicating an underlying geological control, for example meter-scale geological heterogeneities in carbonate facies and mud content. Zones of heterogeneity show strong correlation to traditional fluid flow zonations, and by applying the same statistical measures of heterogeneity to established flow zone units it is possible to rank these in terms of their internal heterogeneity. Increased reservoir quality correlates with both increased and decreased heterogeneity depending on the type of wireline measurement and can be related to underlying geological heterogeneities and measurement types.

Key Topics to be covered include:

• What does Heterogeneity mean?
• Types of heterogeneity in carbonate reservoirs
• Characterizing heterogeneity in a dataset
• Examples of heterogeneity measures
• Basic application to a reservoir succession
• The Heterogeneity Log
• Characterizing geological features
• Insights into reservoir compartments
• Insights into physical properties
• Heterogeneity and reservoir quality / flow units