Exploring for a wide range of hydrocarbon reservoirs, including carbonate systems is increasingly important in times of higher resource demand and progressively dwindling reserves.
Exploration for carbonate systems is generally more difficult than siliciclastic reservoir exploration because of intrinsic heterogeneities, which 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 intrinsic complexities can be related to geological processes controlling carbonate production and deposition, and to changes during their subsequent diagenesis. The term "heterogeneity" is rarely defined and almost never numerically quantified in petrophysical analysis, although it is widely stated that carbonate heterogeneities are poorly understood.
This work in carbonates of the Tal block has investigated how heterogeneity can be defined and how we can quantify this term by describing a range of statistical heterogeneity measures (e.g., Lorenz and Dykstra-Parsons coefficients). These measures can be used to interpret variation in wireline log data, allowing for comparison of their heterogeneities within individual and multiple reservoir units. Through this investigation, the Heterogeneity Log has been developed by applying these techniques to wireline log data, over set intervals of 10, 5, 2 and 1m, through a carbonate reservoir.
Application to petrophysical rock characterization shows a strong relationship to underlying geological heterogeneities in carbonate facies, mud content and porosity (primary & secondary porosities) in the Tal block. Zones of heterogeneity identified through the successions show strong correlation to fluid-flow zones. By applying the same statistical measures of heterogeneity to established flow zones it is possible to rank these units in terms of their internal heterogeneity. Both increased and decreased heterogeneity are documented with high reservoir quality in different wireline measurements; this can be related to underlying geological heterogeneities.
Heterogeneity Logs can be used as a visual indicator of where to focus sampling strategies to ensure intrinsic variabilities are captured.
Carbonate lithology and mineralogy can be highly variable, both vertically and horizontally through a succession. Carbonate depositional environments produce a diverse range of sedimentary facies which contain different porosity types with varying degrees of connectivity, producing complex and irregular pore networks. Minerals such as calcite, aragonite, and dolomite may co-exist within a single rock unit in varying proportions. Carbonate minerals have different stabilities and are susceptible to the many postdepositional processes of diagenesis.
Porosity-permeability relationships in carbonate reservoirs are notoriously poorly defined, although work by authors such as Lucia (1995; 2000) suggest correlations can be derived from pore type and grain size relationships. The ability to predict porosity and permeability relationships in carbonates continues to be an area of industry research interest. Reservoir zonation is often established using poroperm features through complex statistical analysis, although traditional placement of flow zone boundaries comes down to visual assessment and an analyst's experience and expectations.
This study therefore focuses on developing these techniques and applying them to carbonate petrophysical and geological data including borehole image and cre data in the Tal block, which can have further application to characterizing poroperm relationships, fluid flow zone identification and sampling strategies.