The Carbonate Analogs Through Time (CATT) Hypothesis – A Systematic and Predictive Look at Phanerozoic Carbonate Reservoirs

James R. Markello, ExxonMobil Upstream Research Company, Houston, TX

The Carbonate Analogs Through Time (CATT) Hypothesis defines an approach for developing systematic evaluations and predictive models of Phanerozoic carbonate systems and reservoirs for use in exploration, development, and production businesses. The CATT hypothesis simply stated is: "insightful, high-confidence, age-specific predictive models for carbonate systems and reservoir occurrence, composition, stratal attributes, and reservoir properties can be developed by summing the ambient conditions of the carbonate processes and Earth processes at any geologic age." We term these models age-sensitive patterns. The hypothesis is built upon the knowledge that demonstrates carbonate and Earth processes have differentially varied throughout Phanerozoic time. These processes include: 1) ecologic, oceanographic, sedimentologic process-based controls on carbonate factory development; 2) stratigraphic and accommodation process-based controls on carbonate stratal architecture; 3) secular trends of evolution, grain mineralogy, tectonics, climate, eustasy, ocean circulation, and ocean chemistry. Two key research products are a poster of secular varying geologic controls synchronized to the time-scale, and a global atlas containing 29 paleogeographic maps with details of known Phanerozoic carbonate systems/reservoirs and age-sensitive patterns.

1. Developing an “age-sensitive pattern” is when the paleogeographic map-view configuration and spatial relationships of carbonate systems are convolved with the ambient states of the carbonate and earth processes for that time period. The Ellenberger formation and reservoirs of west Texas are representative carbonate systems/reservoirs basis for the Cambro-Ordovician time-based theme. Expectations for typical Cambro-Ordovician carbonate reservoirs are 1) meter-scale peritidal mud-dominated cycles, 2) thin bedded, heterogeneous layering, 3) thrombolitic/ microbial buildups only, 4) moderate reservoir quality from dolomitization, 5) karst porosity beneath the top-Sauk unconformity, and 6) locally fracturing.

2. Sometimes there are significant differences between carbonate systems and reservoirs within a geologic time period or age. The CATT Hypothesis and Atlas provide an approach and tools for comparative analysis between coeval systems that gives insight for causes of differences. An example is contrasting Late Jurassic systems/reservoirs of the Arabian Basin (Arab Formation fields) with those of the northern Gulf of Mexico (Smackover Formation Fields).

3. The utility of these tools for analog selection is illustrated by explaining the heritage-Mobil example of farming-into Tengiz field in the mid-1990's. Buying equity in a field under development requires knowledge of field value (working-interest EUR) and measure of investment return. Typically, these numbers are derived by simulation. Mobil engineers asked for the best field/reservoir analog on which to base a Tengiz simulation. Would Arun field (Miocene) in Indonesia be okay? We answered absolutely not! Based on our CATT approach, the best analogs would be Devonian/Carboniferous fields in the Volga-Ural trend or North America.

4. The CATT Hypothesis coupled with basic concepts of carbonate geology, sedimentology, and stratigraphy can be used to construct many different types of predictive concepts. These can range from very simple to quite complex. A simple CATT-based predictive concept is Late Permian ramps will lack major framebuilt boundstones, be peloid/ooid-dominated, and be mostly dolomitized with associated evaporites. A more complex predictive concept is for platforms formed during icehouse times (Late Carboniferous to Early Permian; Late Tertiary), 4rd-order high-amplitude, high-frequency sea level changes result in vertically discontinuous sequences with internal lateral facies heterogeneities; marginal boundstones will be vertically separated.