AAPG Annual Convention and Exhibition

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The Less Conventional Side of Carbonates


Carbonates are a significant component of many unconventionals—a diverse group of continuous and tight reservoirs. Though they differ in trapping mechanism, these reservoir types share complex pore systems and a need for stimulation. As such, facies heterogeneity plays an important role through its impact on mechanical behavior and reservoir properties. Here, we present a model for facies heterogeneity in low permeability carbonate-rich reservoirs, using case studies to illustrate common characteristics. Platform top reservoirs may have low permeability due to low porosity, cemented grain-rich facies or to microporous, mud-rich facies. Platform top facies typically constitute tight, conventionally trapped reservoirs, though may rarely be continuous reservoirs ‘coupled’ to adjacent source rocks. Facies belts are typically broad, with stacking of m-scale cycles influencing the mechanical stratigraphy. In platform margin and slope settings, cementation commonly generates low matrix permeability, but these facies are prone to natural fractures, creating conventional reservoirs. Growth of these margins typically leads to stratigraphically thick mechanical units with high lateral heterogeneity. Reservoirs in deep basins typically constitute continuous accumulations, with character depending largely on margin type and proximity to the shelf margin. More proximal settings include channelized and debris-rich facies that generate higher lateral heterogeneity. Facies geometries and stacking relate to margin type, stability, slope angle, and accommodation changes. Such deep basin reservoirs are dominated by sediment gravity flow processes. A final class of unconventional reservoirs constitutes drowned platforms, which typically are continuous reservoirs. These deposits form when a widespread carbonate platform is drowned during a second order flooding event. Platforms commonly backstep, and subsequently interfinger with basinal facies during regressions. Such deposits are typically influenced by storms or shelf currents.