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Influence of Inundated Erosional Landscapes in Localizing Coarse-Grained Heterozoan Carbonate Reservoir Facies

Abstract

Paleotopographic features may provide an opportunity for predicting the character and distribution of heterozoan carbonate reservoir facies. Deposition and reservoir character of heterozoan carbonates respond positively to currents and elevated nutrients, resulting in the localization of higher quality facies at paleotopographic features acting as straits between water masses, rather than in isolated paleovalleys or embayments. To evaluate this depositional model, we examine outcrops of Miocene carbonates in the Cabo de Gata region, SE Spain. In this area paleotopography is preserved with little structural deformation, and facies distribution is controlled by antecedent erosional topography that is seismically resolvable in scale. Miocene heterozoan deposition unconformably overlies eroded volcanic basement near the intersection of the Agua Amarga, Níjar-Almería, and Rodalquilar depocenters. Digital outcrop models were constructed from field mapping, stratigraphic observations, and UAS photogrammetry, illustrating 200 meters of relief on the unconformity across the 30 square kilometer area. Observable features include a discontinuous ridge, isolated highs, a broad low-relief surface, and topographically low features that slope away from the highs and in some places converge. These features are interpreted as subaerial erosional incision forming paleovalleys and drainage networks. Carbonates overlying the unconformity are coarse-grained heterozoan facies including mollusk-bryozoan packstone, bryozoan-rich packstone, serpulid framestone, and mixed volcaniclastic/carbonate facies. The most extensive paleovalleys would have flooded during the late Miocene forming straits that connected water masses and focused currents, resulting in extensive heterozoan deposition. Thickness is highly variable with facies onlapping the paleotopographic highs, but reaches a maximum of 50 meters in the center of these straits. Other less laterally extensive flooded paleovalleys terminated against basement highs and did not focus currents between different water masses. Although these features had similar substrate slopes and accommodation, they did not accumulate large amounts of coarse-grained heterozoan facies. This study provides a conceptual model to inform more accurate subsurface prediction of carbonate reservoir facies using seismically-resolvable paleotopography, and identification of laterally extensive paleovalleys that focused currents may be a key predictor.