AAPG Geoscience Technology Workshop

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Carbonate Play Models From Miocene Outcrops, Western Mediterranean: Part I - Setting and Stratigraphy

Abstract

The Almeria region of southeastern Spain is known for exceptional exposures of Upper Miocene heterozoan, coral reef, and oolite-microbialite carbonate reservoir analog systems that developed regionally in the western Mediterranean. Miocene paleotopography is largely preserved and 3D exposures allow direct documentation of sequence architecture and facies distributions along proximal-to-distal transects in a variety of paleotopographic settings. Prior to carbonate deposition, Neogene volcanic and Paleozoic-Mesozoic metamorphic rocks were subaerially exposed and eroded, forming highly variable substrate paleotopography upon which the carbonates were deposited. The settings for carbonate deposition include archipelagos, with closely spaced steep-sided highs surrounded by straits and flooded paleovalleys, and marginal basins surrounded by carbonate platforms. Our studies for more than three decades have isolated and quantified the effects of sea level, paleotopography, oceanography and climate on sequence architecture, facies distribution, and reservoir character. Five major 3rd and 4th order sequences are identified in the study area. Marine inundation of the volcanic and metamorphic substrate resulted in deposition of two heterozoan carbonate sequences during temperate climate conditions. The heterozoans are mostly grainy deposits consisting of molluscs, red algae, foraminifera, bryozoans, and echinoderms (+/- siliciclastics). The loose grainy nature of these systems made them susceptible to reworking and transport, which depended on sea-level changes, water energy, and substrate slope angles. Where shallow water intersected gentle substrate slopes, shallow-water autochthonous sediments were preserved. Deposition and distribution was tied to sea-level fluctuations and where currents and nutrients were focused. Where shallow water intersected steep substrate slopes (> 15 o), processes of downslope bypass dominated. Shallow water heterozoan sediments were not preserved in situ, and instead, they were transported as sediment gravity flows to areas of lower slope, where they are interbedded with hemipelagic deposits. Two overlying photozoan coralgal reef sequences indicate a transition to a tropical climate. Porites and Tarbellastrea corals are the major framebuilders. When steep-sided volcanic slopes (>15o) were in shallow water, processes of bypass dominated. Initially, fringing reefs developed on the steep slopes, but those steep slopes and relative sea-level falls promoted their transport as sediment gravity flows into deep water, without in place preservation of reefs. Where shallow water intersected substrates with gentle slopes (<15 o) fringing and isolated reefal platforms developed, preserving reef to forereef slope to basin facies transitions over lateral distances of 0.5 to more than 2 km. Aggradational, progradational, and downstepping geometries preserve much of the sea-level cycle. Downstepping in the latest stages of reef development indicate a major sea-level fall that culminated in subaerial exposure and erosion of the reefal platforms, forming an unconformity that is likely equivalent to the Messinian lower evaporite unit in the Mediterranean Basin. Sea-level rise after, or perhaps partially during, upper evaporite deposition resulted in deposition of an upper stratigraphic unit in margin areas composed of oolite, microbialite (thrombolite and stromatolite), and minor coralgal reefs. Within this unit, four sequences were deposited in association with high-amplitude glacioeustasy and evaporitic drawdown. At intermediate substrate positions (relative to sea-level history), each of the four sequences drape paleotopography and maintain relatively equal lateral thickness. Internally, the sequences have a build-and-fill architecture, characterized by a relief-building phase and a relief-filling phase. Microbialites dominate during the relative sea-level rises and build topographic relief. Oolites dominate during relative sea-level falls and fill topographic relief. The studied carbonate systems are excellent analogs for Miocene carbonate reservoirs, such as those in eastern and central Mediterranean areas, and the Indo-Pacific region. The results of our studies provide predictive capabilities for reservoir model development of thirteen play types (see Part 2 abstract).