--> --> Carbonate Play Models from Miocene outcrops, Western Mediterranean: Part 2 – Stratigraphic and Diagenetic Plays

AAPG Geoscience Technology Workshop

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Carbonate Play Models from Miocene outcrops, Western Mediterranean: Part 2 – Stratigraphic and Diagenetic Plays

Abstract

Miocene (Tortonian and Messinian) carbonate outcrops in SE Spain are reservoir analogs potentially useful for plays in the Eastern Mediterranean. The stratigraphic succession of heterozoan (lower), reefal platform (middle), and oolite-microbialite (upper) was affected by complex paleotopography. Thirteen distinct play analogs fall into 6 categories: (1) shallow-water heterozoans; (2) deep-water heterozoans; (3) coralgal reefal platforms; (4) deep water equivalents to coralgal platforms; (5) oolite-microbialite sequences; and (6) diagenetic plays. (1) Three shallow water play analogs developed as heterozoan systems during times of cooler climate or nutrient excess. For one, a strait linking two basins concentrated current energy, depositing cross-bedded grainstone in the strait’s paleotopography. An angular unconformity on top led to isolated paleohills, sealed after transgression. Another play analog consists of shallow-water grainy heterozoan carbonates deposited as a distal wedge during transgression. The third play analog consists of ramp deposits with seven draping and onlapping transgressive-regressive cyclothems; bryozoan, bivalve, and red algal facies dominate. (2) In areas with steep substrate paleotopography, shallow-water heterozoan sediment was bypassed downslope as sediment gravity flows. The sediment accumulated in channels, minibasins, and straits. Internal stratigraphy is cyclic, and sediment gravity flows alternate with hemipelagic sediment deposited during high sea-level stands. Sediment gravity flow deposits onlap against paleotopography and are encased in deeper water hemipelagic sediment deposited as part of a long-term transgression. (3) As conditions became favorable for photozoan carbonates, shallow-water coralgal reefal platforms dominated. One reefal play analog consists of isolated and fringing platforms. The reefal facies consists of corals, Halimeda, and associated components. Reefs are laterally discontinuous, with framework and moldic pores that may be variably connected. In comparison, a forereef slope play analog is volumetrically more important than the in-place reef. Forereef slope deposits are progradational and downlapping, consisting of breccias and grainy facies with better-connected porosity than the in-place reef. (4) Three different types of deep water play analogs developed at the same time as coralgal reefs. The most prospective play consists of focused flow deposits, where reefal debris was bypassed downslope by sediment gravity flows and focused into a deep water channel. The paleotopographic focus led to a reservoir with high porosity and low heterogeneity. In areas lacking such a paleotopographic focus, grainy sediment gravity flows accumulated in isolated lobes, leading to high heterogeneity. In the most distal areas, hemipelagic chalks, diatomites, and low density turbidites led to deposition of high volumes of fine-grained deposits with petrophysical properties consistent with conventional chalk reservoirs, or with burial diagenesis, properties consistent with unconventional reservoirs or source rock. (5) After initial deposition of Messinian evaporites, four sequences of oolite and microbialite draped basin margin paleotopographic highs. The oolite preserves abundant primary and moldic porosity, and the microbialite preserves variable porosity, depending on original fabrics and later cementation. (6) Diagenetic processes contribute to three play analogs. The most important is dolomitization from ascending freshwater-mesohaline mixing, which is highly correlated with moldic porosity. The best reservoir porosity is created in distal settings close to the base of the carbonate platform. Other diagenetic plays are either not associated with, or are negatively associated with, subaerial exposure along seven subaerial unconformities. Most exposure events had little impact on reservoir porosity. Along one unconformity, however, significant amounts of calcite cement precipitated because of increased rainfall. This led to porosity preservation in areas away from water table cementation. Finally, hypogene karstic porosity creates another play type, where sulfuric acid-rich chemically corrosive fluids were injected from below. The thirteen play analogs illustrate the complexity of predicting reservoir porosity in Eastern Mediterranean carbonate reservoirs. Despite such complexity, each play is either seismically or geologically predictable.