--> Abstract: Unusually Thick Microcodium Occurrences in Early Permian Carbonates, Timan Pechora Oil Field (CIS): Origin and Significance for Reservoir Properties, by Philippe A. Lapointe, Andrew J. Barnett, and Pavel Kabanov; #90039 (2005)

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Unusually Thick Microcodium Occurrences in Early Permian Carbonates, Timan Pechora Oil Field (CIS): Origin and Significance for Reservoir Properties

Philippe A. Lapointe1, Andrew J. Barnett1, and Pavel Kabanov2
1 TOTAL E&P, 64018 Pau Cedex, France
2 Paleontological Institute of Russian Academy of Sciences Moscow, Moscow 117647, Russia

The Studied oil field, located in the Timan Pechora Basin (Komi Republic, CIS), is a multi-horizon oil-bearing reservoir. The Permian carbonates are one of the reservoir targets. Core and thin section observations reveal an extraordinary abundance of Microcodium as suggested by close macro- and micromorphological similarity to the Cretaceous-Tertiary Microcodium type. It shows an unusual thickness and density. In one well, near pervasive replacement of metre- to decametre-scale units by Microcodium occurs up to ca. 30m below the sub-Artinskian boundary.

The carbonate platform suffered long term sub aerial exposure evidence of which occur below the sub-Artinskian disconformity in several wells. These include calcrete development at the top of the Microcodium invaded zone which is indicated by rhizocretions, secondary laminar-cloudy fabric, alveolar-septal fabric, brownish micrite, strongly degraded or completely obliterated relics of marine limestone texture, and the negative shift of δ13C.

Microcodium morphology within the calcrete is distinct from that below by its small isodiametric or transverse grains, loose texture, and degraded generations of Microcodium grains. Non-selective Microcodium infestation of the Asselian-Sakmarian carbonate platform started each time the platform top was sub-aerially exposed.

Actinomycete bacteria are suggested to be responsible for Microcodium formation. The main reorganization of marine carbonates into Microcodium took place during sub-Artinskian pedogenic event.

Microcodium replacements and their burial diagenesis are responsible for considerable changes in rock properties. Microcodium distribution shows weak to moderate, non-steady negative correlation with porosity and permeability properties. Microcodium horizons are reservoir prone only where they have been leached by meteoric fluids.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005