--> Abstract: Facies Architecture and Diagenetic Control on Middle Eocene Tide-Dominated Deltaic Sandstones in Lagunillas Field, Maracaibo Basin, Venezuela, by J. Maguregui; #90987 (1993).

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MAGUREGUI, JESUS, Intevep, S. A., Caracas, Venezuela

ABSTRACT: Facies Architecture and Diagenetic control on Middle Eocene Tide-Dominated Deltaic Sandstones in Lagunillas Field, Maracaibo Basin, Venezuela

Sedimentological interpretation of Middle Eocene "B-2-X Sandstone" in Eastern Lagunillas Field suggests that several aggradational tide-dominated deltaic cycles were deposited during a general transgression at the end of middle Eocene. Facies present are estuarine distributary channel sandstones, tidal sand ridge sandstones, prodelta/shelf assemblages, and tidal channel facies. Geometries of sandstone bodies are highly dip-elongate and strike lenticular. These shoestring sands maintain their widths in landward direction (west) and splay outwards at their seaward limits (east) where loci of sand deposition occurred. Individual, correlatable depositional units are interpreted to be discrete delta lobes established during progradation of an estuarine distributary channel complex. Associa ed interdistributary areas consist of tidal delta plain and tidal channel deposits. Seaward, the facies tract merges into tidal sand ridge and marine shale facies.

Diagenetic modification changed substantially the mineralogical composition and the pore space characteristics of these sandstones. At present, sandstones are subarkosic to sublitharenitic, within the very fine to lower medium grain size range and highly-cemented mainly by quartz overgrowths, and in some cases, by calcite cement. Diagenetic processes transformed the original, highly-interconnected pore space distribution (all primary porosity), into a pore-disseminated, irregular, poorly-interconnected pore space framework (minor primary and mainly secondary porosity). Porosity was initially reduced by mechanical compaction and quartz/calcite cements and subsequently enhanced by cement and grain dissolution. Permeability, on the other hand, is highly sensitive to the irregular porosit framework and to the presence of illite and mixed layer illite/smectite authigenic clays.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.