Abstract: A Facies Analysis of the Misoa Formation (Eocene) of Venezuela

Roger Higgs

The Misoa Formation, the main oil reservoir of the Maracaibo Basin, is a northeast-thickening wedge of sandstones and mudstones (minor limestones) reaching 5 km in thickness. Outcrops and published core descriptions show three main facies: (1) sandstone bodies 3-20 m thick; (2) mudstone with trains of symmetrical sandstone ripples; and (3) mudstone. Sand bodies are tabular (non-channeled) and comprise stacked, decimetric tabular cross-sets, variably masked by bioturbation; some sets are capped by symmetrical ripples and draped by a thin (mm-cm) mud layer. Fossils are rare but diverse, including echinoids, bivalves and gastropods, occurring in both sandstones and mudstones. Sand bodies contain a Skolithos ichnofacies, of Arenicolltes, Ophiomorpha, ?Skolithos and Thalassinoides. The three facies can occur in any vertical order, such that Facies 1 sand bodies can appear to hav either sharp or gradational bases and tops.

The depositional environment is interpreted as below fairweather wavebase in a permanent water body (sea or lake), based on: (1) the lack of evidence for emergence, such as coats or paleosols; and (2) the tabular cross-bedding, uncharacteristic of the beach or shoreface. The Skolithos ichnofacies and echinoids suggest a sea rather than a lake. Shelf depths are implied by the pervasive symmetrical (wave) ripples, while their intimate association with cross-bedding, rather than hummocky cross-stratification, suggests interaction of tides and storms. Thus, the Misoa sand bodies are interpreted as tidal-shelf sand sheets influenced by storms, comparable to published models for the Jura Quartzite and other formations.

The tidal-shelf model is crucial for predicting reservoir geometry and supercedes the antiquated deltaic model.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela