--> ABSTRACT: Offshore Bars or Incised Valleys? A Sequence Stratigraphic Approach - The Tocito Sandstone (Cretaceous), New Mexico, by Clive R. Jones, David C. Jennette, John C. Van Wagoner; #91020 (1995).

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Offshore Bars or Incised Valleys? A Sequence Stratigraphic Approach - The Tocito Sandstone (Cretaceous), New Mexico

Clive R. Jones, David C. Jennette, John C. Van Wagoner

The Tocito sandstone is the largest oil producer in the San Juan basin, New Mexico. Production is from long (>20 miles) linear sandstone bodies trending NW-SE across the basin.

Most previous publications on the Tocito interpret the basinal distribution, geometry and facies associations to represent an offshore bar complex. Sandstones are usually encased in shale above, below and apparently laterally, giving rise to the interpretation that shelfal processes had created sand build-ups, rooted in offshore muds.

To investigate the nature of these 'offshore bars', a detailed sequence stratigraphic analysis of the Tocito, both in outcrop and the subsurface was initiated. The data set consists of 15 outcrop measured sections, 11 cores/core photograph sets and 14 well log cross-sections utilizing 1200 wells, datumned above the Tocito using bentonite markers. Interpretation of these data, supported with paleontological evidence shows that the Tocito comprises at least three sandstone rich sequences. Each sequence was deposited in a fluctuating high to low energy tidal setting, within linear, structurally influenced paleo-valleys. The geometries of the Tocito sandstone bodies are controlled by truncation and onlap. No lateral facies changes to shelfal mudstones have been observed at valley edges. B sal Tocito contacts are abrupt and erosional, often with coarse pebble lags lining valley bottoms. Paleosols are occasionally developed on interfluve areas, which rest directly on underlying offshore mudstones. These contacts fulfill three major criteria for sequence boundary recognition: widespread truncation and incision of underlying strata downward shift in facies and evidence of subaerial exposure.

A sequence-stratigraphic approach is fundamental to the understanding and exploitation of these types of stratigraphically trapped hydrocarbon accumulations.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995