--> ABSTRACT: Fluvial Mudstone Breccias and Their Petroleum Significance, by Peter E. Putnam; #91038 (2010)

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Fluvial Mudstone Breccias and Their Petroleum Significance

Peter E. Putnam

The classic fining-upward model of fluvial deposition places mudstone breccia fragments as basal channel lag deposits. Basal breccias can form by bank erosion and collapse by migrating channels and channel down-cutting into preexisting mudstones. However, mudstone breccias associated with fluvial sediments display much wider distributions and can be found at the top of channel fills. Some formative mechanisms for breccias found toward the tops of fluvial sequences are (1) gravity sliding down point bar surfaces; (2) bank erosion and collapse by migrating underfit streams found within abandoned channel reaches undergoing vertical accretion and; (3) oversteepening and collapse of channel banks in response to stage fluctuations. Thus, breccia deposits can be located above or adjacent to well-sorted porous and permeable sands.

In the subsurface, fluvial breccias are difficult to recognize in core if individual clasts are larger than the borehole diameter and flat lying. Dense concentrations of clasts also influence log readings by displaying high gamma-ray and relatively positive spontaneous potential responses. Core analyses commonly give misleadingly low indications of porosity and permeability because of the relatively small sample sizes available. It is very easy to mistake thick, dense concentrations of mudstone breccia for the deposits of shale-filled channels. Breccias found at the top of fluvial sequences are commonly overlooked reservoirs because hydrocarbons will be found in zones characterized by very large impervious blocks formed of muddy sediment. Recognition of the presence and distribution o breccias is crucial in the exploration and development of channel reservoirs.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.