Abstract: Low Resistivity Pays in J Sandstone, Deep Basin Center Accumulations, Denver, Basin

R.J. Weimer, S. A. Sonnenberg

Low resistivity low contrast (LRLC) pays in the J Sandstone (Lower Cretaceous) occur in the deep part of the Denver basin. The productive potential of these anomalous intervals is overlooked in log analyses by most operators because low resistivities are considered indicative of wet or tight non-productive reservoir, interpretations now regarded to be incorrect.

One area of low resistivity pays in the J Sandstone is in the Radar-Jamboree-Wattenberg fields (T2S, R64W; T1S, R64, 65W). The reservoir sandstones are fine-to medium-grained and are interpreted as fluvial channels deposited in a J₂ valley-fill trend. These fields are in the deep portion of the Denver Basin where a continuous oil and gas column is present, and no water legs are found in the oil and gas pools. The LRLC intervals are shown by core analyses and density logs to be porous and permeable. They occur interstratified with higher resistivity sandstones that are tight mainly because of silica cementation. The high resistivity zones are interpreted and perforated as the main reservoir rock, and the low resistivity intervals, generally 5 ohms or less, are not perforated in wells because of fear of water production. However, in 5 wells perforated in the low resistivity intervals, oil and gas have been produced with only minor water. An additional 38 wells were identified with potential LRLC pay that were not perforated.

Low resistivity sandstone pays may result from clay occurrences within sandstone as: thin shale laminations; shale clasts; burrows lined with clay; pore fillings or linings with clay minerals; and clay grains mixed with sand grains (Darling and Sneider, 1993). Based on SEM photographs and core observations, the low resistivity pays in the J Sandstone are caused by pore fillings and linings of clay minerals, but the clays are not in sufficient quantity to prevent petroleum production.

In the search for "sweet spots" in otherwise tight sandstones in deep basin petroleum occurrences, LRLC pays may be overlooked reservoirs. Log interpretation procedures must be continually evaluated and revised in light of production history and detailed petrophysical data. For example, in the Denver Basin LRLC pays also occur in the Upper Cretaceous Codell and Terry sandstones. In early exploration programs, these pays were overlooked by log analysts only to later become important petroleum producing reservoirs. Thus, new ideas may flow from re-interpretations to offer opportunities for future exploration and development.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994