Drainage Anomalies in Gulf Coast Tertiary Sandstones

Abstract

Drainage anomalies in Gulf Coast Tertiary sandstones are common phenomena. They can be present in reservoirs deposited in one geologic environment or several. Recognizing where these undrained areas occur leads to drilling of new wells or recompleting existing wells. Five examples of drainage anomalies in three offshore Louisiana fields are presented. Introduction Unanticipated drainage patterns, here called drainage anomalies, occur more frequently in Gulf Coast Tertiary sandstone reservoirs than commonly has been assumed. Although we may show on our maps and cross sections that all sandstone stringers in a given interval are in communication, this is often not correct. Sometimes potential areas where such anomalies may occur can be predicted during the primary development of a field. Usually these primary development of a field. Usually these drainage anomalies become apparent, however, only after considerable production has been obtained. Such anomalies are indicated by a detailed analysis of production and pressure data, by pulsed neutron production and pressure data, by pulsed neutron logs, and by logs in new wells drilled later in the life of the field.One of the best ways to ferret out areas where such anomalies may occur is by undertaking comprehensive subsurface engineering reviews that incorporate all geological, petrophysical, reservoir, and production data from the field. These reviews require a great deal of time to organize, analyze, and synthesize large amounts of engineering data; however, performed properly, they can be extremely rewarding. During these detailed studies, internal boundaries within a reservoir are often recognized. Many of these internal boundaries result because the sandstones were deposited in different depositional environments - e.g., channel sandstones cutting into fringe deposits. Drainage anomalies can occur however, even in sandstones deposited in on depositional environment. If these sandstones are delta fringe deposits, their vertical continuity is interrupted by shale layers, and in these cases the drainage anomalies are understood easily. Drainage anomalies, however, frequently occur in massive channel sandstones with no apparent shale layers These are more difficult to explain. JPT P. 1313