Abstract

Quantitative and qualitative electric log interpretation utilizing multiple log responses can be used as a tool to identify Fall River point bar reservoirs. Numerous authors have shown how S.P. curve shapes can be used to identify point bar and fluvial deposits. However, few have attempted to characterize and identify point bar deposits through quantitative and qualitative interpretation using several electric log responses in combination. Multiple log responses are utilized to identify the sharp basal contact, high permeability and larger grain size typical in the lower portion, and the gradation into finer grain size and lower permeabilities typical in upper portions of Fall River point bar deposits. The technique works best in producing wells which are at irreducible water saturation. In these, porosity can be plotted versus water saturation, and relative estimates of vertical grain size changes can be determined. Gamma ray and density logs are utilized to aid in identifying the narrow, channel filling "clay plugs" associated with point bars. Reservoirs from two fields in the Powder River Basin are used to demonstrate the technique. The Coyote Creek Field includes reservoir rocks interpreted in several ways by previous authors. The second field is a more recent Fall River Third Bench reservoir in the Lonetree Creek Field, where many of the techniques applied in this study were developed. Observation of porosity and resistivity development in vertical sequences, and position on porosity and water saturation crossplots are used to aid in distinguishing between regional depositional environments, point bar deposits, and channel fill.