Sand-Rich Deltas

Abstract

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. The preceding article in the Technology Today Series (Jan. 1989 JPT. Pages12–13) described the general processes of building deltas. The present articlefocuses on the reservoir characteristics of high-energy sand-rich deltas. Theseform when the wave and tidal action is sufficiently strong to rework thedeposits brought in by distributary channels before their burial. The principaltypes of sand bodies and highest-quality reservoir rocks in a delta arechannels and stream-mouth bars. Also present are the sheet sands deposited on adelta front and localized sand bodies left in minor crevasse deposits adjacentto channels during floods. While these bodies have definite areal relationshipsto one another during the process of building a lobe on the delta, theirthree-dimensional relationships become complex as the deltas build seaward orshift positions along the shoreline. Thus, a channel active during a laterbuilding cycle may cut into or may lie over stream-mouth bars, delta-frontsheets, or crevasse sands left earlier. Fortunately, the various sand bodieshave different distributions of permeability and different responses to logs sothat they can be identified. The coarsest sand is left at the base of a channeland silt and clays are left at the top. Typical gamma ray log responses andpermeabilities measured in cores show depth profiles in Fig. permeabilitiesmeasured in cores show depth profiles in Fig. 1a through c in sands leftupstream, onshore, and near mouths of distributary channels, respectively. Thelow gamma ray log responses at the bases show that little clay or shale ispresent. The increased gamma ray log response toward the tops of the channelsreflects the presence of more clay and shale. Similarly, the largest grainsizes and highest permeabilities are measured in cores from the base anddecrease upward. p. 89