Sand Injectites in Deep-Water Depositional Environments, Detection and Interpretation using Borehole Electrical Images

By Peter Schlicht, Schlumberger Technical Services Inc., Angola, with Oskar Yepes and John Crowe, Chevron Angola

Conventional core data from Miocene deep water turbidite channels in Angola show common occurrence of sand injections. Structures formed by these elastic intrusions are called injectites and they can have both negative as well as positive impact on the reservoir performance. They occur as sub-vertical dikes, horizontal and br bedding parallel sills. Early detection and recognition of these features, which can range from millimeter to kilometer scales. can considerably impact the development strategy of for a given reservoir. Borehole electrical images allow to detect injection features and to differentiate from the surrounding geological contexts, so their impact can be quantified. In our case study we examine the resistivity content of the borehole electrical image in combination with the interpreted directional data, the formation dip. Based on user-input contrast cutoffs, conductive and resistive events that do not correlate across the borehole along the bedding direction, and that show high enough contrast with respect to their background, are detected by performing a heterogeneity analysis. Assuming ‘pure’ elastic systems we infer a directly proportional relationship between non- conductive mud invasion to porethroat- and grainsize in order to separate sand, silt and clay proportions. Detecting relative grainsizes combined with evaluation of non- correlating bedding events lead us to the recognition of sand intrusion events and their orientation.