Litho-flow facies prediction in an alluvial fan/fluvial system, central North Sea

An existing generic facies scheme consisting of a channel, sheetflood and floodplain facies was available from the cored wells. Bivariate and multivariate analysis of various wireline and core data indicated that each generic facies exhibits wide variations and significant petrophysical overlap with other facies. This scheme cannot therefore reliably be applied using wireline data, negating its application in uncored intervals through discriminant analysis. Due to the large variation in porosity and permeability of each facies, the existing scheme does not permit adequate determination of the probable reservoir performance of the sediments. In order to address these problems a new "litho-flow" facies scheme has been constructed which can clearly be identified from wireline data thereby enabling it to be reliably extended beyond cored intervals. Each facies in the scheme exhibits a discrete range in porosity and permeability by which the probable flow characteristics of the sediments can be assessed and has successfully been used by reservoir engineers as input to a probabilistic reservoir modelling package. Finally, the predicted facies are valid in terms of the environment of deposition, assisting geologists in the correlation of units between wells for the establishment of a depositional model. The new facies scheme was formulated from a careful selection and processing of data to achieve the above. This included core descriptions, core porosity and permeability measurements, mineral volume analysis and porosity estimation from wireline data, permeability predictions and electrofacies from cluster analysis.