The Shuaiba Formation is a complex carbonate reservoir characterized by small-scale geological heterogeneity primarily related to rudist macrofauna. The textural heterogeneity corresponds to extreme permeability variation that is the controlling factor in reservoir production. Because of the large vertical resolution contrast between cores (actual scale) and conventional logs (averaged responses over a few feet), extrapolation of small-scale heterogeneity into uncored wells using a traditional approach is unreliable. High-resolution log data, like dipmeters and image logs, are required to characterize small-scale heterogeneity that is essential to a good three-dimensional (3-D) geological model that predicts true reservoir behavior. Our study illustrates an approach to improve the reservoir characterization of the Shuaiba by quantifying small-scale heterogeneity using dipmeter and image logs.
A methodology is proposed to use the existing high-resolution (High-resolution Dipmeter Tool [HDT], Stratigraphic High-resolution Dipmeter Tool [SHDT], and Formation MicroScanner [FMS]) and conventional log data to characterize and extrapolate geological heterogeneity. Texture and neural network analyses, derived from microresistivity variations and a multiregression approach, have been used in two wells to demonstrate the methodology. Both rock types and permeability are estimated for extrapolation into uncored wells. Although differences between the conductivity of conductive and resistive areas when normalized for background conductivity are an indirect measure of permeability for HDT and SHDT, the connectivity coefficient of conductive anomalies derived from textural analysis is related to permeability in the case of FMS. Reservoir rock types (RRT) obtained from the methodology correlate well with RRTs derived from the integration of core, special core analysis (SCAL), and conventional log data in two test wells. Permeability estimations, including small-scale extreme variations from less than 1 md in tight mudstones to greater than 1 d in caprinid rudstones, are in good agreement with the core plug permeability measurements after a simple calibration. In one example, permeability trends in rudist RRTs, which are not discernible using core plugs and minipermeameter data, are clearly resolved using SHDT data. The degree of correlation between estimated and core permeabilities is greater with increased vertical resolution of the tool (HDT, lowest; FMS, highest).
Distinct RRTs resulting from small-scale geological heterogeneity can be classified and permeability can be estimated with a high degree of confidence, providing a better pathway of extrapolating from core data to conventional logs for 3-D modeling. The methodology has potential application to other carbonate and siliciclastic reservoirs with appropriate calibration and scaling. Underused HDT, SHDT, FMS, and Fullbore Formation MicroImager (FMI) databases are available in many other areas that could be analyzed. An order of magnitude improvement in the ability to characterize small-scale heterogeneity has significant implications for future coring, logging programs, and reservoir characterization efforts.