Question 5 of 12: Limitations of the Hingle Plot

Question 5: Limitations of the Hingle Plot (4 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The Hingle (resistivity-vs-porosity, non-linear-grid) cross-plot is a fast graphical way to find R_w and φ simultaneously without iterating Archie's equation by hand, but it carries four significant limitations:

• Requires a single, constant R_w across the whole plotted interval. If formation-water salinity (or temperature) varies over the section — common across a thick reservoir or multiple sands — every point no longer lies on one "100% water" line and the fitted R_w is a poor average.
• Assumes fixed Archie parameters (a, m, n). The plot's non-linear resistivity axis is drawn for one assumed cementation exponent (usually m=2); rocks whose true m departs from that (vuggy carbonates, poorly sorted sands) plot off the assumed water line even at S_w=100%, producing a biased φ-R_w fit.
• No correction for shale/clay content. Like a raw Archie calculation, the plot has no built-in V_sh term; shaly points scatter toward apparently lower resistivity for a given porosity and are easily mistaken for higher water saturation rather than for clay conductivity.
• Requires an a-priori, correctly-chosen matrix travel-time/density endpoint. The porosity axis is built from an assumed matrix parameter (Δt_ma or ρ_ma); an incorrect lithology assumption shifts every point's porosity, and because the porosity scale is compressed at low φ, reading error is magnified exactly in the tight, often most important, part of the reservoir.