Ternary Diagram

Key Takeaways

• Reading a ternary diagram requires understanding that each point's composition is determined by its perpendicular distance from each side — the percentage of component A at a given point equals the perpendicular distance from the point to side BC (the side opposite corner A), expressed as a percentage of the triangle's height; the sum of the three perpendicular distances always equals the triangle's height regardless of point position, which is why the three percentages always sum to 100%; gridlines parallel to each side show constant percentages of the component in the opposite corner, so following a gridline parallel to BC traces all compositions with the same percentage of A, regardless of how B and C are distributed between themselves.
• Phase behavior ternary diagrams for EOR design are among the most practically important applications in petroleum engineering — in CO2 or hydrocarbon miscible flooding, the ternary system pseudo-component 1 (light components, C1-C4) / pseudo-component 2 (intermediate components, C5-C12) / pseudo-component 3 (heavy components, C12+) is used to map the phase envelope, the critical point, and the tie-line directions that determine whether injected gas achieves miscibility with reservoir oil; the shape and position of the two-phase region within the ternary, and particularly the composition of the plait point (critical point), determines the minimum miscibility pressure and the efficiency of the displacement process; reservoir engineers design injection compositions and pressures to keep the displacement path outside the two-phase region, achieving miscibility that dramatically improves oil recovery compared to immiscible displacement.
• Folk's sandstone classification ternary (Q-F-L: quartz, feldspar, lithic fragments) is one of the most widely used geological classification tools — the relative proportions of quartz, feldspar, and rock fragment grains in a sandstone are plotted on the ternary, and the position of the point within the triangle defines the sandstone name (quartzarenite, subarkose, arkose, lithic arkose, lithic arenite, and subtypes within each field); the Q-F-L composition is a function of the sediment provenance (where it came from) and diagenetic history (how much modification has occurred since deposition), making the ternary plot not just a classification tool but a window into the tectonic setting and diagenetic maturity of the reservoir rock.
• Geochemical correlation uses ternary diagrams to fingerprint oil families and source rock contributions — when multiple oils of uncertain source are analyzed for specific biomarker ratios or compound group percentages, plotting them on ternary diagrams allows visual identification of compositional clusters that reflect common source rock origins; oils derived from marine carbonate source rocks, marine shale sources, and lacustrine sources have characteristic positions on various biomarker ternary diagrams; mixing of oils from different source rocks produces data points along mixing lines between the source end-member fields; ternary fingerprinting is used in production allocation (determining what fraction of oil in a commingled production stream comes from each contributing reservoir) and in exploration to correlate discovered oils to potential source formations.
• Ternary diagrams are inherently insensitive to absolute concentrations and only reflect relative proportions — two samples with very different total amounts of the three components but the same proportions plot at exactly the same point on the ternary; this makes ternary diagrams ideal for compositional characterization where ratios matter more than absolute amounts (as in geochemical fingerprinting and petrographic classification) but inappropriate for applications where the absolute amount of each component matters; additionally, ternary diagrams become impractical when there are more than three significant components, requiring either projection techniques (reducing a multi-component system to three pseudo-components) or alternative multi-dimensional visualization methods.