Scattergram: Crossplot Analysis, Petrophysical Log Interpretation, and Reservoir Property Mapping
A scattergram, also called a scatter plot or crossplot in petroleum geoscience, is a two-dimensional graphical representation in which individual data points are plotted on Cartesian axes without connecting lines, allowing visual identification of correlations, clusters, outliers, and trend patterns between two measured variables. In petroleum applications across the Western Canadian Sedimentary Basin and globally, scattergrams form the foundation of petrophysical log analysis, core-log integration, reservoir characterization, and production data analysis at every major operator and service company. The two variables on the x and y axes typically represent measurements such as bulk density and neutron porosity, gamma ray and resistivity, water saturation and permeability, or cumulative production and well depth, with each data point representing a single depth interval, a discrete well, or a single measurement event. A third dimension is commonly encoded through color, symbol shape, or symbol size, allowing the visualization to convey three-dimensional relationships such as a porosity-permeability crossplot color-coded by lithology or grain size class. In WCSB petrophysical work performed by service companies including SLB, Halliburton, and Baker Hughes, scattergrams of neutron porosity versus bulk density (the well-known neutron-density crossplot) distinguish gas zones from oil and water zones in Cardium tight oil reservoirs, identify dolomitization in the Slave Point and Leduc carbonates, and differentiate Montney organic-rich siltstone from clean siltstone facies. A second canonical petroleum scattergram, the Pickett plot, displays formation resistivity (Rt) versus porosity (phi) on log-log axes and is used to calculate Archie cementation exponent (m), saturation exponent (n), and formation water resistivity (Rw) for WCSB conventional and unconventional reservoirs. The Hingle plot, a related crossplot of resistivity versus porosity on specialized axes, provides similar parameter estimation without iteration. Modern petrophysical software including Geolog, Techlog, Petrel, and IP all generate scattergrams interactively with cutoffs and filters, supporting reservoir engineers at companies like Cenovus Energy, ARC Resources, and Tourmaline Oil in evaluating Duvernay liquid-rich gas plays, Montney horizontal completions, and Viking light oil pools. Scattergram interpretation is taught as a foundational skill in University of Calgary and University of Alberta petroleum geology programs and remains an indispensable tool in any reservoir engineer or petrophysicist's daily workflow, whether analyzing core plug data, well log digital files, decline curve analyses, or production allocation results.
Key Takeaways
- Neutron-Density Crossplot: The neutron porosity versus bulk density scattergram is the canonical lithology identification tool used by every WCSB operator. Sandstone, limestone, and dolomite each plot along distinct lithology lines, with gas-bearing intervals deflecting toward low neutron and low density values. WCSB Cardium analysts identify gas zones, oil zones, and water-wet zones with greater than 85 percent accuracy from this single crossplot alone in routine evaluation workflows.
- Pickett Plot Petrophysics: Resistivity versus porosity on log-log axes constructs the Pickett plot, a scattergram that yields formation factor F equals a divided by porosity raised to the cementation exponent m, and water saturation via Archie's equation. WCSB Cardium reservoirs typically show cementation exponent m of 1.85 to 2.05, saturation exponent n of 1.9 to 2.2, and formation water resistivity Rw of 0.04 to 0.08 ohm-metres at reservoir temperature.
- Porosity-Permeability Correlation: Scattergrams of core porosity versus core permeability (Phi-K crossplots) define reservoir quality classes (RQI, FZI) used for flow unit identification. Cardium tight oil reservoirs typically show 6 to 14 percent porosity correlating to 0.01 to 5 millidarcy (mD) permeability. The Winland R35 method extends the crossplot with iso-pore-throat lines color-coded for visual flow unit identification and reservoir compartmentalization analysis.
- Three-Variable Encoding: Color, symbol size, and shape encode a third variable on the scattergram. Common encodings include lithology by color, depth by gradient, productivity index by size, or well operator by symbol. This converts a two-axis plot into a three-dimensional information display without requiring perspective rendering, preserving readability for printed reports and digital screens used in board presentations and AER hearings.
- Outlier and Cluster Detection: Scattergrams reveal data quality issues that statistical summaries hide. A bimodal cluster suggests two distinct reservoir zones, while isolated outliers indicate sample contamination, log measurement errors, or unique geologic features. Reservoir engineers at WCSB operators routinely use scattergrams as the first quality check on any new digital well log or core plug dataset before any quantitative interpretation is attempted.
Neutron-Density Crossplot Application in Montney
For a Tourmaline Oil Montney horizontal in the Karr area at 2,650 metres true vertical depth, neutron porosity versus bulk density crossplots distinguish clean Montney C siltstone (15 to 20 percent porosity-equivalent, plotting near the sandstone line) from organic-rich Doig phosphate intervals (with high gamma ray and density excursions toward 2.6 grams per cubic centimetre) and the underlying Belloy sandstone reservoir. Color-coding by gamma ray produces a three-variable visualization that geosteering teams use during horizontal drilling to keep the wellbore in the C sub-unit. The integrated petrophysical workflow at SLB or Halliburton Calgary runs at CAD 25,000 to CAD 45,000 per well evaluation, including crossplot generation and zone-by-zone interpretation.
Production Data Scattergrams for Decline Analysis
Operators use scattergrams of cumulative oil production versus time-to-50-percent-decline, or initial production versus estimated ultimate recovery (EUR), to benchmark WCSB Montney and Duvernay wells against peer offsets. ARC Resources, Tourmaline Oil, and ConocoPhillips Canada each maintain proprietary EUR scattergrams for over 1,000 horizontal wells in the Dawson, Pouce Coupe, and Karr trends, identifying type curves by completion intensity (proppant per metre) and lateral length. These visualizations support bench valuations, public reserve disclosures filed under National Instrument 51-101 in Canada, and acquisition due diligence at CAD 25 to 80 million per typical Montney transaction.
Fast Facts
The first published petroleum crossplot dates to a 1942 paper by Gus Archie of Shell Oil Company that established the resistivity-porosity relationship, codifying what became Archie's equation for water saturation calculation. The Pickett plot, introduced by George R. Pickett of Marathon Oil in a 1966 SPE paper, formalized the log-log scattergram methodology that remains in use over six decades later at every major WCSB operator, with modern software simply automating Pickett's original hand-drawn graphical method.
Related Terms
Scattergrams underpin several specialized petroleum analytical methods. The Pickett Plot is a particular scattergram of resistivity versus porosity on log-log axes used to determine Archie parameters for water saturation calculation. Crossplot is a synonym used interchangeably in petroleum geology and reservoir engineering. Porosity and Permeability are commonly crossplotted to define reservoir quality classes, with each scattergram representing the integrated petrophysical understanding of a WCSB pool or formation.
Real-World WCSB Scenario: ARC Resources Sunrise Montney Petrophysics
At an ARC Resources Sunrise Montney evaluation well drilled in 2024 near Dawson Creek British Columbia, the petrophysics team at SLB Calgary generated a neutron-density scattergram of 247 wireline depth points across the 2,400 to 2,650 metre TVD interval. Points clustered into three groups: a high-porosity cluster (15 to 22 percent) plotting near the sandstone line representing Montney C reservoir; a low-porosity cluster (4 to 8 percent) at elevated density representing tight Montney B; and a third cluster with significant gas effect (neutron suppression and density reduction) indicating dry gas potential at 2,485 to 2,530 metres depth.
The team recommended a 1,200 metre lateral targeted within the high-porosity gas-bearing interval, with completion intensity of 2,500 kilograms of proppant per metre. The well achieved a 30 day initial production rate of 11.2 million standard cubic feet per day equivalent (317 e3m3/day), placing it in the 88th percentile of nearby Montney wells. Petrophysical evaluation cost was CAD 38,000 for the comprehensive workflow including digital deliverables filed with the BC Energy Regulator under their well data submission requirements.