Crossplot Porosity

Key Takeaways

• The neutron-density crossplot is one of the fundamental interpretation tools in petrophysical analysis because it provides simultaneous information on lithology and porosity: when neutron porosity (x-axis) and density porosity (y-axis) values from a formation interval are plotted, they fall in predictable locations depending on the mineral composition of the rock matrix; sandstone (quartz) plots in one zone, limestone (calcite) in another, dolomite in a third, and anhydrite and salt in distinct separate zones far from the carbonate-clastic trend; mixtures of minerals (silty carbonates, calcite-cemented sandstones) plot between the end-member positions; the interpretation engineer draws mineral composition lines on the crossplot and uses the position of the data points to estimate both the mineral mix and the porosity simultaneously, without needing independent mineralogy data from X-ray diffraction or core thin sections; this dual-interpretation capability — lithology and porosity from two measurements — is the reason the neutron-density crossplot has remained a standard tool in log analysis for six decades despite enormous advances in wireline logging technology.
• The gas crossplot effect is one of the most reliable qualitative indicators of a gas-bearing formation available from conventional wireline logs: because gas lowers neutron reading and raises density reading simultaneously, the two curves cross or separate in the direction of neutron less than density (called negative separation when the neutron log is plotted in apparent limestone porosity units), creating a visual signature in the log track that experienced log analysts can identify immediately; the magnitude of the separation is proportional to the gas saturation and the gas effect on each tool's measurement, with high-GOR light oil producing a smaller separation than dry gas at the same porosity; this crossplot signature is routinely used as a first-pass gas indicator before quantitative saturation analysis, and zones with significant neutron-density separation in a section expected to be oil-bearing are flagged for detailed fluid characterization because the separation may indicate gas cap influence, solution gas coming out of solution near the wellbore, or an unexpected change in GOR.
• Shaly sandstones and clay-rich formations significantly complicate crossplot porosity interpretation because clays contain large amounts of bound water in their structure that registers as apparent hydrogen (and therefore apparent neutron porosity) that is not movable pore space; the clay minerals themselves have variable density (2.4-2.9 g/cc depending on clay type) that differs from the clean sand matrix density assumed in standard density porosity calculations; the net effect is that the neutron porosity reads too high (because of clay-bound water), the density porosity reads inconsistently depending on clay type, and their average overestimates effective porosity by amounts that can be large enough to make a tight shale look like a porous reservoir; correcting crossplot porosity for clay content requires estimating the volume of clay from the gamma ray log or a spectral gamma ray, then subtracting the clay's contribution to each tool's response before computing the crossplot combination; the shale-corrected crossplot porosity is a more reliable effective porosity estimate for reservoir quality evaluation than the raw crossplot average in shaly intervals.
• The density-neutron combination is complementary to the acoustic (sonic) porosity log in mixed-lithology formations because the sonic velocity responds differently from density and neutron to certain rock fabrics and fluid types: in vuggy carbonates, the sonic log responds primarily to the matrix velocity and underestimates secondary porosity (vugs and fractures), while the density log responds to the total bulk density including all pore types; comparing sonic porosity to density-neutron crossplot porosity reveals secondary porosity by the systematic difference between the two estimates; this secondary porosity index (SPI) is used in carbonate reservoir description to distinguish intergranular porosity (sonic and density-neutron agree) from vuggy or fracture porosity (density-neutron exceeds sonic), which is important because vugs and fractures connect the reservoir in ways that intergranular matrix porosity does not, and the distinction affects both reservoir simulation modeling and completion design.
• Modern advanced log interpretation programs compute crossplot porosity automatically as part of a multimineral petrophysical model that inverts the responses of six or more log measurements simultaneously (neutron, density, sonic, photoelectric factor, spectral gamma ray, resistivity) to solve for the fractional volumes of multiple minerals and fluids in a joint inversion; the crossplot porosity computed in this multimineral context is the pore space left over after all minerals are accounted for, and it is more accurate than the simple two-curve crossplot in complex lithologies because the simultaneous inversion uses more constraints; however, the simple neutron-density crossplot remains in routine use for quick-look interpretation, well-to-well correlation, and situations where only a basic logging suite is available, demonstrating the enduring practical value of a technique that can be applied with a pencil and a crossplot chart as easily as with sophisticated software.