A well log is a strip chart that records the physical properties of rock formations encountered while drilling. Reading one means understanding the layout, recognizing what each curve measures, and combining multiple curves to identify rock types, fluids, and porosity. Once you know the conventions, the same visual patterns repeat across nearly every log you’ll encounter.
The Basic Layout: Tracks and Depth
Every well log is divided into vertical columns called tracks, separated by a narrow depth track in the center. The depth track shows measured depth in feet or meters, typically referenced from the Kelly bushing (the point on the drill floor where depth counting starts). Depth increases downward, just as you’d expect when drilling into the earth.
By convention, Track 1 sits to the left of the depth column and contains the gamma ray log, the spontaneous potential (SP) log, and the caliper log. Track 2 typically holds resistivity curves, and Track 3 displays porosity measurements like the density and neutron logs. This standard arrangement means that once you’ve read one well log, you can orient yourself on almost any other. Some modern logs add extra tracks or rearrange curves, but this three-track format is the baseline you’ll see most often.
Before reading any curves, check the header at the top of the log. It lists the well name, location, date logged, drilling mud properties, and critical values you’ll need later for calculations. The mud resistivity (Rm) and mud filtrate resistivity (Rmf) recorded in the header are essential for interpreting resistivity and SP curves correctly. Header-derived values can sometimes be inaccurate, so experienced interpreters treat them as starting estimates rather than absolute numbers.
The Gamma Ray Log: Identifying Rock Type
The gamma ray curve is usually the first thing you read because it tells you what kind of rock you’re looking at. It measures natural radioactivity in the formation, displayed in API units. Shales are rich in radioactive minerals and typically read around 120 API units. Limestones and clean sandstones have much lower radioactivity, often around 20 API units or less. Dolomites also tend to read low, though exceptions exist in every rock type.
In practice, the gamma ray log works as a “shale log.” High readings mean shale; low readings mean cleaner reservoir rock like sandstone or carbonate. You can also estimate how much shale is mixed into a reservoir zone by seeing where the reading falls between the clean baseline and the shale baseline. A sandstone reading 70 API units in an area where clean sand reads 20 and pure shale reads 120 contains a significant shale fraction, which affects how much fluid the rock can hold and produce.
The gamma ray log is also the universal reference curve for aligning different logging runs. Because it’s recorded on nearly every tool string, analysts use its distinctive peaks and troughs to match depths between separate passes. Depth discrepancies between runs are common, caused by cable stretch, tides, or incomplete heave compensation. If you notice that features on one log seem shifted relative to another, depth matching using the gamma ray is the standard fix.
The SP Log: Spotting Permeable Zones
The spontaneous potential log measures tiny natural voltages generated where drilling mud contacts formation water. It appears as a single curve in Track 1 with no units on the scale, just millivolts of deflection from a shale baseline. The key information is the direction and size of the deflection.
When the mud filtrate resistivity (Rmf) is much greater than formation water resistivity (Rw), the SP deflects strongly to the left (negative) across permeable zones. When Rmf is much less than Rw, the deflection goes to the right (positive). If the two resistivities are equal, you’ll see no deflection at all, regardless of permeability. This means the SP doesn’t measure permeability directly. It responds to the contrast between mud and formation water, and that response only appears where the rock is permeable enough for the two fluids to interact.
Impermeable zones like shale show a flat, featureless SP line. Across a thick, clean, permeable sand with good contrast between Rmf and Rw, you’ll see a clear rectangular deflection called the static SP. In thin or shaly beds, the deflection is reduced. Low-permeability zones can also suppress the SP because mud filtrate invades so deeply that the natural voltage can’t develop fully.
Resistivity Curves: Finding Hydrocarbons
Resistivity logs in Track 2 measure how strongly the formation resists electrical current, displayed on a logarithmic scale in ohm-meters. The core principle is simple: salt water conducts electricity well (low resistivity), while hydrocarbons do not (high resistivity). A water-saturated sandstone reads low on resistivity. The same sandstone filled with oil or gas reads much higher.
Most resistivity tools record three curves simultaneously: shallow, medium, and deep. The shallow curve reads the zone closest to the borehole, where drilling mud has flushed into the rock. The deep curve reads farther out, closer to the undisturbed formation and its original fluids. By comparing these curves, you can learn two things at once. First, separation between the curves indicates that mud filtrate has invaded the formation, which means the rock is permeable. Second, the deep reading approximates the true resistivity of the formation fluids, giving you the best indication of whether those fluids are water or hydrocarbons.
In a hydrocarbon-bearing sandstone, expect to see the resistivity curves climb noticeably higher than in the water-saturated zone below. This contrast between the hydrocarbon zone and the water zone is one of the most reliable visual indicators on any well log.
Porosity Logs: Density, Neutron, and Sonic
Track 3 usually contains two porosity curves plotted together: the density log and the neutron log. Each measures porosity through a different physical property, and their relationship reveals both how porous the rock is and what fluid fills those pores.
In a liquid-filled formation (oil or water), the density and neutron curves track each other closely or overlay depending on lithology. The powerful signal comes when they cross over, with density porosity reading higher than neutron porosity. This crossover pattern is the classic indicator of gas in the formation. Gas has much lower density and fewer hydrogen atoms than liquid, which causes the two tools to disagree in opposite directions. When you see this crossover in a zone that otherwise looks like consistent sandstone on the gamma ray, gas is the likely explanation. Corrected porosities in gas zones commonly fall in the 24% to 28% range, though this varies by formation.
When the curves separate in the other direction (neutron reading higher than density), it usually signals a lithology change rather than a fluid effect. Shales, for instance, cause the neutron log to read artificially high because of the hydrogen bound in clay minerals.
The sonic log measures how fast sound travels through the formation, recorded in microseconds per foot. Faster travel times mean denser, less porous rock. You can calculate porosity from the sonic log using a straightforward relationship: porosity equals the difference between the measured travel time and the rock matrix travel time, divided by the difference between the pore fluid travel time and the matrix travel time. The sonic log responds primarily to primary (intergranular) porosity and tends to undercount fractures and vugs, which makes it useful for comparison. If the density-neutron porosity is significantly higher than the sonic porosity, the difference often represents secondary porosity from fractures or dissolution.
The Caliper Log: Checking Borehole Quality
The caliper log measures the actual diameter of the borehole and appears in Track 1 alongside the gamma ray. It’s easy to overlook but essential for quality control. A borehole diameter equal to the drill bit size indicates a stable, consolidated formation. Enlargements (washouts) occur in weak, fractured, or weathered zones and tell you that other log readings in that interval may be unreliable, since most tools are calibrated for a specific borehole size.
A diameter slightly smaller than the bit size points to either swelling clays or mudcake buildup. Mudcake forms when drilling fluid filters into permeable rock, leaving a thin layer of solids on the borehole wall. This is actually a positive sign: it confirms permeability. The type of caliper arm matters too. Rod-type arms under high pressure tend to cut through mudcake and read the true borehole wall, while pad-type arms ride over it. Knowing which caliper was used helps you interpret whether a thin reading reflects real borehole narrowing or just the tool skimming over filter cake.
Putting It All Together
Reading a well log is a process of pattern matching across multiple curves simultaneously. Start with the gamma ray to identify your rock types. In clean (low gamma ray) intervals, check the resistivity for hydrocarbon indicators. Look at the density-neutron pair for gas crossover. Confirm that the caliper shows a good borehole before trusting any readings in a given zone. Use the SP to flag permeable intervals.
A practical workflow looks like this: scan the gamma ray from top to bottom and mark the boundaries between shale and reservoir rock. Within each reservoir interval, note the resistivity level and whether the deep curve reads higher than the shallow curve. Check the density-neutron relationship for gas signatures. Look at the caliper to see if any zones are washed out, which would make you skeptical of the readings there. Finally, use the SP to confirm which intervals are permeable.
For quantitative work, the industry standard is the Archie equation, which calculates water saturation from porosity and resistivity. The basic form states that water saturation equals a function of formation water resistivity, true formation resistivity, porosity, and a few empirical constants (the cementation exponent, saturation exponent, and tortuosity factor). The accuracy of the result depends heavily on getting these constants right for your specific reservoir. A water saturation of 100% means the pore space is entirely water. Anything significantly less means hydrocarbons are present, and the lower the number, the more hydrocarbon-rich the zone.
The visual patterns come quickly with practice. High gamma ray plus low resistivity equals shale. Low gamma ray plus high resistivity plus density-neutron crossover equals gas sand. Low gamma ray plus moderate resistivity plus no crossover equals wet sand or oil sand, depending on how the resistivity compares to a known water zone. These combinations become instinctive after working through a few logs, and they form the foundation for every more advanced interpretation technique.