composite log, Oil & Gas Glossary

What Is a Composite Log?

Composite log (also called a mud log, wellsite log, or drilling log) is a continuous graphical record created at the wellsite throughout drilling that combines on a single log-format strip the lithology column derived from drill cuttings descriptions, gas chromatograph readings for total gas and individual hydrocarbon components (C1 through C5), rate of penetration (ROP), drilling parameters (weight on bit, rotary speed, flow rate, mud weight), and formation tops, providing the first integrated picture of the formations penetrated and the first indication of hydrocarbons encountered before any wireline logs are run.

Key Takeaways

The composite log is produced in real time at the wellsite by a mud logging company under contract to the operator; mud loggers work 12-hour shifts around the clock throughout drilling, maintaining a continuous record from spud to total depth.
All gas readings and cuttings descriptions must be depth-corrected by a lag time calculation — the time required for mud and cuttings to travel from the drill bit to the surface shale shakers — which ranges from 15 minutes to several hours depending on wellbore depth and mud pump rate.
Gas chromatography measures five hydrocarbon components (C1 methane, C2 ethane, C3 propane, i-C4/n-C4 butane, i-C5/n-C5 pentane); the relative ratios of C1 through C5 help differentiate oil-associated gas (heavy C3-C5 ratios) from dry gas condensate (predominantly C1-C2).
Rate of penetration is one of the most sensitive early indicators of formation changes; a sudden ROP increase (drilling break) while drilling through a shale sequence into a porous sandstone or carbonate is often the first signal of a potential reservoir, triggering a flow check to test for a kick before continuing.
Composite logs from offset wells are used for formation top prognosis, correlation between wells in a field, and calibration of seismic interval velocities to actual formation depths in the same area.

Components of the Composite Log: Tracks and Their Meaning

The composite log is formatted like a wireline log, with depth running vertically and multiple data tracks arranged horizontally. The leftmost track typically contains the lithology column: a graphical representation of drill cuttings descriptions using standard lithology symbols (dots for sandstone, bricks for limestone, horizontal lines for shale). The mud logger collects cuttings samples from the shale shaker screens at regular depth intervals — typically every 5 to 10 feet in shale sections, every 2 to 5 feet when approaching a potential reservoir — washes them, and examines them under an ultraviolet (UV) light to detect crude oil fluorescence, then describes the mineralogy, texture, grain size, porosity, cement type, and any visible oil staining or fluorescence under magnification. The UV examination is particularly valuable: many oils fluoresce distinctly under UV light (crude oils can show yellow, orange, or white fluorescence depending on API gravity), providing a direct visual indicator of oil presence in cuttings before any gas readings confirm it.

The gas track displays total gas (TG) and the individual C1 through C5 component curves from the continuous gas chromatograph. Mud gas is extracted from the return mud stream by a gas trap — a motorized agitator that degasses the mud into a collection hood — and the gas stream is piped continuously to the chromatograph unit in the mud logging cabin. Total gas is expressed in units or percentage, scaled from zero to a maximum that the mud logger adjusts to keep the gas show clearly visible on the log. Background gas is the baseline level of total gas while drilling lithologies with no significant hydrocarbon content; a gas show is any excursion above background, classified as slight (1.5 to 3 times background), fair (3 to 10 times background), or good (more than 10 times background). The chromatograph cycle time — typically 2 to 4 minutes per full C1-C5 analysis — means that fast drilling can advance several feet per cycle, so fast ROP requires either a faster chromatograph or acceptance that gas readings may alias thin beds.

The drilling parameter tracks — weight on bit (WOB), rotary speed (RPM), standpipe pressure, flow rate, mud weight in and out, and pit volume — are recorded either manually by the mud logger every 15 to 30 minutes or automatically from a surface data acquisition unit that logs sensor outputs continuously. ROP is calculated as the time required to drill each foot and is typically plotted in minutes per foot (min/ft) or feet per hour (ft/hr). A drilling break — a sudden step change in ROP as the bit enters a more porous or softer formation — alerts the driller and mud logger to examine cuttings and gas more closely and may trigger a flow check: the pumps are shut off and the well is observed for flow to ensure formation pressure is not exceeding wellbore pressure. The drilling break combined with a gas show and UV fluorescence in the cuttings constitutes a reservoir indication that is immediately reported to the company man and can influence decisions on whether to run a formation test, set casing, or adjust mud weight before continuing.

Fast Facts: Composite Log

Also known as: Mud log, wellsite log, drilling log, hydrocarbon log
Cuttings sample interval: 5–10 ft in shale; 2–5 ft approaching reservoirs
Lag time range: 15 minutes at shallow depth to 4+ hours at 15,000 ft; calculated as annular volume divided by pump output
Chromatograph components: C1 (methane), C2 (ethane), C3 (propane), i-C4/n-C4 (butane), i-C5/n-C5 (pentane)
Gas show classification: Slight (1.5–3x background), fair (3–10x), good (>10x background total gas)
UV light wavelength: 365 nm (long-wave UV); crude oils fluoresce at 400–600 nm visible range
Mud logging crew: 2 mud loggers per well, 12-hour shifts, 24/7 coverage from spud to TD
Log scale: Typically 1:500 (1 cm = 5 m depth) for well correlation; 1:200 or 1:100 for detailed reservoir sections

Wellsite Operations Tip:

Calculate and update the lag time every 500 feet of drilling depth, or any time the pump rate changes significantly. Lag time determines the depth offset applied to all gas and cuttings data — if the lag calculation is wrong by 10 minutes and the ROP is 30 ft/hr (0.5 ft/min), your cuttings descriptions are mislocated by 5 feet of depth. In thin-bed reservoirs or tight formations where the company is correlating sand tops between offset wells to 5-foot precision, a stale lag time calculation can cause the formation top to appear 10 to 20 feet too high or too low on the composite log, leading to incorrect casing seat selection or missed pay.

Composite log is also referred to as:

Mud log — the most common field term; refers to the fact that the log is produced from analysis of the drilling mud returns (gas extracted from mud, cuttings carried by mud to surface) rather than from downhole sensors
Wellsite log — emphasizes that the record is created at the wellsite in real time during drilling, distinguishing it from wireline logs run after drilling or logs processed in a remote data center
Hydrocarbon log — used when the emphasis is on the gas chromatograph and UV fluorescence data that provide direct evidence of hydrocarbons; common in exploration contexts where the primary purpose is detecting oil and gas shows
Drilling log — broader term encompassing all drilling parameter records, sometimes used when the ROP, WOB, and mud weight tracks are more important than the geologic description, as in development drilling where formation tops are already known

Frequently Asked Questions About Composite Logs

How is lag time calculated and why does it matter?

Lag time is the travel time for a parcel of mud — and the cuttings and gas entrained in it — to travel from the drill bit at the bottom of the hole up through the annulus to the surface shale shakers. It is calculated by dividing the annular volume (the volume of the wellbore annulus from bit to surface, accounting for drill string OD and casing ID at each section) by the mud pump output rate in barrels per minute or gallons per minute. A typical calculation: if the annular volume is 500 barrels and the pump is delivering 20 barrels per minute, the lag is 25 minutes. All cuttings descriptions and gas readings on the composite log are shifted upward by this lag time to assign them to the correct depth at which they were generated, rather than the depth at which they were observed at surface. If the lag time is incorrect — stale by 30 minutes because the pump rate changed or the depth calculation was not updated — every formation description from that point onward is mislocated, which corrupts formation top correlation and can cause incorrect casing setting depth decisions that cost hundreds of thousands of dollars to correct.

How do mud loggers use C1 through C5 ratios to classify hydrocarbon shows?

The ratio of individual hydrocarbon components (C1 through C5) in gas shows provides a rough indication of the type of hydrocarbon present in the formation. Dry gas reservoirs (methane-dominated) show very high C1 percentage relative to C2-C5, with a wetness ratio (C2+C3+C4+C5)/total gas often below 5 percent. Oil-associated gas and solution gas from oil reservoirs show greater proportions of C2 through C5, with wetness ratios above 20 percent and significant n-pentane and isopentane in the chromatograph. Condensate reservoirs show intermediate wetness with elevated C5 relative to C3-C4. These ratios — plotted as the "gas character" track on some composite log formats — help the geologist distinguish a free-gas zone (clean dry gas show, potential gas reservoir) from a solution-gas show in an oil reservoir (wet gas signature), informing decisions on whether to run a formation test and what type of wellbore test equipment to use. The ratios are directional indicators, not definitive reservoir fluid classification, which requires pressure-volume-temperature (PVT) analysis of collected downhole fluid samples.

How does the composite log relate to wireline logs run after drilling?

The composite log and wireline logs are complementary: the composite log provides a real-time record of formation characteristics and hydrocarbon shows as the well is drilled, while wireline logs (gamma ray, resistivity, neutron-density, sonic) provide precise quantitative measurements of formation properties run after the drill string is pulled from the hole. The composite log is used to select the depth intervals where wireline logging is most important, to flag potential problem zones (lost circulation, gas kicks, tight formation breaks), and to confirm formation tops that wireline logs will characterize in detail. Formation top depths identified on the composite log — based on changes in cuttings lithology and ROP — are cross-checked against wireline gamma ray and resistivity character to verify the formation boundary and measure its exact depth. In exploration wells, the composite log gas shows guide the selection of formation intervals for pressure testing (MDT or DST) with wireline or drillstem tools, maximizing the information returned from expensive downhole sampling operations.

Why Composite Logs Matter in Oil and Gas

The composite log is the earliest and most continuous record of subsurface geology obtained from a well, created in real time during drilling before any other evaluation tool is deployed. In exploration wells where the formation tops, pressures, and fluid content are unknown, the composite log is the primary safety tool as well as the geological record: gas shows and flow checks detected from the mud log prevent well control incidents by alerting the drilling crew to unexpected reservoir pressures before they become kicks. In development drilling, composite logs are used for rapid formation top correlation between new wells and offset producers, confirming that the well has reached its target formation and enabling real-time steering decisions in horizontal wells. The composite log is also the permanent geological record that accompanies the well file throughout its producing life, providing essential context for workover decisions, recompletion targeting, and field-wide geological modeling decades after the well is drilled. Regulatory submissions to energy regulators in most jurisdictions — including the AER in Alberta, COGCC in Colorado, and BOEM for US offshore — require composite log submission as part of the well completion report.

Composite Log: The Wellsite Mud Log and Integrated Drilling Record