Top Log Interval
The top log interval in well logging operations is the uppermost depth section recorded on a particular logging run, defined by the shallowest depth at which the logging tool begins recording data as it is pulled upward from its bottom depth (the tool depth) toward the surface; in practice, the top log interval is determined by the casing shoe depth (logging is limited to the open-hole section below the last casing string), by rig floor and kelly bushing heights (the surface reference elevation above which wireline tools cannot be pulled), by rig restrictions on wireline tension at shallow depths, or by the operational decision to log only a subset of the open-hole interval for efficiency reasons; the top log interval is recorded on the log header along with the bottom log depth, the bit size, mud properties, casing shoe depth, and tool identifiers, providing the depth boundary framework for the entire suite of log curves recorded on that run; quality-control practice requires that consecutive logging runs in the same well overlap their depth intervals (the top of the deeper run extends above the bottom of the shallower run by at least 50 to 100 feet) so that depth correlation between runs can be verified by matching characteristic gamma ray or resistivity curve features in the overlap zone, detecting depth discrepancies from wireline stretch, tide corrections in offshore wells, or reference-point measurement errors before the logs are delivered to the geological team for interpretation.
Key Takeaways
- The choice of top log interval affects the completeness of formation evaluation and must be planned before each logging run to ensure critical reservoir and cap rock intervals are not omitted from the log coverage: in a well with multiple reservoirs at different depths, the top log interval for the main reservoir logging run should be set shallow enough to include the lowermost cap rock or seal interval above the reservoir (to evaluate seal integrity) and the transition zone between the reservoir and overlying shale (to calibrate the gamma ray baseline for shale content calculation); if the top log interval is set too deep (because of an operational error in tool depth measurement or a decision to save rig time by shortening the log run), a reservoir zone near the top of the open hole may be unlogged and missed in the formation evaluation, potentially causing an unrecognized bypass pay situation that would require a logging sidetrack or re-entry to diagnose; in exploratory wells where the reservoir location is uncertain, the top log interval is set at the casing shoe (logging the full open-hole interval) to ensure that any unexpected reservoir encountered shallower than prognosed is captured on the log suite.
- Depth control and top log interval verification are essential elements of wireline quality control because systematic depth errors in the wireline measurement (from wireline stretch, magnetic anomalies in drill pipe that affect the casing collar locator used for depth reference, or measurement wheel slippage on the logging unit) can shift the apparent top log interval away from its actual formation depth by amounts ranging from a few feet to tens of feet in very deep wells: the standard depth reference for wireline logging is the kelly bushing (KB) elevation above mean sea level (or above a defined datum in offshore wells), and all log depths are measured as feet below KB from the surface wheel counter on the logging unit; the actual cable length in the hole is measured by the counting wheel, but the effective tool depth differs from the cable length by the cable stretch under tension (which is proportional to the effective cable weight in the drilling fluid times the cable elastic modulus and cross-section) and the heave correction in floating vessels; the top log interval is reported on the log header at the nominal KB-corrected depth, but a discrepancy of 10 to 20 feet between the reported top log interval and the actual formation depth at which data recording began is not unusual in deep wells without explicit depth correction applied.
- Rathole and conductor pipe logging at the top log interval presents special challenges in wells where the rathole (the section of wellbore drilled below the planned total depth to allow BHA room) or the conductor pipe (the large-diameter shallow casing used for structural support and shallow gas protection) is the shallowest section logged: when a sonic or density tool is pulled through a shallow, large-diameter casing section near the top log interval, the tool readings in the casing are obviously unreliable (the casing and cement rather than the formation are measured), but the transition zone immediately below the casing shoe where the tool begins reading formation (rather than casing cement or wash-out zone) may take 10 to 20 feet of logging depth before the tool reaches full formation response; this transition interval near the casing shoe is the uppermost part of the open-hole log data and must be interpreted with caution because cycle-skipping in sonic tools, density pad contact issues in rugose washout zones, and mud filtrate invasion in the near-casing-shoe zone often distort the log response relative to undisturbed formation values away from the wellbore.
- Overlapping log intervals for depth correlation between consecutive runs depends on selecting the top log interval of the shallower run to extend at least 50 to 100 feet above the bottom depth of the shallower run logged previously, creating a depth interval recorded twice on two separate logging trips into the wellbore from which depth shift corrections can be calculated by cross-correlating the gamma ray or spontaneous potential curves from the two runs: the gamma ray curve is preferred for depth correlation because its response to formation lithology is largely independent of borehole condition (mud invasion, rugosity, and tool speed effects are smaller for gamma ray than for density, neutron, or sonic) and because it shows distinctive short-wavelength variation in most formations; a systematic depth offset between the gamma ray curves from the two runs (where all curve features are shifted by the same constant depth amount in one run relative to the other) indicates a uniform depth error from cable stretch or wheel calibration that can be corrected by a constant depth shift applied to the shallower run's data; a non-systematic depth mismatch (where some features match and others do not) indicates either differential cable stretch (tool motion during logging), formation heterogeneity across the overlap zone, or logging speed variation that changes the depth sampling characteristics of the tool.
- Completion and perforation planning references the top log interval as the upper bound of the formation evaluation data available to the completion engineer, and intervals recorded above the top log interval (in casing or in rathole) must not be perforated without additional through-casing logging (using pulsed neutron or carbon-oxygen logs) or pressure testing to confirm the nature of the unlogged interval: in wells where time or cost pressures result in a top log interval that is too deep (missing a potential upper reservoir zone), the completion engineer must decide whether to accept the unlogged zone as potentially uneconomic (and move forward with perforating only the evaluated intervals below the top log depth) or to acquire additional log data before making the completion decision; this decision is particularly consequential in gas wells where the density contrast between a thin gas-charged sand and the surrounding shale is small enough that the zone would be identifiable only on logs with adequate vertical resolution and formation evaluation through the full open-hole interval.
Fast Facts
The convention of specifying both the top and bottom depth of every wireline logging run on the log header was established as a quality standard by the American Petroleum Institute (API) in its logging documentation guidelines and by the Society of Petrophysicists and Well Log Analysts (SPWLA) in its recommended data recording practices. The top log interval, along with the casing shoe depth and bit size at total depth, forms part of the mandatory header information required by most regulatory agencies (including the U.S. Bureau of Land Management and state regulatory bodies) when reporting well log data in support of drilling permit compliance and resource assessment filings.
What Is the Top Log Interval?
The top log interval is the shallowest depth recorded on a wireline or logging-while-drilling run, defining the upper boundary of the formation evaluation data collected for that logging trip. It is determined by the casing shoe depth (open-hole logging begins below the last casing string), the surface reference elevation, and operational decisions about which depths require logging on a given run. The top log interval is recorded on the log header and must be planned to include all critical reservoir and seal intervals, overlapping with previous runs for depth correlation, and extending shallow enough that no potential pay zone above the primary target is inadvertently omitted from the evaluation program.
Synonyms and Related Terminology
Top log interval is also called the log top, the upper logging limit, or the top of logged interval in well records and in regulatory filings. Related terms include wireline logging (the suite of downhole measurements made by instruments lowered into the wellbore on an electrical cable, which are recorded from the bottom depth to the top log interval as the tool is pulled upward through the open-hole section to characterize formation lithology, porosity, fluid content, and mechanical properties), log header (the page of well identification, depth reference, borehole condition, and tool specification data that accompanies every wireline or LWD log and which records the top log interval, the bottom log depth, the casing shoe depth, the mud weight, and the bit size that define the logging program parameters for that well run), casing shoe (the bottom of the casing string, which defines the upper boundary of the open-hole section available for logging and which typically establishes the maximum allowable top log interval for the logging run in the interval below that casing string), depth correlation (the process of matching characteristic features in log curves recorded on overlapping depth intervals from different logging runs or different log types, used to verify that the top log intervals of successive runs are accurately depth-registered so that multi-run log composite displays show correct formation depths), and kelly bushing (the reference datum from which all wireline log depths are measured, equal to the elevation of the rotary table above mean sea level or a defined offshore datum, which determines the absolute depth coordinate for the top log interval and all other logged depths in the well).
Why Top Log Interval Planning Is a Critical Well Engineering Decision
Every foot of open-hole formation left unlogged above the top log interval is a foot of reservoir potential that the formation evaluation program has abandoned without data. In exploration wells where the reservoir location is uncertain, setting the top log interval at the casing shoe ensures that any unexpected reservoir encountered above the prognosed target is captured in the evaluation. In development wells where the reservoir location is well understood, setting the top log interval to include the full seal-reservoir system above the known pay zone ensures that the integrity of the overlying cap rock can be confirmed before perforating. The few hundred dollars saved by reducing a log run by 100 feet can be vastly exceeded by the cost of a subsequent through-casing logging run, a sidetrack, or a missed completion interval when the top log interval was set too deep. Getting the top log interval right before the logging tools go in the hole is one of the simplest and most consequential decisions in well evaluation planning.