Detail Log: Depth Scale Selection, Thin-Bed Resolution, and WCSB Formation Evaluation

A detail log is a well log printed or displayed on a depth scale chosen fine enough to show sufficient detail of the formation for close analysis. The two most common detail scales are 1:200 and 5 inches of log paper per 100 feet of hole, which are near equivalents: at 1:200, one metre of borehole occupies 5 mm of log, and at 5 inches per 100 feet, 100 feet of hole occupies 5 inches of paper. Both spread the data out enough that a geologist or petrophysicist can resolve individual beds, thin porosity streaks, fluid contacts and bed boundaries that would be crushed together and lost on a coarser scale. The idea of a detail log is best understood against its counterpart, the correlation log, which is printed at a compressed scale such as 1:500 or 1:1000, or 1 or 2 inches per 100 feet. The correlation log squeezes hundreds or thousands of metres of section onto a manageable strip so a geologist can line up the same marker beds from well to well across a field and build a picture of structure and stratigraphy. The detail log does the opposite job: it zooms in so the same tools, the gamma ray, resistivity, density, neutron and sonic curves, can be read bed by bed to pick net pay, calculate porosity and water saturation, and place perforation intervals precisely. In Western Canadian Sedimentary Basin evaluation this distinction is routine. A Cardium or Viking sand may carry productive porosity in laminations only a fraction of a metre thick, and the difference between seeing those beds and missing them can decide whether a zone is completed or bypassed. On a 1:200 detail print those thin beds are legible; on a 1:1000 correlation print they blur into a single deflection. Logging service companies deliver both scales from the same recorded data because the digital measurement is scale-independent; only the plotting changes. Modern log analysis is done on screen where an interpreter can zoom continuously, but the named scales persist because regulatory submissions, mudlogging deliverables and cross-section work still reference them. In Alberta, logs submitted to the AER and archived through the public well data system follow standard presentation conventions so that any later analyst can pull a detail log at a known scale and read porosity and pay exactly as the original interpreter did. The detail log, then, is not a different measurement from the correlation log; it is the same suite of curves plotted at a resolution chosen to let the fine structure of the reservoir be read directly.

Choosing 1:200 Detail Versus 1:1000 Correlation

The scale an analyst calls for depends on the question. To map a field and tie a new well into known stratigraphy, the correlation log at 1:1000 or 1:500 is right, because it puts hundreds of metres of section in view so marker beds can be traced from offset to offset. To evaluate a specific reservoir for completion, the detail log at 1:200 is right, because it magnifies the target interval until individual beds, porosity streaks and fluid contacts can be read and measured. Many WCSB well files carry both, the correlation print for the geologist building cross-sections and the detail print for the petrophysicist calculating net pay, water saturation and perforation intervals in the pay zone.

Reading Thin Beds and Fluid Contacts on a Detail Log

The practical value of the detail scale shows in thinly bedded reservoirs. A half-metre porosity streak that appears as a single small kick on a 1:1000 log resolves into a clear, measurable bed on a 1:200 detail print, letting the interpreter assign it porosity from the density-neutron pair and saturation from resistivity. Fluid contacts, the depth where oil gives way to water, are also sharper at detail scale, so the analyst can place a completion above the contact with confidence. This resolution is what turns a raw logging run into specific perforation depths handed to the completions engineer.

Related Terms

A detail log is the fine-scale counterpart to the correlation log, which is printed at a compressed scale for tracing marker beds across a field; the two are the same curves at different resolutions. The curves themselves come from tools such as the gamma ray log, which reads shale volume and helps pick bed boundaries, and the density and neutron logs used to compute porosity, the pore volume fraction that decides how much fluid a bed can hold. Reading these curves at detail scale is what allows precise net-pay and perforation-interval selection.

Real-World WCSB Scenario: A Thinly Bedded Cardium Completion Call

A geologist evaluating a horizontal Cardium well near Pembina first reviews the vertical pilot hole logs at 1:1000 to correlate the top of the Cardium against three offset wells and confirm the well is in zone. That correlation print serves its purpose but shows the reservoir as a single broad porosity response. To decide the completion, the petrophysicist pulls the same gamma ray, density, neutron and resistivity curves replotted at 1:200. At detail scale the Cardium resolves into several sand laminations separated by tight streaks, two of which carry good porosity above the water saturation cutoff.

Reading the detail log, the analyst counts roughly 4 m of net pay across the productive laminations rather than the 2.5 m a coarse read would have suggested, and hands precise perforation depths to the completions team. The extra pay identified at detail scale, worth meaningful incremental production over the CAD 4 to 8 million cost of the horizontal well, is exactly the value a fine depth scale is meant to capture.