Shandal Geological Field Survey System · geologicalfieldsurvey.com
Composite log and interpretation
Crossplots
Layers and their interpretation
Calculate at a depth, step by step
Type a depth or tap the composite log. Every equation is written out with the readings of that depth, as worked in the lectures: gamma-ray index, shale volume, density, sonic and neutron porosity, total and effective porosity, secondary porosity, Sw, Sxo, the moveable hydrocarbon index and the hydrocarbon saturations.
Comparison and correlation of the loaded wells
Read a log from an image
Load a scan or photo of a printed log (from a report or a book). Mark two depths and the scale of the track, pick the colour of the curve, and the system traces it into readings. Hold the camera square to the page: a tilted photo skews depths.
Log response explorer
Choose a log and a rock or fluid: the system shows the typical reading, where it sits among the other rocks, and why the log reads so there.
How each log responds to rocks and fluids
Guide to well logs
Accuracy and limits of error
The calculations follow the published equations exactly; the error of the results comes from the logs themselves and from the parameters chosen. The typical measurement accuracy of the logs, from service-company specifications, and its effect:
| Log | Typical accuracy | Effect on results |
|---|---|---|
| Density | ± 0.01–0.025 g/cm³ | ± 0.6–1.5 porosity units |
| Neutron | ± 1 p.u. (φ < 10 %) · ± 5 % rel. | Lithology and gas separation |
| Sonic | ± 1–2 µs/ft | ± 1 porosity unit |
| Deep resistivity | ± 2–5 % | ± 1–3 % in water saturation |
| Gamma ray | ± 5 % (statistical) | ± 0.03–0.05 in shale volume |
The largest error is in the parameters, not the logs. An error of 0.2 in the cementation exponent m, or 20 % in Rw, changes water saturation by 10–20 saturation units. For every hydrocarbon layer the table of layers gives the range of Sw when m and n vary by ± 0.2, Rw by ± 20 % and porosity by ± 0.02.
Lithology: the mineral solution is checked by rebuilding the measured logs from the solved volumes; the misfit is shown in the results. A misfit above 1 (in units of the log error) means a mineral or fluid is missing from the model: change the formation setting or check for gas, coal or salt.
Permeability from the Timur equation is good to within about one order of magnitude; calibrate it with core or well tests.
Bad hole: where the caliper exceeds the bit by more than 1 inch or the density correction exceeds 0.1 g/cm³, density and neutron readings are unreliable; those levels are marked and kept out of the gas flag.
Logs read from an image: the error is about one pixel in depth and in value; the tool states it in units after tracing. Scan at 300 dpi or more and keep the page flat.