{"id":"well-log-depth-md-tvd-datum-engineering-guide","canonicalSlug":"well-log-depth-md-tvd-datum-engineering-guide","url":"https://wellficent.com/en/news/well-log-depth-md-tvd-datum-engineering-guide","language":"en","title":"Well-log depth: MD, TVD and datum explained","summary":"A petroleum-engineering guide to depth references, well logs and hydrostatic pressure, with a verified SODIR example and reproducible calculations.","body":["A depth without its measurement path, reference and unit is an incomplete engineering statement. Three numbers can describe the same bottomhole position while answering different questions: how far the borehole travels, how far down it goes, and where it sits relative to sea level. Confusing those questions can distort a geological correlation, a pressure calculation or a drilling comparison even when every copied number is accurate.","This guide separates published observations from calculations and fictional teaching cases. The real example is Norwegian Offshore Directorate wellbore 16/11-1 S, source identifier 112, checked on 8 November 2026. The geometry and drilling-fluid cases are independent mathematical examples. They do not reconstruct that borehole, estimate its present pressure or prescribe an operating programme."],"keyPoints":["MD follows the borehole; TVD measures a vertical separation. A common unit does not make them interchangeable.","For SODIR 112, 3,050 m MD and 3,020 m TVD are reported from RKB. The 2,993 m depth below mean sea level is calculated using the reported 27 m RKB elevation.","Unknown datums, missing log samples and unavailable historical vintages remain unknown; they are not zero or observed measurements."],"sections":[{"heading":"1. Start with the coordinate, not the largest number","paragraphs":["Measured depth, MD, describes distance along a borehole. True vertical depth, TVD, describes the vertical separation from a stated reference. With the same starting point, a straight vertical hole has equal MD and TVD; a deviated path generally has a longer MD. A published total depth should therefore retain its depth type instead of being reduced to a generic depth field.","SLB’s terminology distinguishes these uses, while BOEM’s borehole field definitions explicitly reference total MD and TVD to the rig kelly bushing. That is evidence about those fields, not permission to assign the same reference to every other dataset. A comparable record needs the quantity, unit, zero point, sign convention and source identity together."]},{"heading":"2. A datum shift is a vertical calculation","paragraphs":["Define elevation as positive upward from mean sea level, MSL, and depth below MSL as positive downward. If the rotary kelly bushing, RKB, has elevation z_RKB, then depth_MSL = TVD_RKB − z_RKB. The subtraction changes the vertical reference; it does not convert MD into TVD. A different zero point or sign convention requires a separately stated transformation.","The historical UKOOA P7/2000 Revision 5 format distinguishes zero measured depth, zero true vertical depth and the permanent vertical datum. Its October 2002 terminology is a useful explanation of why a rig change or sidetrack can change the reference. It is not presented here as a new or mandatory reporting standard. Energistics’ WITSML 2.0 coordinate definition likewise carries datum and unit explicitly."]},{"heading":"3. Read one documented borehole carefully","paragraphs":["SODIR reports 16/11-1 S with total MD 3,050 m RKB, final TVD 3,020 m RKB, RKB elevation 27 m above MSL and water depth 67 m. These are source-reported values, not four measurements newly taken by Wellficent. The matching Well Atlas record is no-sodir-112: its sourceRecordId is 112 and its MD/TVD fields preserve the RKB reference. A text search also returns 7216/11-1 S, so a similar name alone is insufficient identification.","Under the sign convention above, the reported TVD gives 3,020 − 27 = 2,993 m below MSL. Subtracting the 67 m water depth gives a calculated 2,926 m vertical separation below the sea floor. Neither derived value replaces the source fields or constitutes an independent survey. In particular, subtracting 27 from the 3,050 m MD would not produce a valid subsea TVD."]},{"heading":"4. A final inclination cannot replace a deviation survey","paragraphs":["For an ideal straight segment of length L inclined by angle i from vertical, the vertical increment is L × cos(i). At L = 3,000 m, inclinations of 0°, 30° and 60° give vertical increments of 3,000 m, approximately 2,598.1 m and 1,500 m. The chart reproduces these fictional straight-segment calculations, rounded to 0.1 m. They are geometry checks, not drilled trajectories.","A real borehole changes inclination and azimuth along its path. Reconstructing TVD requires the appropriate survey stations, references and calculation method, with uncertainty and quality checks. Applying the maximum or final inclination to the entire MD invents a trajectory. The reported 3,050 and 3,020 m totals do not reveal all intermediate depths, lateral displacement, formation dip or true reservoir thickness."]},{"heading":"5. Hydrostatic pressure uses vertical fluid height","paragraphs":["For a static, continuous fluid column of uniform density, the pressure increment is density × gravitational acceleration × vertical height. In conventional oilfield units the rounded relation is pressure in psi = 0.052 × mud weight in pounds per US gallon × TVD in feet. This is an increment from the column’s reference pressure, not automatically an absolute reservoir pressure.","A fictional 12 ppg column spanning 10,000 ft vertically gives 6,240 psi using that coefficient. Substituting an unrelated 15,000 ft MD gives 9,360 psi, an overstatement of 3,120 psi or 50%. The erroneous result illustrates the reference mistake; it is not a second observation. Circulating friction, surface pressure, compressibility, temperature, gas and changing density require additional treatment. No mud-weight or well-control instruction is inferred from this classroom case."]},{"heading":"6. A LAS file is a data contract","paragraphs":["The USGS explanation of LAS identifies separate version, well, curve and ASCII-data sections. Read the file’s actual version and wrapping declaration, start/stop depths, step, curve order, mnemonics, units and null definition before interpreting the samples. A file extension or familiar curve name does not establish a complete depth reference, calibration history or permission to reuse the material.","If a teaching file declares NULL = −999.25, that value means missing in that file; it is not a physical log response or a universal sentinel to impose on all other files. Preserve genuine zero responses separately. A negative step can describe a decreasing-depth sequence, so reordering samples without preserving the source relationship is also a transformation that needs documenting. Unknown units remain unknown."]},{"heading":"7. Depth matching and thickness require more evidence","paragraphs":["Formation tops, cuttings, core intervals, wireline logs and drilling measurements need not share the same depth acquisition or correction history. Before overlaying them, establish the wellbore branch, run, reference and any documented depth shift. Keep the original values and the transformation separately. A visually attractive overlay does not validate a correlation.","An interval measured along hole is not automatically its true vertical thickness, and a vertical thickness is not necessarily true stratigraphic thickness in dipping beds. The well path and structural geometry matter. A gap between two reported formation tops can be a derived interval within a compatible reference; it is not proof that a contact was directly measured at every intermediate depth."]},{"heading":"8. Keep the atlas snapshot separate from today’s source page","paragraphs":["The Well Atlas record used here retains its existing source-check date of 2 October 2026. This article’s source reading on 8 November does not refresh the whole registry, certify the well’s present operational state or change the historical availability of its measurements. The current primary page and the retained reviewed edition are separate evidence objects, even when their displayed depth values agree.","Use the source-scoped record identifier to retrieve the documented borehole, preserve its attribution and terms, and follow its source record URL. The Norwegian Offshore Directorate attribution and NLOD 2.0 terms travel with the registry response. Public access does not settle every downstream reuse question. An absent temperature, depth datum or original available-at timestamp must not be filled by assuming today’s download was historically known."]},{"heading":"9. Build an auditable engineering decision","paragraphs":["A useful review package records the original document and revision, source access time, exact field or curve, unit and reference, branch identity, calculation inputs, sign convention, rounding and unresolved limitations. Store derived depths alongside their formula rather than silently overwriting the source total. A second calculation should reproduce the result from those inputs without needing an undocumented plotting adjustment.","These distinctions have economic value because interval placement, completion design and comparison of drilling progress depend on consistent coordinates. This guide assigns no invented dollar benefit to correcting a datum. A financial assessment would also need the actual programme, costs, uncertainty and consequences. The supported conclusion is narrower and useful: establish the coordinate contract before converting a depth into an engineering claim."]}],"text":"Well-log depth: MD, TVD and datum explained\n\nA petroleum-engineering guide to depth references, well logs and hydrostatic pressure, with a verified SODIR example and reproducible calculations.\n\nMD follows the borehole; TVD measures a vertical separation. A common unit does not make them interchangeable.\n\nFor SODIR 112, 3,050 m MD and 3,020 m TVD are reported from RKB. The 2,993 m depth below mean sea level is calculated using the reported 27 m RKB elevation.\n\nUnknown datums, missing log samples and unavailable historical vintages remain unknown; they are not zero or observed measurements.\n\nA depth without its measurement path, reference and unit is an incomplete engineering statement. Three numbers can describe the same bottomhole position while answering different questions: how far the borehole travels, how far down it goes, and where it sits relative to sea level. Confusing those questions can distort a geological correlation, a pressure calculation or a drilling comparison even when every copied number is accurate.\n\nThis guide separates published observations from calculations and fictional teaching cases. The real example is Norwegian Offshore Directorate wellbore 16/11-1 S, source identifier 112, checked on 8 November 2026. The geometry and drilling-fluid cases are independent mathematical examples. They do not reconstruct that borehole, estimate its present pressure or prescribe an operating programme.\n\n1. Start with the coordinate, not the largest number\n\nMeasured depth, MD, describes distance along a borehole. True vertical depth, TVD, describes the vertical separation from a stated reference. With the same starting point, a straight vertical hole has equal MD and TVD; a deviated path generally has a longer MD. A published total depth should therefore retain its depth type instead of being reduced to a generic depth field.\n\nSLB’s terminology distinguishes these uses, while BOEM’s borehole field definitions explicitly reference total MD and TVD to the rig kelly bushing. That is evidence about those fields, not permission to assign the same reference to every other dataset. A comparable record needs the quantity, unit, zero point, sign convention and source identity together.\n\n2. A datum shift is a vertical calculation\n\nDefine elevation as positive upward from mean sea level, MSL, and depth below MSL as positive downward. If the rotary kelly bushing, RKB, has elevation z_RKB, then depth_MSL = TVD_RKB − z_RKB. The subtraction changes the vertical reference; it does not convert MD into TVD. A different zero point or sign convention requires a separately stated transformation.\n\nThe historical UKOOA P7/2000 Revision 5 format distinguishes zero measured depth, zero true vertical depth and the permanent vertical datum. Its October 2002 terminology is a useful explanation of why a rig change or sidetrack can change the reference. It is not presented here as a new or mandatory reporting standard. Energistics’ WITSML 2.0 coordinate definition likewise carries datum and unit explicitly.\n\n3. Read one documented borehole carefully\n\nSODIR reports 16/11-1 S with total MD 3,050 m RKB, final TVD 3,020 m RKB, RKB elevation 27 m above MSL and water depth 67 m. These are source-reported values, not four measurements newly taken by Wellficent. The matching Well Atlas record is no-sodir-112: its sourceRecordId is 112 and its MD/TVD fields preserve the RKB reference. A text search also returns 7216/11-1 S, so a similar name alone is insufficient identification.\n\nUnder the sign convention above, the reported TVD gives 3,020 − 27 = 2,993 m below MSL. Subtracting the 67 m water depth gives a calculated 2,926 m vertical separation below the sea floor. Neither derived value replaces the source fields or constitutes an independent survey. In particular, subtracting 27 from the 3,050 m MD would not produce a valid subsea TVD.\n\n4. A final inclination cannot replace a deviation survey\n\nFor an ideal straight segment of length L inclined by angle i from vertical, the vertical increment is L × cos(i). At L = 3,000 m, inclinations of 0°, 30° and 60° give vertical increments of 3,000 m, approximately 2,598.1 m and 1,500 m. The chart reproduces these fictional straight-segment calculations, rounded to 0.1 m. They are geometry checks, not drilled trajectories.\n\nA real borehole changes inclination and azimuth along its path. Reconstructing TVD requires the appropriate survey stations, references and calculation method, with uncertainty and quality checks. Applying the maximum or final inclination to the entire MD invents a trajectory. The reported 3,050 and 3,020 m totals do not reveal all intermediate depths, lateral displacement, formation dip or true reservoir thickness.\n\n5. Hydrostatic pressure uses vertical fluid height\n\nFor a static, continuous fluid column of uniform density, the pressure increment is density × gravitational acceleration × vertical height. In conventional oilfield units the rounded relation is pressure in psi = 0.052 × mud weight in pounds per US gallon × TVD in feet. This is an increment from the column’s reference pressure, not automatically an absolute reservoir pressure.\n\nA fictional 12 ppg column spanning 10,000 ft vertically gives 6,240 psi using that coefficient. Substituting an unrelated 15,000 ft MD gives 9,360 psi, an overstatement of 3,120 psi or 50%. The erroneous result illustrates the reference mistake; it is not a second observation. Circulating friction, surface pressure, compressibility, temperature, gas and changing density require additional treatment. No mud-weight or well-control instruction is inferred from this classroom case.\n\n6. A LAS file is a data contract\n\nThe USGS explanation of LAS identifies separate version, well, curve and ASCII-data sections. Read the file’s actual version and wrapping declaration, start/stop depths, step, curve order, mnemonics, units and null definition before interpreting the samples. A file extension or familiar curve name does not establish a complete depth reference, calibration history or permission to reuse the material.\n\nIf a teaching file declares NULL = −999.25, that value means missing in that file; it is not a physical log response or a universal sentinel to impose on all other files. Preserve genuine zero responses separately. A negative step can describe a decreasing-depth sequence, so reordering samples without preserving the source relationship is also a transformation that needs documenting. Unknown units remain unknown.\n\n7. Depth matching and thickness require more evidence\n\nFormation tops, cuttings, core intervals, wireline logs and drilling measurements need not share the same depth acquisition or correction history. Before overlaying them, establish the wellbore branch, run, reference and any documented depth shift. Keep the original values and the transformation separately. A visually attractive overlay does not validate a correlation.\n\nAn interval measured along hole is not automatically its true vertical thickness, and a vertical thickness is not necessarily true stratigraphic thickness in dipping beds. The well path and structural geometry matter. A gap between two reported formation tops can be a derived interval within a compatible reference; it is not proof that a contact was directly measured at every intermediate depth.\n\n8. Keep the atlas snapshot separate from today’s source page\n\nThe Well Atlas record used here retains its existing source-check date of 2 October 2026. This article’s source reading on 8 November does not refresh the whole registry, certify the well’s present operational state or change the historical availability of its measurements. The current primary page and the retained reviewed edition are separate evidence objects, even when their displayed depth values agree.\n\nUse the source-scoped record identifier to retrieve the documented borehole, preserve its attribution and terms, and follow its source record URL. The Norwegian Offshore Directorate attribution and NLOD 2.0 terms travel with the registry response. Public access does not settle every downstream reuse question. An absent temperature, depth datum or original available-at timestamp must not be filled by assuming today’s download was historically known.\n\n9. Build an auditable engineering decision\n\nA useful review package records the original document and revision, source access time, exact field or curve, unit and reference, branch identity, calculation inputs, sign convention, rounding and unresolved limitations. Store derived depths alongside their formula rather than silently overwriting the source total. A second calculation should reproduce the result from those inputs without needing an undocumented plotting adjustment.\n\nThese distinctions have economic value because interval placement, completion design and comparison of drilling progress depend on consistent coordinates. This guide assigns no invented dollar benefit to correcting a datum. A financial assessment would also need the actual programme, costs, uncertainty and consequences. The supported conclusion is narrower and useful: establish the coordinate contract before converting a depth into an engineering claim.\n\nTechnical and economic analysis\n\nAnalysis as of: 2026-11-08\n\nEvidence cutoff: 2026-11-08\n\nOutlook horizon: Illustrative straight-segment geometry\n\nIllustrative sensitivity\n\nSame segment length, different vertical increments\n\nFictional ideal straight segments: L = 3,000 m; i measured from vertical; vertical increment = L × cos(i). Rounded to 0.1 m. Not real trajectories, measurements or a reconstruction of SODIR 112.\n\nThese calculations illustrate stated assumptions; they are not observations or a calibrated forecast.\n\ni = 0°: 3,000 vertical increment in metres\n\ni = 30°: 2,598.1 vertical increment in metres\n\ni = 60°: 1,500 vertical increment in metres","category":"energy","region":"world","topics":["oil","gas"],"eventDate":"2026-11-08","eventDateBasis":"analysis-as-of-date","publishedAt":"2026-11-08T14:54:15Z","modifiedAt":"2026-11-08T14:54:15Z","publicationBasis":"first-publication","preparedAt":"2026-11-08T14:42:43Z","sourcePublishedAt":null,"translatedAt":"2026-11-08T14:34:18Z","sources":[{"url":"https://factpages.sodir.no/en/wellbore/pageview/with/welllogs/112","name":"Norwegian Offshore Directorate — wellbore 16/11-1 S, source record 112","licence":"NLOD 2.0 — source terms apply","checkedAt":"2026-11-08","licenseUrl":"https://data.norge.no/nlod/en/2.0","attribution":"Norwegian Offshore Directorate (SODIR); source values distinguished from Wellficent calculations."},{"url":"https://glossary.slb.com/en/terms/m/measured_depth","name":"SLB Energy Glossary — measured depth","checkedAt":"2026-11-08"},{"url":"https://glossary.slb.com/en/terms/t/true_vertical_depth","name":"SLB Energy Glossary — true vertical depth","checkedAt":"2026-11-08"},{"url":"https://glossary.slb.com/en/terms/d/depth_reference","name":"SLB Energy Glossary — depth reference","checkedAt":"2026-11-08"},{"url":"https://glossary.slb.com/en/terms/h/hydrostatic_pressure","name":"SLB Energy Glossary — hydrostatic pressure","checkedAt":"2026-11-08"},{"url":"https://www.data.boem.gov/Main/HtmlPage.aspx?page=borehole","name":"BOEM Data Center — field definitions for boreholes","checkedAt":"2026-11-08"},{"url":"https://www.iogp.org/wp-content/uploads/2016/12/P7.pdf","name":"UKOOA — P7/2000 Revision 5, October 2002; historical well-deviation exchange format","checkedAt":"2026-11-08"},{"url":"https://docs.energistics.org/WITSML/WITSML_TOPICS/WITSML-500-449-0-R-sv2000.html","name":"Energistics — WITSML 2.0 MeasuredDepthCoord","checkedAt":"2026-11-08","publishedAt":"2016-11-11"},{"url":"https://www.usgs.gov/programs/national-geological-and-geophysical-data-preservation-program/las-format","name":"USGS — LAS format","checkedAt":"2026-11-08"},{"url":"https://wellficent.com/en/wells/record/no-sodir-112","name":"Wellficent Well Atlas — retained SODIR source record 112; additional internal reference","checkedAt":"2026-11-08"}],"translations":{"en":"https://wellficent.com/en/news/well-log-depth-md-tvd-datum-engineering-guide","tr":"https://wellficent.com/tr/news/well-log-depth-md-tvd-datum-engineering-guide","ar":"https://wellficent.com/ar/news/well-log-depth-md-tvd-datum-engineering-guide","fr":"https://wellficent.com/fr/news/well-log-depth-md-tvd-datum-engineering-guide","es":"https://wellficent.com/es/news/well-log-depth-md-tvd-datum-engineering-guide","ru":"https://wellficent.com/ru/news/well-log-depth-md-tvd-datum-engineering-guide","pt":"https://wellficent.com/pt/news/well-log-depth-md-tvd-datum-engineering-guide"},"corrections":[],"locations":[],"type":"analysis","analysis":{"asOf":"2026-11-08","kind":"technical-economic","chart":{"kind":"illustrative","note":{"ar":"قطع مستقيمة مثالية افتراضية: L = 3,000 م؛ i من الاتجاه الرأسي؛ الزيادة الرأسية = L × cos(i). التقريب إلى 0.1 م. ليست مسارات أو قياسات حقيقية أو إعادة بناء SODIR 112.","en":"Fictional ideal straight segments: L = 3,000 m; i measured from vertical; vertical increment = L × cos(i). Rounded to 0.1 m. Not real trajectories, measurements or a reconstruction of SODIR 112.","es":"Segmentos rectos ideales ficticios: L = 3.000 m; i desde la vertical; incremento = L × cos(i). Redondeo a 0,1 m. No son trayectorias, mediciones ni reconstrucción de SODIR 112.","fr":"Segments droits idéaux fictifs : L = 3 000 m ; i depuis la verticale ; incrément = L × cos(i). Arrondi à 0,1 m. Ni trajectoires réelles, ni mesures, ni reconstitution de SODIR 112.","pt":"Segmentos retos ideais fictícios: L = 3.000 m; i desde a vertical; incremento = L × cos(i). Arredondamento a 0,1 m. Não são trajetórias, medições ou reconstrução de SODIR 112.","ru":"Условные идеальные прямые отрезки: L = 3 000 м; i от вертикали; приращение = L × cos(i). Округление до 0,1 м. Не реальные траектории, измерения или реконструкция SODIR 112.","tr":"Kurmaca ideal düz parçalar: L = 3.000 m; i düşeyden ölçülür; düşey artış = L × cos(i). 0,1 m’ye yuvarlandı. Gerçek yörünge, ölçüm veya SODIR 112 rekonstrüksiyonu değildir."},"rows":[{"label":{"ar":"i = 0°","en":"i = 0°","es":"i = 0°","fr":"i = 0°","pt":"i = 0°","ru":"i = 0°","tr":"i = 0°"},"value":3000},{"label":{"ar":"i = 30°","en":"i = 30°","es":"i = 30°","fr":"i = 30°","pt":"i = 30°","ru":"i = 30°","tr":"i = 30°"},"value":2598.1},{"label":{"ar":"i = 60°","en":"i = 60°","es":"i = 60°","fr":"i = 60°","pt":"i = 60°","ru":"i = 60°","tr":"i = 60°"},"value":1500}],"unit":"m","title":{"ar":"طول واحد وزيادات رأسية مختلفة","en":"Same segment length, different vertical increments","es":"Misma longitud, distintos incrementos verticales","fr":"Même longueur, incréments verticaux différents","pt":"Mesmo comprimento, incrementos verticais diferentes","ru":"Одинаковая длина, разное вертикальное приращение","tr":"Aynı parça uzunluğu, farklı düşey artışlar"},"unitLabel":{"ar":"الزيادة الرأسية بالمتر","en":"vertical increment in metres","es":"incremento vertical en metros","fr":"incrément vertical en mètres","pt":"incremento vertical em metros","ru":"вертикальное приращение в метрах","tr":"metre cinsinden düşey artış"}},"horizon":"Illustrative straight-segment geometry","horizonLabel":{"ar":"هندسة توضيحية لقطع مستقيم","en":"Illustrative straight-segment geometry","es":"Geometría ilustrativa de segmentos rectos","fr":"Géométrie illustrative de segments droits","pt":"Geometria ilustrativa de segmentos retos","ru":"Условная геометрия прямых отрезков","tr":"Örnek düz parça geometrisi"},"evidenceCutoff":"2026-11-08"},"relatedArticles":[{"canonicalSlug":"oil-gas-production-data-rates-boe-decline-engineering-guide","url":"https://wellficent.com/en/news/oil-gas-production-data-rates-boe-decline-engineering-guide","title":"Oil and gas production data: rates, boe, decline and value","language":"en"},{"canonicalSlug":"refinery-capacity-calendar-stream-day-engineering-guide","url":"https://wellficent.com/en/news/refinery-capacity-calendar-stream-day-engineering-guide","title":"Refinery capacity: calendar day, stream day and actual throughput","language":"en"}],"image":{"url":"https://wellficent.com/media/bccfafeca328d8659e473763179eb4230006430da6abd5dd938d461b7acd18f4.webp","width":1600,"height":900,"alt":"Numerical graphic for SODIR wellbore 16/11-1 S: reported 3,050 m MD RKB and 3,020 m TVD RKB; 2,993 m below mean sea level calculated as 3,020 minus the 27 m RKB elevation. No well path is drawn.","credit":"Wellficent — original programmatic graphic and datum calculation. Data: Norwegian Offshore Directorate, NLOD 2.0, checked 8 November 2026. Not a photograph or independent survey.","licenseUrl":"https://data.norge.no/nlod/en/2.0"}}