The purpose of this exercise is to develop an understanding of down hole, measur

Question

The purpose of this exercise is to develop an understanding of down hole, measured depth that is the standard reporting format from the rig floor back to the office and when reading depths directly from well logs. In most cases the measurements begin from the drill floor (DF) or from the Kelly bushing (KB) mounted on the drill floor. DF and KB values are essentially unique to each well given that they are referenced to the ground elevation at the drill site. In order to correlate from well-to-well and generate cross sections between multiple wells, it is essential to establish a normalized depth reference or "datum". In most cases this is datum ties to sea level. Part of the cross-section exercise is based on a measured section from an outcrop in southwestern Montana. This will be tied to data from four wells from the Williston Basin of central Montana. The elevation of the Top Mississippian Unconformity is at an elevation of +2000ft relative to sea level. The cross section extends from the measured section to Well D. *It is important to know that the evaporitic Potlatch Formation is the eastern equivalent of the Wabamum Dolomite. The wells are already positioned as vertical lines on the attached templates. You do not have to construct a column for each well, just use lines connecting each formation top positioned along the well bores. Fill in the blanks on the attached data table. The formation tops are reported as measured depths. You will need to calculate the subsea depth for each formation top as well as an interval thickness for each stratigraphic unit. A.

Explanation / Answer

SOLUTION

Calculating the subsea elevation:

Subsea elevation = (KB elevation or DF Elevation) - Measured depth

WELL (A) DRY and Abandoned (D&A) Elevation (DF) 4291 Total depth - D 6642 Formations Measured Depth Subsea Depth Interval thickness A SS=DF -A I = D/A Mississippian Unconformity 4622 -331 1.44 Sun River Fm 4622 -331 1.44 Lodgepole / Mission Canyon 4780 -489 1.39 Three Forks 5850 -1559 1.14 Potlatch Not seen Nisku 5850 -1559 1.14 Superow 6070 -1779 1.09 6090 -1799 1.09 WELL (B) DRY and Abandoned (D&A) Elevation (DF) 4347 Total depth - D 4692 Formations Measured Depth Subsea Depth Interval thickness A SS=DF -A I = D/A Mississippian Unconformity 2872 1475 1.63 Sun River Fm 2872 1475 1.63 Lodgepole / Mission Canyon 3023 1324 1.55 Three Forks 4050 297 1.16 Potlatch 4050 297 1.16 Nisku 4075 272 1.15 Superow 4295 52 1.09 4338 9 1.08 WELL (C) Completed BPOD Elevation (DF) 3750 Total depth - D 3450 Formations Measured Depth Subsea Depth Interval thickness A SS=DF -A I = D/A Mississippian Unconformity 1910 1840 1.96 Sun River Fm 1910 1840 1.96 Lodgepole / Mission Canyon 1980 1770 1.89 Three Forks 3050 700 1.23 Potlatch 3050 700 1.23 Nisku 3115 635 1.2 Superow 3290 460 1.14 3350 400 1.12 WELL (D) Dry and Abondoned (D&A) Elevation (DF) 3714 Total depth - D 3590 Formations Measured Depth Subsea Depth Interval thickness A SS=DF -A I = D/A Mississippian Unconformity 1950 1764 1.9 Sun River Fm Not seen Lodgepole / Mission Canyon 1950 1764 1.9 Three Forks 3070 644 1.21 Potlatch 3070 644 1.21 Nisku 3158 556 1.18 Superow 3298 416 1.13 3350 364 1.11

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