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Molly

Ferreting out bad stratigraphy data points

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We received a tech inquiry from a client, regarding a problematic stratigraphic structure map:

I'm making a 2D structural elevation map for one of my project formations. There is a bad datum in the data set as almost all the posted subsea datums are between -800 and -1000. My contour lines are honoring a point in excess of -1600. I've posted the datums near the wellbores and for the life of me, I cannot find that errant point.

The first thing to do is to check what RockWorks is "seeing" when it's modeling a formation top: Use Stratigraphy / Export / Stratigraphy -> ASCII XYZ File. Pick the unit name, choose Upper Elevation, and the resulting list are the points that are being sent to the gridding program when you select Stratigraphy / Structural Elevations. Note that the XYZ points are actual downhole point locations, and may not match the XY surface locations for deviated holes. The listing is pulled for enabled holes only.

If you see problematic elevations in the listing (e.g. -1600), you can try searching the database for the closest borehole, using View / Locate Closest Borehole and typing in the XY coordinates. For the closest borehole:

1) Check the surface elevation defined for the hole (Location table). If this looks ok, then...

2) Check the top (and base) depths entered in the Stratigraphy data table, for that unit. If the depths look ok, then...

3) Check the downhole survey data in the Orientation table.

If the holes are inclined or deviated, and you are having difficulty determining which well is contributing the bad point, try using Striplogs / Multi-Log Plan View to create a bird's-eye-view map of your log traces. (You can omit all vertical holes, and use the 2D striplog designer tab to activate only the log titles and strat patterns in the log displays.) Save the log plan-view map, then append it to the problematic contour map to determine which well coincides with the bad point.

Molly Mayfield

RockWare Inc.

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