Tuesday, May 12, 2009

No Dip or Low Dip?

We recently finished a Geodes analysis of the dip data from a Marcellus well. It was a nice example of "No Dip or Low Dip?", the title of a 1989 oral paper presented by Jim Morse and Andy Bengtson. Here's the abstract:

Can we distinguish between zero planar dip (no dip) and low planar dip (low dip) using just dipmeter tadpole plots? No, but we can accomplish this using SCAT. Moreover, SCAT lets us test interpretations, whereas tadpole plots do not.

Because of scatter, most dip angles from no-dip settings are not exactly zero; rather, they are low, just like those from low-dip settings. Therefore, no-dip and low-dip settings cannot be distinguished on the basis of dip angle. Accordingly, they cannot be distinguished by examining the patterns of the tadpole-plot "heads."

No dip and low dip should be distinguishable, however, on the basis of azimuth: no-dip settings should have uniform azimuth distributions, whereas low-dip settings should have a subtle concentration of data at the true azimuth. It is difficult if not impossible to detect this subtle concentration by examining the tadpole-plot "tails." Therefore, we cannot confidently distinguish between zero and low dip using tadpole plots.

In contrast, the subtle azimuth concentration we expect in low-dip settings is clearly evident on SCAT's A-Plot (dip azimuth vs. depth). Moreover, no dip and low dip almost always exhibit different patterns on dip vs. azimuth and tangent plots, azimuth-frequency histograms, and apparent dip vs. depth plots. Thus, once we choose between the two settings on the basis of the A-Plot pattern, SCAT lets us test our hypothesis by examining these other displays.

--from Morse, James D., and C. A. Bengtson, 1989, No dip or low dip? (abstract): AAPG Bulletin, v. 73, p. 1167-1168 (http://search.datapages.com/data/doi/10.1306/44B4A7B7-170A-11D7-8645000102C1865D).

For the Marcellus well in question, in addition to determining that it had drilled a low-dip setting (and not a zero-dip setting), we were able to estimate both the dip and the azimuth with confidence.

No comments: