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The industry spends a fair bit of money on geosteering—the process of matching logging-while-drilling data to open-hole log data to ensure that the wellbore is drilling through the most productive rock.

It can be a nerve-wracking process—I know because I did some geosteering on Austin Chalk wells around Dilley, Texas, in the early 90s, mostly by sample descriptions. The technology for doing what we do today was rudimentary, novel, and raw.

Today’s operators can, by-and-large, trust their geosteering contractors or in-house staff to keep them pretty much in their defined target or landing zone, and the geosteering mavens routinely deliver great results.
I’ve taken it as an article of faith that the more the wellbore is in zone, the better the well will be.
However, now that I’ve looked at a fair bit of data, I’m not so sure that’s true.

I looked in DI Play Assessments at wells in the Delaware Basin with a landing zone = Wolfcamp A XY.

There doesn’t seem to be a clustering of out-of-zone wells. Instead they are spatially distributed in the same manner of wells that have higher in-zone percentages.

With the help of our geology team, I got this cross section of the Wolfcamp A XY landing zone. The orange line is the top of the Wolfcamp A XY, the blue marker is the top of the Lower Wolfcamp A.

Note that the section gets shalier as you move from north to south, and the section thins by about 90’.
The map below generally confirms the trend and shows that both out-of-zone wells and in-zone wells are generally targeting the same lithologies.

However, it’s very hard to see meaningful differences in well performance as measured by Peak BOE. The map below compares Peak BOE for wells with less than 5% of wellbore in zone to wells with greater than 75% of wellbore in zone.

Graphing BOE by % in zone shows no meaningful difference in median First 12 months BOE—139,000 First 12 months BOE for less than 75% in zone versus 143,000 First 12 months BOE for more than 75% in zone.

Bulk correlating log metrics like density or neutron porosities doesn’t yield a great correlation with production performance.

There’s little correlation between gross perforated interval and First 12 months BOE.

There is, however, what looks to be a good correlation between lateral length and First 12 months BOE.

So, if your company man calls in and says the geosteering job didn’t stay in zone as much as hoped…don’t worry too much…unless you only drilled 3000’ of lateral.

Agree? Disagree? Send your thoughts to me at mnibbelink@drillinginfo.com.

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Mark Nibbelink

Drillinginfo Co-Founder, Director Of University Outreach. Before co-founding Drillinginfo in 1999, Mark had a long career as a prospect geologist at Gulf Oil before beginning work as an independent geologist. Mark is responsible for quality control and data integrity. He received his Bachelor of Arts in geology and his master’s in geology and geophysics from Dartmouth College.

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