SURPRISE, SURPRISE
Paleotopography can throw some interesting curves. This project started 35 years ago as a pool study in support of unitization, a process where the operators of various wells use science to come to an agreement as to ownership and cost sharing so that the oil field can be produced as a single unit, instead of one well at a time. This is especially important in anticipation of some form of secondary recovery, such as a water flood, from which all operators should benefit. During the initial work, we used the first desktop computer system for log analysis, developed by the author.

The pool was duly analyzed, pressure transient and decline curves were studied by the engineers, and meetings were held monthly for several years. But no agreement was reached. The operators in the west had better pressures and better production than those on the east. No one had a good answer as to why this should be so. After a while, the meetings fell off and people stopped talking about forming a unit.

Several years later, the project was revived by different operators and the petrophysical job returned. I recognized the project immediately as some of our original work was in the well files. But more wells had been drilled, more pressures taken, and of course more oil had been produced. The objective was still to unitize the field.

But the catching point was the newest well, drilled based on seismic attributes that suggested a higher than average porosity near the center of the field.  But it was a sandstone channel incised into the carbonate. The east and west sides still appeared to be in separate pressure systems and the channel appeared to be in a third system. So there were three separate oil fields, not one large field. The channel had to be isolated from the east and west by shale, not seen on any logs.

The original correlation between wells also could not predict the channel, and even with the new seismic, it was predicted as a higher porosity (which it was), but not as a sandstone channel bisecting the original field. Sample descriptions and log analysis lithology calculations solved a 10 year old mystery.

My first logging job without supervision was in a similar environment about 20 miles south of this one. In those early days, we took the nearest offset well log from our "private" files to the job site. If the new logs looked something like the offset, we were home free. My new log was OK above a certain depth but below there was no similarity. Frantic radio-phone calls and a visit by the boss fixed it - it was "geology", not me, that was at fault. You can see why Integrated Petrophysics is so important. One science at a time will not solve very many problems.

  
A west to east cross section showing the original correlation of the carbonate reservoir.


The west to east cross section 10 years later, with an incised sand channel separating west from east.
 

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