Confidence in Data A Winning Strategy
By MARLAN W. DOWNEY
In the mid-1990s, Arco International reviewed a farm-out proposal from Enterprise Oil for drilling on a block held by Enterprise in the Romanian Black Sea. Enterprise already had completed a seismic program and had mapped a variety of structures on the block.
Sediments on the block were Tertiary sands and shales, and a review of scanty records of past drilling suggested that hydrocarbon charge may be a concern in the area.
Arco's review of the seismic data noted some subtle amplitude anomalies in off-structural settings. Careful analysis of the data convinced Tom Velleca, Arco VP of exploration, that the amplitude anomalies probably were indicating the presence of hydrocarbons in thick, high porosity sands, at relatively shallow depths.
Detailed integration of the regional petrophysical data to the seismic data indicated that hydrocarbons should be present in good reservoirs, but light oil could not be differentiated from gas in the reservoir sands with the control available.
Velleca's group accepted the farm-out proposal from Enterprise - with the caveat that Arco would select the drilling locations for the obligation wells.
Seeing Is Believing
Arco selected the first location as a test of one of the amplitude anomalies. Great confidence and a high certainty of success was expressed by the exploration team.
The drilling rig was located over the seismic anomaly and drilling began.
At the anomaly level, no sand and no hydrocarbons were seen by the shocked exploration team. A careful repeat of the logging run showed, again, no sand and no hydrocarbons at the seismic anomaly level.
The first reaction of the staff was to say "Sorry, Marlan, something must have gone wrong. We'll release the rig, (it's on stand-by and costing us money), and we'll move on."
Instead, Velleca held the rig and directed that the staff take a last look at the comparison of the seismic traces to the synthetic created from the new well log.
There was a mismatch, at the objective level, between the two sets of acoustical data. Velleca pointed out that seismic doesn't lie; that well logs don't lie; that an explanation has to be found that honors both sets of real data.
Velleca suggested that the staff review the character of the near traces versus the far traces - and an analysis by project manager Mike Richter and project geophysicist Mark Ward noted some distinct differences in character when near traces were compared to far traces.
"Perhaps we drilled a shale plug in the middle of the alluvial sandstone?" suggested Tom O' Brien.
"Move the rig 300 feet and re-drill the well!" Velleca directed. "I believe my seismic data."
Three days of drilling brought the drill bit back to the objective horizon, and the relocated wildcat found a thick, porous, gas-bearing sand - precisely where it had been predicted by the seismic data.
It's one thing to see data; it's another to believe data.
And what did I think I learned from this discovery?
I think I learned that confidence in technology is often as important as knowledge of technology.
I think I was reminded that one person can make the difference between a success and a terrible failure.
At least, that's the way I remember it.