Venezuela’s Onado-51:

Wellsite Geologist Puts His Job - and Chin! - On the Line

American Association of Petroleum Geologists (AAPG)

Exploration geologists are scientists and dreamers, observers and analysts. Our minds wander in the dark, underground worlds of ancient and deeply buried rocks where, in slow motion, sediments were broken, bent and twisted to form traps for oil and gas.

Even though we may be unsure where these traps are hidden, we’ve been tasked with finding them – but knowledge of earth science is not sufficient to find them. We also need experience, judgment, a certain philosophy and intuition.

We are used to taking professional risks – and sometimes we may have to take more personal ones.

The Concession and the Well
The Onado-50 Block lies 30 kilometers northeast of the Aguasay Field in the Maturin Sub-basin in Venezuela’s Eastern Basin. In 1971 the concession block was held 50/50 by Mobil Oil Company de Venezuela, the operator, and Texaco de Venezuela.

That year the concessionaires decided to drill the block’s first well – a wildcat that at a planned total depth of 14,500 feet would be Mobil’s deepest in the country.

The site had been selected on rather poor seismic that showed a possible, interesting reflector at depth, interpreted as the acoustic boundary between the massive sands of the Oligocene Merecure Formation and the overlying, much shalier Miocene Oficina Formation.

The nearest wells were tens of kilometers away.

That seismic and those distant wells were the information available to predict the geological section to be encountered in the new well – nothing unusual for an exploration well in a new area.

The main objective was the “U” sand of the Merecure Formation; the Basal Sands of the Oficina Formation were the secondary objective.

Drilling took place between June and September of 1971.

At that time I was Mobil’s only geologist at its eastern Venezuela office in the town of Anaco, and thus I found myself appointed wellsite geologist for wildcat well Onado-51. Three well-checkers, with whom I had worked before and I trusted, would assist me – Victor Arias, “Pachico” Figuera and Ramon Azocar.

A Dry Hole?

We drilled for three months essentially without problems, except for the usual minor ones plus the occasional sticking of the drill pipe in the thick shales of the Freites Formation.

I did the mud logging in the field lab, looked at the forams and kept the customary geological penetration graphs – and whenever there was no drilling, such as when the drill bit had to be changed, I would relax by drawing or playing the guitar, an instrument I had always kept close by.

By mid-July 1971 we had reached the planned total depth of 14,500 feet. We had drilled through the Mesa, Las Piedras, Freites-La Pica and Oficina formations; their lithologies and thicknesses we encountered were not the typical ones of the Eastern Venezuela Basin.

We had found a very shaly Oficina Formation and the few sands that were encountered bore no oil. And we had not found the Merecure Formation, our primary objective.

The Mobil and Texaco exploration departments in Caracas met and discussed the situation. According to the seismic interpretation the Oficina-Merecure contact should have been found at a depth of 14,000 feet, well above the depth we had reached by then.

What had happened to the shallow marine to deltaic Merecure sands? Had they not been deposited here?

Were we dealing with a shalier facies in the deeper part of the basin?

Abandon the Well!

The well cost had been more than planned, and Texaco, which paid for half the costs, didn’t want to invest more money in what they had decided was a dry hole. Both companies agreed on abandonment.

Meanwhile at Onado – unaware of the decision taken in Caracas, and for the umpteenth time – I looked at all my information: the stratigraphic column made with ditch cuttings, the list of microfossils, the drilling time log, the seismic sections, the electric logs and my basinwide stratigraphic sections.

Once more, in my mind, the data I had confirmed my suspicion: We had not reached the Merecure Formation.

Why not? Very likely because the basement was deeper than envisaged and the rock units in Onado were thicker than elsewhere in the basin.

I was reminded of a quote I had read in Ellen Sue Blakey’s book, “Oil on Their Shoes:”

“If you see a man walking down the street with oil on his shoes, where it shouldn’t be, and no oil on his hair, where it should, that’s an oil man. If he has a faraway look in his eye and seems to be contemplating the depth of the first Jurassic sandstone in Persia, that’s a geologist.”

At that moment I felt that I had oil on my shoes, and that I was contemplating the first sands of the Oligocene Merecure Formation. I felt sure that the Merecure was there, close by; we just had to drill more.

Shortly thereafter the toolpusher walked into my trailer and told me that he had been ordered to abandon the well. I immediately called exploration in Caracas on the radio, emphatically disagreed with that decision and explained why.

Another Five Hundred Feet!

I don’t remember how, but I won the first round and Mobil’s management decided to postpone abandonment and circulate the mud.

Texaco in Caracas considered this ill-advised and sent Jose Ortega, one of their best geologists, to look at my data. Ortega left convinced but he was unable to convince his bosses. I insisted with my stand, “We have to continue!”

Mobil and Texaco disagreed. What? Spend another million dollars? Why should we trust a young geologist with only six years experience?

M.C. Parsons, Mobil’s exploration manager in Venezuela, ordered his deputy, Foster Smith, to go urgently to Anaco. As soon as he landed I showed him and Mobil’s Anaco manager my data and explained why I was pushing for further drilling. They agreed and accepted my recommendation. I had won the second round!

While all this was going on, the rig continued on standby, circulating the mud and, of course, spending money.

At noon Smith boarded the company’s DC-3 and returned to Caracas. In the afternoon he talked it over with Parsons and convinced him the company should accept my recommendation.

Taking a Risk

Later that same afternoon Parsons phoned me from Caracas and, without much greeting, told me, “You convinced Foster and Foster convinced me.”

He then asked, “Are you prepared to put your job on the line? You realize that if we fail, you will have cost the company a great deal of money.”

I swallowed hard and answered, “Yes!”

He then replied, “Go ahead! I’ll take care of Texaco,” and hung up.

A few minutes later the Anaco manager told me that Caracas had decided to continue drilling and ordered me back to the well. I went by my house, explained to my wife, Luisa, what had happened and that I was risking my job now, but that I felt I had to do it, kissed her and my young son and got ready to leave.

Just before driving off I told her, “I will make a promise. I’ll let my beard grow, and if we find oil, I’ll never shave it off.”

That night I returned to Onado, a distance of 140 kilometers from Anaco, and when I arrived at the wellsite I heard the unmistakable engine hum that confirmed that we were once again drilling – drilling slowly, about five to ten feet an hour, but drilling.

I decided to catch ditch cuttings every foot instead of the usual every five feet, and hour after hour sat glued to the microscope and the fluoroscope. My guitar stayed discreetly silent.

We drilled on and kept finding shale, shale and more shale! Every now and then a few sand stringers. Good, porous sand but invariably without oil shows.

I could only wonder whether I had been reckless in risking my job and my career.


Gradually we approached the agreed-upon extended depth of 15,000 feet finding more shale and just a few sand stringers, all water-bearing. My morning “no shows” radio reports to the Caracas head office were like buckets of cold water for Mobil and Texaco and optimism kept sinking as we drilled on.

  • 14,650 feet: no sands, no shows.
  • 14,700 feet: no sands, no shows.
  • 14,750 feet: shale and more shale.

A beard started to darken my face, a beard that I didn’t want to have to shave away soon. I could feel the disappointment and tension – not only that of my bosses both in Anaco and Caracas, but also among the drilling crew with which I shared my days. I’m sure that more than one thought, “We shouldn’t have listened to the wellsite geologist!”

Suddenly, in the early morning of the fifth day, while drilling at 14,850 feet, things turned around completely.

At 3:30 a.m., Victor Arias came to my trailer shouting, “Orlando, Orlando! We have oil!”

His trembling hands stained mine as he handed me oil-bearing sand that he had brought from the shale shaker. I looked at it and there was no question: It was sand with shiny, dark oil! What a beautiful sight!

Victor and I ran to the shale-shaker and the happy drilling crew surrounded us, shouting, jumping and hugging each other.

Dawn never seemed further away than on that morning as I counted the passing minutes. After what seemed like a century the clock struck six, the time to submit my morning radio report:

“Caracas, this is Onado 51. Over.”

“OK, Onado 51, come in. Over.”

“Caracas, this is Orlando Méndez reporting from discovery well Onado 51.”

I don’t remember what else I said, but I know that at the other end they were jumping with joy!

Three hours later the excited Anaco managers arrived at the well. There were many calls to Caracas. We had drilled only 30 feet into the oil-bearing section and, of course, management authorized us to keep on drilling.

A few days later we had gone through the Merecure Formation and then stopped at a total depth of 15,383 feet. The well was logged and completed. On tests it flowed 500 b/d of 26-degree gravity oil, and on Sept. 4, 1971, it was officially completed as a producer.


Fourteen appraisal and development wells were drilled in the months and years ahead and the total oil reserves were calculated to be 100 million barrels.

Forty-two years later I still have my beard and the Onado Field is still producing – but time has taken its toll and we’ve both aged: My beard is now gray and the wells are on pumps.

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Historical Highlights - Orlando Méndez

Orlando Méndez is a geology professor at Universidad Central de Venezuela. He was an exploration and development geologist for Mobil Oil de Venezuela (1965-76); PDVSA LLanoven S.A. (1976-78); PDVSA Lagoven S.A. (1978-85); and PDVSA Public Affairs (1985-98).

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Hans Krause is an AAPG Honorary Member, Distinguished Service Award winner and former chair of the AAPG History of Petroleum Geology Committee.

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A history-based series, Historical Highlights is an ongoing EXPLORER series that celebrates the "eureka" moments of petroleum geology, the rise of key concepts, the discoveries that made a difference, the perseverance and ingenuity of our colleagues – and/or their luck! – through stories that emphasize the anecdotes, the good yarns and the human interest side of our E&P profession. If you have such a story – and who doesn't? – and you'd like to share it with your fellow AAPG Members, contact the editor.

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