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Volume 17, Number 1January/February 1966

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How They Find The Oil

"WANTED: Earth scientist with rugged physique, excellent health, strong nerves and inquiring mind. For outdoor job involving constant travel, exacting work, irregular hours. Those afraid of snakes, jungle fevers, foreigners, frostbite, sunstroke and solitude need not apply."

Written by Daniel Da Cruz
Photographed by T. F. Walters

Anyone answering such an advertisement with complete confidence would be a promising candidate for oil exploration work. If he also happened to be a young man who bounded out of the house without jacket or tie in his haste to answer it he would be nearly the perfect candidate, for youth, informality and enthusiasm are valuable qualifications as well in a profession that blends imagination with intense physical and mental activity.

At the present moment there are over 25,000 "explorationists" roaming six continents in an unremitting search for oil. They include geologists, geophysicists, paleontologists, mineralogists, stratigraphers, geochemists, hydrologists and many other specialists. Also today there are proved world-wide oil reserves estimated at about 350 billion barrels already located underground. This would be enough to last more than 30 years—at the present rate of consumption. But oil economists, looking into the future, predict that consumption rates will not remain steady. Taking into account population growth, heightened economic activity, rises in the overall standard of living, as well as increased use of petroleum in petrochemicals, new protein food additives and other products, they foresee continuous growth in demand.

A senior vice president of New York's First National City Bank, member of a business fraternity noted for its conservatism, places the annual energy demand growth of the free world at 2½ per cent, and points out that oil and gas supply three-quarters of all energy requirements. The chairman of British Petroleum, speaking before a meeting of the American Petroleum Institute, saw free-world oil demand as doubling by 1980 and increasing another 50 per cent by the year 2000. He warned against complacency over the future supply picture, remarking that "our descendants will be unlikely to thank us if all our geologists, geophysicists and drilling crews take a vacation."

In earlier days oilmen could use relatively simple but wasteful methods of finding oil and getting it out of the ground and still remain in a sound financial condition. Now, even though demand for its products is climbing, fierce competition in the world oil industry leaves little room for the casual operator. Locating oil sources by surface or near-surface indications or by random wildcat drilling is being replaced by ever more sophisticated exploration techniques: subsurface geology, core drilling, deep drilling and geophysics. Even if oil is found, however, there is still no guarantee that the story will have a happy ending. It is vital nowadays for oil industry management to weigh carefully such considerations as the size of a new field and the cost of developing it efficiently, the length of pipe required to connect it to an outlet and the proximity of an adequate shipping port before an exploration man can be certain that the results of his discovery will ever reach the marketplace. The answer to the question no longer rests solely on the existence of oil in a given spot on earth, but on a multitude of economic, political, social and logistical factors which would wrinkle the brow of a horny-handed prospector of old who sought oil with a gleam in his eye, a divining rod in his hand, and unshakable optimism in his heart.

Our old-time prospector went about looking for oil blissfully ignorant of where it came from and the physical laws governing its underground movement. He probably couldn't have cared less. Even today there is no universal agreement as to how petroleum came into being—or when. It is generally acknowledged, however, that oil is found in nearly all ages of rock, going back some 500 million years to the Cambrian Age, when the organic debris of innumerable tiny invertebrate animals or simple vegetable matter carpeted ocean floors. As the layers of dead cells were buried deeper and deeper beneath deposits of sand and ooze, physical and chemical forces still but dimly understood converted part of the organic substances into oil. In some places an overlying deposit of material such as clay, silt, sand or lime effectively sealed in the petroleum.

Though the organic theory of oil's origin is now generally accepted, a minority of geologists still espouse the inorganic theory of petroleum formation. In this view, carbon emanating from the core of the earth, in passing through sedimentary rocks, reacted with the hydrogen and oxygen they contained to produce petroleum, a theory which fits nicely with that of the ancient Greeks who gave the substance its name: petros—"rock," and elaion—"oil."

In any case, geologists agree that oil can occur in quantities ranging from a few hundred to over 100 billion barrels in a single reservoir, in deposits from a few inches in thickness to more than 1,500 feet (as in Kuwait's Burgan field), and from depths of at least 20,000 feet to the surface of the earth, where the largest known body is the Trinidad pitch lake, which once contained 200 million barrels of tarry oil and asphalt. Petroleum may also occur in sandwich-like strata separated from each other by impermeable rock: Venezuela's West Guara field has 47 such producing strata, one above the other, and there, as elsewhere, the practice has been to drill through low-yield strata, if necessary, to get at the richer ones beneath.

If the same rules applied to science as to politics, in which the majority view usually prevails, the petroleum geologist's lot would be far happier, for the underground oil reservoir's configuration would neatly reflect the layman's impression that it is something akin to an underground swimming pool—larger and deeper certainly, but requiring only a pump of adequate dimensions to drain it dry.

Alas! Like most stereotypes this one has only a germ of truth. Far from being a hollow chamber, the oil reservoir generally is hard sandstone or limestone. This rocky substance, even though solid in appearance, actually has interstices between the separate granules that form it, giving it pore space. These microscopic pores in reservoir rock can contain petroleum in its natural state, much as the pore spaces in an ordinary sponge can hold water.

The three essentials for the underground accumulation of petroleum are a source of oil, a porous and permeable reservoir rock in which it may gather, and an impermeable layer of cap rock to trap it. The impermeable cap rock is the geological analogue of the diving bell which, flaring down and out, sustains the diver with its trapped air. Since petroleum first began to be formed far below the earth's crust it has migrated inexorably upward, slowly and steadily, impelled by the heavier water below. Astronomical quantities may have thus percolated to the surface and evaporated into the atmosphere. Where it comes up against a layer of impervious rock such as shale or anhydrite, for example, its vertical progress may be arrested and if other conditions are right it may accumulate in the underlying reservoir rock and form a "pool." Such "pools" are, of course, the object of the petroleum ex-plorationist's quest.

The active search for crude oil underground dates back little more than a century; before that the stuff was considered pretty much of a nuisance. Oil was doubtless regarded as such by Shadrach, Meshach and Abednego, who were thrust into the "fiery furnace" by King Nebuchadnezzar, fortunately to emerge unsinged. That "furnace," fueled by natural gas seeping out of the ground, probably ignited aeons ago by lightning, burns on today above one of the world's richest oil fields, in Kirkuk, Iraq. Although American Indians used crude oil from surface seeps to caulk their canoes and for medicinal purposes, it wasn't until the second half of the 19th century that science learned that refined petroleum products made excellent fuels, lubricants and illuminants.

In the first rush for oil, explorers searched out surface seeps of crude oil, set up their primitive cable tool rigs, and started drilling, on the reasonable assumption that there must be more where that came from. The results were entirely satisfactory—until the seeps were exhausted. But already a sharp-eyed Canadian named T. Sterry Hunt had been observing the topographical distribution of oil seeps, and had devised a theory to account for them. He noted that many seeps originated on elongated dome-shaped structures and reasoned that if structures were repeated in the rock layers below, like a nest of inverted mixing bowls, then petroleum rising from the depths toward the surface would be trapped under or between these layers. Hunt termed this type of structure an "anticline." There remained this question: where had the seeps come from, if the oil was trapped in the anticline. Simple. It has oozed up through minute cracks in the rock.

In 1888, I. C. White put Hunt's theory to a test by drilling four "wildcats"—wells outside known productive areas—and promptly brought in three producers. Ever since, the anticlinal theory has been a gospel that has lost none of its appeal or validity to exploration geologists. It has worked just as well for the Arabian American Oil Company (Aramco) in comparatively recent times as it did a century ago, when Hunt first propounded it. Saudi Arabia's huge Ghawar field, about the size and shape of Long Island, with oil-bearing strata some 250 feet thick, is a classic example of an oil field that was found on an anticline.

Such well-defined surface anticlines, like the bulge in a gunman's jacket, often betrayed the existence of more anticlines underneath, and these unseeable structures required far more advanced techniques than mere observation to locate. Whereas in the past explorationists had been guided by surface features such as seeps, domes and anticlines, they now began to probe beneath the surface of the earth with instruments, attempting to discern patterns in the structure of the earth's crust which would tell them: "Drill here!"

One of the first really scientific devices perfected in the search for oil was the torsion balance, used as early as 1901 in mapping lake bottoms. Based on Newton's principle that gravitational pull depends on the weight of an object and its distance from the measuring device, the torsion balance was able to map such subterranean features as an anticline because the gravitational pull of the rock at the bottom of the dome, being nearer to the instrument on the surface, was greater than that of the same rock on the anticline's edges. The variations in the gravitational field led geologists not only to anticlines but to lodes of iron ore, which are heavier than the surrounding stone, and to salt domes, which are relatively light.

The discovery of salt domes by gravimetry turned out to be a profitable application of the method. Salt will flow plastically under differential pressure and thrust itself upward through a weak spot in the overlying rocks. Following the formation of these salt domes, migrating petroleum often is trapped on the flanks and beneath impermeable rocks capping the dome. Oilmen have exploited this phenomenon by drilling above and on the periphery of salt plugs, some of which are miles in diameter, and discovering oil fields. The gravity meter (a refinement of the torsion balance) is now allied with a new generation of instruments, including the airborne magnetometer. This instrument points like a large finger from the tail of an aircraft and can map the magnetic field of hundreds of miles of terrain a day without exposing the crew to so much as a mosquito bite.

With field parties which seek to fathom the earth's mysteries by sonic and electronic methods, however, it's quite another story. On wide-swept deserts they must apply the same concentration, the same scientific approach that more comfortable men practice in quiet, dust-free laboratories. Somewhere in the world at this moment oil explorationists are wading knee-deep in steamy marshes, rolling across the desert in trucks loaded with high explosives, paying out cable from the stern of seismic ships in rough seas, flying precise patterns over empty veldt, or just swatting ineffectually at swarms of voracious sand flies.

It's almost becoming an aphorism among veteran petroleum geologists now that oil exploration never stands still. Practitioners are forever probing new locations and trying new techniques. Consider as one instance the present scope of exploration: where once efforts were concentrated on promising areas like Texas and Venezuela and the Middle East, the search for petroleum has extended to the ends of the earth, not excluding the high seas. Where once the geologist needed little more direction than the axiom, "Find the anticline and drill," he now has to be able to distinguish fault traps, lenses, pinch-outs, reefs (oddly enough, just what the term suggests—ancient coral reefs whose pores provide an excellent habitat for oil), stratigraphic traps, unconformities and combination traps, none of which, of course, are visible on the surface. The new concepts, in a sense, are simply an extension of Hunt's fundamental thesis that for oil to accumulate there must be a permeable reservoir rock and, above it, an impermeable cap rock. The irony is that despite a quantum jump in our knowledge of subsurface geology since early in this century we still have no surefire way of knowing whether a promising trap actually contains oil except to drill into it.

Because there are no sure answers, the explorationist never stops asking questions. Better than anyone else he knows that only one out of ten wildcats drilled today will yield petroleum in commercial quantities. He knows too that a wildcat 6,500 feet deep will cost an average of $200,000 in the United States. In the Middle East the cost would run more than twice that figure. Simple arithmetic, therefore, dictates caution, a hedging of bets, the use of every weapon in his arsenal of exploration tools, for massive failure could spell corporate hard times.

The search for oil today begins, prosaically enough, in the research library, where all available literature on the area to be prospected is combed for clues which may elucidate the chosen region's geology. In the United States the next step is to obtain exploration leases in the area; in foreign lands this process often becomes a full-scale diplomatic operation, requiring adequate compensation to the host nation in return for an exploration concession, and development rights if oil should be found. Geologists, cartographers, aerial photographers, magnetic and gravity survey teams are then mobilized, often working simultaneously in their special fields to obtain a broad picture of the geological character of the surface and what lies below, which will serve as a guide for the more intensive exploration that follows.

The search for petroleum has become so sophisticated a science that many oil companies engage outside specialists to supplement their own exploration efforts. One such survey organization has been exploring for Aramco the flat, almost featureless northeast corner of Saudi Arabia. Backed by the oil company's logistical support, the team of 12 Americans and 62 Saudi Arabs comprises a close-knit group that works seven days a week at such a rapid clip that its mobile camp must be moved every few days merely to keep within commuting distance of the work at hand.

It is hard to conceive of more grueling, lonely labor anywhere. The desert in this part of Arabia offers limitless plains of stone-strewn, tight-packed sand, interrupted here and there by patches of low scrub, and absolutely nothing else, from horizon to horizon, but hot wind and relentless sun. The survey organization's mission can be simply stated: go out into this empty desert and come back with a series of accurate cross sections, or profiles, of the subsurface structure along straight lines stretching almost (it must seem at times to them) to infinity.

There being initially no lines, straight or otherwise, to follow, it is the duty of the surveyor to provide one. The survey party is the first out in the morning in a four-wheel-drive truck, speeding up the line to the last stake it planted the day before. The surveyor hauls out his transit, his rodman and other assistants hurry forward, and so begins a day that will see them lay out markers some 30,000 feet along lines which when tied to other lines will describe a series of quadrangles whose sides must have a linear accuracy of better than 999 parts in 1,000. Elevations, sometimes carried for 100 miles without the points, must be just as close. Each day takes the surveyors farther from camp, so that by the time the camp is finally moved up to the vicinity of their last stake they may have to drive back and forth some 60 miles a day each way without, however, having to worry about the traffic.

Behind the surveyors, moving at a more ponderous pace, lumber three large truck-mounted drilling rigs. At intervals of 3,600 feet they stop 110 feet apart, raise their masts, and bore 90 feet into the earth, using blasts of air to cool their bits and disperse the cuttings. They don't tarry long. Usually within an hour they have lowered their masts and moved on to the next location marked by the surveyors, leaving behind three neat holes in the desert floor and a set of tire tracks soon to be obliterated by the wind.

Long after the drillers have vanished over the horizon the seismograph crew pulls up and begins to unload at each hole sacks of what appears to be fertilizer. Actually it is fertilizer—ammonium nitrate—but after 50 pounds of the substance are sprayed with one gallon of diesel oil the mixture becomes highly explosive. The shooter carefully inserts an electric blasting cap into a can of blasting agent primer, wires three more powder cans to it, and lowers the string to the bottom of the hole by means of the electric wire which will detonate the charge. Then he pours three sacks—150 pounds—of the loose explosive into each hole on top of the primer charge, adds a few buckets of sand as ballast, and finally attaches his three wires to the firing mechanism in his truck.

Meanwhile, others in the seismograph party have been working on a line parallel to the shot holes, laying down a string of geophones. Using the seismograph truck as their point of reference (it is parked opposite the middle shot hole, 450 feet away), the crew sets groups of geophones on the ground at hundred-yard intervals in each direction from the truck, so that the extreme ends of the line are 7,200 feet apart. In effect, they have rigged 24 separate groups of receivers, all able to feed any signals they receive back to the seismograph truck in the middle of the string. When all the receivers are ready for the shot, the shooter signals his men for silence, for geophones are so sensitive that even a herd of goats grazing 500 feet away can produce distorting sound waves. At a nod the shooter presses his plunger and a muffled boom! stirs the earth underfoot. Four seconds later, the goats may safely graze once more.

Minutes pass before the door of the seismograph truck, a vehicle crammed with $150,000 worth of electronic equipment and a single arresting pinup picture, swings open and the operator holds up his thumb to indicate a perfect shot, in the other hand displaying a limp strip of photographic paper representing four seconds in the life of a shock wave. Fresh from the developing solution, the strip is a hand's breadth wide and about the length of the one that got away. It is crowded from one end to the other with irregular wavy lines. At the end of an average day the seismograph crew will have six or seven such seismograph traces, and these wavy lines on strips of photosensitive paper are why the crew is there.

Moving along up to five miles a day, the seismograph crew sets off explosive charges that ignite simultaneously and send shock waves deep into the crust of the earth, which then rebound upward from each successive layer of rock encountered, to be recorded one by one as they reach the surface and the geophones. To the layman the wavy lines are an utter, unintelligible mishmash. They are little more to the geophysicist until the "noise" has been removed. For it must be noted that since shock waves are propagated in all directions, some of them race from the shot holes directly across the intervening topsoil to the geophones during the short interval that other waves are penetrating into the earth and echoing back from as far as 20,000 to 30,000 feet down. The horizontal "noise" waves tell the geophysicist nothing he wants to know, and elaborate means, including the use of data processing computers, are invoked to remove their traces. What is left, provided interpretation is expert, is a pattern of up to six significant reflections, each representing a reflecting stratum of rock. When plotted on graph paper the results of the shot will appear as a 3,600-foot cross section of the earth, as much as four miles deep, along the surveyor's line.

Variations in seismographic technique are enormous and most of them are designed to reduce surface noises by a suitable combination of shot pattern and geophone array. The vexatious variable that complicates this task is the speed of shock waves, which depends mainly upon the density of the substance through which it travels: about 1,100 feet per second in air, from 6,000 to 8,000 fps in compacted soil near the surface, and up to 18,000 fps in limestone, to list a few of the media encountered. The problem of interpretation would be eased considerably if the velocities of the first 500 to 800 feet of the subsurface were known. Seismic explorers attempt to bridge this knowledge gap by drilling 300-foot holes and shooting to determine the "near-surface velocity."

Even if reflection seismography were a flawless technique it would tell the geologist no more than at what levels, down to approximately four miles, he would expect to find the principal formations in which, hopefully, oil is trapped. He would learn much about the geological structure but next to nothing about the geological composition of the area he was investigating, including, of course, the possible presence of petroleum-bearing formations. To fill in the gaps in the seismologist's geological model, therefore, geologists have recourse to structure and stratigraphic drilling. Structure drilling, which calls for wells one to 20 miles apart, goes only to relatively shallow depths of 500 to 3,000 feet, seeking key horizons or strata which will indicate the deeper structure. Stratigraphic wells search for lithologic data from deeper horizons, of 12,000 feet or beyond, and are spaced farther apart than in the case of structure drilling.

At "Strat Five," a stratigraphic rig that has been drilling near the Saudi Arabia-Iraq Neutral Zone, the primary purpose of the well, as in all stratigraphic wells, is to discover the lithologic character of all rock units at the point being drilled, and to note the differences in composition and texture of potential reservoir rocks to obtain trends in those which are improving.

At the well site, as the drilling bit passed the 11,200-foot mark, the drilling foreman of Strat Five explained the procedure. "In areas of interest we take 60-foot cores which are analyzed by the geologist on the spot and then taken to our Oil Operations Laboratory in Dhahran where core plugs are cut out and tested for permeability and porosity. This gives us a pretty good idea of the composition of the subsurface at various levels. Between coring runs, of course, we screen out cuttings from the drilling mud, which is pumped down through the bit, lubricating and cooling it, then back up to the surface. Microscopic examination of these samples provides sort of a moving picture of the material through which we're drilling."

Some of the most vital information to come out of a stratigraphic well is obtained by logging, an inch-by-inch examination of the hole by means of mechanical and electric instruments. The quantity and variety of useful data made available by logs is enormous. Electronic gear lowered by cable into the hole measures the electrical self-potential, resistivity and other characteristics of the rocks penetrated by the drill, from which it is possible to calculate the rocks' porosity and the nature of reservoir fluids present. Radioactivity logging, in use for only about 25 years, has the advantage of being able to bring out useful information through several layers of steel casing, or even through cement. Gamma-ray logging, one variety of this relatively new branch, takes advantage of the fact that all rocks contain varying amounts of radioactive material; the information it provides is similar to that obtained by lithologic logging. Since shales generally have more radioactive material in them than do sandstone or limestone, presence of the former type of rock is revealed in the gamma-ray curve. In a second variety of radioactivity logging, an artifically-induced bombardment of wall rocks within a well bore produces a neutron curve which reveals the presence of liquids in the surrounding rock and therefore acts as a kind of porosity log.

The men on a stratigraphic crew are fortunate. They can walk to the rig from their living quarters, and generally have less of a gypsy existence than their colleagues on seismographic crews. On Strat Five, the crew may be on one location for as long as two and a half months, working around the clock, to complete the stratigraphic well and the shallow well necessary to supply water for the drilling operation.

Sun-darkened, dehydrated, isolated and eye-sore from wind-blown sand, the drillers and seismograph crews find relative comfort in their camps at the end of a day's work. Sometimes a seismo truck doesn't quite make it and spends the night wandering up and down the desert looking for the twinkle of light that means home. Usually, however, crew members get in by the last light of day, shower away the accumulated dust and fatigue, and gather in the air conditioned (desert shade temperatures can soar to 125° F.) dining trailers. Over plain but expertly-cooked fare, the table talk is an easy blend of the day's problems, reminiscences of home and family, and recollections of possibly amusing, perhaps hair-raising experiences on assignment in faraway lands—Sumatra, Borneo, Mexico and Brazil, Africa or the Canadian Arctic. But the talk is brief because these are bone-weary men, and dawn and another round of hard work suddenly seem very near. In ones and twos they drift off to their bunk trailers to think solitary thoughts before they fall asleep, of their next vacation near rivers and green forests, or perhaps of a camp move on the morrow, which will leave their present site as achingly empty as it had been before they came there.

Many believe that all the great fields of the world have already been discovered. As early as 1650 Rumania was producing oil. Russia's Baku fields go back to 1871, Mexico's to the 1880's, Texas' Spindletop to 1901, Kuwait's and Saudi Arabia's to 1938. Some of the most important newer sources of oil were located in Libya, Algeria and Nigeria in the 1950's. Veteran petroleum consultant B. W. Beebe might have had all these in mind when he said, "Oil and gas are not found by flashes of genius but are the product of rigorous observation and tenacious, dogged, often dreary work and study." The frenetic activity all over the world of structure drill and stratigraphic crews and seismographic teams certainly bears him out. Each new field, however, means not only more oil for now, but a contracting world in which the explorationist must exercise his unique talents, a stiffer challenge to him to discover oil in places where it has never been thought to exist before.

To all of which the oil explorationist, a man of boundless faith, has but one answer: "If it's there—we'll find it."

Daniel da Cruz is a regular contributor to Aramco World and other Middle East publications.

This article appeared on pages 1-11 of the January/February 1966 print edition of Saudi Aramco World.


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