Geology of petroleum. Levorsen, A. Arville Irving , Books for People with Print Disabilities. San Francisco, W.
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Abstract Discovery. Geologic factors. Economic factors. Personal factors. This is in part because the geologist is accustomed to thinking in terms of many variables, and the management of the oil industry, possibly more than that of most others, must deal in variables, many of which are not known.
It is also because the geologist understands better than anyone else the hazards upon which much of the exploration money and effort is spent, and can therefore better appraise the chances of its bringing in a fair return.
The result is that the petroleum geologist is now the key in a complex interlocking of many specialized abilities, ranging from those of persons doing wholly scientific work to those of persons engaged wholly in management. The final chapter of this book is therefore devoted to an analysis of the place of the geologist in the petroleum industry and to some of the factors involved in the end point of his effort—the discovery of new pools or the extension of old ones.
These factors might be thought of as centering on the petroleum prospect, which is that combination of geology, economics, and personal opinion that justifies the drilling of a test well. First, a prospect is located that is thought to be worth drilling; second, a test well is drilled into the potential reservoir rocks.
Each prospect is unique. In giving thought to the various factors that go to make up a petroleum prospect, it should be remembered, first of all, that a favorable prospect is the immediate objective of the petroleum geologist and is a prerequisite for every wildcat well that is drilled.
But the value of a prospect is partly determined by the varying needs of operators. Each prospect has a different value and appeal to each operator who might consider spending time and money in testing it. What seems like a good drilling prospect to X Company, for example, may hold no interest for Y Company with its different experience and needs. And what makes a first-class drillable prospect to an individual may have no appeal to a major company.
It should also be remembered that many prospects are found, and that many pools have been discovered, without the benefit of formal geology. By the very nature of the occurrence of oil and gas, especially in areas of stratigraphic traps, if enough holes are poked into the ground, discoveries will be made—even as the plum will be found if enough cuts are made into the pudding.
The efficiencies of these methods are low; that is, the ratio of dry holes to discoveries is three or four times higher than with the orthodox methods of geology and geophysics; but in the aggregate they find many pools and furnish many needed well records for more scientific work.
The discovery of oil is very much like fishing: you have to keep a baited hook in the water to catch fish, and you have to drill wells to find oil. For many years, the record in the United States has been that one out of every nine new-field wildcat wells produced some oil or gas. Of more significance, however, is that during the past 18 years it has required 38 new-field wildcat wells for the discovery of one oil field containing one million barrels or more. The continuing need for locating many new prospects is readily seen.
While the petroleum prospect is first and fundamentally a geologic feature, the ultimate decision whether it justifies drilling depends upon many economic and personal factors along with the geologic factors. These three kinds of factors are useful headings for examining the question of what constitutes a prospect. Of these, the trap is the most important, for it is the factor that localizes both the depth and the areal location of the prospect.
The trap is a result of various combinations of deformation, such as folding, tilting, and faulting, together with various stratigraphic variations, such as lenticular porosity, facies changes, erosional truncation and depositional overlap, and solution cementation.
The structural elements are generally the most readily determined before drilling because in many places they extend through a wide vertical distance and can be mapped by geological or geophysical surveying from the surface of the ground.
After holes have been drilled and data concerning the potential reservoir rocks have become available, the stratigraphic information becomes increasingly important as a means of predicting traps, and many new combinations of structure with stratigraphy are revealed. Probably two out of every three traps thought to be present are found to be nonexistent after drilling.
The nonexistence of predicted traps may be due to the unreliable nature of the data that were used or to a misinterpretation of reliable but inconclusive data. Thus poor outcrops, facies changes, unconformities, alluvium cover, and miscorrelations all add hazards to the presence of the surface fold.
Where holes drilled in the past have been widely scattered, the well logs and samples poor, and the correlations difficult, the subsurface maps are correspondingly unreliable. An actual showing of live oil and gas in a potential reservoir rock in the vicinity is significant, but if the evidence is obscure and the information has passed through several interested persons in the telling, it becomes of little importance. Seismic measurements may be thrown off by facies changes, buried faults, discordances associated with unconformities, and surface weathering conditions.
Finally, when the trap is drilled for, it may not be there at all! Thus the hazards of drilling for a trap are considerably increased by the unreliable nature of the information that often must be used to predict whether a trap exists and, if so, just where it is.
Even after a trap has been located in a potential reservoir rock, an average of probably only one out of three or four traps will prove to be productive of oil or gas. The ratio will vary widely, however, for different regions. Some regions may produce from only one out of a great many porous and permeable formations that are found to occur as traps and are tested.
Other regions contain only one or a few potential reservoir formations in the geologic column, but these may all produce wherever a trap occurs. The drilling of dry holes— holes that find only barren traps or no traps at all—is a calculated risk of petroleum exploration. A geologist attempts to show his opinion of the validity of the geologic data on his maps. If he considers the information sufficient and is confident of its validity, he draws continuous contours and formation contact lines.
As he becomes more and more doubtful of the data and their interpretation, he draws long-dashed lines, then short-dashed lines, and finally widely spaced dots. So, the shorter the continuous lines, the greater the risk, in his judgment. Frequently he has no clear idea what is happening underground but knows that the condition is not normal—that something anomalous is present.
He can express such an idea on a map not only by using dotted and dashed contours, but by using special geologic symbols and by putting in short written descriptions on the map. The geophysicist, also, uses solid and dashed lines to indicate his opinion of the data, and, in addition, he frequently marks the seismic record G for good, F for fair, P for poor, and VP for very poor.
Thus both the geologist and the geophysicist endeavor to give, on their maps, not only their idea about what kinds of traps are present but also their estimate of the validity of the evidence upon which their opinion is based. Another variable besides the evaluation of the geologic data enters into the problem of intelligently predicting the position of a trap. It might be called a confirmation of the data. This means that if belief in the presence of a trap at a certain point is based on but one kind of evidence, the trap is not as likely to be found, and therefore not as attractive a prospect, as it would be if two kinds of evidence confirmed each other.
It is still better if three independent kinds of evidence all point the same way. A surface terrace fold alone may indicate a trap at depth, but the probability of a trap below that terrace may be doubled if the surface evidence is supported by a seismic anomaly underlying the surface structure.
And it might be more than trebled if, in addition to the surface and seismic evidence, a subsurface fold were indicated by good well logs. Three independent kinds of evidence indicating the presence of a trap at a certain location in a known reservoir rock would provide strong assurance to the operator who expected to drill the test well—provided, of course, the evidence for each was reliable.
The problem finally resolves itself into a matter of personal judgment as to what weight should be attached to each bit of evidence and what confirmation one kind of evidence gives to the other kinds. In general, the more varied the kinds of evidence and the more certain the data that indicate a trap at a certain locality, the better the prospect geologically.
There is a wide range in the attractiveness of prospects. Suppose, on the one hand, that we have a well-defined surface dome, accompanied by ample seismic and subsurface evidence of deep-seated folding in harmony with the surface fold, all in an area containing well-authenticated showings of oil and gas in a thick potential reservoir rock.
Such a prospect would represent a minimum risk. On the other hand, we may have the same trap potential except that all of the data are sketchy, hazy, and inconclusive.
In such a case many new hazards are introduced. We find every gradation between an A prospect, which might be considered ideal, down to an F prospect, of such small interest as to be only slightly better than any random location in the surrounding region. The geologic considerations boil down to an estimate of the chances of finding oil or gas in the prospect if a test well is drilled. The petroleum geologist is in many ways like a detective—he is forever following up and evaluating clues that might lead to the discovery of the pool of oil or gas.
The location of traps is a deductive process. Some clues give direct evidence of the location of a pool at some specific spot, while others merely suggest the presence of a pool in some general area. In the latter case additional evidence is needed in order to localize the search. Sometimes this can be obtained by reworking the geology with greater detail, by closer spacing of the seismic shot points, by re-examining the geophysical measurements, by shallow core-drilling, or by employing more experienced men.
At other times there is enough evidence to justify the drilling of a test well, which, even if it is not productive, will give valuable added information so that the next well location will have a better chance of being productive.
Many operators expect that to make a discovery on a prospect may require two, three, or even more test wells, and in their planning they take this possibility into consideration.
The ingenuity of the geologist in finding clues and putting them together in their proper relation has much to do with the solution of the mystery—the location of the pool. The petroleum geologist must realize and understand the economic elements involved in exploring for oil, and he is more and more required to show the chances for a potential profit along with his recommendation of a favorable drilling site.
Discounting of a potentially small pool is especially frequent among the larger oil companies, whose crude oil requirements run into many millions of barrels per year and whose operating expenses are proportionately large. They are not much interested unless the prospect has some chance of being a substantial help in their supply problems. The amount of oil or gas in a reservoir cannot be predicted in advance of drilling, it is true, but where the trap is small and where the sands are known to be thin and deep, the chances of a profit being realized become progressively smaller.
Such prospects may be attractive to the individual operator, who has lower overhead and can operate more cheaply than the major company, or to the operator whose oil and gas requirements are not as large as those of the large, integrated company. The desirability of a prospect is generally in direct ratio to the potential profit, but profits for the large company are on a totally different scale than profits for the individual.
One operator may drill a prospect without hoping to make any profit beyond what would come from the refinery operations after the oil is produced—with no hope, that is, of direct profit from production. The requirements of another operator can be satisfied only by large and directly profitable production. Every geologic prospect, then, must face the profit requirements of the operator when the drilling of a test well is being considered.
The greater the potential profit, the less vital it is that all known geologic conditions be favorable, and the greater the sum that can justifiably be risked in testing the possibilities.
The economic evaluation of a prospect also depends on the total money outlay required to acquire the land and then test it. If the probable outlay for a single prospect is large in terms of the budget or in comparison with the resources of the operator, he considers the reports on its geology and geophysics much more critically than if it is small.
He may be less critical if he plans to sell a part of the prospect and share the costs as well as the risk and thereby to have funds to participate in the testing of several prospects. The first consideration is to form some idea of the size of the pool—if it should exist within the prospect. Is its potential content one, fifty, or one-hundred million barrels of productive oil? Usually the experience gained from the discovery of pools in areas of similar geology serves as the best guide, both to the amount of oil recoverable by primary methods of production and to the amount recoverable by secondary methods—fire-flooding, steam treatment, water-flooding, and others.
The relative importance of the factors that go into locating the prospect is more or less determined by the size of the pool that might be expected. The Price of Petroleum The price paid for oil and gas at the well constitutes the entire income of the producer; there is ordinarily no scrap, no by-product, nor any accessory product to sell. The price of oil and gas at the well varies with the demand, with the character of the oil and gas, with the distance to refineries, and with the kind of transportation available—truck, boat, railroad, or pipe line.
Each prospect is therefore evaluated with respect to these factors, which in turn control the price. A prospect in a heavy-oil area, where truck transportation is the only method of carrying the oil to the refinery, and where the chief demand is for gasoline rather than for the lower-priced heavy oils, requires much more favorable geological reasons to justify the risk than a prospect located where the oil is of high API gravity and meets the prevailing demand, and where transportation costs are low, all favoring a high price for the oil and potentially larger profits.
As the price of oil and gas at the well fluctuates with the demand, wildcatting activity also fluctuates. Even the prospect of a drop in price will cause a lessening of interest in exploration. When the price drops, drilling tools are stacked, leases are canceled, and operating costs are cut as much as possible.
This means that during such a period many operators drill only the best prospects or those that must be drilled in order not to lose an attractive lease.
Geology of Petroleum
Get Permissions Abstract Geologic framework. Short or long migration. Primary migration: water squeezed out of clays — normal water circulation — sedimentary oil — recycled oil. Secondary migration: entrained particles — capillary-pressure — displacement-pressure phenomena — buoyancy — dissolved gas effects — accumulation — tilted oil-water contacts — stratigraphic barriers — vertical migration — time of accumulation — petroleum supply.
LEVORSEN GEOLOGY PETROLEUM PDF
Pratt Reprinted from AAPG Bulletin, volume 50 , number I January It is with a sense of inestimable privilege that I speak on this occasion in acknowledgment of the debt I know every member of this audience feels all of us owe, and this Association owes, to our cherished friend, Honorary Member and former President-the late Arville Irving Levorsen. I do not exaggerate when I assert that all around the earth petroleum geologists held him in highest esteem. Among us who knew him personally, a similar high esteem blended with our warm affection. Auden insists that among criteria of worth in a man, "no documents, no statistics, no objective measurements can ever compete with the single intuitive glance. Are his values to be discerned only in the personality of the man himself? Or are they reflected also by the fruits of his labors? I can not pretend to an intimate acquaintance with A.
Faujin The Plover Formation source rock was a poor-to-good hydrocarbon generative potential and reached the geolohy to late mature oil window in the Sunset-Loxton Shoals field whereas in the Chuditch field, it was an overall fair-to-good hydrocarbon generative potential, and attained the Late mature oil window. Amazon Inspire Digital Educational Resources. Browse titles authors subjects uniform titles series callnumbers dewey numbers starting from optional. Scientific Research An Academic Publisher. More tools Find sellers with multiple copies Add to want list.
Amazon Drive Cloud storage from Amazon. Arville IrvingPublication date Topics Petroleum. W h ither the Full Season: English Choose a language for shopping. Page 1 of 1 Start over Page 1 of 1. Cite this Email this Add to favourites Print this page.