The Dream in the Bottom Drawer

Published October 31st, 2000 - 02:00 GMT

Infallible techniques to identify oil fields from the land surface. All oil companies would like to possess them. But concrete results are still a long way off. A workshop on the subject organized at end-December by the Agip Division and EniTecnologie.  

 

Among other adventures and events, those of Patillo Higgins are described in one of the finest and most fascinating books on the history of oil prospecting, "The Prize", by Daniel Yergin. 

 

He was a one-armed Texas mechanic, who, on a school excursion, spotted dozens of small patches of swirling gas on a hillside.They were the years of the great oil rush at the end of the 19th century, and he was immediately convinced that there was a huge oil field down there, and that what could be seen at the surface was no more than the direct manifestation of what was trapped in the subsurface.  

 

Only his doggedness enabled him to pursue his project of drilling a well, in spite of the mocking comments of contemporary geologists. In the end he discovered an oil field able to produce seventy-five thousand barrels a day, a real record for that time. 

 

The occurrence of surface hydrocarbons was a decisive guiding element in oil prospecting for many years until the anticline theory emerged in the '20s. According to this concept, hydrocarbons, which are lighter than water, tend to accumulate in the subsurface in the upper part of uplifted convex structural folds, namely anticline.  

 

Attention started to be focused on seeking these structures, and later on, with the evolution of knowledge, on the identification of all those geometries that could act as an efficient trap for the accumulation of hydrocarbons. 

 

Thus the development of geophysical techniques able to provide indications on the traps present in the subsurface took on more and more importance in the course of time, while the presence of hydrocarbons on the surface assumed a more marginal role. 

 

At the end of the Sixties, coinciding with one of the all-time lows in prices of crude oil, the pressing need to increase the rate of success in exploration wells led finally to the emergence of the concept that, to discover an oil accumulation, the existence of a potential trap in the subsurface is a necessary but not a sufficient condition. In fact, the presence of hydrocarbons, which can be trapped, is also important. 

 

This concept is emphasized in one of the most important books for oil exploration, by John Hunt, a scientist famed worldwide and the father of modern geochemistry, which stressed that the majority of oil fields have been discovered in areas where oil seeps were present, thereby confirming the close connection between what is seen at the surface and what is down below. 

 

The idea of using what is 'seen' at the surface to understand whether a potential trap could be filled with oil thus again gained credence toward the end of the Sixties. 

 

However, the occurrence of hydrocarbons at the surface, such as those observed by Patillo Higgins, is often linked to the presence of faults, which enable the oil and the gas to seep to the surface following pathways that are sometimes tortuous and hard to reconstruct.  

 

Thus while on the one hand they are valuable for understanding whether there are hydrocarbons in the subsurface, on the other hand their direct use in understanding whether a trap has been mineralized can be highly problematic. 

 

Another hypothesis then became popular: if even very small quantities of hydrocarbons could escape along the vertical of the field and seep to the surface, detecting them would enable important elements to be added in assessing the possible targets to be drilled. 

 

Two snags immediately opposed this hypothesis, one theoretical and the other of a practical nature. The theoretical one starts from the assumption that underground traps, although the result of specific formations of the reservoir rocks, such as anticlines, are completed by what are known as cap rocks, which are sediments of low permeability overlying the reservoir.  

 

These cap rocks have the basic task of preventing the hydrocarbons from continuing their upward movement, thus making possible their accumulation.  

 

Their efficiency is of fundamental importance for a reservoir of industrial interest to exist. Perplexity, linked to the hypothesis that even small quantities of hydrocarbons can penetrate them, stems from the fact that these pools formed over millions of years and remained intact for millions more years before being detected and exploited.  

 

Hence the quantities that could possibly have penetrated the cap rocks must be truly infinitesimal for not having impoverished the accumulation. It might also be wondered: what can the physical mechanisms be which allow the hydrocarbons to get through the cover, eluding the well known forces which instead hinder their progress? And again, in the case of very deep accumulations, how can the different properties of the whole column of overlying sediments and its possible lack of homogeneity deflect the possible flow of hydrocarbons that  

 

might have passed through the covering rock? 

As far as practical difficulties are concerned, in reality the quantities of hydrocarbons could be infinitesimal, and the problem would be to detect them at the surface, considering also the possible anthropic pollution and disturbances due to animal and plant activity. The risk is of having to seek the proverbial needle in a haystack. 

 

Despite these doubts and uncertainties, activity in the field of surface geochemical surveying has been increasing, with growing investments which may currently be estimated as greater, globally, than $40 million a year. 

 

It should in fact be remembered that in the oil exploration budget the cost of wells is the most expensive item, and hence it has become ever more urgent to decrease drastically the number of dry wells. This justifies the increasing interest in these methodologies, which in the event of success promise high economic returns. 

 

Many service companies in the upstream business are presently engaged in the setting up of different technologies in this sector.  

 

The techniques of detecting hydrocarbon traces at the surface can indeed in a different way be aimed either at directly quantifying the hydrocarbons (direct methods) or at measuring the effects induced by them (indirect methods), such as for example the growth of specific bacteria that feed on petroleum or the presence of given chemical elements in particular states of oxidation.  

 

In particular, direct methods have benefited in the last few years both from the increasing sensitivity of instrumental analysis, which has reached hitherto inconceivable levels, in the order of milligrams of hydrocarbons per ton of soil, and from the possibility of using special substances which if left in the ground for a few days enable the hydrocarbons to be concentrated, adsorbing them on the surface, thereby allowing the detection of even smaller quantities in the laboratory. 

 

All of this is opening up new horizons and fresh opportunities unthinkable in the past, even though great caution must be used regarding the real scope of these methodologies. Also because while the capacity of analysis has increased to an incredible extent, the problem remains of removing those spurious signals linked with pollution and plant and animal activity. 

 

This problem brings up another particularly important aspect: the interpretation of the data acquired in the field with statistical methods, able to remove the so-called 'background noise' depending on what goes on in the upper layers of the soil, so as to be able to recognize what really stems from the depths of the subsurface.  

 

It is obvious that in this context of great interest, the oil companies are not standing by and twiddling their thumbs but have organized activities on their own account, both of scientific research and of experimentation, to sound out the reliability of the already existing technologies.  

 

Certainly, Elf's much-publicized experience in the '70s with their 'sniffing planes' which, in spite of their high cost, proved quite ineffective in locating oil fields, has in some way weighed heavily on all the companies.  

 

More generally however it can be observed that there have been ups and downs in the progress of these techniques, and that at times some companies have offered services, claiming performances well beyond the limits of the methodologies available.  

 

While the oil companies' activity in this area remains considerable, it is hard to obtain a real assessment of the results and of the real progress achieved, since very little information is forthcoming because the subject is regarded as confidential in view of its potentially strategic nature. 

 

Naturally the Agip Division in collaboration with EniTecnologie has already been active in this field for some time with various activities, including both research and experimentation in the field. 

 

An interesting internal workshop was also organized last December on this theme, with the participation of numerous researchers, of explorationists from various Agip geographical Units and of representatives of service companies particularly in the forefront in this field.  

 

The guest of honor of this successful event was Professor Ronald W. Klusman of the Colorado School of Mines. Prof. Klusman, a world renowned expert in this field and the author of many articles, went into a number of theoretical aspects linked with the propagation of hydrocarbons from the field to the surface, passing on elements of optimism to the numerous followers of Patillo Higgins present.  

Source:E.N.I. 

 

© 2000 Mena Report (www.menareport.com)

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