Abstract This paper presents the Microbial Prospection for Oil and Gas (MPOG) method, which uses microbiological techniques to explore for oil and gas. These techniques are based on the principle that light hydrocarbons from oil and gas fields escape to the earth’s surface, and this increased hydrocarbon supply above the fields creates conditions favorable for the development of highly specialized bacterial populations that feed on the hydrocarbons. This leads to significant increases in the microbial cell numbers and cell activity of these specialized microbes. By developing methods to establish the separate activities of methane-oxidizing bacteria (a gas indicator) and those bacteria that oxidize only ethane and higher hydrocarbons (oil indicators), it is possible to differentiate between oil fields with and without a free gas cap, and gas fields. This microbial method of surface prospection has been applied in Germany since 1961. The technique has also been used offshore in the North Sea since 1995. The total area that has been investigated to date is 6000 km 2 , most of which has undergone wildcat prospection. To investigate the influence of geologic structure on the results of microbial prospection, model investigations were carried out, with the close cooperation of exploration geologists, on fields that had already been developed. Investigations were conducted on an unfaulted field, on fields with faults within salt, on fields with faults in the supra- or subsalt, and on a completely destroyed field. The case histories presented here support the following conclusions. (1) In microbial prospection, the results do not show a fault-related dependence. (2) The microbial anomalies occur vertically above the respective field. (3) Thick salt seals (Zechstein salt as thick as 800 m) have no effect on microbial prospection. (4) Tectonically destroyed fields without hydrocarbons do not create any microbial anomalies. (5) Recent biogenic methane formation in marsh areas does not lead to significant MPOG anomalies. To date, a total of 17 oil and gas fields, as well as several seismically identified structures without hydrocarbons—both onshore and offshore—has been confirmed subsequently by the results of 220 wells drilled in the area. The success rate of microbial surface prospecting has therefore reached 90%; i.e., the probability is about 90% that MPOG can indicate the occurrence or absence of hydrocarbons. Seven case histories will be discussed in this paper. In unexplored areas, MPOG represents a cost-effective method for preliminary exploration work. In mature areas, the method is helpful for ranking seismically defined geologic structures by indicating possible infill locations, as a contribution to reservoir characterization.

Abstract Prior to BP Exploration’s drilling the well Antufash-1 in the Yemeni waters of the Southern Red Sea, reservoir quality was estimated to be poor; it was dry, plugged, and abandoned. The Miocene sandstones encountered were tight, with a mean porosity of 4% in the cored section and a permeability of only 0.07 md. The prediction of low quality for the reservoir section of Antufash-1 was based on very few core analysis data. The diagenetic history of potential reservoir sands in the Antufash acreage was calculated from data on depth to prospect, burial and thermal history of the area, reservoir sand provenance, and depositional environment. An initial assessment, using limited local well data, led to the conclusion that only at depths <0.5 km was it reasonable to expect high reservoir quality (>100 md). However, at depths <1.5 km, permeability was likely to be as low as 10 md. Throughout this depth range, the chances of halite cementation were also reasoned to be high. The rapid deterioration of reservoir quality with depth was attributed to the instability of the original volcaniclastic detritus. Such detritus was predicted to have converted to a mixture of zeolites and smectitic clay soon after deposition. The reactivity of the assemblage was also predicted to have been exaggerated by the high thermal gradients in the area. The recommendation was to avoid large parts of the license area known to have received input of volcaniclastic sediment, and to develop prospects in the few areas thought to have had arkosic sand input. These sands, it was reasoned, would suffer less degradation of reservoir quality. The Antufash-1 well successfully proved the existence of such arkosic sands in the basin, and their diagenetic history was as predicted. Unfortunately, the sandstones were tight. Halite cement filled, as predicted, all remaining porosity.

Abstract Discovery. Geologic factors. Economic factors. Personal factors. THE PETROLEUM GEOLOGIST is the center pier in the foundation of the petroleum industry. The industry is completely dependent upon a continuing discovery of new pools, or extension of old pools, for maintenance of its supply of crude oil and thereby of itself; and most certainly “the discovery and production of oil is a geological enterprise.” 1 Because of its dependence upon geologists to guide it in its search for petroleum, the industry has entrusted many geologists with much greater business and management responsibilities than fall to the lot of the average scientifically trained person. 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. DISCOVERY The discovery of petroleum generally follows a more or less similar pattern. First, a prospect is located that is thought to be worth drilling; second, a test well is drilled into the potential reservoir rocks. The answers to the questions “What is a prospect?” and “What makes a prospect worth drilling?” combine not only geology but also economics and many personal attitudes in an almost infinite number of combinations and variations. 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. Some of these discoveries come from what might be termed “magic geology”: the area “looks like Pennsylvania,” the creek “makes a bend like the one at Walnut Bend,” or “any prospect near a town named Eldorado is good enough for me—remember the rich oil fields at Eldorado, Kansas, El Dorado, Arkansas, and Eldorado, Texas?” Others come from the thinking of the “lay geologist,” who knows he should drill on a “high” or “get up against a show of oil” and remembers that “twenty years ago ole Joe Doakes said there was an oil field under the pile of rocks in the far forty”; and other discoveries come from the advice of “doodle-buggers”—those persons of mystery who continually seem to have some new method of locating oil and gas pools(?). 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. Whenever a wildcat well * is drilled, it means that someone, for some reason or other, has located a prospect that he believes worth testing. The fact that most wildcats result in dry holes is not a deterrent to the effort, just as the real fisherman is not discouraged if he doesn’t get a fish every time he goes fishing. 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. 2 A field containing a million barrels would supply only about one-tenth of the daily demand for the United States and for the operator may be either nonprofitable or barely profitable. 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.