SEARCH FOR OIL

Evening Post, Volume CXXV, Issue 70, 24 March 1938, Page 11

SEARCH FOR OIL

HIDDEN RESERVOIRS

POSSIBILITIES IN DOMINION

GEOLOGIST'S REVIEW

Now that the Government has taken unto itself ■ powers to possess all the oil which may be hidden beneath New Zealand's -rugged surface, and redoubled efforts are being made by scientists, equipped with the most modern apparatus, to locate this hidden treasure, the possibility of New Zealand springing into supreme importance as an Empire oil reservoir is not entirely beyond the bounds of possibility. .• Oil is known to exist in many parts of the country: no one. can dispute that. But the important question is whether it exists anywhere in large enough quantities to make it worth while spending large sums in sinking bores and erecting refineries. For well over fifty years now oil" in small quantities has been taken from beneath New Zealand's surface, but" in many of the early borings it was a case of hit or miss, more often the latter. But in recent years improved methods of prospecting have helped to eliminate the misses, and these new methods, coupled with the findings of geologists, will; it is hoped, prove shortly, and once and for all, whether. New Zealand possesses the vast reservoirs of oil which are the dream of optimists. A survey of the Dominion's probable and possible oilfields -is sketched by Dr. J. Henderson, Director of the Geological Survey. Branch of' the Department, of Scientific and Industrial Kesearch, in a recent number of "The New Zealand*' Journal of Science and Technology." After dealing at length with the geological factors involved in oil accumulation, . and with the geology of New Zealand in detail, Dr. Henderson says- that there were at least two considerable oil-forming periods in New Zealand. The younger occurred in early Pliocene and late Miocene times and the older in the early Tertiary and, late Cretaceous. To the former belong the oil-yielding rocks of New Plymouth and Kotuku, and to the latter those of the East Cape-Gisborne district, Hawke's Bay, and East Wellington, Marlborough and ■North Canterbury, and Moeraki. The middle Tertiary limestones also contain a trace of oil at many points, though no seepages can •be attributed to them. There is some evidence that the thick mudstones of slightly younger age are in places. petroliferous. These dark-argillaceous beds in the Gisborne and Hawke's Bay regions smell of oil at many points, and they may well be the source of the oil in the Murchison basin, though here black carbonaceous shales of early Tertiary age also

occur. ' . . . . . ■ ] In the petroliferous districts^ there are numerous examples of the kinds of ' structure that in o.ther, parts of , the world contain oilrpools. In the flatlying beds of T.aranaki structural .terraces and noses in addition to gentle anticlines and domes have been described. Here also monoclines sealed by unconformities, faults, or igneous intrusions may occur as well as isolated sand lenses at unconformities or elsewhere in the Tertiary sequence.; Similar structures,, together with anticlines formed'by the setting and compaction of strata over buried hills, are possible in -North .Westland and. the Gr.ey-Inangahua grabeh. In the- latter' sharp-fractured folds are to be expected, such as are present in the Murchisqn basin. '"r There is a wide range 'of structures in the' East Coast ■ districts. In the Wairoa. and Akitio synclines the simple surface structures ■ may become more pronounced below ; each set of beds. In the Gisborne district similar, folds are likely and the crude arrangement of faulted Tertiary : blocks in disordered anticlinals or syn- ; clinals may well overlie intensely , plicated anticlinoria and synclinora in the Cretaceous rocks beneath. : GEOPHYSICAL PROSPECTING. 'I'A few years i ago," continues Dr. Henderson, "the geologist,had to rely on what he could see!on the surface, supplemented with what information he could.get from bores; today he has ' many geophysical instruments to assist him. Geophysical instruments measure or record physical phenomena depending directly on the physical propertied of the rocks below the surface. The magnetometer records differences in the earth's magnetic field. The seismograph measures the velocity of propagation of elastic waves through the different formations. ■ The ratiometer and other electric appliances gauge the relative electrical "conductivities of rocks and of ' the fractures in ' them. The torsion balance directly evaluates the rate of change of the intensity of gravity. None of them-positively determines the presence or absence of a valuable deposit or of a particular structure. The geologist interprets the data of the geophysicist, combines them with facts observed in the locality, and with his knowledge of the rocks and structure of the region, and then draws conclusions that only the driller can test. The.immediate and impressive success of geophysical prospecting for oil in Texas and Louisiana demonstrated the value of the methods. In that region they have markedly reduced the hazard, risk of development and have given to prospecting campaigns that in former years only the most reckless 'wild catter' would j have seriously considered a fair hope i of success.1 On the other hand, in some areas long-continued and costly geophysical work has yielded nothing that careful surface observation and logical deductions had not . already , ascertained. Many geological problems, however, on account of the imperfect data, are susceptible of more than one interpretation, ' and geophysical methods iriay add the little more required to convert doubt to reasonable probability; but until the geophysical work is actually carried out one may not be able positively to state that it will add anything to visual observation. "In New Zealand there are some oil problems to which geophysical methods are obviously applicable. Volcanic •; debris, thickly covers a large part of . Taranaki; which part includes the petroleum seepages ' and the semicommercial wells of New Plymouth as well as most of the belt yielding emanations of combustible gas. -The nature and attitude of the subsurface rocks can be inferred with' strong probability from geophysical evidence, and this will enable sites to be chosen for bores that.have a reasonable chance of determining whether the district is worth further prospecting or not. Without the geophysici'st's aid, the task of exploring the ashcovered part of Taranaki would involve an enormously costly programme of core-drilling,and would indeed be starkly futile. Similar remarks apply to North Westland, though here the area worth prospecting is smaller, the detrital cover less complete, and the history of the search for oil less encouraging. Gravels mask other probable considerable areas of tertiary beds under the Canterbury and Southland plains, but there is no direct evidence that such rocks, if present there, do contain oil." A HOPEFUL NOTE. > A hopeful note 'as to the future is struck by Dr. Henderson when he says:—"During the past seventy years many attempts have been, made to. find

commercial oil in New Zealand, most success being achieved at New Plymouth, where a few wells yield a semicommercial output. Other areas showing seepages and other indications of oil arc known to be underlain by rocks that may well contain valuable oil deposits. In some areas suitable structures are apparent. Though the work hitherto undertaken did not prosper, the results are inconclusive; much of it was ill-directed and futile, and of the remainder some holes did not reach their objectives owing to poor technical equipment or lack of funds. But enough is known to encourage the hope that commercial output is not improbable. Better understanding of the geological problems and improved technique in drilling and production engineering combined with adequate finance may well yield _a rich hardest." __