Art. XXXVIII.—Note on the Occurrence of Petroleum, in New Zealand.
[Read before the Philosophical Institute of Canterbury, 2nd December, 1914.]
The question of the origin of the petroleum known to occur in New Zealand has a scientific interest apart from its economic importance, but, unfortunately, the first is almost entirely dependent on the results which are furnished by attempts to prove the capacity of the fields commercially, and therefore, till a mass of evidence has accumulated as the result of boring operations, conclusions as to its origin must be largely of a tentative nature
In the following parts of New Zealand indications of the presence of petroleum certainly exist: (1) Taranaki; (2) Gisborne and the East Coast in the neighbourhood of Poverty Bay; (3) east coast of Wellington, between the Tararua Ranges and the sea; (4) in the valley of the Grey River, especially near Kotuku.
There are, in addition, other places of minor importance, such as the Clarence Valley and Cheviot, which have given indications of less positive nature. It must be clearly understood that, except, perhaps, in the first of the above-mentioned districts, the present prospects of the oil industry are doubtful; but the difficulties may eventually be obviated, and the actual result may prove more satisfactory than the present state of affairs perhaps indicates. Some of the difficulties are no doubt connected with drilling rather than the absence of oil in commercial quantities.
In all the districts cited the prevailing beds associated with the occurrence of oil consist of marine mudstones, marls, sands more or less coherent, with shell beds, all of Miocene or Mio-Pliocene age. The predominating strata are, however, mudstones and marls, locally known as “papa,” and it is only where these exist in great thickness or continuity that the indications are strong.
The most important problem to be considered is whether these beds are the actual place of origin of the petroleum, or whether it is derived from a more deep-seated source and has migrated to the overlying beds along various lines of weakness or dislocation. A correct answer to this question would prove of the highest value in framing estimates for the adequate exploration of the petroleum-bearing areas. I therefore give the following brief summary of the position in accordance with the evidence now available; and it will be more convenient to refer first to the Taranaki area, since it is the best known and, according to present evidence, the most promising.
The strata which cover the greater part of the Province of Taranaki consist of the typical Miocene rocks of the North Island—viz., mudstones, sandstones, shell beds, and occasional bands of conglomerate which dip west and south-west at low angles in the neighbourhood of New Plymouth, but when followed towards the lower course of the Waitotara River change their direction of dip to the south-east, the line of strike turning approximately through a right angle. The thickness of these beds certainly exceeds 4,000 ft., as is disclosed by the bores at New Plymouth, and may be much greater, and they are apparently little disturbed by dislocations or other
deformations; but owing to the infrequency of good sections this statement is subject to revision; in one or two places undoubted faults of small throw are existent. Rising through these sediments and covering them over considerable areas are the volcanics from Mount Egmont and numerous small parasitic cones round the base of the mountain, as well as the igneous masses of the Kaitaki and Pouakai Ranges. The age of these mountains must be considerably greater than the cone of Egmont, but there is no strong evidence that they are of pre-Miocene age, although that is extremely probable. A notable constituent of the sandstone beds interstratified with the mudstones and marls of this age are fragments of hornblende crystals similar to the mineral occurring in the Taranaki volcanics. These would seem to indicate that a considerable area of igneous rocks was existent in the neighbourhood while the petroleum-bearing beds were being laid down. This fact may have some bearing on the date of the intrusive rocks at Moturoa, but, owing to the obscuring of the contacts between these and the intruded rock by the overburden of fragmentary volcanic matter, no conclusion can be come to at present; all the same, it is remarkable that bores could have been put down to 4,000 ft. within but a short distance of these masses. It seems, therefore, reasonable to suppose that they were intrusive into the Miocene rocks as well as into the pre-existing masses.
The interstratification of sand with close impervious beds of mudstone and marl would undoubtedly furnish satisfactory conditions for the storage of hydrocarbons were they introduced into the beds, but they do not furnish prospects for great concentration. If the field proves satisfactory it will have the character of one which will produce steady yields rather than sensational returns for a short period.
The evidence for the presence of petroleum, apart from the actual vicinity of Moturoa, where flowing wells occur, is based on the following:—
(1.) The existence of slight seepages of oil in various places remote from New Plymouth, and distributed over an area stretching from the sea near Waitara to the Waitotara River.
(2.) The existence of gas emanations over the same area, including those recorded by Clarke.* Outside the area, therein referred to, they are widely distributed, the most important occurrences being near Inglewood, at Huiroa, in the Mangaone Valley, in the Waitara River. Such gas discharges are known to occur as far to the east as Whangamomona.
(3.) Beds of oil-bearing shale of lenticular shape, but not extensive in area, occur in several places, interstratified in the papa rocks.
(4.) In addition, there are sulphuretted-hydrogen and saline springs in various places, these not being in themselves evidence of the occurrence of petroleum, but are frequently associated with it.
(5.) The occurrence of mud volcanoes, which are frequently found in oil regions, has been recorded from various parts of the district, but the evidence on which these reports have been made is somewhat doubtful.
These various lines of evidence show that oil is found over a fairly wide area, but they do not show that it is found in sufficient quantities to be commercially valuable: that can be demonstrated only by boring. The original theory for the origin of Taranaki petroleum was based on the supposition that its distribution was very local, and entirely confined to the neighbourhood of Moturoa; but the evidence that it is widely distributed on the coast and inland between the Waitotara and the Mokau Rivers is
[Footnote] * Bulletin. No. 14 (new series), N.Z. Geol. Surv., pp. 43–45.
very strong, so that any explanation of its origin must account for its occurrence near New Plymouth as well as in parts distant from that town, and must also be considered in connection with its distribution in beds of similar age and lithological character elsewhere in New Zealand where the conditions obtaining at Moturoa do not exist.
The first explanation of its origin was that advanced by Sir James Hector, who suggested that the petroleum was formed by the distilling action of the heated rocks of Paritutu and the Sugarloaves on the brown coal, which, it was supposed, continued from the Mokau River till it came in close contact with the igneous masses near Moturoa.
This hypothesis has been accepted by most geologists who have written on New Zealand geology, including Park and Bell. McKay has, however, considerable doubt about it; and Clarke, too, hesitates to accept it, but does not advance any theory of his own. That the volcanic rocks have had some effect is no doubt true, judging from the remarkable amount of carbon dioxide present in the gas given off in the wells (see analyses quoted, p. 46, Bull. No. 14, N.Z. Geol. Surv.), these results differing markedly from those given for other wells in the Taranaki District. The chemical constitution of the oil itself shows a notable percentage of constituents distilling at a high temperature, and points either to the more volatile matter having been driven off by the heat of the volcanic masses or its having escaped through fractures in the rocks. Both of these circumstances may occur in the same locality, and in this case may be attributed to the igneous masses of Paritutu and the Sugarloaves, which have provided the requisite heat, and have also in all probability fractured and dislocated the surrounding beds when the intrusion took place.
In criticizing Hector's theory it may be pointed out,—
(1.) There is no evidence that the Mokau brown-coal measures do extend from the outcrops on that river to the neighbourhood of New Plymouth—a distance of approximately seventy miles—and, judging from the form of the coal areas in other parts of New Zealand, this is extremely unlikely to be the case. Most of these areas in New Zealand are marginal in character, and rarely continue over any wide expanse, although their length may be considerable; the seams, too, are lenticular in shape and are markedly discontinuous. If, however, there were other concealed coal areas lying on a pre-Miocene land-surface in the neighbourhood of northwest Taranaki, this objection would not apply.
(2.) Seeing that the action of volcanic heat cannot be responsible for the occurrence in districts remote from the centres of volcanic activity, some other cause must be looked for in order to account for its presence; and because, the petroliferous beds near Moturoa are of the same age as those containing oil in other parts of the country, it seems reasonable that they owe it primarily to the same cause, even if local variations of conditions exercise a modifying effect. In advancing any theory for the origin, there is an important question to be considered—viz., whether the oil has originated in the papa beds or has migrated into them from some other beds, necessarily of more deep-seated position. It is likely, too, that both of these circumstances may be in existence at once, and that the beds owe part of the amount to accumulation in situ and part to migrations from other strata.
The fact that the Miocene mudstones are usually associated with petroleum indications, and also that where these beds do not occur the indications are scanty or absent, leads one to regard them as the locus of origin. As
far as present knowledge allows a conclusion to be made, indications of the occurrence of gas or oil are met with at horizons distributed throughout the whole thickness of these beds. It is true, nevertheless, that the amounts are greater at lower levels; but this is to be expected, since the more perfect cover afforded by the increased thickness of overlying strata naturally reduces the chances of the escape of oil. If it has originated in situ, the only possible explanation of its origin is that it is due to the distillation of the animal remains. The amount of vegetable matter included in these beds is small and seemingly insufficient to account for the phenomena, but a large amount of marine-animal matter does indeed occur. This includes remains of Foraminifera, and above all of Mollusca, whose shells are widely distributed, and at times form banks several feet in thickness. By the distillation of the animal matter they formerly were associated with, sufficient hydrocarbon would be furnished to account for the oil in the strata, and their wide distribution at various levels would account for the occurrence of oil through great depth. In this connection it may be noted that the oyster-beds associated with the coal-measures in other parts of New Zealand frequently give out a strong smell of petroleum when freshly broken. A reference has been made previously to the existence of beds of carbonaceous shale, which are interstratified in the papa rocks, but the origin of the carbonaceous matter is at present unknown. When analysed these show the presence of oil, and they furnish absolute proof that it does exist in situ in the Miocene beds.
The sandstone layers which are interstratified with the mudstones of this series would undoubtedly be suitable for the storage of the oil, and thick covering mudstones would favour its retention; but the structure of the country, as far as is known at present, hardly favours its concentration.
As regards the existence of a deep-seated source, it must be admitted that there is no direct evidence at present available from Taranaki. The most suggestive fact is that the surface indications and the flow of oil are specially strong in the immediate neighbourhood of the intrusive rocks near Moturoa. This may be explained by supposing that leaks have occurred up the surface of contact of these masses and the beds into which they have been intruded, and that the disruptions which would occur when the volcanic rocks were being forced into the surrounding sedimentaries would afford a relatively easy path for leakage through them from lower levels. There appears to be a close parallel between this occurrence and the sensational wells of the East Mexico field in everything but volume of flow.
Further, in the valley of the Waitotara River, along a line of fault, the surrounding clays are impregnated with hydrocarbons, and smell strongly of petroleum. This is certainly due to a leak, but whether it is a leak from a lower level in the Miocene beds or to a leak from a still lower horizon it is impossible to say. The strongest evidence of the deep-seated origin of the oil is, however, obtained from other localities.
In referring to the Kotuku district of the Grey Valley, Morgan says* that the Cobden limestone underlying the marls of that locality is charged with oil; unfortunately, nothing certain is known of its origin in that region, but the evidence undoubtedly points to a source below the limestone, and the impregnations of the overlying clays are due to upward migrations.
[Footnote] * Bulletin No. 13 (new series), N.Z. Geol. Surv., p. 143.
In summing up the position, Morgan says (p. 148), “It seems not unlikely that the source of the Kotuku petroleum may be at considerable depth, in a horizon corresponding to the Omotumotu beds, Kaiata mudstone, or Island sandstone. There is also the possibility of the oil originating in the Brunner or Paparoa coal horizons. The chief evidence, however, in favour of the view that the oil occurs at depth is based on the logs of the Brunner Company's Nos. 2 and 9 bores. These logs state that a little oil and gas occur all through, the conglomerates that lie below the limestone. Since the bores were cased, a mistake as to the occurrence can hardly have been made, and therefore a deeper source than the conglomerate for all or much of the Kotuku oil must be regarded as the most probable. The small amount of benzene in the oil hitherto obtained supports the hypothesis of a deeper source than the argillaceous sandstone or the limestone.”
In the Gisborne district, according to Adams,* the oil which occurs on the surface as well-marked seepages at Waitangi is derived primarily from a bed of oil shale which lies at the base of the Whatatutu series of the Survey. The author says (page 40), “As a result of the examination that has just been completed, it seems certain that the clay shales which form the lowest members of the Whatatutu series are the upper portion of or form the impervious cover for the oil zone of this area. Every sample of this rock which was obtained gave, when finely pulverized and subjected to strong heat, a decided smell of hydrocarbons. From its character the clay shale is advisedly suited for a cover for a stratum containing oil.”
These beds of the Whatatutu series are classified by Adams as of Miocene age, but it seems reasonable that McKay's classification of them as Cretaceous is more correct, since he found Inoceramus shells in them; and the report has been recently confirmed by the discovery of these fossils in boulders in the bed of the Waipaoa River in a matrix similar to that of the lowest beds of the series. This discovery apparently confirms McKay's classification, and we may therefore look on the shale, or the beds associated with it—probably the sandy beds—as the place of origin of the oil, and the fact that indications are to be found freely in the overlying clays which may be of Miocene age is attributable to migrations from a lower to a higher horizon. In this locality the only possible origin for the oil is an organic one, and almost certainly the oil has been derived from the remains of animal-life of a former sea-bottom.
In the East Coast district of Wellington the Miocene marls and clays are well developed, and under them lies, in all probability unconformably, a Cretaceous series.† Surface indications, such as gas emanations, are frequent in the overlying beds, but in all probability the major portion of the oil is derived from the lower series, in which black slaty shales are interbedded with clays and sandstones. These are well exposed near the coast, and have a general inland dip. The overlying marls are very thick—certainly over 3,000 ft., and probably over 5,000 ft.—and since they are marine in origin and contain traces of marine organisms it is quite possible that a part of the hydrocarbon matter they contain, and in some cases the whole of it, has been derived from the beds themselves and has not been accumulated by migration from the petroliferous beds underneath.
If we consider, therefore, that a deep-seated source of oil exists, then its probable mode of origin should be briefly referred to. The only acceptable
[Footnote] * Bulletin No. 9 (new series), N.Z. Geol. Surv., Geology of the Whatatuta Subdivision.
[Footnote] † See Park, Geol. Surv. Report for the year 1887–88, p. 20.
explanation is that it is due to the alteration of organic matter. This exists in two forms at the base of the Cretaceo-tertiary series—viz., as coals and lignites of vegetable origin, or as animal matter of marine origin Although the former is a possible source of the supply, it appears extremely unlikely that these coals have been depressed so far that the earth's internal heat has been sufficient to convert them into hydrocarbons. Our coals show little change unless they have been placed in close proximity to volcanic intrusives, in which case they are converted into altered brown coals or into mineral cokes or anthracites. The marine-organism explanation, therefore, is apparently the best solution of the problem. Below the limestones of Lower Tertiary or Upper Cretaceous age there lie almost invariably Greensands and other sands which contain a considerable amount of matter of organic origin. It has been noted previously that the black oysters associated with the coals of this series, when broken or struck with the hammer, give off a pronounced petrolaceous smell, and the remains of Foraminifera and other marine organisms are widely distributed through the Greensands. A thick band of these contains very generally a large quantity of sulphur, whose formation can certainly be attributed to the former presence of organic matter; and the frequent association of petroleum with sulphur compounds in other fields is also very suggestive in this connection.* A very illuminating paper by Daly in the “American Journal of Science,” vol. xxiii, 1907, entitled “The Limeless Ocean of Pre-Cambrian Time,” draws attention to the probable accumulation of organic matter which may occur under conditions similar to those obtaining in the Black Sea at the present time, and to the existence of large quantities of sulphur compounds associated with these accumulations. We have thus an explanation of the connection of sulphur with the large masses of organic material which could yield petroleum if buried under an impervious covering and subjected to the slow but sure chemical changes which go on in the crust of the earth, whether these changes be due to the transference of heat from lower levels in the crust, or to the action of bacteria, or to any other cause.
If, then, these sands and their associated beds be the locus from which the supplies of petroleum are derived, it becomes increasingly important when prospecting for oil in this country to know what thickness of the Miocene marls has to be penetrated in order to reach, the underlying Cretaceous sands. In the North Island the marls are very thick—up to 5,000 ft., and perhaps much more. In the South Island the marls, with the exception of those in the Marlborough district, are comparatively thin; but they are usually folded with broken anticlinal crests, and it is probable that the greater part of the oil they may have once contained has escaped. There are places, however, such as Cheviot, which do undoubtedly give indications of the presence of oil in small quantities, and in these cases it may be reasonably supposed that the conditions favoured its retention for a longer period than elsewhere.
[Footnote] * See U.S. Geol. Surv., Bull. 398: “Geology and Oil Resources of Coolinga District, California,” p. 187.