THE TARANAKI PETROLEUM DISTRICTS.

Press, Volume XII, Issue 1585, 5 December 1867, Page 3

THE TARANAKI PETROLEUM DISTRICTS.

[From Dr Hector's Report.] The mode of occurrence of the petroleum oil at Taranaki was the first subject of investigation, and from an examination of the natural sections afforded by sea cliffs for thirty miles north of New Plymouth, I arrived at the conclusion that the views I previously expressed respecting the deepseated source of the rock oil in this locality are substantially correct. The borings are being made in one of the most modern and superficial rocks of the district, resting on older strata, from beneath which in other parts of the island bituminous products have been found to escape ; and the only reason which can be suggested for the oil reaching the surface at the Sugar Loaf point is its vicinity to a centre of volcanic disturbance, and the consequent disturbed and dislocated condition of the rock formations. The formations which enter into the structure of this district are best displayed in the section between the White Cliffs and the Waitara Kiver ; and in this section we may safely conclude, owing to the dip of the strata, that we have somewhat the same succession of formations which would be encountered in a vertical boring near the Sugar Loaves, namely :-— 1 Eeet. (a) Volcanic breccia ...250 to 300 (b) Newer tertiary clays ... 400 (c) Conglomerate and quartz cement ... 100 (d) Older tertiary marlstones 900 The above estimates of the thickness of the strata to be passed through are taken from the coast sections, and are, therefore, only approximate. The rock in which the boring for petroleum is in progress is the breccia (a) in the above section. This breccia or agglomerate consists of a variety of trachyte and basaltic fragments, cemented by a tufaceous clay and calcareous marl or ash of red, yellow, and green colours, but some times there occurs a thick bed of tufaceous clay of a green tint with but few included fragments. Between the innermost island and the north headland the fragments from this breccia, where it has been denuded by the sea, have formed a natural causeway, which is dry at low water; and it is on this principally, and also round the shore of the headland, that the indications of the presence of petroleum prevail. Masses and seams of ferruginous tufa or clay ironstone, which intersect the tufaceous clay in the neighbourhood of the great dyke, yield the oil when broken, but whether this indicates the outlet of fissures which are vents for the oil from a deepseated source, or whether they are merely masses of bituminous ironstone, had not been ascertained. At the time of my visit borings were in progress at three places. Close to the main Susrar Loaf and to the foot of the cliffs is the Taranaki Company's bore, No. 1 A, which has been sunk with much trouble to a depth of 300 feet. The derrick stands at ten feet above high water, and for some time the water in the bore maintained the level, but after a time it sunk suddenly to thirty-two feet, whicli probably indicates the existence of subterranean channels communicating with chambers where there is less than the external atmospheric pressure, owing to the condensation of oil vapours. At 254 feet a patch of grey ferruginous tufa, charged with oil like j the seams at the surface, was passed through by the boring rods and was the only trace of petroleum obtained. In the bore some hard basaltic rock was encouutered, but indicating no decided change in the character of agglomerate. The Taranaki Company's bore, No. 2 C, is on the island off the north, headland, and is commenced on a shelf twenty feet above the water level. This island consists of ferruginous sand fifty feet, false bedded like the sand dunes at New Plymouth, and resting on forty feet of r ed tufaceous sand, which, again rests on the agglomerate. In the section which is shown m the lower part of the cliffs of this island the agglomerate is seen to be very distinctly inter-stratified with beds of indurated tufa and water-worn fragments of rock. The bore was, in October, sunk to a depth of 145 feet, 1 being ten feet in the sandstone, ninety-

five feet in the agglomerate breccia, thirty feet in consolidated tufa, and a few feet more into the agglomerate again. A few oil patches have been passed through, but no appreciable quantity has been obtained. Tiie third bore is that of the Alpha Company, which is situated a short distance from the north headland (b). It is from this locality the most of the oil which has been chemically examiued I has been obtained. At ten feet above high water* and close to the bouldercovered shore, under a high sandy cliff, a shaft was sunk for sixty feet in the agglomerate, from the sides of which, at forty-four feet from the surface, oil was found to ooze. This shaft was continued by a bore hole to a depth of 180 feet, oil. being got at eighty feet, and again at the extreme depth. When allowed to remain at rest, a considerable quantity of oil collects both on the surface of the water in the well, and also in the bore tube, accompanied by an escape of gas. This oil was pumped into a tub along with the water, from the surface of which it was afterwards skimmed off. Recently, the well has been pumped more regularly, and yields I am informed about fifty gallons a week. The escape of gas in large quantities from under the sea at various spots among the islands would seem to indicate that the oil oozing from the ironstone seams is only an after deposit, and as we now know from what has been observed respectiug the occurrence of bituminous products on the East Coast, that inflammable gas [and oil escape from the surface in localities in which the most superficial bed observed is the marlstone (d. in the section), we must conclude that its source is from some more deep-seated stratum than any that have been j described, and that its reaching the surface at the Sugar Loaf Point through the superincumbent formation's must be dependent in some way upon the dislocations in the neighbourhood of the great igneous dyke which forms the promontory. As there is every appearance of the formations a, b, and c being more recent than the eruption of this dyke, and therefore not dislocated by it, it is just possible that the oil and gas may have escaped upwards as far as the base of these formations, in which case it would be collected in reservoirs at a depth I should eurmise of from 500 to 700 feet below the surface, although by following fissures such as those marked by the ironstone veins it may rise in small quantities to a much higher level, and even to the surface. What I would wish to impress with regard to these oil wells, as I did in my former report on the subject, is that there is no indication in the Taranaki district of the occurrence near the surface of the regular alterations of- sandstone and shales which characterises the best oil-bearing formations. Under the marlstone formation, judging from analogy with other parts of the colony, there probably occurs a thick group of ferruginous clay and sandstone, which again rest on the same coal formation as on the west coast of Nelson. Great interest will therefore attach to the investigation of the geology of the Mokau district, and of the country lying between it and the range of hills terminating on the coast at the White Cliffs, as in that area the underlying formations may be expected to reach the surface, and I think it very probable that among them extensive carbonaceous strata will be discovered. Respecting the prospect of gold occurring in the Province of Taranaki, concerning which many inquiries were addressed to mc, I am not able to give much encouragement. If it does occur it will be in some area where older rocks than any I saw are superficial, or where such rocks form high and steep ranges, as on the west coast of the South Island, where the streams have been enabled by their rapid fall to shed the auriferous drifts on the surface of the marine tertiary rocks. I do not know enough of the interior of the northern districts of the province to say whether such conditions exist, but the analogy to the western district of the Nelson province enables mc to say that were the thick coating of the tertiary and volcanic rocks removed, the underlying paloeozoic rocks would be more likely to prove auriferous than any of the slate ranges of the eastern part of the North Island. The North Island must always be considered to be at the present epoch, as compared with the South depressed, probably to the extent of several thousand feet, the level of the land being brought up above the sea by formations almost wholly wanting in the South Island, so that the chance of finding drift deposits formed of the detritus of the auriferous rocks is not great. I had not an opportunity of visiting Mount Egmont, but in February last my assistant, Mr Buchanan, visited it principally with a view of examining its botany, but he also collected information respecting its geological features in the course of the ascent of this most striking volcanic cone. From his notes and specimens I gather that the surface of the cone is deeply grooved or fluted in radiating lines, and that in following up one of these grooves the party encouutered great difficulties in the ascent owing to the loose debris. This groove was confined by precipitous cliffs 150 feet in height, which entirely prevented any ascent on to the crest of the spurs, so that if any vegetation grows on these rocky ridges it must escape notice. He describes the sections afforded in these cliffs as showing in the lower part of the cone a parallel disposition of lava floes, coinciding with the slope of the mountain, which is about 37deg. to the horizon. The specimens .of these parallel floes prove to be a close-grained trachyte, speckled with a few minute crystals of hornblende or augite. Above these, in his particular line of ascent, for more than 1000 feet, the rock is massive grey tra-

chyte- or greenstone, which he describes as several hundred feet thick, and having a tabular structure with numerous horizontal rents. This rock is continually falling away in large masses, which, thundering down the side of the cone, are rather alarming to travellers, especially when, as is often the ease, they become enveloped in dense fogs. The specimens of rock from the summit are porous trachyte, of a yellow and reddish colour, but withou: any approach to the structure of pumice. Tbe rocks of the outer range appear to be similar to those at the Sugar Loaves, which are compact trachytes, rendered porphyritic by crystals of hornblende, sometimes mixed with glassy felspar, in the form of crystalline aggregations of large size. On the whole Mr Buchanan seems to have been disappointed with the results of his expedition as regards the number and variety of the forms of vegetation; but before it is concluded - that Mount Egmont is much less interesting than had been anticipated, it will be necessary to make a thorough examination of the south-east or shady side of the cone also, as it is there that the conditions more favourable to plant life may be expected to occur. The account of Mr Buchanan's trip, and the botanical results be obtained, will be found in a future part of the reports.