TARANAKI OILFIELDS.

Stratford Evening Post, Volume XXXV, Issue 98, 2 May 1913, Page 2

TARANAKI OILFIELDS.

The following interesting article was specially written for the Shipping and Commerce Journal by Mr Augustus E. Wtkins, Assoc. M. Aus., I.M.E.

SITUATION

The Taranaki oilfield is situated on the western side of the North Island of New Zealand at about its widest part, and may be said to be the most compact and fertile portion of the Dominion, for, with the exception of the upper half of Mount Egmont and of the uanges adjoining (Egmont National Park), which absorb 36.000 acres, the whole of the area is suitable for settlement, and certainly two : thirds is good land. The gross are of the district is 2,417,299 acres. The principle rivers are the Waitara, Mokau, Patea, Wanganui and Waiwakaiho. The oilfield is in close proximity to the town of New Plymouth, the principal town of the district.

TOPOGRAGPHY. The most striking physical feature in Taranaki is the splendid symmetrical cone of Mount Egmont (8260 feet), the crest of which is perpetually snow-clad. To the west the cone is flanked by a short range of mammillated hills rising to a height of 4387 feet, and known as the Pouakai Ranges. To the north and west of thesp rise, to a height of 2240 feet, the bushcovered hills known as the Patua or Kaitoko Ranges. North of the ranges the land slopes gently to the sea northward and westward, rising towards the eastward perceptibly to near and beyond the Waitara River. The coast-line for many miles is flanked by low cliffs, marking the denudation of the plain by the sea. A few miles west of New Plymouth the volcanic hills, both on the mainland and on the islands in the sea—known as the Sugar Loaves—form part of an old-dissected volcano of earlier age than Mount Egmont.

THE OILFIELD. The description of an oilfield now-a-days usually resolves itself into a mass of figures and statistics, of great interest and value to the engineer and commercial man, but practically indigestible to the general public. In an oilfield, such as Taranaki, which has never yet been fairly or adequately tested in any part (except, perhaps, at Moturoa), the interest lies more in the working out of the scientific problems it presents, in elucidating the answers to such questions as—Why ife there any oil? How was it fbund? Where may it most easily and profitably be obtained? This is where we have to 1 seek'the aid of the geologist. A complete ‘examination 'of the tertiary strata, so 'well exposed eastward and northwards from Moturoa to the Mokau, gives the observer a very fair idea of what may be expected in the drill-holes. Overlying the 'M6k.au coalbeds (the south-westerly extension of which by coming in contact with the volcaliic heat or mere* increas'd of heat towards the earth’s interior; triay have been the source of: the petroleum), are thick beds of porous green sandstone. It is thought that, until those are cut by drilling, no very extensive pool of oil will be struck, unless, perchance, a large fissure rising from the great depths is reached •above. Consequently' it follows that the dvili-holes so far put down at Moturoa ore not sufficiently deep to meet the porous strata seen at Mokau. From what has been said concerning the structure and arrangements of the miocene rocks, it is evident that in all probability strata of tho same horizon as that reached at Moturoa will bo found near the surface in the Waitara Survey District and still nearer the surface further to the east. The writer believes it will be found that in the eastern part of this district greater earthmovements have taken place, which have forced tho solid rocks into more pronounced folds than in the western portion. It is worthy of note that nearly all the younger fields of the world have been found in miocene and other tertiary strata.

THE TERTIARY SERIES OF TARANAKI.

The greater part of the district of Taranaki is formed of tertiary rocks, which have accumulated to the thickness of 3500 to 4000 feet, and during the great interval of time which such accumulation of strata represents, the physical conditions were much the same as at the present day. Under the restless, though gradual and imperceptible movements to which the surface of the earth was subjected, there wore constant relative changes of level between land and sea, so that fine clays of marine origin were formed above the sand and gravel banks and swamp deposits. These alterations are preserved for us in the record of the rocks throughout part of the tertiary period. We find that all the area now being considered is underlain by the same strata, viz., claystones (locally called “papa”), sand stones, fine conglomerates and marls. This series must merely be classified as tertiary in age. The tertiary rocks lies generally almost flat or dip at low angles. They are thought to form part of a great monoclinal fold, dipping southerly and westerly from the Mokau country. Minor cross swells in the form of anticlines and synclines are in evidence in places, and are thought to bo of sufficient importance to markedly influence the position of subterranean reservoirs of petroleum. Government Geological Bulletin, No. 14, says;—“A careful plotting of the most reliable strikes and dips has failed to reveal the existence of any

persistent anticlines and synclines. The strata under consideration dip to all points of the compass, but, on | sTimrnarising, it is found that the I westerly dips outnumber the easter-

ly in the proportion, of three to one. The dips are always at low angles, usually less than 10deg., only ten or twelve instances of dips of 15deg. or more, and about the same number of dips at angles between lOdeg. and 15dcg. have been observed. hi. recently drilled wells in the Bell Block district about 10 miles from the oil-producing wells at Moturoa, a decided anticline nipping about lodeg. N.fi. and S.)V. is found, and the line of strike is north-west and south-east. For the formation and natural storage of petroleum, these essential conditions must be present. There must he a quantity of material from which the on can be formed by chemical processes; there must be strata sufficiently porous to contain the oil; and there must be. impervious strata above to seal up the natural laboratory m widen these little-known chemical processes take place, ;and to prevent the escape of gas or volatile material. All these conditions are found in the tertiary strata of the Taranaki disu'Ct.-i as above outlined.