The principal evidence of an unconformity in the lower division of the Miocene period is the occurrence of the volcanic agglomerate and a rather abrupt steepening in the inclination of the strata below the Kanguinsa sandstone relative to the overlying beds. The conglomerate at this horizon is bedded conformably with the Bacau shale beneath it. The Bacau shale is very fine grained, bluish to brownish-black and indistinctly bedded. It is mainly a compact, indurated clay, but contains also thin sandy lenses. The beds immediately above and below it are carbonaceous, but the Bacau shale contains little carbon although it emits a strong odour of petroleum. The fossil shells in it are covered with a film of oil or grease and appear upon fresh exposure to be remarkably well preserved. Any attempt to remove them, however, reveals a tendency to crumble, similar to the effect produced upon limestone by calcination. Fresh surfaces in the shale itself appear to be greasy. The material weathers into concretionlike ellipsoidal pieces, which break down further through the splitting off of concentric layers into small fragments with conchoidal surfaces. The formation may be regarded as the upper 50 to 100 metres of the Vigo shale, since the two are not sharply distinct. areas. The Vigo shale is characteristically thin-bedded and the successive beds are varyingly more and less sandy. In spite of these evidences of changing conditions of deposition, the formation is extensive and uniform over large It grades insensibly into the Bacau shale above, while its base is not exposed within the area under discussion. Occasional strata appear to be identical in character with the material in the Bacau shale, but more generally the Vigo shale is sandy and yellow to brown in colour. Thin beds of sandstone occur in it and appear to be more abundant in the upper portion. An apparent thickness of at least 1,000 metres of Vigo shale is exposed in the limbs of the principal anticline in the field and the section does not include the base of the formation. Structure. Bondoc peninsula is anticlinal in structure but the arch is not accomplished in a single, simple fold. A principal, central anticline follows the axis of the peninsula and may be traced for a distance equal to one-half of the length of the oil field. Flanking the central anticline on either side is a minor fold marked by small anticlines that occur at intervals along a line roughly parallel to the axis of the main anticline. The central anticline and the line of anticlines along each coast are sharp flexures, while the intervening synclines are broad and shallow, so that, although the peninsula as a whole is an anticlinorium in structure, the greater proportion of the land mass is contained within synclines. The central anticline is inclined to the east and pitches to the south. The beds near the axis dip almost vertically in the eastern limb and at an angle of about 40° in the western limb. The eastern limbs of the anticlines along the eastern coast are similarly steep and are likewise more highly inclined than the western limbs, while along the western coast both limbs of the anticlines are flatter, with the western limbs steeper. In the northern part of the region erosion has cut away the crest well down into the. Vigo shale on the central anticline, while farther south only the upper beds of the Vigo formation are exposed; and at the extreme southern end of the peninsula, owing to the southerly plunge of the anticlinal axis, even these are intact. The minor anticlines are eroded less deeply than the central anticline. PETROLEUM. Occurrence.-Petroleum is indicated on Bondoc peninsula by seeps of gas and oil at widely separated points throughout the region. The seeps occur invariably near the crests of anticlinal folds and always at about the horizon of the Bacau shale. In several instances the oil oozes directly from the Bacau shale, but in at least one case it appears to come from the Vigo shale. Gas, unaccompanied by oil, is emitted at a number of places. No discolouration of the strata by the escaping petroleum can be detected, but an odour of light oil is noticeable in the neighbourhood of a petroleum seep. The wells which have been drilled obtained their oil from the Bacau shale. Physical properties of the petroleum.-The first analyses of Tayabas petroleum made by RICHMOND,' showed so large a proportion of the fraction distilling below 150° C. that suspicion was aroused as to the authenticity of the sample. Later examination of samples collected at one of the wells by the Division of Mines confirmed the first results and proved that the petroleum contained an unusually large percentage of light oils. Tayabas petroleum is light brown to wine-red in colour by transmitted light and blue by reflected light. It loses part of its lighter constituents readily upon exposure, and because of this fact, specific gravity determinations on different samples have shown variation. One sample tested as low as 0.805 in specific gravity while the heaviest recorded sample had a specific gravity of 0.845. A representative sample subjected to fractional distillation in the Division of Inorganic Chemistry, Bureau of Science, by Mr. E. R、 DOVEY, gave the following results: 1 Tayabas petroleum; Phil. Journ. of Sci., Vol. V. (1910), A, No. 1, p. 1. 3.6 per cent. The lighter fraction of this oil proved to be lævorotatory to the extent of 0.55°. RICHMOND found 30 per cent. of unsaturated hydrocarbons in the crude oil, 16 per cent. in the gasoline fraction and 24 per cent. in the kerosene fraction. The same authority reports 8.1 per cent. of paraffin in the crude oil. Origin of the petroleum.—There is some evidence that the Bacau shale is the source of the petroleum encountered in it. SMITH has identified numerous Globigerina in the shale and considers it probable that petroleum may have originated from the decomposition of the fleshy part of these organisms. The Globigerina are not present, however, in as great adundance as would seem to be demanded to account for even the limited quantity of petroleum in these beds, and it has been suggested that a lower horizon, probably in the unexposed portion of the Vigo shale, may be the real source of petroleum, some of which, having diffused upward, is now encountered in the Bacau shale. The strata below the Bacau shale, so far as they are exposed, show evidence of petroleum only rarely. However, an oil with the character of the Tayabas product might leave in the rocks little or no trace of its passage through them. The Bacau shale, for example, loses every indication of its petroliferous character after a short period of exposure to air. Hence it is possible that petroleum exists in the unexposed lower beds of the Vigo shale, from which small quantities have migrated along favourable channels to the surface, where the oil has been completely dissipated from all the rocks except those which, like the close grained Bacau shale, are suited to retain it most tenaciously. From the well-known principles of fractionation by diffusion through a porous medium, the light clarified nature of the oil might be taken to confirm the theory of migration. Economic considerations: possible development. The existence of petroleum on Bondoc peninsula is established, although the quantity which may be available is undetermined. Judging from the situation of oil seeps in Tayabas relative to the structure, and from experience in the productive field of Palembang, Sumatra, where the geologic conditions apparently are very similar to the conditions in the Tayabas field, the petroleum would be expected to have accumulated in the anticlines in accordance with the general anticlinal theory. If oil has tended to collect in the anticlines, then the structure in parts of the peninsula must be considered favourable (anticlines intact above the oil horizon) for the retention of the oil under conditions which would permit of commercial exploitation. If no petroleum is found other than the small quantity which occurs. in the Bacau shale, a large commercial production is not to be anticipated. However, it appears that wells in the Bacau shale could be made to yield small individual flows. This shale is petroliferous and probably the unbroken sandstone lenses which it contains are saturated with petroleum. The thinness of the sandstone beds indicates that their lateral extent is also limited. Consequently it should be possible to space wells closely without mutual interference. The area over which such wells could be located is large, and the aggregate possible production from the Bacau shale might be of commercial importance. If petroleum is encountered in the unexposed base of the Vigo shale, the chances of a large production are better. Under the circumstances, Bondoc peninsula must be classed as an unproven oil field where there is a chance of obtaining petroleum economically. Owing to the reluctance of adequate capital to engage in Philippine enterprises, it has been suggested that, on the chance of developing a valuable natural resource, the government undertake the initial exploration of the region. If this plan prevails, wells will be drilled upon public land by the Bureau of Public Works at sites to be determined by the Division of Mines, Bureau of Science. Contributions diverses: Paléontologie et physiographie. 1. PAUL BERTRAND, Étude du slope de l'Asteropteris noveboracensis (page 909). 2. PIERRE PRUVOST, La faune continentale du terrain houiller du nord de la France; son utilization stratigraphique (page 925) 3. A. E. DAY, The age of the Nubian sandstone (page 939). 4. CHARLES R. KEYES, Certain features of eolic gradation (page 941). |