Oligocene Sedimentary Rocks Research Articles

Global sea level changes or local tectonics? Pliocene, Miocene and Oligocene biomarkers in cored sedimentary rocks from IODP Expedition 317, Canterbury Basin, New Zealand

The various influences of global sea level (eustasy) and local tectonic changes on sedimentation processes in continental margin deposits are important topics in sedimentary research. International Ocean Discovery Program Expedition 317 to the Canterbury Basin, on the eastern margin of the South Island of New Zealand, provided an opportunity to study sediment geochemistry in contrasting depositional settings, from mid-shelf to upper continental slope sedimentary rocks from the Eocene to the Holocene. This paper provides the first examination of the organic geochemical record from Pliocene, Miocene, and Oligocene sediment samples recovered during the expedition from the U1351, U1352, and U1353 sites, using bulk geochemistry and hydrocarbon distributions, including biomarkers. The main aim of this research was to correlate changes in organic accumulation with local tectonic activity and global climate changes that occurred in the Miocene. There is good preservation of C11 to C35n-alkanes, with varying predominance of odd-over-even chain length n-alkanes, and much variation in the terrigenous/aquatic ratio over the cores. Based on biomarker and Rock Eval parameters, thermal maturity varies from low in the Pliocene, to early oil window in the deeper Miocene and Oligocene sedimentary rocks. The pristane/phytane ratios for the three cores indicate anoxic to oxic depositional environments. Marine organic matter input is indicated by the high C30 sterane index, whilst high relative amounts of oleanane, C24 tetracyclic terpane, and C29 steranes indicates major terrigenous organic matter input. Local tectonic activity is suggested to have had a significant influence on the accumulation of the organic matter in the Canterbury Basin. The influence of eustasy and global sea temperature variations are suggested for the periods < 14 Ma, 12–7 Ma, and ∼ 6 Ma. An increase in terrigenous organic matter input during the late Miocene correlates with the uplift of the Southern Alps and an increase in the continental slope angle for the South Island of New Zealand.

Grain size analysis of the Oligocene Nari Formation sandstone in the Laki Range, southern Indus Basin, Pakistan: Implications for depositional setting

This study investigates the depositional setting of the Oligocene sedimentary rocks of the southern Indus Basin according to grain size analysis of sandstones. Twelve sandstone samples were collected and studied from the Oligocene Nari Formation that distributes along the N‐S‐trending Laki Range in the Kirthar Province of Pakistan. The analysis results specify very fine to fine‐grained sediments and unimodal nature of grain size distribution. The sorting values of the studied sandstones vary from 0.66φ to 0.90φ, which indicates that the sediments are moderately to moderately well‐sorted. The skewness values of investigated samples vary from −0.04φ to 0.25φ, which indicate nearly symmetrical to fine‐skewed nature. The kurtosis of the studied samples varies between 0.73φ and 1.44φ which shows platykurtic, mesokurtic, and leptokurtic nature. Linear discriminant function and interpretation diagrams of sediments demonstrate that the Oligocene Nari Formation sediments from Lal Bagh section were being transported as bedload and suspended load with a bimodal to unimodal depositional mode that were deposited in a fluvial to shallow marine deltaic environment of a passive margin.

A rapid shift in the sediment routing system of Lower-Upper Oligocene strata in the Qiongdongnnan Basin (Xisha Trough), Northwest South China Sea

Abstract The Oligocene is a period of continental breakup in the South China Sea and of earth surface topographic change in the southeastern margin of the Tibetan Plateau. In previous studies, Oligocene sediment from the Qiongdongnan Basin (QDNB) in the NW South China Sea has seldom been recovered, which limited understanding of basin evolution at this time. Here, we apply detrital zircon U-Pb dating to Oligocene sedimentary rocks from the QDNB. The U-Pb age populations in Lower Oligocene highlight the distinctive signatures of several competing sources providing material from nearby structural highs within the basin and Hainan island. In contrast, detrital zircon U-Pb ages of the Upper Oligocene in the central QDNB shows similarities with the Red River and/or coastal rivers in northern Vietnam. These sediments are significantly different from the Lower Oligocene. This abrupt change in the QDNB may imply that its distal sedimentary sources were initially established in the Late Oligocene. Our reconstruction of sediment discharge from proximal to distal sources in the Oligocene improves understanding of the sediment distribution in the QDNB, which will implicate the river drainage evolution and benefit the petroleum exploration of the Northwest South China Sea.

Description of a new toothed mysticete from the Late Oligocene of San Juan de La Costa, B.C.S., México

This paper describes a toothed mysticete that belongs to a basal family found in Oligocene sedimentary rocks deposited in the North Pacific Ocean. The material that is described here belongs in the Chattian stage of the Oligocene, and it was collected near Arroyo El Saladito from the San Juan Member in the El Cien Formation, Baja California Sur, México. The sedimentary environment of this area has been interpreted to be a continental platform with nutrient-rich waters combined with anoxic conditions. Therefore, the specimens that have been found in the phosphorite layers lived in an environment of shallow coastal waters. The material described is a dentary fragment, a skull and fragments of the teeth. The new taxon is located within the Superfamily Aetiocetoidea due to the presence of functional teeth, which have a slightly globose shape with eight slightly triangular apices. Niparajacetus palmadentis presents a morphology that contrasts with other toothed whales. For example, it shows an elongated and wide skull in the posterior region with a triangular shape, and its estimated total length is 3.2–3.5 m.

Age and provenance of detrital zircons from the Oligocene formations of the Marseille–Aubagne basins (SE France): consequences on the geodynamic and palaeogeographic evolution of the northern Gondwana margin

Eight samples from Oligocene sedimentary rocks of the Marseille–Aubagne basins have been analysed for their detrital zircon age spectra. These age spectra provide information about the regional evolution, from Oligocene to Archaean times. The Carboniferous Variscan and the Late Cretaceous to Eocene Pyreneo-Provencal belts represent the latest main tectonic, magmatic, and volcanic events that formed the major zircon age populations found in studied sediments. The obtained detrital zircon age record of the Marseille–Aubagne basins comprises eleven detrital zircon age clusters. They reflect the long and complex geologic history of the sediments source areas and can be ascribed to the opening of the western Mediterranean, the Variscan, Cadomian and Pan-African belts, to an unknown Mesoproterozoic event, to the Eburnean orogeny of West Africa and to the different tectono-metamorphic events that took place in Archaean times. In general, the Palaeo- and Mesozoic events are ascribed to the dispersal of Western and Eastern Gondwana and the Pangaean supercontinent cycle. Thus, the successive recycling of zircon grains from older and the incorporation of them to younger belts lead to new geodynamical models for the northern Gondwana margin evolution. Significant amounts of Mesoproterozoic detrital zircon are at odds with previous hypotheses and re-open the question of the provenance of these zircon age populations. Therefore, this tiny tertiary basin is a natural archive which records the main geological events in SE France and its vicinity.

Constraining provenance, thickness and erosion of nappes using low‐temperature thermochronology: the Northland Allochthon, New Zealand

AbstractThe Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto the in situ Mesozoic and early Cenozoic rocks (predominantly Late Eocene–earliest Miocene) in northwestern New Zealand. Using low‐temperature thermochronology, we investigate the sedimentary provenance, burial and erosion histories of the rocks from both the hanging and footwalls of the allochthon. In central Northland (Parua Bay), both the overlying allochthon and underlying Early Miocene autochthon yield detrital zircon and partially reset apatite fission‐track ages that were sourced from the local Jurassic terrane and perhaps Late Cretaceous volcanics; the autochthon contains, additionally, material sourced from Oligocene volcanics. Thermal history modelling indicates that the lower part of the allochthon together with the autochthon was heated to ca. 55–100°C during the Late Oligocene and Early Miocene, most likely due to the burial beneath the overlying nappe sequences. From the Mesozoic basement exposed in eastern Northland, we obtained zircon fission‐track ages tightly bracketed between 153 and 149 Ma; the apatite fission‐track ages on the other hand, generally young towards the northwest, from 129 to 20.9 Ma. Basement thermochronological ages are inverted to simulate the emplacement and later erosion of the Northland Allochthon, using a thermo‐kinematic model coupled with an inversion algorithm. The results suggest that during the Late Oligocene, the nappes in eastern Northland ranged from ca. 4–6‐km thick in the north to zero in the Auckland region (over a distance &gt;200 km). Following the allochthon emplacement, eastern Northland was uplifted and unroofed during the Early Miocene for a period of ca. 1–6 Myr at the rate of 0.1–0.8 km/Myr, leading to rapid erosion of the nappes. Since Middle Miocene, the basement uplift ceased and the erosion of the nappes and the region as a whole slowed down (ca. 0–0.2 km/Myr), implying a decay in the tectonic activity in this region.

THE SMALL MAMMALS (INSECTIVORES, BATS AND RODENTS) FROM THE HOLOCENE ARCHAEOLOGICAL SITE OF VALLONE INFERNO (SCILLATO, LOWER IMERA VALLEY, NORTHWESTERN SICILY)

The Vallone Inferno rock-shelter is an archeological site located at 770 m a.s.l. in the Madonie massif in Sicily. This massif is modeled into the Triassic and Oligocene sedimentary rocks of the Imerese Basin. Thearchaeological excavations conducted since 2008 have provided a long prehistoric and historic sequence from the Neolithic to the medieval period. From the four sedimentary complexes identified, only levels 3.4 to 3.1 from complex 3 and 4.2 from complex 4 have yielded small-mammal material. Level 4.2 is poor in remains and as yet without cultural ascription, though it has a radiocarbon age of 9450±50 years BP. Level 3.4 has yielded fragments of ceramic characteristic of the Middle Neolithic-Bronze Age period, with a radiocarbon age between 3948±35 and3244±42 years BP. Levels 3.3 to 3.1 have provided ceramic fragments ascribed to the Late Roman-Byzantine period, with a radiocarbon age between1332±26 and 1260±34 years BP.The small-mammal assemblages recovered from the sieving-washing of all the sediment from the excavation campaigns include a total of at least 14 taxa (three insectivores, four chiropterans and seven rodents). The materials from this locality provide the first mention in the fossil record of Sicily for Suncus etruscus , Muscardinus avellanarius , Eliomys quercinus and Rattus norvegicus , as well as the last occurrence for Arvicola amphibius . The scarcity of the remains recovered from stratigraphic levels 3.1, 3.2, 3.3 and 4.2 makes it difficult to undertake a palaeoenvironmental and palaeoclimatical interpretation of them. However, the richness in small mammal contents from level 3.4 allow us to show that this level is dominated by dry meadows and woodland areas with a temperature and precipitation range that lies within the current values for the surrounding area, coinciding with the dry and temperate phase detected previously by marine surveys and pollen and microcharcoal studies.

Source variability of sediments in the Shihmen Reservoir, Northern Taiwan: Sr isotopic evidence

High turbidity in water reservoirs during the monsoon season has caused serious problems for drinking water quality in Taiwan. In this study, we collected stream waters and streambed sediments, including bed loads and fresh landslide deposits, in the Dahan Stream and analyzed 101 87Sr/ 86Sr ratios in different grain-size fractions in the streambed sediments from the upper catchment and 3 sediment cores from the Shihmen Reservoir, as well as dissolved major and trace elements in stream waters, to identify possible changes of sediment sources in the reservoir. The 87Sr/ 86Sr ratios in stream water and streambed sediments ranged from 0.71296–0.71349 and 0.71714–0.71843, respectively. The large Sr isotopic difference between the paired stream water and streambed sediments are evidence for disequilibrium water/rock interaction in the weathering limited regions. The Sr isotopic compositions in bulk sediments and various grain-size fractions showed no clear spatial variability. However, there is an interesting relationship between grain sizes and Sr isotope ratios, implying effects of mineral sorting during sediment transportation. The illite crystallinity and the illite chemistry index of streambed sediment suggest moderate to strong chemical weathering in this regions. The dissolved constituents in the Dahan stream support that silicate weathering is the predominated controlled mechanism and only minor carbonate dissolution occurred near the Central Mountain Range. The 87Sr/ 86Sr ratios in the sediment cores suggest that modern reservoir sediment is mainly derived from the upper watershed, composed of primarily Oligocene sedimentary rocks. However, the sediment sources have changed significantly since the reservoir built 37 years ago, the 87Sr/ 86Sr ratios were spread widely outside the present-day observations in the Dahan catchment. The homogeneous distribution of 87Sr/ 86Sr in the upper reservoir cores reflected disturbance due to recent turbidity events within the reservoir. These new chemical and isotopic data provide useful spatial and temporal information of weathering sources in a high denudation sub-tropical mountainous watershed.

Geology, Geochemistry and Chromite Mineralization Potential of the Amnay Ophiolitic Complex, Mindoro, Philippines

AbstractThe Amnay Ophiolitic Complex in Mindoro, the Philippines, is considered an emplaced Cenozoic South China Sea oceanic lithosphere as a result of the collision between the Palawan microcontinental block and the Philippine mobile belt. Middle Oligocene sedimentary rocks intercalated with dominantly MORB‐like pillow lavas and volcanic flows suggest the generation of this ophiolite complex in an intermediate spreading ridge within a back‐arc basin setting. The volcanic rock suite geochemistry also manifests a slab component suggesting that it is a supra‐subduction zone ophiolite. Petrography of the gabbros shows a plagioclase‐clinopyroxene crystallization order consistent with a back‐arc basin setting. Spinel and pyroxene geochemistry shows that the lherzolites and aluminous‐spinel harzburgites are products of low degrees of partial melting. The chromitites hosted by the harzburgites could have not been associated with the MORB‐like volcanic suites, gabbros, lherzolites and aluminous‐spinel harzburgites. The chromitites are products of mantle sources that have undergone higher degrees of partial melting that would have involved the presence of water. The study of this ophiolitic complex gives us a glimpse of the characteristics of the South China Sea.

Provenance and palaeogeographic implications of Eocene–Oligocene sedimentary rocks in the northwestern Basin and Range

A thick sequence of uppermost Eocene to lower Oligocene volcaniclastic and sedimentary rocks is exposed at the base of the Warner Range in northeastern California. This isolated exposure provides insight into the palaeogeographic setting of the northwestern Basin and Range during this time period. Significant thinning of the unit over 35 km of lateral exposure and predominantly volcanic clast compositions suggest that the sequence was deposited in an alluvial plain adjacent to a volcanic arc. Palaeocurrent indicators in the conglomerates define a NNE transport direction. Detrital zircon analysis on coarse sandstones and dating of individual granite cobbles show a range of ages consistent with a local, volcanic source area primarily from the SSW with some far-travelled input from northern Nevada; the far-travelled component increases in influence as the unit thins to the north. Comparison with other sedimentary sequences of Eocene age and integration with palaeofloral and geophysical data help to define drainage divides, and suggest that this sequence accumulated in a relatively isolated, intra-arc basin. This localized accumulation differs markedly from contemporaneous drainages to the south that transported material westwards from central Nevada to the palaeoshoreline, and suggests that ongoing volcanism had a strong influence on palaeogeography in this region during the Eocene and Oligocene.

Geochemistry of the Mirador Formation (Late Eocene-Early Oligocene), southwestern Venezuela: Chemostratigraphic constraints on provenance and the influence of the sea level

A geochemical study was carried out on Eocene-Lower Oligocene sedimentary rocks (the Mirador Formation) from the southern Lake Maracaibo Basin, western Venezuela. The goal of this work was the identification of source rock composition and tectonic settings of source areas, and their temporal changes. The geochemistry was also used to investigate attributes of the paleoenvironment. Eighty-nine rock samples were collected and the bulk inorganic geochemistry (Al2O3, SiO2, TiO2, Fe2O3, MnO, CaO, MgO, K2O, Li, B, Sc, V, Cr, Ni, Zn, Rb, Y, Mo, La, Ce and Th) was determined by ICP-AES; total sulfur was determined in a LECO SC-432 analyzer. Multivariate statistical tools were employed to evaluate the correlation between the different variables. Through cluster-constrained analysis, four subdivisions, or main chemofacies, were defined and correlated with the significant lithologic units. The tectonic setting for the source rock could be a passive margin, and a mixed source was inferred, consisting of recycled sedimentary rocks and igneous and/or meta-igneous rocks of predominantly felsic composition. The most marine-influenced lithological units occur at the bottom of the section.

Differential rotations of north Kyushu Island related to middle Miocene clockwise rotation of SW Japan

Paleomagnetic measurements were made on samples of Cretaceous granites and Tertiary sedimentary rocks from north Kyushu Island in order to clarify the kinematics of clockwise (CW) rotation of the SW Japan tectonic block at 15 Ma. A paleomagnetic direction from Oligocene sedimentary rocks in the northern part of north Kyushu Island indicates a significant CW rotation of the region (R=35.9°±11.0°) relative to eastern Asia, while the direction from Oligocene to early Miocene sedimentary rocks in the western part of north Kyushu Island shows no significant tectonic displacement. In situ directions of secondary magnetization from Eocene sedimentary rocks in the southwestern part of north Kyushu Island, and stable remanences from Cretaceous granites, provide CW deflections of declination. These paleomagnetic data, combined with those previously reported, suggest that the main part of north Kyushu Island (Main Kyushu) was rotated CW (36°) between 30 and 14 Ma relative to eastern Asia, whereas no significant tectonic displacement has taken place since 20 Ma in the western part (Western Kyushu). Considering regional geological structures, the paleomagnetic results indicate that the western extent of the SW Japan tectonic block which experienced middle Miocene CW rotation is truncated west of Main Kyushu. The paleomagnetic data also suggest that Main Kyushu rotated counterclockwise (28° ± 13°) relative to the main part of SW Japan between 30 and 14 Ma. This result implies that the westernmost part of the SW Japan block (Main Kyushu) did not experience as much rotation as its main part during CW rotation of the whole block.

Bicadinanes and other terrestrial terpenoids in immature Oligocene sedimentary rocks and a related oil from the Surma Basin, N.E. Bangladesh

The aliphatic fractions and one aromatic fraction of outcropping Oligocene rocks from the NE Surma Basin of Bangladesh were studied by GC-MS and compared with their subsurface equivalents and one oil (Sylhet) from boreholes further south in the basin. Terrestrial biomarkers frequently encountered in these rocks whose biomass is dominantly woody include the triterpanes oleanane and bicadinane, several tetracyclic terpanes and the sesquiterpenoids cadinane and cadalene. The bicadinanes and their aromatized counterparts are found as major triterpenoid components only in the Upper Jenam Formation where the richest horizons are interpreted to contain locally-derived concentrations of dammar-type resin excluded from Dipterocarpaceae or similar tropical angiosperm trees. Vitrinite reflectance and geochemical maturity parameters indicate pre-oil window maturity for the Upper Jenam outcrop, allowing a temperature of less than about 90°C to be estimated for the formation of bicadinanes from resinous polymer. Cis-cis-trans-bicadinane, which is abundant in Sylhet oil, was not positively identified in any of the rock extracts, although the distribution of bicadinanes is otherwise very similar.

First paleomagnetic evidence for rotation of the Ionian Zone of Albania

A paleomagnetic study of 170 samples obtained from 15 sites from Cretaceous, Eocene and Oligocene sedimentary rocks documents a ∼45° clockwise rotation of the Albanian Ionian zone. This rotation is certainly post‐Eocene and most probably post‐Oligocene. These results show that the entire Ionian zone has undergone a semi rigid rotation about a pole of rotation situated farther north than previously believed.

Poway Fan and Submarine Cone and Rifting of the Inner Southern California Borderland

Eocene and Oligocene sedimentary rocks of the northern Channel Islands, San Nicolas Island, Santa Monica Mountains, Simi Hills, and northwest Peninsular Ranges of southern California include conglomerate beds characterized by the distinctive Poway clast suite of gray and red siliceous tuffs. The beds containing Poway clasts in the offshore islands and the Santa Monica Mountains are separated from beds containing similar clasts in the Peninsular Ranges by blueschist and greenschist of Santa Catalina Island and the Los Angeles basin; these schists are basement rocks unrepresented in the clasts. The offshore islands taken together and the Santa Monica Mountains–Simi Hills are interpreted as being two separate blocks or small plates which broke away from the Peninsular Ranges ∼16 m.y. ago. These blocks originally comprised the upper plate of a large thrust fault (the southern California subduction zone of Hill, 1971) over blueschist and greenschist. Boundaries of these blocks are identified in part by the narrow belt of Miocene San Onofre Breccia along their edges, in part by basement contrasts in wells in the Los Angeles basin, and in part by the 400-fm isobath on tne western side of Santa Cruz basin. The island block drifted relatively westward for 3 m.y. during Miocene time at a rate of ∼5.3 cm per yr, thereby opening up the inner basins of the southern California borderland. If a palinspastic translation of the Santa Monica Mountains 90 km east is combined with a 155- to 160-km eastward translation of the island plate, the paleocurrent indicators of the Poway-bearing sequence radiate from a point east of the Santa Ana Mountains near Lake Elsinore. Northwest, west, and southwest from this point, the percentage of conglomerate decreases and, in the marine rocks, water depth at the time of deposition increases. The sedimentary body thus defined is called the Poway fan and submarine cone, analogous to the Mississippi delta and cone but now fragmented by Miocene rifting. The palinspastic restoration also places a north-trending lower Miocene shoreline in the Santa Monica Mountains on a line with a similar shoreline in the Santa Ana Mountains, and it places lower Miocene rocks of shallow-marine facies in the western Santa Monica Mountains adjacent to rocks of similar age and facies on Santa Cruz Island. Presumably, the drifting of these small blocks represents a brittle separation of Peninsular Ranges basement and attenuation of the underlying blueschist and greenschist accompanying east-west spreading of the underlying lithosphere. The east-west relative motion necessary to explain the distribution of these small Poway-bearing microplates suggests that the motion was entirely within the North American plate and not a consequence of the northwest-southeast relative motion between the Pacific and North American plates. The southern California borderland may have formed as an intracontinental interarc basin as the East Pacific Rise and the West Coast subduction zone were annihilated at the western edge of the continent, and the northern boundary of the borderland may represent an intracontinental transform structure analogous to the Garlock fault as postulated by Davis and Burchfiel (1973).

Inter-Discipline Approach to Paleoenvironmental Interpretations: ABSTRACT

In subsurface studies, unlike surface studies, specific environments of deposition can not be established reliably on the basis of sediment geometry because a sediment pattern can represent several environments. Clues from various disciplines can be integrated to identify regional and local environments--deltaic plain, open shelf, distal bar, longshore bar, lagoon, swamp, littoral zones, etc. The accuracy of interpretations is governed by the types of samples available, types of disciplines applied, spacing of control wells, and the experience and imagination of the interpreters. The confidence level ordinarily ranges from 80 to 95% in Tertiary and younger strata but is in few cases more than 85% in older sediments. Because paleoenvironments resulted from the interaction of climatic, physical, chemical, and biotic factors, each factor must be evaluated in part and in combination. The physical-chemical system is determined from the mineralogy, textures, sedimentary structures, and trace chemical gradients ascertained with petrologic, X-ray mineralogy, and geochemical methods. These methods are applicable to all environments. Palynology utilizes pollen and spores and other plant and animal micro-entities; their acid-resistant nature and abundance permit statistical applications to paleoecologic and stratigraphic problems. Evaluations principally are botanical with an environmental range of terrestrial to open-marine shelf. Fossil faunal assemblages chiefly are marine and contribute information abou water depths, salinity values, and turbidity. For each specific environment, one discipline End_Page 2320------------------------------ dominates the interpretation and other disciplines increase the confidence level. Interpretive procedures illustrated are: a regressive marine wedge in the Miocene sedimentary rocks of Terrebonne Parish, Louisiana; supralittoral, littoral, and sublittoral sequences in the Oligocene sedimentary rocks of Starr County, Texas; and a channel system in the Cretaceous lower Tuscaloosa sedimentary rocks of Amite County, Mississippi. End_of_Article - Last_Page 2321------------

Geological History and Petroleum Possibilities of the Philippines

The Philippine Archipelago comprises 10 major island areas and a great many small islands and islets in a triangular area between Formosa on the north and Borneo and Celebes, of Indonesia, on the south. The basement includes mainly volcanic and sedimentary rocks of unknown age, and probably of considerable time range, that have been intruded by ultrabasic to intermediate plutonic rocks as well as by younger volcanics. Fossiliferous Cretaceous, Eocene, and Oligocene sedimentary rocks are found, but it was not until Miocene and Pliocene time that recognizable geosynclinal conditions developed to produce the thick Upper Tertiary sequences now exposed as a result of the late Tertiary and Quaternary disturbances. Considerable geological mapping and exploratory drilling have be n done in areas of higher prospective value. However, the substantial differences between the intra-archipelagic environment and its resulting complex stratigraphy and geologic structure, which are in great contrast to the normally regular relations in intra-continental areas, have not been properly evaluated. Certain areas in the Philippines may possess reasonably attractive petroleum possibilities although exploratory efforts to date have been negative.