Benjamin S. Austria Raymundo A. Reyes Jr.
Trans-Asia Oil & Mineral Development Corp.
Manila
The West Batangas basin is one of the few remaining untested basins on the Philippine side of the South China Sea.
It lies about 100 km southwest of Manila, off and straddling the west coast of Batangas province, Luzon island (Fig. 1).
About 60% of the basin offshore lies in water 200 m deep or less.
This promising frontier basin is probably part of the North Palawan block, which underlies the petroleum production area off Palawan about 350 km to the southwest.
Recent discoveries in this petroleum province include 1 West Linapacan, which is estimated to contain more than 100 million bbl of oil recoverable, 1 Camago, a major gas/condensate find with reserves of more than 1 tcf, the 1 Calauit oil discovery, and the 1 Octon condensate deposit.
The volume of recoverable hydrocarbons in the West Linapacan and Camago reservoirs demonstrates that sizable reserves can be discovered in the relatively underexplored North Palawan block, whose geologic evolution is closely linked with the rifting and development of the South China Sea.
Similar features and plays can be observed in the frontier area of West Batangas, which has a basin with as much as 9,000 m of Mesozoic and Cenozoic section and similar sandstone and carbonate reservoirs as the Northwest Palawan terrane.
EXPLORATION HISTORY
Initial exploration data were acquired by companies under nonexplosive geophysical permits granted by the Philippine government.
CDCP Mining, a Philippine company, acquired 84 line km of seismic profiles in 1979 as part of a large reconnaissance program along the western shelf of Luzon island and the east coast of Mindoro island.
ARCO shot a 375 line km reconnaissance survey in 1980 and followed up with a 179 line km semidetailed (2 km by 3 km grid) program the next year.
A group of local companies that included Trans-Asia as operator, Balabac, and Unioil was awarded a geophysical survey and exploration contract (GSEC 55) over the area in September 1989. The group recorded 161 line km of seismic data and reprocessed ARCO's 1980-81 lines.
The same consortium reapplied for and obtained in February 1992 a new geophysical contract (GSEC 62) over essentially the same area covered by GSEC 55. The group has committed to undertake a new seismic program intended to pursue potential drilling targets delineated under the old GSEC.
TECTONICS, STRUCTURE
The West Batangas basin is found within the active Philippine archipelagic zone, which is presently undergoing oblique compressive deformation between the Eurasian plate to the west and the Philippine Sea plate to the east (Fig. 1).
The convergence that has taken place between the two plates since early Cenozoic is thought to have caused amalgamation of allochthonous terranes onto the main north-south archipelago.1
These terranes include rifted continental block(s), magmatic arcs, accretionary wedges and oceanic crusts. The best documented evidence of the accreted continental fragments is the "North Palawan block,"2 which is believed to extend from Reed Bank to Palawan, Cuyo Shelf, and Mindoro.
This block (or possibly blocks) is widely regarded to have rifted from the South China continental margin in the early Tertiary and subsequently drifted in a southward-southeastward direction beginning in late Oligocene time until its collision with the main archipelago during Middle Miocene.2-3
Although the northern and northeastern boundaries of this terrane have not been established, recent magnetic and seismic data have delineated a branch of the Philippine fault system, which extends into the West Batangas basin.4
This structure may correspond to the northern boundary of the "North Palawan block."
The proximity of the depocenter to Mindoro raises the distinct possibility that the basin fill could have significant continental provenance. Consider the distribution of rocks with continental affinity in southern Luzon, Mindoro, Lubang, and adjacent areas (Fig. 2).
It appears that the major northwest-trending crystalline basement ridge in Mindoro island, which acts as the northern margin of the Southwest Mindoro basin, extends to the Lubang island group and forms the southern boundary of the West Batangas basin.
Fig. 3 is a simplified structural framework of the West Batangas basin. The paleo-basin boundaries to the north, west-northwest, and east are unconstrained by available data. An east-west striking ridge, which is expressed as a present-day bathymetric high, divides the basin into two subbasins.
Line PB 80-46 (Fig. 4), a regional dip profile, depicts a stable "basement" block to the south overlain by a mildly to moderately deformed probable Tertiary slope front fill in the southern subbasin.
The "basement" is correlatable to the Paleozoic-Mesozoic exposures in the Lubang island group and northern Mindoro.
The acoustic basement horizon in this area (late Mesozoic unconformity?) plunges to more than 5 sec TWT towards the north, indicating a sedimentary fill possibly more than 9,600 m thick.
Based on the trend of this horizon's contours and aeromagnetic data, the late Tertiary and Quaternary sedimentary and volcanic cover of Batangas to the east masks a thick sedimentary succession at depth.
A possible structural analog of the southern subbasin in the Northwest Palawan shelf is presented (Fig. 5). The southeast half of the seismic section shows a large rift-stage half-graben filled with mildly-deformed Paleogene and younger deposits formed over a stable basement interpreted to be of Mesozoic and older age.
Hydrocarbons found in the discovery well to the southeast must have been generated by source rocks occurring within the half-graben. The rift, drift onset, and collision unconformities recognized in this section could be correlated to major stratigraphic breaks mapped in Line PB 80-46.
The northern subbasin apparently strikes northeast and is likewise interpreted to contain sections more than 5 sec TWT thick.
Gross seismic character of acoustic reflections in this sector is markedly different from that in the southern depression. Reflections do not display the coherence and continuity found in the latter subbasin. The difference could be a function of contrasting geology.
The nature of the mid-basin ridge is uncertain. It could either represent a northward tilted Mesozoic-Paleozoic basement block related to the Lubang-Mindoro basement block related to the Lubang-Mindoro basement or the edge of an of an intermediate crust juxtaposed to the continental fragment comprising the southern subbasin.
In any case, the ridge appears to have had a complex geological history, probably starting as a paleohigh of moderate relief in the early stage of basin development, then later on subjected to pulses of uplift, erosion, subsidence, and intrusion.
The feature seems fault-bounded to the south with a few individual faults hinting oblique strike-slip movements. The boundary fault may form part of the major splay of the sinistral Philippine fault system, motion along which may have caused compressive deformation of the basin fill (particularly in the southern subbasin, and regional uplift.
This tectonic episode (Coeval with microplate clocking/collision?) is considered to be the main phase of basin deformation. Pliocene strata are observed to be generally flatlying to gently dipping in the surrounding onshore areas.
Seismic sequences assigned this age and younger in the offshore are mostly undeformed. Outcropping Middle Miocene and older sediments and their correlative seismic sequences offshore, on the other hand, are invariably folded, faulted, and-or tilted to varying degrees.
PREDICTED STRATIGRAPHY
In the absence of well information, the stratigraphy of the West Batangas basin is predicted based on regional correlation of surface geological information on the surrounding onshore areas, i.e., Mindoro island, Batangas, and the Lubang island group, and available seismic records.
The very thick basin fill indicated on seismic profiles suggests sedimentation for much of the Cenozoic and Mesozoic, at least comparable in extent to that in Southwest Mindoro, where more than 7,000 m of sedimentary strata have been measured.
Exposures in Northwest Mindoro, the Lubang island group, and Batangas represent incomplete successions mainly useful in prognosticating the upper basin fill.
The generalized stratigraphy of the basin has been synthesized from geological and geophysical data on hand (Fig. 6). Due to poor seismic data quality in the northern subbasin, reliability of the prognosis in that area might be poor.
PALEOZOIC [BASEMENT]
Low- and medium-grade metamorphics cropping out in the Lubang island group, Northwest Mindoro, and southern Batangas are correlatable based on identical lithologies, associated felsic intrusives, and inferred age.
The extension of the same or equivalent crystalline basement is projected to floor at least the southern part of the West Batangas basin.
JURASSIC
Jurassic rocks found extensively in Mindoro probably extend to the area of interest.
These strata may correspond on seismic sections to layered sequences below the main slope-front fill and directly overlying the Paleozoic crystalline basement.
Lithologies anticipated are principally shale, quartzose, and arkosic sandstone deposited in transitional marine to non-marine environment, and limestone.
EOCENE-L. OLIGOCENE
Rocks of Eocene to early Oligocene age are thought to represent the first phase of Tertiary sedimentation and comprise the main rift-valley fill.
Shelfal limestone, deeper marine shales, and quartzofeldspathic sandstones cropping out in Southwest and Northwest Mindoro are expected to be present in the subsurface of the target area.
Deeper water limestone facies may occur with associated clastics in submarine/turbidite fan complexes indicated on seismic profiles. Likewise, shelfal clastics may have been laid on paleo-highs and platforms at or near the basin and subbasin margins.
U. OLIGOCENE-M. MIOCENE
Turbidite sandstones, mudstones, and limestone equivalent to outcrops in Mindoro probably comprise the bulk of the interpreted "drift" sequence in the area.
Major carbonate trends with associated reefal buildups have been identified on seismic along the margins of the southern subbasin. These may be correlatable to the petroliferous Oligo-Miocene Nido limestone offshore Northwest Palawan.
Shallow marine sands are anticipated to occur in prograding shelves recognized in the west-central part of the southern subbasin.
U. MIOCENE-PLIOCENE
In surrounding onshore areas, this stratigraphic interval is characterized by deep marine conglomerates, sandstones, marls, and mudstones.
These deposits might be expected offshore although shelfal limestones and clastics may have been locally deposited over and around bathymetric highs and platforms.
Pliocene clastics deposited near the coast of Batangas are likely to be partly tuffaceous as in outcrops onshore.
PLIO-PLEISTOCENE
Tuffaceous clastics and limestone found onshore most probably occur in the offshore part of the basin.
PETROLEUM POTENTIAL
OIL, GAS OCCURRENCES
Oil or gas seeps have not been reported in the onshore areas comprising the basin margins.
However, in the Southwest Mindoro basin south of the common basement high three oil seeps and one gas seep have been documented.
The surface emanations occur along a northwest-southeast trend, the oils associated with carbonates of Eocene age and the gas with Eocene to Miocene (?) clastics (Fig. 2). All three oils are classified as paraffinic and light.
Oils in the "Mianao" and "Sibiawan" seeps are of 40-44 API gravity and show only slight biodegradation. Oil from the "Mianao" seep is further characterized as low in sulfur, metals, and asphaltics.
The composition of the oils suggests derivation from nonmarine organic matter.
The "Pandayan" gas seep consists of 95% methane. Carbon isotope ratios imply thermal rather than biogenic origin.
Four wells drilled in the southern part of Mindoro recorded oil and gas shows.
North of Busuanga Island, the 1 Calauit discovery flowed at the rate of 3,000 b/d of oil through a 1 1/2 in. choke, whereas the recently drilled followup well, 1 Calauit South, flowed 2,059 b/d of oil through a 1 in. choke (Fig. 2).
SOURCE ROCKS
In the absence of well control and lack of suitable samples in the immediate surrounding areas, the source potential of sediments in the contract area could not be conclusively evaluated.
The nearest available data were derived from samples collected in Southwest Mindoro. Eocene and Jurassic samples generally exhibited average organic carbon contents, whereas Oligocene and Miocene specimens were organically lean.
Pyrolysis data mostly indicated humic (Type III) kerogens, which are regarded as capable of generating dominantly gas at proper maturation levels. Although exceptions were noted in a few samples, none was considered to have significant oil source potential.
It is concluded that the precursors of the surface shows in Mindoro were not sampled. The lone gas seep, on the other hand, could not be correlated with the gas shows encountered in the 1 Progreso well in the absence of samples from said well.
On a regional basis, West Batangas basin being likely a part of or related to the continental terrane which drifted from the South China margin during the early Tertiary, holds promise in terms of source rock development.
Source rocks have been identified in the Eocene (regional source facies), Oligocene and Lower Miocene sections in five sedimentary basins in the northern part of the South China Sea.5
Potential source facies of these ages have likewise been recognized in the oil productive Northwest Pala wan shelf. These source rock suites may likewise occur in the area of interest.
Jurassic source rocks may also be present inasmuch as in the 1 Mindoro well, minor oil shows were recorded in sandstones of probably Jurassic age.
MATURATION
Based on spore coloration indices, Eocene and Jurassic field samples in Mindoro are considered thermally mature for oil generation.
Miocene samples are classified as immature to marginally mature.
These findings could give an indication as to the maturation levels of sediments in the West Batangas basin, although the sedimentary fill in the target area is apparently thicker on seismic than that in Mindoro.
The geothermal gradients of three wells drilled in southern Mindoro are 1 Progreso 2.90 C./100 m, 1 Mindoro 2.81 C./100 m, and 1 Semirara 3.08C./100 m. The average is 3.08 C./100 m or 1.61 F./100 ft.
In view of the probable tectonic affinity of the contract area with southern Mindoro, broadly similar geothermal gradients could be reasonably assumed.
Accordingly, an oil generation threshold of 115 C. (Miocene-Pliocene of Los Angeles basin) may be attained in the area of interest by source burial depths of about 3,000 m.
This depth, corresponding to the "conservative" top of the principal zone of oil formation for a Paleogene source horizon, must have been achieved in the depo-centers of the basin which certainly have more than 6,000 m and possibly as much as 9,000 m of sedimentary section.
RESERVOIRS, SEALS JURASSIC
The Jurassic formation as exposed in southern Mindoro is approximately 5,000 m thick and consists mainly of arkosic and quartzose sandstone alternating with shale.
Gross sandstone thicknesses ranging from 700-1,200 m have been measured.6 The lower part of the formation displays thick, massive or poorly bedded sand units that have well consolidated and poorly consolidated varieties.
Interbeds of conglomerate, fossiliferous limestone and tuff occur occasionally within the Jurassic formation. A massive, partly re crystallized limestone with a thickness of about 400 m crops out in Pagbahan River in south-central Mindoro.
In view of their extent and vertical development, Jurassic sandstones and carbonates of reservoir quality might occur in the area of interest. Intercalated shale beds of the same age and mudstones of Eocene to early Oligocene age could provide adequate seals.
EOCENE-L. OLIGOCENE
Three documented oil seeps emanate from carbonates of Eocene age, probably in association with fractures caused by faulting.
Thin sections of these limestones show fair primary and secondary porosity. Though no reefal buildups of this age have been identified in onshore Mindoro, such bioherms could not be discounted in the West Batangas basin where suitable paleogeographic and paleo-ecological conditions permit.
Eocene quartzofeldspathic sands are likewise considered potential reservoirs in the basin. Such sands may be associated with submarine fan complexes at the foot of the slope or with transgressive deposits localized on paleo-reliefs and platforms.
In Mindoro Island, porosities of 7-20% and permeabilities of 0.06-225 md from field samples of these lithologies have been obtained.
Suitable cap rock lithologies are present in the Eocene to Lower Oligocene formation as well as in the overlying Upper Oligocene fine clastics.
U. OLIGOCENE-L. MIOCENE
Oligo-Miocene carbonates in onshore Mindoro indicate deposition at shelfal depths.
As in the Eocene limestone, fair primary and secondary porosities could be observed petrographically. Lithotypes are mostly grainstone and packstone deposited under moderate energy condition in which carbonate mud is either subordinate to allochems, or absent.7
These carbonates, especially in conjunction with fracturing, may hence be regarded as potential reservoirs in the area of interest. Reefal facies (not found onshore) that are interpreted in the target area provide additional reservoir possibilities.
Lower Miocene reefs (Nido, Cadlao, Camago, and others) and fractured, deeper marine carbonates (West Linapacan, Calauit) are among the main productive reservoirs in the Northwest Palawan shelf.
Facies equivalents of Upper Oligocene arkosic sands encountered in 1 Progreso are likewise possible reservoirs in West Batangas basin. Fine clastics of Miocene age could act as cap rocks.
MIOCENE
Miocene sandstones are regarded by exploration companies as the primary reservoir objective in the Mindoro area.
These sands exhibit porosities between 9-22% and permeabilities between 0.09-1,070 md in outcrop.
Miocene nearshore sands exposed in Semirara Island off the southern tip of Mindoro are mature, well sorted, and consist predominantly of quartzite grains.
Similar transitional and shallow marine sands may have developed in the central and western sectors of the southern subbasin, where a major, east-facing prograding shelf is indicated on seismic.
Deeper water equivalents found in Mongpong River and Sablayan area in Southwest Mindoro, though containing substantial quartz and chert grains (20-80%), are thin, fine grained, moderate to poorly sorted, and are texturally immature.
These clastics were probably deposited in the mud-dominated distal turbidite. domain.
Better reservoir characteristics are expected in channel sands and mid-fan lobes of submarine fan complexes or turbidite sand sheets which are interpreted to occur in the south and south-central parts of the southern subbasin.
The prospective depositional facies is probably represented in Maricaban Island off the southern coast of Batangas province, where a 5,000 m thick sedimentary series of upper Lower Miocene to lowermost Upper Miocene age is exposed.8
The series consists of an interbedded sand, shale, conglomerate sequence with a sand-shale ratio of 3:1.9
Individual sandstone beds are commonly tens of centimeters to a few meters thick. Sandstones are fine- to medium-grained and are predominantly arkosic to lithic in composition. Sorting is moderate to very good.
It should be noted that Lower Miocene turbidite sands act as reservoirs in the Galoc and Octon discovery wells in the Northwest Palawan shelf. In the same area, the 1 Esperanza well flowed 7 MMcfd of gas from deep marine Middle Miocene sands.
Intraformational Miocene shales and fine clastics of Late Miocene age could provide seal units.
PLIOCENE
Blanket deposits of deep marine marls and mudstone comprise the regional seal. Deposits near Batangas are probably tuffaceous.
TRAPS
Potential traps mapped in the area consist of a variety of structural and stratigraphic types.
Structural traps include anticlines of gentle and moderate relief. In the southeast sector of the basin close to the shore of Batangas, a major compressional fold may give rise to vertically-stacked reservoirs.
Deep-seated anticlines and thrust folds related to older deformation are likewise present in the west-central part of the southern subbasin.
Stacked submarine fan complexes have likewise developed near the foot of the steep basement slope in the south. A large submarine fan mound is found in the northern subbasin. A northwest trending sinuous submarine fan channel more than 10 km long has been delineated in the southern subbasin.
At least three reefal carbonate trends have been identified in the southern subbasin. A major prograding shelf in the west-central part of the southern subbasin is interpreted to have brought about the development of shelf sand bodies and a delta fan complex.
Other potential stratigraphic traps include wedgeouts, angular unconformity, and buried hills.
CONCLUSIONS
- The West Batangas basin contains a very thick sedimentary fill exceeding 5 sec TWT in the depocenter, equivalent to possibly as much as 9,000 m of Mesozoic and Cenozoic section.
- The area exhibits structural and stratigraphic affinity with the petroleum-productive North Palwan block continental terrane.
- Potential sandstone and carbonate reservoirs are expected to occur in the Mesozoic and Tertiary.
- Hydrocarbon occurrences are present and potential source rocks inferred in the adjacent basin which belongs to the same or related continental terrane.
- Thermal maturity in the basin is projected to have been adequate for significant oil and possibly gas generation.
- Local and regional seals consisting of fine clastic lithologies are indicated at various stratigraphic levels.
- Moderate deformation and suitable paleogeographic settings have resulted in a variety of potential structural and stratigraphic traps.
- None of the plays involving the potential traps has been tested by drilling.
In view of these positive attributes, the basin is considered to have promising potential for significant hydrocarbon accumulation and, therefore, merits exploratory drilling.
ACKNOWLEDGMENT
This paper is published with the permission of the Office of Energy Affairs, Philippines.
REFERENCES
- Bureau of Energy Development Robertson Research (Australia; Pty. Ltd. and Flower Doery Buchan Pty. Ltd., Sedimentary Basins of the Philippines, Their Geology and Hydrocarbon Potential, 1986, 12 vols.
- Hollowan, N.H., North Palawan block, Philippines-its relation to Asian mainland and role in evolution of South China Sea, AAPG Bull., Vol. 66, 1982, pp. 1,355-83.
- Taylor, B., and Haves, D.E., The Tectonic evolution of the South China Sea basin, AGU Geophysical Monograph, Vol. 23, 1980, pp. 89-104.
- Bischke, R.E., Suppe, J., and del Pilar, R.E., A new branch of the Philippine fault system as observed from aeromagnetic and seismic data, Tectonophysics, Vol. 183, 1990, pp. 243-264.
- Wang, S., Depositional Features and Source and Reservoir Rocks of Tertiary Age in Northern Part of South China Sea, Paper delivered at the 7th Circum-Pacific Energy & Mineral Resources Conference, Singapore, 1986.
- Metal Mining Agency of Japan-Japan International Cooperation Agency, Report on Geological Survey of Mindoro Island, Phase 1, 1982, 82 pp.
- Sarewitz, D.R., and Karig, D.E. Stratigraphic framework of western Mindoro Island, Philippines, The Philippine Geologist, Vol. 40, No. 1, 1986, pp. 3-51.
- Oca, G.R., Report on the Geological Reconnaissance of Maricaban Island, Bauan, Batangas, Philippine Bureau of Mines, unpublished report, 1952, 15 pp.
- Pineda, M.Y., Reconnaissance Geologic Investigation of Batangas Province, Trans-Asia Oil & Mineral Development Corp. unpublished report, 1990, 9 pp.
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