Indonesian Association of Geologists will conduct EVENING TALK with :

Theme :
HC Potential of Meteorite Impact Structures, Focus on Sundaland

Speaker :
H. D. Tjia (Petronas)

Wednesday, January 21,  2004

6.00 pm - 9.00 pm

Venue :
Thamnak Thai Resto & Lounge
Jl. HOS. Cokroaminoto No. 78 Menteng, Jakarta

HC potential of meteorite impact structures, focus on Sundaland

Craters abound on the surfaces of the Moon, solid planets and their
satellites. Prior to the manned Apollo missions, views that the craters of
the Moon were products of volcanism or of meteorite impacts were shared
about equally. The collected Moon rocks show definitive features of high
pressure but relatively low-temperature metamorphism that overwhelmingly
favour impact origin. On Earth, the suspected impact depression in the
Southwestern United States known as Meteor crater, was found associated with
high-pressure quartz, or coesite. Currently some 300 terrestrial structures
are considered products of impacts by extraterrestrial objects. Almost two
hundred of these have been proven as such by combinations of the following:
(1) arcuate to circular surface morphology, (2) circular gravity anomaly
patterns, (3) shatter cones, (4) poly-megabreccias containing cleaved
quartz, (5) quartz and feldspars with mosaicism (patchy extinction), (6)
high-pressure quartz polymorphs of coesite and stishovite, (7) anomalously
high Iridium, (8) diaplectic glass, and (9) microdiamonds. The comparatively
low density of terrestrial impact craters on the Earth's surface is
attributable to reworking by exogenous processes of weathering, erosion,
organic activity, burial by younger deposits, and the 70 per cent surface
cover by water. Impact craters should be as common on Earth as on the solid
extraterrestrial bodies. Calculations suggest a mean probability of over
15,000 significant impact craters having hit land since the Archaean. On
land the average depth to diameter ratio of an impact crater is 1 : 0.2 ,
while rim height is about 4 per cent of the total diameter. Also on land,
the diameter of simple, bowl-shaped impact craters probably do not exceed 4
km in igneous rocks and about 2 km in sedimentary rock. Beyond these limits,
complex crater morphologies develop as result of flattening through gravity.

The petroleum significance of meteorite impact structures is -so far- very
minor, but may be more important for several reasons. Renewed attention to
impact structure plays is relatively recent and was fueled by the 1991
single-strike discovery (25 MMBO, 15 BCFG recoverable reserves) in the
vicinity of Ames, Oklahoma, U.S.A. About twenty years earlier other
significant discoveries were made at Red Wing Creek, North Dakota (20 MMBO,
25 BCFG), and at the world-famous Chicxulub region, Yucatan Peninsula,
Mexico (total recoverable reserves of 30 BBO, 15 TCFG). However, the
structures of these earlier finds were not identified as astroblemes and at
the time of their discoveries the respective reservoirs were considered
ordinary fractured carbonates and fractured granite-and-carbonates.

The petroleum occurrences in the vicinity of Ames, a small town in Oklahoma,
U.S.A., were the main topic of a special meeting in the early 1990s. The
reservoir rocks of the Ames structure are lithified "basement" granodiorite
and Cambro-Ordovician Arbuckle dolomite breccia on the crater rim; 2D
seismic indicates an erratic, pocket-like distribution of its porosity and
permeability. Trapping is by subsurface closures with one known structural
closure. A dolomite caprock and overlying Middle Ordovician Simpson Shale
provide the cap and seal. The source consists of the Arbuckle sediments or
rocks that were exposed to the "cracking" environment of the meteor impact,
and the shales that were deposited in the crater. Flow is c. 250 to 500
BOPD; with some of the wells producing in excess of 250,000 bbl oil, while
one particular well has yielded 3 BCFG. Recovery has been in the 10% to 60%

Continental Southeast Asia includes the tectonic platforms of Indosinia and
Sundaland . By virtue of its longer exposure, its thinly covered
pre-Tertiary "basement" areas can be expected to host more impact structures
than regions containing thick Cenozoic sediments. Good reservoir is provided
by brecciated and fractured basement rocks, while traps may comprise rim
anticlines, irregularly distributed breccia pockets within craters, and
central rebound peaks (in the larger impact structures). Internal seals may
be provided by impact-melt cloaks and a top seal by the blanketing
sediments. For the Southeast Asian impact structures, the hydrocarbon charge
is likely to originate in the Tertiary basins.

Several impact structures have been recently discovered in pre-Tertiary
Peninsular Malaysia. In the Langkawi islands three of four arcuate ridges
are associated with cleaved quartz that crops out as a sill and dyke
complex. Other shock-metamorphic features include ribbon quartz and
mosaicism. The two major craters, named Mahsuri Rings, partially overlap and
each is about 2.4 km across. In 280o - 100o direction their centres are 600
metres apart. Bouguer gravity cross sections prove their crater form, one
being 45 m deep, the other attaining 107 m depth into the target rocks of
Carbo-Permian Singa Formation. Both depressions are filled, the top surface
consisting of Quaternary alluvium. The partially encircling hills of Singa
Formation crest at less than 150 m elevation and are open to the west.
Towards SW are two other circular/arcuate topographic features: Temoyong
Ring and horseshoe-shaped Tepor Island. The diameters decrease in the same
direction: approximately 800 m and 500 m, respectively. These four ring-like
structures have been interpreted as products of serial impacts by a flight
of extraterrestrial projectiles arriving from the Southwest. The impact age
is not yet determined and field relations only indicate a post-granite
(Triassic-Jurassic) event. In the case that the morphologically young
Mahsuri Rings are not exhumed features, impact could have occurred within
the last 10 million years.

Another proven impact structure is the Paloh Ring that straddles the state
boundary between Terengganu and Pahang. The proof consists of planar
deformation features (PDFs) and mosaicism in vein quartz intruded into
undivided Carboniferous metasedimentary strata that compose the lower
eastern slope of the 623m high Paloh peak. PDFs are also seen in thin
sections of quartz phenoclasts of polymict-breccia boulders in Sungai
Mengkuang that drains the east side of the hill. Bukit Paloh is the peak on
the high, circular topography surrounding a deep depression, now referred to
as Paloh Ring-1. This ring-like topography is 3.5 km across and has local
relief of the order of 150 to 200 metres. A small -about 0.5 km across-
circular depression is located on the Paloh Ring-1. The south half of the
larger ring consists of felsic igneous rock whose K/Ar age is 243 Ma, while
the north half is composed of Carboniferous metasedimentary rocks. The
youthful morphology of Paloh Ring-1 points to a geologically young impact
event, estimated to be not later than Late Miocene. More than a decade
earlier, PDFs in quartz were discovered in partly weathered granite
underlying the Quaternary basalt at Gebeng, some 40 km SE of Bukit Paloh.
The granite at Bukit Ubi quarry, located in the same area but nearer to
Kuantan town, also contains quartz with PDFs. These two findings are the
initial proofs for impact products in the whole of Malaysia. It is
unresolved if the Gebeng-Bukit Ubi impact products and those at Bukit Paloh
originated from the same event.

The third proven impact products are at Bukit Bunuh near the world-famous
palaeolithic site of Kota Tampan along the Perak River. Recently Mokhtar
Saidin, geo-archaeologist of Universiti Sains Malaysia, verbally reported a
1.74 Ma fission-track age for a volcanic agglomerate composed of unsorted
small to huge fragments of quartz, quartzite, schist, felsic igneous rocks,
and other as yet unspecified rock types. The megabreccia is set in
light-coloured groundmass, superficially resembling volcanic tuff.
Thin-section examination of the quartz turned up parallel planar fractures,
mosaicismal extinction and also several cleavage sets. Detailed studies are
in progress, but there is little doubt that this "agglomerate" is in fact s
u e v i t e , an impact breccia. In spite of alteration by oxidation,
hydrolisis and local secondary mineralisation of fractures and other voids,
porosity estimates by visual inspection are of the order of 10 per cent.

Satellite and synthetic-aperture radar images complemented by topographic
maps and aerial photographs indicate over 60 prominent arcuate and circular
features throughout Malaysia. Craters, geological and geophysical field
studies, presence of suevite, and PDFs in quartz prove the impact origin for
the Mahsuri Rings in Langkawi, the Paloh Ring-1 on the Terengganu-Pahang
border, and the megabreccia at Bukit Bunuh (Kota Tampan) in central Perak.
Thirty seven other arcuate/circular features could be impact products and
will need confirmatory studies; eight other structures appear to represent
domal intrusions or exhumed volcanic complexes, while the remaining sixteen
are meander scrolls and structural basins. Contributing factors of
extraterrestrial impacts to petroleum systems comprise depressions favouring
source-rock development, enhancement of reservoir quality in most of the
target rocks, providing a trapping environment for fluids, and perhaps
accelerate maturation. Given that prominent impact structures are not rare
in Malaysia and that during the Cretaceous-Palaeogene vast regions of
continental Southeast Asia were subaerially exposed, impact-structures
probably occur in significant numbers. The initial steps of testing
impact-structure plays include expansion and verification of the regional
database of impact structures and re-examination of seemingly anomalous
subsurface structures that are already known.

About the Presenter

H.D. Tjia , Ph.D., is a staff structural geologist, Exploration Division of
PCSB in Kuala Lumpur. He is a Foundation member of Ikatan Ahli Geologi
Indonesia since 1960; member American Geophysical Union (since 1968); member
Persatuan Geologi Malaysia (since 1968; conferred Honorary membership in
1986); for 10 years (1977-1987) as secretary of  INQUA sub-commission on
Quaternary Shorelines of Indian and Pacific Oceans.

He was a professor of geology, Head department of geology, at the University
Kebangsaan Malaysia (UKM), in Kuala Lumpur and Bangi, and was a visiting
professor, School of Physics, Universiti Sains Malaysia, in Pulau Pinang.

His Contributions in geology includes Structural Geology and Tectonics,
Fractured Basement Reservoirs, Geomorphology, Quaternary Geology, Regional
Geology of Southeast Asia, Sea-Level Fluctuations, Lunar Geology, and Impact

Fee :
IAGI Member : Rp. 75.000,-
Non IAGI Member : Rp. 100.000,-

Reservations and cancellations will be accepted until 11:00 am on Tuesday,
January 20, 2004.  Please call Ijul / Benz at Phone / Fax  : (62-21)
83702848, 83702577  or E-mail at : [EMAIL PROTECTED]
No Reservations and cancellations Will be accepted after this time.

Indonesian Association of Geologists
Professional Division
Jl. Prof. Dr. Supomo, SH., Gedung Geologi Lantai 4, Jakarta Selatan
Telp/Fax : (021) 83702848, 83702577, Email : [EMAIL PROTECTED]

Geologi & Sumberdaya Mineral Building, 4th Floors
Jl. Prof. Soepomo, No.10
Phone/Facs : (62-21) 8370-2848 / 2577



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