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II. REGIONAL GEOLOGY
2.1. Regional Plate Tectonic Setting
The geological history of the East Java basin is closely related to tectonic
activity of the Southeast Asia especially the Western Indonesia region. The
tectonic activity in this area is believed to be pretty much controlled by
interactions of three major plates; Indo-Australia oceanic plate that moves
northward; westward moving Philippine-Pacific oceanic plate and the relatively
stable Eurasian plate. There are many tectonic models that were trying to explain
the complex tectonic history of the area (Figure 2.1); Non Rotational ModelHamilton, 1979; Indentation Tectonics-Tapponier, 1986; Clockwise Rotation
Model-Daly et al., 1991; Counter Clockwise Rotation Model-Hall, 1996, Micro
Continent Model-Parkinson et al., 1998; Micro Continent Model-Wakita et al.,
1999 and East Java Micro Continent Model-Sribudiyani et al., 2003.
Figure-2.1: The tectonic models of Southeast Asia and Indonesian region
As the consequence, the tectonic evolution of East Java basin is still
subject to the ongoing debate as well. However, in general based on those existing
tectonic models they can be grouped to two main ideas; the first one suggested
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that the East Java area is part of the Sundaland from time to time while the second
idea mentioned that East Java area is a continental fragment detached from the
Gondwana super-continent drifted north-eastward and collided with the eastern
margin of Sundaland in Late Cretaceous-Early Eocene. This collision was
believed to be one of the tectonic development main controls in this area.
2.2. Basement and Basin Configuration
Hamilton (1979) and Barber (1985) mentioned that three types of crust are
present in vicinity of Java; continental, intermediate and oceanic crusts. However,
there are many opinions about the dominant basement composition floored the
East Java Basin, in 1996 Pertamina – BPPKA team, drawn a map showing that the
East Java basement dominantly composed by oceanic and/or highly extended
continental crust. Tognini et al., (2007) mentioned that the basement composition
of East Java basin is dominantly comprised of a mélange of low and high grade
metamorphic, meta-igneous rocks, slightly metamorphosed sediments, volcanic
and rafts of sialic material (Figure-2.2).
Figure-2.2. East Java Basin basement composition (Tognini et al. 2007).
Meanwhile the un-published Robertson Research study results in 2002
believed that East Java Basin is dominantly floored by micro-continental granitic
basement. The same idea has also stated by Sribudiyani et al. (2003). In fact, some
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of wells in the area penetrated the granitic basement such as: JS8-1, JS44A-1, and
NSA-1C proved that the East Java Basin has a strong continental affinity
basement.
The complex tectonic development in this area controlled the basement
grain which influenced the basin trends. In the eastern part of East Java Basin the
dominant basement grain is E-W, as can be particularly well observed controlling
the Kendeng and Madura Troughs. Another type of basin configuration developed
at the collision zone oriented NE-SW, parallel to the direction of the collisional
suture along Lok Ulo – Meratus Complexes (Sribudiyani et al., 2003). The
research area is located in the East-West trending of basement grain structures
lineament (Figure-2.3).
Figure-2.3: Tectonic elements of East Java Basin (Sribudiyani et al., 2003).
East Java basin as a whole encompasses a total area of approximately
250,000 square kilometers onshore and offshore covering East Java and Madura,
the Madura Straits and the East Java Sea. To the west, it is bordered by
Karimunjawa Arc and Sunda Shelf, to the north by Meratus High, and to the east
by Masalembo-Doang High. Java volcanic belt forms the southern boundary. The
configuration of basin, basement high and lows are basically controlled by
tectonic strain which is different from one area to another. Pertamina-BPPKA
team divided the basin into there major distinct structural provinces; Northern
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Platform, Central High and Southern Basin. The NE Java (Kujung)-MaduraKangean-Lombok High is grouped as the Central High, The Bawean Arch-JS-1
Ridge-Northern Madura/Kangean platform is defined as the Northern platform
and the Rembang-Madura Strait-Lombok Sub-basin is determined as the Southern
Basin. Furthermore Sribudiyani et al., in 2003 made a more detailed structural
provinces division as it shown in previous figure-2.3.
East Java Basin has many different geometric configurations in each area,
Pireno and Mudjiono (2001). drawn a map of East Java Basin regional
configuration. From this map it can be inferred there are three main Paleogene rift
trends in NE-SW, NW-SE and E-W orientation (Figure-2.4a). In the Camar area,
Southern Basin area, Central Deep, Lombok sub-basin and etc, those rift or
inverted rift geometries were obvious observed. However, this rift geometry is not
shown in other part of the basin, such as in area near to the Madura Island,
Northern Platform area and etc. As an example, this situation can be seen in a
NW–SE cross-section reconstruction from Camar area to the Madura Island
(Figure-2.4b).
Figure-2.4: a. The East Java Basin regional configuration and b. NW-SE crosssection reconstruction from Camar area to Madura Island (Pireno and Mudjiono, 2001).
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This fact was lead to the multi interpretation about the basin formation
mechanism and basin classification, for example Pertamina-BPPKA team (1996)
and many others mentioned that the East Java basin is a rift basin while in another
hand Robert Hall (2003) stated that it is not a rift basin but a volcano flexural
loading basin.
The research area is located in so called the Lombok sub-basin, the
southeastern portion of East Java Basin. Based on regional mapping of top
basement as well as gravity data interpretation, Tognini et al (2007) mentioned
that the Bali-Lombok sub-basin is interpreted to be the site of an extremely thick
Tertiary depocentre (up to 9000 m) and it is a part of an extensive rift system that
developed during Paleocene and Middle Eocene. The author illustrated the
architecture of East Java and the Bali-Lombok sub-basin in a long regional cross
section and inferred that the basin appears as a series of half graben and basement
highs showing a predominantly eastern polarity. The thickest depocentre is
located between the Baluran 1 and the ST Alpha-1 wells. The synrift section is
thicker in the east but thins out moving westward from the immediate vicinity of
Baluran-1 (Figure-2.5).
Figure-2.5: The Bali-Lombok sub-basin gravity map (a) and regional
crossection (b) (Tognini et al., 2007).
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2.3. Tectono-stratigraphy of East Java Basin
The East Java Basin is an area at the present considered to be a back-arc
basin; however the geologic complexity of the basin has lead to the multiinterpretation about the timing of basin initiation, basin formation and
development as well as the stratigraphy. Bransden & Matthews (1992) believed
that the East Java basin development is started in as a response to plate margin
processes since Cretaceous. Their interpretation about the East Java Tectonostratigraphic history (figure 2.6) is summarized as follow;
2.3.1. Pre-Tertiary (Megasequence I)
Based on outcrop, seismic and well constraints, Bransden & Matthews
postulated the important Pre-Tertiary tectonic event in the area as a collision
between an East Java Sea micro plate and the southeastern part of the Eurasian
Plate occurred in late Cretaceous. This collisional event produced of a widespread
heterogeneous accretionary basement complex and followed by some local
marginal basins formation deposited the well bedded Upper Cretaceous sediment
(Megasequences I).
The Upper Cretaceous Sediments (Megasequence I) shown a fairly
widespread and thickly developed. The sediments sub-crop the basal Tertiary
reflector at a high angle, and internally are well bedded. Numerous wells in the
area penetrate highly indurate and thermally over mature (%Ro 1.3 to 1.8)
sediments with overall section showing a mud-dominated, with lesser siltstone
and some tightly cemented, lithic to sub-lithic sandstone inter-beds. The
stratigraphic relationships are unclear; however the broad correlation suggests
lower sequences are grey to grey-black in color, with rare late Cretaceous marine
microfossils. Several wells penetrate red beds which appear to be higher
stratigraphically. Seismic interpretation has not demonstrated stratigraphic breaks
in the unit; some well data suggests however that the”pre-Ngimbang Fm”
(Phillips, 1991) may represent a separate sequence of possible Tertiary age. The
accretionary complex beneath the base of the Tertiary considered as the
effectively acoustic basement, whereas the bedded Cretaceous represents
economic basement.
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2.3.2. Paleogene (Megasequence II)
Following the accretion of the East Java Sea micro plate during the late
Cretaceous, active subduction proceeded around the margin of the newly modified
SE Eurasian Plate. The reduction in convergence rate around the SE Eurasian
Plate, possibly resulting in subduction roll-back, is a plausible mechanism for
back-arc extension around the margins of the SE Eurasian Plate. Localized
extension was underway and the onset of rifting occurred in early to mid Eocene
(P9/ P10 or older) and rifting was very widespread by late Eocene.
During the Paleogene, sedimentation was relatively restricted aerially and
concentrated in axial rift zones, with thinner sediment deposited on the relatively
stable flanking platforms. The vertical facies development of this sediment is
transgressive with a gradation from non-marine (fluvial and ephemeral lacustrine),
to coastal plain (frequently with coals), and marginal marine. Ngimbang Clastics
is the current litho-stratigraphic term incorporates all Eocene aged clastics
followed by the deposition of Ngimbang Carbonate which is currently the only
regionally recognizable seismic marker within the Ngimbang Fm.
2.3.3. Neogene (Megasequence III)
The next tectonic even in the region is a collision of the Australian
continent with a northerly island arc initiated in the early Miocene. As results,
major thrusts were occurred and considered as the principal driving mechanism of
inversion in the East Java Sea. The Neogene inversion history is most simply
explained by fault movement reversal with the location of the major uplifts
reflecting the location of the main Paleogene depocentres, this being a function of
the Paleogene fault geometry and linkage. All the Neogene uplift structures
currently interpreted can be explained by dominantly dip-slip reverse motion and
subsidiary lateral motion on pre-existing Paleogene structures, with local
generation of new contractional faults.
Neogene’s inversion led to a reversal in basin geometry but not polarity
(i.e. clastic sediment input remained from the west). Major reverse movement on
the controlling faults led to the sediment thick being inverted, and new Neogene
depocentres forming to north and south of the inversion zone. These locally have
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foreland basin geometry, notably in the Madura Strait and Kangean and Lombok
Southern Basins, where locally over 6 second TWT of Neogene is developed.
Reworking of exposed parts of the inversion trend provided some of the sediment
fill. In eastern areas the erosion products were largely mud-prone, whereas in the
west some sands may have been reworked. Despite the complex inversion history,
the eustatic and sediment supply controls on Neogene basin fill can be resolved
from the sequence stratigraphy of more stable areas, such as the northern
platforms which remained net-extensional during the inversion phase.
Figure-2.6: The East Java basin megasequence chrono-stratigraphy, litho-stratigraphy
and petroleum distribution (Bransden & Matthews, 1992)
2.4. Pre-Ngimbang and Ngimbang Formations
As mentioned in the previous section, there are many other interpretations
which tried to describe the East Java basin development scenario, however based
on the literature studies, these differences were mostly triggered by the different
point of view about the geology of Pre-Ngimbang and Ngimbang Formations,
especially in age of the formation and the differences definition about those
formations.
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Based on some well reports summary, the Pre-Ngimbang age were
reported ranging from Cretaceous to Middle Eocene, with sandstones, siltstone,
shale, coal, and limestone intercalation as its dominant lithologies. The
depositional environment varies from deltaic lacustrine, marginal marine, deltaic,
outer shelf and bathyal. While Ngimbang clastic is ranging from Middle to Late
Eocene in age, dominated by the sandstone, siltstone, shale, coal, and limestone
streak lithologies with the depositional environment varies from the alluvial plain,
marginal marine, deltaic, outer shelf and bathyal.
Another aspect that leads to the multi-interpretations controversy is the
geometries of seismic reflection overlaid directly above the basement which
believed to be the Pre-Ngimbang/Ngimbang package. For example in Kangean,
Lombok and surrounding area, some of the seismic lines showing a distinctive of
highly folded geometry at the lower part were truncated by an angular
unconformity on top of it and overlaid by continuous and parallel reflector
packages (Figure-2.7).
Figure-2.7: The seismic reflection geometries of “Pre-Ngimbang/Ngimbang”
(Modified from Bransden & Matthews, 1992)
Those limited well reports and those seismic geometries were used by
authors as a basic fact for their interpretations. Some author interpreted those
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highly folded geometry at the base as part of Pre-Tertiary section , while the top of
angular unconformity considered as the top of pre-Tertiary section horizon and the
sediment above it was consider to be the earliest deposit of the East Java Tertiary
basin, called the Pre-Ngimbang (Paleocene sediment) which next overlaid by
Ngimbang (Eocene sediment). Another authors defined that the folded-truncated
packages as the Pre-Ngimbang unit which is Cretaceous in age and the relative
continues-parallel package above is as the Ngimbang Clastic unit.
It appears that the dispute about the Pre-Ngimbang and Ngimbang units
were because each author has own definition about those formation. Indeed, there
is still considerable uncertainty regarding the existence and distribution of the
“Pre-Ngimbang” sediment throughout the East Java area since up to now the
Paleogene geologic information in this area were scare; there is no Pre-Ngimbang
outcrop present anywhere in East Java, in addition the seismic and well data are
also limited (Sribudiyani et. al, 2003).
However, refers back to the history of the Pre-Ngimbang term, Harper in
1989 defined the Pre-Ngimbang Formation as: “The Paleocene to middle Eocene
Pre-Ngimbang Formation of the Northern Platform-Central High in the Kangean
and Sepanjang PSC’s comprises a sequence of sandstone, siltstones and shales
which uncomformably overlies Cretaceous basement and is unconformably
overlain by the Late Eocene Ngimbang Clastics”. This is the earliest documented
Tertiary sediments in this region and it is defined as synrift deposits which mostly
consist of inter-bedded thin sands and shales, with some coals. The Fluvial-deltaic
clastics part filled in depositional lows, approximately during Paleocene-Middle
Eocene time. It overlies unconformably the Cretaceous but generally absent on
Cretaceous paleo-highs (Pertamina-BPPKA, 1996). In the eastern of the East Java
Basin, the top of the Pre-Ngimbang is picked typically at the base of the last clean
sand of the Ngimbang Clastics and the base by a sharp jump in vitrinite values
from 0.5 to 1.2 and greater and by an angular unconformity recognized from
dipmeter (Phillips, et. al., 1991; op.cit. Pertamina-BPPKA, 1996).
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