II. REGIONAL GEOLOGY

Transcription

1 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 Model- Hamilton, 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., 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 II-1

2 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 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 II-2

3 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 II-3

4 Platform, Central High and Southern Basin. The NE Java (Kujung)-Madura- Kangean-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). II-4

5 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). II-5

6 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; 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. II-6

7 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 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 II-7

8 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. II-8

9 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 II-9

10 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). II-10