VII. NORTH EAST JAVA SEA BASIN VII.1 Introduction The North East Java Sea Basin is one of the Tertiary back-arc sedimentary basins situated in Java Sea. The Bawean Arc separates the North East Java Sea Basin with the Muriah Trough and the Pati Trough. To the west basin is terminated by North Madura Platform, and limited by Masalembo High in the north (Figure 1). VII.2 Regional Geology VII.2.1 Tectonic Setting The North East Java Sea Basin area has evolved structurally in response two major episodes of fault reactivation which followed the accretion of the East Java Sea Microplate during the Late Cretaceous (Figure 2). The first phase of reactivation
involved
Paleogene
extension
on
Pre-Tertiary
thrusts
which
produced low-angle low-angle and locally listric extensional extensional geometries. Elsewhere much
variability of fault trends and the probable influence of inherited structural grain within the East Java Sea require that some faults will inevitably have experienced oblique-slip during the Paleogene, irrespective of extension vector. Early Miocene and younger uplift has affected a large area of the East Java Sea. This was developed by episodic reserved movement on faults which had accommodated subsidence in the Paleogene, some of which can also be seen to reactivate pre-Tertiary thrusts. The graben and half-grabens which existed at Early Miocene times have been inverted, resulting in several of the regional structural dip and altering the sediment fill pattern which had existed prior to uplift. VII.2.2 Stratigraphy In contrast to the East Java and Madura basins which are dominated by deepwater sediments the basins of the East Java Sea are filled almost entirely by subaerial to shallow marine shelfal sediments (Figure 3). Facies boundaries occur along a diffuse zone of faults and flexures located close to the north
K u j u n g F o r m a ti o n
The overlying early Kujung Formation continued to transgress basin margins, with the deposition of argillaceous shelfal carbonates, and local shoreline sands on the margins of highs. To the south shelf/slope break occurred along the Kujung Fault system, with thick bathyal muds deposited in the East Java and Madura areas. Carbonate content increased in the increasingly open marine Kujung limestones with continued transgression. Best reservoir facies are found in patch reefs located on the platform areas to the north of the Kujung Fault (eg Camar, Poleng and KE fields). Kujung limestones deposition culminated in deposition of the massive Kujung I platform sequence over a very wide area in the earliest Miocene. This is equivalent to the Prupuh Limestone of the other East Java basins. The platform is locally karstified over the Northwest Bawean Arch resulting in a highly diffractive seismic event. Neogene
-
A facies change occurs to Unit II which includes sandy turbidites and hemipelagic mudstones which accumulated in the basin to the south.
-
Unit III, which transgresses both of the lower units. Unit III contains sandy bioclastic limestones in the north (Platten or Bulu limestones) and the equivalent channelised sandy turbidites, hemipelagic mudstones and contourites in the south.
Late Miocene to Recent The remainder of the Miocene is transgressive in character and is mudstone /carbonate dominated. The Upper Miocene/Lower Pliocene Karren Formation (Upper OK) limestones are widely developed and may in part be the platform equivalent of the hemipelagic Paciran chalks which form the reservoirs at MDA and Sirasun. A shelf edge between these two facies is rarely developed due to the
widespread
structural
inversion.
More
usually,
the
northern
shelfal
limestones onlap the northern flanks of inverted structures and on the southern flanks thin reefal limestones pass rapidly into deepwater marls and globigerinid
sands represent the deepest reservoir targets in the area and such quartz-rich sands are present in this basin. Carbonates have variable reservoir quality but in the Bawean area are deeply buried with poor reservoir potential. Kujung III (Late Oligocene) This interval consists of mixed shelfal clastics and carbonates. Sands are medium to coarse, and locally tuffaceous, and are interbedded with muds, micritic limestone and lignite. The Kujung III Formation reservoir performance is enhanced considerably by fracturing. Kujung II (Late Oligocene) These limestones form one of the main targets in the Central Deep and provide the reservoirs at the Camar Field. Limestones of high energy and reefal facies overstep basement on the flanks but are fine grained in the basin centre. Primary reservoir quality id fair and performance is enhanced by fracturing adjacent to reactivated fault zones.
Additional top seals are provided by the intraformational Kujung shales and dense limestones that act as good top seal for the underlying Ngimbang & Kujung reservoirs. VII.3.4 Trapping Mechanism Proven plays are largely extensional drape of faulted dip closures, and reefal build-ups. The two often combine in the same feature to give multiple play horizons. The failure of inverted traps is possibly due to their late development and the limited amount of adjacent subsidence post-dating inversion. Largely structural traps include the KE-2 (Madura) Field and Camar Field. The KE-5/6 and Poleng Fields are largely patch reef complexes with some structural control which probably initiated reefal growth. VII.4 Hydrocarbon Play The Muriah Trough and Central Deep are characterized by multiple producing horizons due to stacked reservoir/seal couplets. Accumulations may produce
References Bransden P.J.E., and Mathews S.J., 1992, Structural and Stratigraphic Evolution of the East Java Sea, Indonesia, Proceed. Indon. Petrol. Assoc.21st Ann. Conv. pp 417-453. Darman H., and Hasan Sidi F., 2000, An Outline of The Geology of Indonesia, Published by IAGI-2000, pp 54-59. IEDS, 1995, An Evaluation of Proven and Potential Gas Reserves Section 2 Java, Vol.1. Kaldi J.G., Macgregor D., and O’Donnell G.P., 1997, Seal Capacity in Dynamic Petroleum Systems; Example from Pagerungan Field, East Java Sea, Indonesia, Proceeding of the Petroleum Systems of SE and Australian Conferences, pp 829-836.
Northeast Java Sea Basin
FIGURE 1. Location Map of North East Java Sea Basin
o
115E
L EG END : STRUCTURALHIGH
N
BARITO PLATFORM
BASINMARGIN/TERRACE STRUCTURAL/BASINALLOW FAULTZONE
0
NORMALFAULT
125
THRUSTFAULT
Kil ometer s
STRIKESLIPFAULT
AROSBAYAHIGH KUJUNGHIGH NGIMBANGSUB-BASIN
Surabaya PORONG SUB-BASIN
o
115E
CHRONO STRATIGRAPHY
EPOCH
ONSHORE AREA AND MADURASTRAIT
NORTHEASTJAVA SEA
LIDAH
PLEIS KAWENGANFM.
MT OL
KARREN
WONOCOLOFM.
(UPPEROK)
NGRAYONG TUBANFm. (LOWEROK)
R
S RANCAK
S UNIT-I
R
UNIT-II R
S
MADURAISLAND
JAVASEA
NNW
JS-20AREA
JS 1 - 1
JS19-1
A RO SB AY AA RE A
BD-1
10
0
12 11
9 1000
MADURASTRAIT MS1-1
G I GI R -1 A RE A
1
5
6
4
2000
8
3
3000
2 7
4000 5000
S PRE-TERTIARYBASEMENT
6000
1.KujungIIU.Oligocene 2.NgimbangL.Oligocene 3.KujungII-U.Oligocene 4.KujungII-U.Oligocene 5.KujungI-L.Miocene 6.Rancak-M.Miocene
7.KujungI-L.Oligocene 8.Tuban-M.Miocene 9.Ngrayong-U.Miocene 10. Wonocolo-U.Miocene 11.Lidah-Pliocene 12. Lidah-Pleistocene
FIGURE 4. Hydrocarbon Play of North East Java Sea Basin
R
SSE
