delineation of sedimentary subbasin and subsurface

Bulletin of the Marine Geology, Vol. 34, No. 1, June 2019, pp. 1 to 16

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Delineation of Sedimentary Subbasin and Subsurface Interpretation East Java Basin in the Madura Strait and Surrounding Area Based on Gravity Data Analysis

Delineasi Subcekungan Sedimen dan Interpretasi Bawah Permukaan Cekungan Jawa Timur Wilayah Selat Madura dan Sekitarnya Berdasarkan Analisis Data Gayaberat

Imam Setiadi1, Budi Setyanta2, Tumpal B. Nainggolan1, Joni Widodo1

1 Marine Geological Institute, Jl. Dr. Djundjunan No. 236, Bandung, 40174, Indonesia.2 Centre for Geological Survey, Jl.Diponegoro No.57. Bandung, 40122, Indonesia.Corresponding author: [email protected](Received 08 January 2019; in revised form 15 January 2019 accepted 27 March 2019)

ABSTRACT: East Java basin is a very large sedimentary basin and has been proven produce hydrocarbons,this basin consists of several different sub-basins, one of the sub-basin is in the Madura Strait andsurrounding areas. Gravity is one of the geophysical methods that can be used to determine geologicalsubsurface configurations and delineate sedimentary sub-basin based on density parameter. The purposes ofthis study are to delineate sedimentary sub-basins, estimate the thickness of sedimentary rock, interpretsubsurface geological model and identify geological structures in the Madura Strait and surrounding areas.Data analysis which used in this paper are spectral analysis, spectral decomposition filter and 2D forwardmodeling. The results of the spectral analysis show that the thickness of sedimentary rock is about 3.15 Km.Spectral decomposition is performed at four different wave numbers cut off, namely (0.36, 0.18, 0.07 and0.04), each showing anomaly patterns at depth (1 Km, 2 Km, 3 Km and 4 Km). The sub-basins that can bedelineated from the gravity data analysis are 10 sedimentary sub-basins, while the structural patternsidentified are basement high, graben and fault. 2D modeling results indicate that the basement is acontinental crust with a mass density value of 2.7 gr/cc. Sedimentary rock from modeling result consecutivelyfrom the bottom to up, the first is Paleogene sedimentary rock with mass density value of 2.4 gr/cc and abovethis layer is Neogene sedimentary rocks with mass density values of 2.25 gr/cc. The results of the subsurfacegeological modeling analysis show that based on the graben pattern and the basement high of the East Javabasin in the Madura Strait and surrounding areas there are many structural patterns that support thedevelopment of petroleum systems like at the western part of the East Java basin that have already producedhydrocarbon.

Keywords : Gravity, spectral analysis, spectral decomposition filter, 2D Modeling, East java basin

ABSTRAK : Cekungan Jawa Timur merupakan cekungan sedimen yang sangat besar dan telah terbukti memilikikandungan minyak dan gas bumi. Cekungan ini terdiri atas beberapa sub-cekungan yang berbeda-beda, salahsatunya adalah sub-cekungan yang ada pada wilayah selat Madura dan sekitarnya. Gayaberat merupakan salahsatu metoda geofisika yang dapat digunakan untuk mengetahui konfigurasi bawah permukaan serta mendelineasisub-cekungan sedimen berdasarkan parameter rapat massa (densitas). Tujuan dari penelitian ini adalah untukmendelineasi sub-cekungan sedimen, memperkirakan ketebalan sedimen, menginterpretasi geologi bawahpermukaan serta mengidentifikasi struktur yang ada pada wilayah selat madura dan sekitarnya. Analisis data yangdigunakan yaitu analisis spektral, filter spektral dekomposisi serta pemodelan maju (forward modeling) 2D. Hasilanalisis spektral menunjukaan bahwa tebal batuan sedimen rata-rata adalah sekitar 3.15 Km. Spektral dekomposisidilakukan pada empat bilangan gelombang cuttoff yang berbeda beda yaitu (0.36, 0.18, 0.07 dan 0.04) yangmasing-masing menunjukkan pola anomali pada kedalaman (1 Km, 2 Km, 3 Km dan 4 Km). Sub-cekungan yangdapat didelineasi dari analisis data gayaberat ini adalah sebanyak 10 sub-cekungan sedimen, sedangkan polastruktur yang teridentifikasi yaitu berupa tinggian, graben dan patahan. Hasil pemodelan 2D menunjukkan bahwabatuan dasar adalah berupa kerak kontinen dengan nilai rapat massa 2.7 gr/cc. Batuan sedimen hasil pemodelansecara berturut turut dari bawah ke atas yang pertama yaitu batuan sedimen yang berumur Paleogen dengan nilairapat massa 2.4 gr/cc dan di atasnya adalah batuan sedimen berumur Neogen yang mempunyai nilai rapat massa

2 Imam Setiadi et al.

2.25 gr/cc. Hasil analisis model bawah permukaan menunjukkan bahwa berdasarkan pola graben dan tinggiancekungan Jawa Timur segmen selat Madura dan sekitarnya cukup banyak terdapat pola struktur yang mendukungberkembangnya petroleum system seperti pada wilayah sebelah barat cekungan Jawa Timur yang sudah berproduksihidrokarbon.

Kata Kunci : Gayaberat, spektral analisis, filter spektral dekomposisi, pemodelan 2D, Cekungan Jawa Timur

INTRODUCTIONIndonesia region located at the zone of three

tectonic plate which still actively move namely theEurasian plate, the Indo-Australian Plate and the Pacificplate. Interaction of the three plates resulting theemergence of volcanoes, faults and sedimentary basinsin the Indonesian area. One of oil and gas basin whichhas produced in Indonesia is East Java Basin, East Javabasin located in the southeast part of Sunda shelf whichbounded by Karimunjawa arc at the western part,Meratus high at the northern part, and Masalembo highat the eastern part, and volcanic series at Southern partof East Java (Sribudiyani, 2003). The purposes of thisresearch are to delineate Madura Strait subbasin and thesurrounding area, to know structural patterns andsubsurface geological configurations by using thegravity method. Gravity is one of the geophysicalmethods that can be used to determine the geologicalsubsurface condition based on the physical parametersof density. There are several filter methods that can beused to analyze gravity data including a Gaussian filterto separate regional and residual anomalies fromgravity data (Karunianto et al., 2017), (Obasi et al.,2016) use the trend surface analysis method to separateresidual and regional anomalies from gravity data.Zahra and Oweis, (2016) use high pass filter on gravityand geomagnetic data to obtain short wavelengthanomalies associated with shallow anomaly sources. Inthe processing of gravity data, anomaly will appear as

the target of a study that makes it easy to interpret thegeological subsurface conditions, the anomaly in thegravity method is known as the Bouguer anomaly.Bouguer anomaly is a superposition of anomaliescomponent from various depths, information aboutdepth and position of the object anomaly sourcebecomes very important at the interpretation stage.Spectral analysis is a method that can be used toestimate the depth of anomalies in the frequencydomain of gravity data. In this study, spectral analysis,spectral decomposition filter and 2D modeling ofgravity data will be carried out. The purposes of spectralanalysis are to estimate basement depth and to obtainoptimum window width to determine regional andresidual anomalies in the study area, while spectraldecomposition is carried out to determine structureanomalies pattern based on spectral analysis at severaldifferent depths. 2D modeling is carried out to find aquantitative interpretation of subsurface geologicalmodel so it is easier to be interpreted. The interpretationresults of both qualitative and quantitative gravity datawill be expected to add subsurface and geologicalstructure information which can be used to enrichexploration activities references in the East Java basin,especially in the Madura Strait and surrounding areas.Geographically, the location of the study area is in theMadura Strait and surrounding areas of East JavaProvince at coordinates (112 - 113.5) East and (7-8)South as shown in Figure 1.

Figure 1. Research area in the Madura Strait and surrounding areas, East Java Province

Delineation of Sedimentary Subbasin and Subsurface Interpretation East Java Basin in the Madura Strait and Surrounding Area Based on Gravity Data Analysis 3

GEOLOGY OF RESEARCH AREAThe East Java Basin geologically was formed as

the result of uplift and unconformity process as well asother processes, such as sea level subsidence andtectonic plate movement. The initial step of forming thebasin is characterized by the presence of half grabenwhich is influenced by the geological structure formedpreviously. The geological structure of the East JavaBasin generally are reverse fault, normal fault, strike

slip and folding with relative East-West trend due to theinfluence of the compression force from the South toNorth direction. The development of tectonics in theEast Java Basin is inseparable from the tectonic activityof the Southeast Asian region to wit the movement ofthe Indo-Australian Ocean Plate northward, thePhilippine and Pacific Oceanic Plate moving westward,and the relatively stable Eurasian Plate. The East JavaBasin is classified as a back arc basin and located at the

Figure 2. Geological map of study area, adopted from geological map scale 1 : 100.000 Published by GeologicalResearch and Development Centre (GRDC).


southeast boundary of the Eurasian Plate (Mudjionoand Pireno, 2001) and situated on the edge of a stableSunda continent. Geological map of study area can beseen at Figure 2, this map adopted from geologicalmaps scale 1 : 100.000 namely geological map ofMojokerto quadrangle (Noya et al., 1992), geologicalmap of Kediri quadrangle (Santosa and Atmawinata,1992), geological map of Malang quadrangle (Santosaand Suwarti, 1992), geological map of Probolinggoquadrangle (Suharsono and Suwarti, 1992), geologicalmap of Tanjungbumi and Pamekasan quadrangle (Azizet al., 1992), and geological map of Surabaya andSapulu quadrangle (Supandjono et al., 1992) whichhave been published by Geological Research andDevelopment Centre (GRDC).

Generally, volcanics rock are situated in theSouthern part of the study area and characterized byhigh topography contour. Alluvial deposits can befound at north, central and eastern part east java alongthe beach and characterized by flat topography.Sedimentary rocks are distributed at many areas such asin the Northeast part of study area especially at MaduraIsland namely Ngrayong Fromation, Tawun Formation,Madura Formation and Pamekasan Formation. Manysedimentary rocks also found at Northwest part of studyarea such as Kalibeng Fromation, Ledok Formation,

Mundu Formation, Paciran Formation, LidahFormation, Kabuh Formation, Pucangan Formation,Kerek Formation, Sonde Formation, NotopuroFormation, Tambakromo Formation and KlitikFormation.

Stratigraphy of East Java Basin

The basement of the East Java basin generally iscontinental (granitic), melange and metamorphic rock.The stratigraphic sequence of the East Java basin(Sribudiyani et al., 2003) is described seen in Figure 3.

Figure 3 shows the oldest sedimentary rock at theage of Early Eocene which deposited above thebasement is the Pre Ngimbang Formation, thisformation consist of sandstone, shale, siltstone and coaland found in the Kangean area in the eastern part of theEast Java Basin. Above the Pre Ngimbang Formation isthe Ngimbang Formation which is Middle Miocene agecharacterized by clastic sediments and consist ofinterspersing sandstone, shale, limestone and coal.Above the Ngimbang Formation was deposited theKujung Formation consist of shale with limestone andsandstone insertions, the upper part of the shaleinsertion and clastic limestone is also known as Prupuhlimestone. Above the Kujung Formation was depositedthe Tuban Formation composed of clay layer, some

Figure 3. Regional stratigraphy of the East Java Basin (Sribudiyani et al., 2003)


insertion of limestone and shale was formed at thebeginning of the Miocene and deposited in the deep seaenvironment. Inter fingering with the Tuban Formationnamely the Ngrayong Formation consist of sandstone,shale, clay, siltstone and limestone insertions. At theage of the early Miocene to Middle Miocene the TawunFormation was deposited consisting intermittent ofshales carbonat with sandstone and limestone. Abovethe Tuban Formation at the age of Middle Miocene toLate Miocene deposited the Wonocolo Formation wascomposed of marl and clay, at the bottom composed ofsandy limestone. Above the Wonocolo Formation wasdeposited Ledok Formation consist of interspersingbetween sandstone and marl insertions. Above LedokFormation was deposited the Mundu Formation consistof marl and sandy limestone. Above the MunduFormation, the Selorejo Formation was depositedcomposed of interspersing between the marly limestoneand sandy limestone. The uppermost formation at theage of the Plistocene was deposited by the LidahFormation consist of black clay and marl.

Petroleum System

Most of the source rocks in the East Java Basin isshale which is rich of organic material and coal thatcame from the Pre Ngimbang, Ngimbang, Kujung andTawun Formations. The results of the geochemicalanalysis of several wells indicate that the source rock inthe East Java Basin is quite mature and has twopossibilities as a source of oil or gas production. Themain reservoir tagets in the East Java Basin are thesandstone Ngimbang Formation, Poleng limestone theKujung Formation, Rancak limestone the TawunFormation, sandstone the Ngrayong Formation, Bululimestone the Tawun Formation, and Paciran limestonethe Mundu Formation. Hydrocarbon migration occursat close distance through fault and fracture structuressuch as migration into younger formations filled by gasand oil. The structural framework of the East Java Basinis very complex, formed by two main basins with themain orientation northeast-southwest and east-west,and some sub-basin whose initial establishment ischaracterized by rifting filled by clastic sedimentPaleogen then followed by the main inversion structure,this major structural inversion (compression) eventinvolves all components of the sequence stratigraphy(Sribudiyani, et.al., 2003). In general, almost all basinsin East Java are classified into basins formed bycomplex structures, characterized by folds and reversefault. The two main types of traps that can be identifiedin the East Java Basin are structural and stratigraphictraps. The sealing rock consists of clay, shale and marl,which is a good capacity sealer to hold a gas or oilcolumn, the rock is believed as a seal for sandstone as

hydrocarbon reservoir that is lies under the sealing rock(Mudjiono and Pireno, 2001).

RESEARCH METHODOLOGYThe data used in this study has been published by

Geological Research and Development Center(GRDC), while the data for the sea area is taken fromsatellites (Topex, 2019). Bouguer anomaly data used inthis study covering some quadrangle map namelyBouguer Anomaly Blitar quadrangle map (Nainggolanet al., 1995), Mojokerto quadrangle (Nainggolan et al.,1994a), Tuban quadrangle (Nainggolan et al., 1994b),Turen quadrangle (Tasno et al., 1995), Malangquadrangle (Buyung et al., 1994), Surabaya and Sepuluquadrangle (Syarif et al., 1994), Bawean andMasalembo quadrangle (Tasno and Ermawan, 1997),Lumajang quadrangle (Suharyono and Sucihati, 1994),Probolinggo quadrangle (Nainggolan and Iryanto,1995), Tanjungbumi-Pamekasan quadrangle (Ermawanand Tasno, 1994). The bouguer anomaly dataprocessing comprise of griding and contouring,continued by spectral analysis to estimate the basementdepth and find out the optimum window width, and thenspectral decomposition to determine the patterns ofstructure anomaly at a certain depth. 2D modeling wasalso carried out to determine subsurface geologicalmodels in the study area. The combined results ofgravity data analysis and geological information arethen used to interpret the geological conditions of thestudy area. The complete research flow diagram can beseen in Figure 4. Spectral analysis can be used toestimate the optimum window width and depth of thegravity anomaly source, spectral analysis was done byfourier transforming of the Bouguer anomaly path thathas been determined previously in the Bougueranomaly map. In general, fourier transform isrearranging or decomposing arbitrary waves into sinewaves with varied frequencies where the sum of the sinewaves is the original waveform. The spectrum isderived from gravity potential and observed in ahorizontal plane (Blakely, 1996), where the Fouriertransform is as follows:

(1)

(2)

(3) Where A is Amplitude, C is Constant, z isdepth, and k is wave number.

The resulted equation can be analogized to astraight line equation :

,


(4)

While ln A act as the y axis, as the x-axis,and as the slope (gradien). Therefore,slope of the lines indicate the depth of shallowand deep planes. as the x-axis as defined asthe magnitude of the wave number in cycle/meter unit, while is the wavelength. Windowwidth can be formulated as follows:

(5)

where ’x is the space domain to be used in FastFourier Transform (FFT), and kc is the wave numbercut-off.

The value of wavenumber (k) is linear withfrequency (f), relation between wavenumber (k) andfrequency expressed as equation or

, it means the low frequency comes from

regional anomaly sources and high frequency comesfrom residual anomaly sources.

Fast Fourier Transform (FFT) results can beplotted as graphical correlation between Amplitude andWave Number as shown in figure 5. The graph consistedof three zone namely noise zone usually located in thehigh frequency/wavenumber, residual anomaly zone inthe middle wavenumber, and the regional anomaly zonelocated in the low wave number.

The signals results of the Fourier transform (FFT)which consist of wavenumber and amplitude spectrumreflecting a combination of subsurface anomalies fromlong wavelength (deep anomalies) to short wavelength(shallow anomalies). This signal anomaly can bedecomposed to find out the anomaly pattern at a certaindepth by determining the wavenumber cutoff, thetechnique to decompose spectrum of the FFT signalresults is known as the spectral decomposition. Spectraldecomposition analysis on gravity data can be done byparsing or making several wave number cut off and

Figure 4. Flow Chart of gravity data analysis at the East Java Basin in the Madura strait and surrounding area


window width by dividing in different depths whichwill be analyzed one by one (Dicky et al., 2017). Thisspectral decomposition analysis intended to obtaininformation about the geological structures at anydetermined depth.

RESULT AND DISCUSSION

Bouguer Anomaly

Bouguer anomaly map which generated fromcombined terrestrial data mapped by the GeologicalResearch and Development Center and the sea dataoriginating from satellites (Topex, 2019) can be seen inFigure 6, from these images, it can be seen that theanomaly values range between (-38 to 43) mGal. Highgravity anomalies are shown in red colour occupies in

the south and north part, with theanomaly value between (10 to 43)mGal. In the southern part, thehigh anomaly may be due to theinfluence of the southernmountain volcanic rocks, whilethe high anomaly in the northernpart may be caused by theinfluence of the basement whichis lifted more upward. Lowgravity anomaly which is showedby blue and green colour occupiesin the middle part that is extendsfrom east to west, this anomalyrange between (-38 to 9) mGal.Low anomaly in the middle part isprobably caused by sedimentaryrocks with lower mass density

values, areas with low gravity anomalies in the middlearea known as Kendeng zones which have thicksedimentary rock. Bouguer anomaly is a combination(superposition) from short wavelengths which isrepresent near surface anomalies and the longwavelengths which comes from the rocks in the deeplocations. To see the more detail geological structurespattern, the Bouguer anomaly needs to be filtered toseparate the influence of residual anomalies that iscome from the rocks in the shallower position withregional anomalies which is originated from the rocks atthe deeper position. Rocks with a deeper position areusually as a bedrock (crust to moho), usually have agreater density it will influence the Bouguer anomaliesvalue that measured on the surface, therefore it isnecessary to be separated regional and residual

anomalies from Bouguer anomaly, and tosee the shallow geological structures canbe used from the result of residualanomaly.

Spectral Analysis

Spectral analysis was done bymaking some line section of the Bougueranomaly map, then from the line sectionthe Fast Fourier Transform (FFT) weredone to determine the signal content alongthe trajectory to know the Amplitudespectrum and wave number. In thisanalysis, a Fourier Transform (FFT) willbe done on 12 line as shown in Figure 6,Spectral analysis was carried out at theselected line (P1-P12) to determine thesignal content at each line of the Bougueranomaly. The signal content in the

Figure 5. Graphical correlation between Amplitude and Wave Number in SpectralAnalysis

Figure 6. Bouguer anomaly patterns and spectral analysis line sections of theEast Java Basin


Bouguer anomaly line section is a combination of highfrequency and low frequency signals.

Graph example of wave numbers (k) versusamplitude (LN A) can be seen in Figure 7. Figur 7.a thatthe depth of residual discontinuity plane in the line P1 is3.14 Km while the depth of regional discontinuity planeis 38.88 Km. While on Figure 7.b shallow depth ofdiscontinuity plane is at 3.18 Km and depth ofdiscontinuity plane is 34.11 Km. The results of spectralanalysis can be used to determine the wave numbercutoff (kc), window width (N) and to estimate the depthof the shallow and deep discontinuities plane, theresults of the spectral analysis can be seen in Table 1.

From the table it can be seen that the average ofwave number cutoff (kc) is 0.099, this wave numbercutoff (kc) will be used to calculate the optimal windowwidth (N). The average of regional discontinuity depthis 36.3 Km, the depth reflect the regional anomalywhich is probably caused by undulation of the lowercrust or Moho plane.The average depth of the localdiscontinuity plane is about 3.2 Km, this depth indicatesthe shallow discontinuity plane which probably due to

the basement undulation pattern in this area. Beside toknow regional and local discontinuity plane, spectralanalysis can also be used to determine the optimalwindow width that used for the Bouguer anomaly datafiltering to produce residual and regional anomaly, fromthe table appears that the optimal window width is 12.7or rounded to N = 13 so that the optimum wavelengthfilter grid used is (60 x 60) Km. By using GeosoftOasismontaj software, a regional and residual anomalyis separated by entering the cutoff wavelength(optimum window width obtained from spectralanalysis results). Regional and residual anomalypatterns can be seen in Figure 8.

Figure 8(a) is a regional anomaly pattern whileFigure 8(b) is a residual anomaly pattern in the EastJava basin of Madura strait and surrounding areas.Regional anomaly patterns in Figure 8(a) showanomalies with longer wavelengths, this anomaly isprobably due to the influence of rocks with high densityvalues, and based on the results of spectral analysiscalculations is originated from a depth of about 36 Km,this anomaly may be caused by undulation of lower

Figure 7. Graph example of wave number (k) vs Amplitude (Ln A) Line P1 and P2

Table 1. Depth of the deep discontinuity plane (regional depth), shallow discontinuityplane (local depth), wave number cut-off (kc), and window width (N)


crust or Moho, where in the southern and northern partthe discontinuity plane are relatively curve upwardwhile in the middle part is relatively curve downward.This condition probably caused the formation ofkendeng zone where position is in the middle, relativelyconcave to downward and filled by a layer of sedimentthat is quite thick, Kendeng zone extends with relativeeast-west direction. Figure 8(b) displays the residualanomaly pattern of the study area, this residual anomalyshows an anomaly pattern with a shorter wavelengthand usually shows as a basement undulation pattern.Based on the calculation of the spectral analysis thedepth of this anomaly is around 3.2 Km, so thatthe basement in this area probably 3.2 Km depth.Low anomalies are shown in blue colour, theseanomalies indicate areas with lower densityvalues and filled by thick sedimentary rocks.High anomalies are shown in red colour, theseanomalies spread in the south and north area.High anomaly in the southern part may be causedby the presence of volcanic rock in the southernmountains, while in the northern part probablydue to the presence of the basement high which isrelatively rises to the top so that responds of theanomaly is higher.

Spectral Decomposition

Gravity anomaly that measured on thesurface is combination of anomalies whichoriginate from shallow sources (near surfaceposition) and usually has short wavelength, anddeep sources come from subsurface rocks that aredeeper position and have longer wavelength. Byusing spectral analysis that is by doing the FourierTransform (FFT) of the trajectory from selectedgravity data, it will known the signal contained ineach gravity trajectories data. Spectral

decomposition is a technique to parse the spatialdomain signal of the FFT so that anomalous sources canbe found from short wavelengths (shallow sources) tolong wavelengths (deep sources).

This technique is used to know the pattern ofstructure anomaly at a certain depth that we want toknow the geological structure related to the sedimentarybasins in the study area. In this study, spectraldecomposition will be conducted at depths of 1 Km, 2Km, 3 Km and 4 Km. The processing starts from FFT ofthe selected gravity data trajectory (P1-P12), then Wedetermine the wave number cut-off from signal (graph

Figure 8. Regional and residual gravity anomaly map of the East Java Basin in the Madura strait and surroundingarea

Figure 9. Graph and table of spectral decomposition analysis at depthof 1 Km


wave number (k) and amplitude (A)) until certain depthlevel that we want. Figure 9 shows an example graphand table of spectral decomposition at depth of 1 km. Inaccordance with the theory previously that thefrequency (f) has linear relationship with the wavenumber (k), so the higher frequency (wave number) thewavelength anomalous will shorter, otherwise thelower frequency (wave number), the wavelengthanomalous is longer. From Figure 9 it can be seen that todetermine a certain depth value of 1 Km by shifting thevalue of the wave number cutoff (kc). The wave numbercutoff results are used to calculate the optimum windowwidth (N) so that anomaly patterns will be found at adepth of 1 Km. Processing is performed on eachselected line and at other specified depths, so thatwavelength anomaly patterns at a certain depth will beobtained as shown in Figure 10.

Figure 10 shows anomaly patterns of spectraldecomposition results at several depths, namely 1 Km,2 Km, 3 Km and 4 Km. In general, the anomaly patternfrom top to bottom shows that the anomaly wavelengthis getting wider and closer to the regional structure. Thetop picture is the anomaly pattern of spectraldecomposition results at depth 1 Km, this anomalyshows a pattern with shorter wavelength and a morecomplex anomaly structure, it appears that the highanomaly is shown in red colour while the loweranomaly is shown in blue colour. High anomaly in thesouthern part at a depth 1 Km may be caused by theinfluence of volcanic rocks that intrude to the top whichcauses a more complex structure anomaly pattern. Inthe northern part of the high anomaly at a depth 1 Kmmay be due to the influence of the basement raised upwhich result high anomaly as basement high. Thepicture below is an anomaly pattern of spectral

Figure 10. Anomaly patterns at depths 1 Km, 2 Km, 3 Km and 4 Km based on spectraldecomposition analysis.


decomposition results at a depth 2 Km, the pictureshows the pattern of anomaly structure at a depth 2 Kmwhere the low anomaly (shown in blue colour) in thenorth part is probably due to the influence ofsedimentary rocks, sedimentary rocks are spreaded inseveral locations which is supposed as a sedimentarysub basin in the northern part of East Java, while in thesouthern part, the low anomaly probably influenced byvolcanoes with magma material so that the anomaly islow. The structure anomaly at a depth 3 Km and 4 Km isrelatively simpler, not as complex as at a depth of 1 Kmand 2 Km. At the deeper structure it is clearly seen thatthe pattern of high anomalies in the southern part showsthe influence of the igneous intrusion of volcanic zonesof the southern mountains of East Java that intrude tothe top which is support with continuous anomaly frombottom to top.

Qualitative Interpretation

The purpose of qualitatif interpretation is to knowthe spread of lateral anomalies pattern, interpretationwas done based on residual anomalies that obtain fromthe of spectral analysis results. Qualitativeinterpretation of residual anomalies is to know thepattern of basement high and delineation of sub basin asshown in Figures 11 and 12.

Figure 11 shows that the pattern of height structureanomaly is relative east-west direction, however thereare some structures having a relative north-southdirection. The structure pattern in the relative east-westdirection is most likely due toregional tectonic influence,which is due to thecompression force of thesubduction from the south ofJava to the north so that causesthe structure height torelatively east-west direction.The basement high pattern asseen from the residual gravityanomaly reflects theundulation of the basementwhich is relatively upwardposition so that of thesedimentary rocks in this areais depleting. In hydrocarbonexploration activities, thelocation of basement high isvery interesting to knowbecause hydrocarbons thathave already mature usuallywill migrate from sedimentdepocenter (lower location) toa higher location.

In addition to seeing the basement high structurepattern, qualitative interpretation was used to know thesedimentary sub-basin distribution patterns in the studyarea as shown in Figure 12. From the picture can be seenthat the sub-basin pattern is spread in the middle andnorth parts which are marked in green-blue color, thesub-basin in blue color is relatively thicker insedimentary rocks than to the green one. The sub-basinpattern reflecting the presence of sedimentary rockdepocentre which is sedimentary rock is accumulated.The thick sedimentary rock will be better, due to thepossibility of the existence of a higher petroleumsystem, although there is no guarantee that thicksedimentary rocks will be found the hydrocarbon.

Quantitative Interpretation

Quantitative interpretation was done by make 2Dmodels of residual gravity anomaly, the purpose ofmodeling is to find out the depth of basement, thicknessof sedimentary rocks, and the density value of rock inthe study area. Quantitative interpretation is carried outin three different line-sections as shown in Figure 13.a.Line section AA Êand CCÊ is relatively east-westdirection while BBÊ line section has perpendicular AAÊand CCÊ that is relatively north-south direction. Figures13b, 13c and 13d are subsurface 2D models createdalong the line section.

Figure 11. The height structure anomaly pattern of the study area based on residual anomaly


The reference of density value that is used in 2Dmodeling is based on stratigraphic information andbook literature density according to Telford et al.,(1990). Figure 13.b shows the line-section of the 2Dsubsurface geological model along the AAÊ profile,from the figure can be seen that the basement in thestudy area interpreted as a continental and metamorphiccrust with a mass density value of 2.7 gr/cc. Above thebasement interpreted as Paleogen-aged sedimentaryrocks with a mass density value of 2.4 gr/cc. ThisPaleogen sedimentary rock is combination of theNgimbang, Kujung and Prupuh Formations consist ofsandstone, shale, limestone and coal. Above itdeposited Neogene-aged sedimentary rocks which arecombination of the Tuban Formation, Ngrayong,Wonocolo, Ledok, Mundu consist of sandstones, shale,marl, limestone, claystone, siltstone with mass densityvalues of 2.25 gr/cc. The BBÊ line section has a north-south direction perpendicular to the AAÊ line section,this cross section passes through the southern MaduraIsland, the Madura Strait, the mainland of Java inPasuruan to the south of East Java area. The subsurfacegeological model of BBÊ line section as shown in Figure13c, the figure shows that the bedrock is continentalcrust with a mass density value of 2.7 gr/cc, the upperlayer is a Paleogene sedimentary rock with a massdensity value of 2.4 gr/cc and the top layer is aNeogene-aged sedimentary rock which has a mass

density value of 2.25 gr/cc.The CCÊ line section islocated in the south of the BBÊline section, same as the AA'and BB' line section, in theCC line section the bedrock isprobably as continental crustthat have to metamorphismprocess with a mass densityvalue of 2.7 gr/cc, upperlayers of bedrock Paleogen-aged sedimentary rock consistof sandstone, shale, limestoneand coal with a mass densityvalue of 2.4 gr/cc, and the toplayer is Neogen-agedsedimentary rock consist of acombination of the Tuban,Ngrayong, Wonocolo, Ledok,Mundu Formationscomprising of sandstone,shale, marl, limestone,claystone, siltstone with amass density value of 2.25 gr/cc. The geological history of

the East Java basin is closely related to the tectonicactivity in the Southeast Asian region and controlled bythe interaction of the Indo-Autralia Ocean plate whichmove to north, the Pacific ocean plate move to the westand the relatively stable Eurasian plate. This tectonicplate movement activity resulting the basement highand graben structure as shown at (Figures 11 and 12).The East Java Basin was formed in the Eocene, startingwith rifting and spreading of the basement then fillingby the sedimentary rock and continued by formed ofanticline structure. Basement configuration that havebeen deformed into heights and lows structure make thebasins in this area rich in hydrocarbon (Panjaitan,2010). The result of spectral decomposition analysis(Figure 10) shows the pattern of anomaly structuresfrom 1Km to 4Km depth, at 1Km depth the structure ofthe anomaly is more complex, it can be seen from thenumber of anomaly closure both high anomaly and lowanomaly. The red colour anomaly in the southern part isprobably caused by the presence of volcanic rocks thathave high density, while in the northern part of the highanomaly is probably due to the lifting of the basementrock. The structure of the basin is clearly visible at 2Kmand 3 Km depth, at this depth the basin shown in bluecolour while the height is shown in green-red colour.This structure pattern at a depth of 2 Km and 3 Km islikely to be interesting for further research from theaspect of petroleum systems because there are manygraben and anticline which are probably as a place for

Figure 12. Delineation of sedimentary sub-basin pattern in the study area based on residualgravity anomaly


hydrocarbon accumulation. The structural pattern atdepth of 4 Km shown a relatively homogeneousanomaly pattern where the low anomaly pattern in themiddle is shown in blue colour which according to(Satyana, 2008) is a low axis (depression) which extendfrom Bogor, North Serayu, Kendeng to Madura Strait.Sedimentary rock with Miopliocene and Plistoceneaged were quickly deposited on this low andcompressed then resulting fold and thrust belt structurein the middle of Java island. Miopliocene andPlistocene-aged sediments were quickly depositedrapidly on this low and compressed resulting in foldsand thrust belt in the middle of Java Island. The lowanomaly in the middle part known as the Kendeng zoneis interesting because sediment deposits in this area arequite thick and based on Satyana (2016), the East JavaBasin and its surroundings area, it is possible forpetroleum system in kenozoic-aged based on deepseismic, geochemical and tectonic data.

CONCLUSIONThe result of spectral analysis from gravity data

shows that the average basement depth of the study areais around 3.2 Km, the number of sub-basins detectedbased on the analysis of residual anomalies is 10 sub-basins. The result of spectral decomposition of gravitydata shows that the subbasin is visible at depths of 1Km, 2Km and 3 Km, whereas at a depth of 4 Km thesubbasin pattern is not clearly visible, this is probablythat at depth of 4 Km reflecting basement undulation.The structural pattern that can be found from the resultsof qualitative and quantitative analysis are basementhigh, graben, and fault. 2D modeling results shows thatthe basement of the study area interpreted as acontinental crust and metamorphic rock with a massdensity value of 2.7 gr/cc. The results of gravityanalysis are useful for knowing graben patterns andbasement high which are interesting for further researchto find out the more detail of petroleum system in thisarea.

Figure 13. Line section direction and subsurface geological model of AA, BB, and CC sections in the East Java basin onthe Madura Strait and surrounding area.


SUGGESTIONDetail geophysical method such as 3D seismic

survey based on different physical properties need to becarried out to know structural patterns and hidrocarbontrap arround Madura Strait subbbasin and surroundingarea, East Java.

Acknowledgements

The Author would like to thanks to the Director ofMarine Geological Institute for his direction, Directorof Center for Geological Survey thanks for his supportto use the land gravity map, Coordinator of Explorationand Site Survey of Oil and Gas Infrastructure for hissupport during preparation this article, and Scientificteam who collaborate to finishing this paper.

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delineation of sedimentary subbasin and subsurface

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