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JCS2005-S029

SEISMOFACIES STUDY IN EARLY FILL TO SOURCE ROCK DEPOSITIONAL ENVIRONMENT, ASRI BASIN

M. Syarif, Bintoro W., and Reno F.

CNOOC SES, LTD

ABSTRACT Asri basin is known as proven oil productive basin since Intan field discovery in 1987. The main exploration and exploitation effort was made on relatively shallow Gita and upper Zelda formation for more than 20 years. However only few of publications was made discussing early basin fill in the Asri basin and it’s well known Banuwati lacustrine shale source rocks. Seismic facies map of early basin fill facies study was done based on 2D seismic interpretation to identify the general boundary of lateral distribution of the defined facies so called “A”, ”D”, “H”, “M” type facies (in alphabetical order). They were calibrated by four well which was drilled penetrate Banuwati formation trough basement in the flank and around Asri depocenter. Two of wells found very good source rock quality with TOC content around 1.6 to 3.7 %weight correlate with type “A” and “H” facies. Others two found the Banuwati formation with different kind of shale (“D” and “M” type facies). The collaborated well log-seismic facies map suggests the sedimentation of Asri basin early fill relatively was in alluvial-fluvial setting with north-south sediment transport direction. Locally, additional sediment transports coming from west to east in the west flank and some alluvial fan and/or slump come from the east flank side. FMI interpretation also suggests that the source of deposition is coming from these directions. As the result, Banuwati Coarse Clastic member divided into alluvial fan facies in the eastern part, braided stream deposits facies in the central depocenter, fluvial and alluvial facies in the eastern part (distal part of alluvial fan), and floodplain and abandonment facies on the western part of the basin. The lacustrine source rocks was deposited in transgressed event, right after deposition of alluvial/fluvial sedimentation formed relatively big lake environment covering all the Asri basin depocenter far away to its flank.

INTRODUCTION Asri basin in southeast Sumatra (SES) has approximately produces half billion barrels oil from several fields including Intan and Widuri oil fields (Figure 1). All of them are producing from Gita and upper Zelda reservoir with depth ranging from 3300ft to 3600ft depth (Figure 2). This Gita and upper Zelda formation reservoir are known as reservoir in the Banuwati-Talang Akar petroleum system of Asri basin (Sukanto et al., 1998). Sukanto et al. (1998) proposed another

petroleum system after Hariet-2 well penetrate oil reservoir in Banuwati Coarse Clastic member below Banuwati Shale member named Banuwati-Hariet member petroleum system. (Figure 3). Many publications are focused on Gita and upper Zelda formation Gardner et al. (1999), Carter (2003), etc. However, deeper section like Banuwati formation is also discussed in some report. Zhu et al. 2005 proposed a tectonic and sedimentary process in Sunda and Asri basin, include Banuwati formation in the first sequence of the basin regional framework from the tectonic

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and stratigraphic point of view. Most of their analyses come from regional line therefore they rely on textbook model. Guritno (2005) works mainly on tectonic evolution and proposed segmented structural development in the early basin development stage. Exploring the deeper section in Banuwati formation is becoming a new strategy to add and enhance reserve. Hariet-2 well, which was drilled in 1995 until 12, 516’ MD (12, 435’ sub sea TVD) until granitic basement, penetrates the early fill sediment, which called Banuwati Coarse Clastic member (Figure 4). This Eocene-Oligocene coarse sandstone and conglomerate is deposited unconformably right beneath granite basement. On the top of Banuwati Coarse Clastic member are laying black-organic rich Banuwati Shale, which has 310 feet thick. This Banuwati Shale is known as source rock of Asri basin (Figure 4). In this paper, we want to focus the discussion mainly on the early fill depositional system, which is the first sequence in Asri basin, extracting from seismic facies analysis. REGIONAL GEOLOGICAL SETTING AND

STRATIGRAPHY OF THE AREA Asri Basin is one of the series tertiary basins located on the Sunda shield. A series of north-south trending extensional faults creating some half-graben depocenters, dividing the basinal area in to some sub-basin (Asri, Sunda and Hera basin) within this area (Sukanto et al, 1998). The syn-rift sediments consist of Zelda member and Banuwati formation (Wight, 1987). The Banuwati formation is divided into two members, Banuwati Shale and Banuwati Coarse Clastic member. This Banuwati formation especially Banuwati Shale, is the only recognized source rock which provides about 1.2 billion barrels of proven oil reserves in this area. There are two petroleum systems which has been recognized in Asri basin, Banuwati-TAF (!) Petroleum system and Banuwati-Hariet/Banuwati clastics (.) (Sukanto et al., 1998). The Banuwati-Hariet (.) petroleum system was not proven until the drilling of Hariet-2 well, which penetrated 310 feet of Banuwati Shale, and 377 feet of Banuwati

Coarse Clastic reservoir. This reservoir had fair to good oil show while drilling and the average porosity is about 13.6% with permeability up to 31 mD. The DST test gives 24 barrel of oil and 16 barrel of water. The reservoir is suspected to be damage since very heavy mud weight is used (up to 15 ppg) and total lost circulation occurred while drilling. On the top of the Banuwati Coarse Clastic member, Hariet-2 penetrates the Banuwati Shale member with 310 feet of thickness. This anoxic black shale has very good TOC (5.12% wt%.) and type 1 oil prone kerogen source rock. This Banuwati Shale is interpreted as deep lake depositional environment. There are four wells in Asri basin already drilled, which has penetrating the early fill Banuwati Coarse Clastic member interval. They are Hariet-2 (1995), Darlene-1 (1996), Mega-1 (2003) and Anastasia-1 (2004). All these wells are targeting Banuwati Coarse Clastic member as reservoir.

SEISMOFACIES

Seismic reflection parameters consist of its configuration, continuity, amplitude, frequency, internal velocity, and some external form associated with seismic facies unit (Mitchum et al., 1977). These parameters are the component of seismic facies analysis as description to the geologic interpretation of seismic reflection in general. Since we will not predict any direct hydrocarbon indicator or related velocity anomaly, we use only reflection configuration, continuity, amplitude, and the external form for our facies analysis. These factors will imply to the interpretation of gross stratification patterns, depositional energy, predicting lithology and depositional setting. We use 2D seismic data from different year survey and processing flow due to illumination effort. Therefore we consider possible different perspective in the seismic image. The seismic covered all Asri basin area by 0.5 Km grids in the west flank area near existing field and 1 to 2.5Km grid at the rest.

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Average interval velocity in the Banuwati Shale and Banuwati Coarse Clastic member is 12000 ft/sec and 9000ft/sec respectively with dominant frequency 25-30Hz. Therefore, seismic resolution for quarter wavelength of both Banuwati Shale and coarse clastic member is 75-120 ft. This is means one seismic wiggle will represent 300 to 480 ft. It is difficult to define depositional environment vertically in detail due to poor seismic resolution. However, we can extract more information and bound the interpretation using well-log data provided. Firstly we picked seismic facies from four key wells that gave four different facies type, and later on the seismic facies is mentioned based on well-type facies. The wells are Anastasia-01, Darlene-01, Hariet-02, and Mega-01 and the correlated seismic facies is simply called “A”, “D”, “H”, and “M” type facies. Discussion will begin with Banuwati Coarse Clastic member followed by Banuwati Shale member. Banuwati Coarse Clastic Member Figure 5 is the “H” type facies. It is conformable with Banuwati Shale member at the top and onlapping surface into basement at bottom. Reflection configuration is mostly parallel-subparallel (a) and divergent basinward direction (b), moderate to high amplitude, and moderate to high continuity. The parallel reflection configuration suggests high energy depositional with uniform rate of sedimentation and change laterally basinward probably due to some more axial influx from the north. The “A” type facies bounded by conformable surface at top and erosional surface at bottom on top of basement. Its reflection configuration is sub parallel if flattened but we like to interpret mostly as contorted configuration, moderate to high amplitude and moderate continuity (Figure 6). We interpret this facies in relatively high depositional energy regime. The “H” and A” facies is distributed widely in Asri basin. Figure 7 shows “M” type facies, which bounded by conformal surface at bottom and erosional surface at the top. Parallel reflection configuration, moderate to high amplitude and moderate to high continuity. We suggest that “M” facies is in relatively low

depositional energy regime with uniform rate of sedimentation. They distributed mostly in southwest part of the Asri. The last example is “D” type facies bounded by eroded basement at bottom and erosional surface of overlying Banuwati Shale formation at the top. It has chaotic reflection configuration, low to moderate amplitude, and low continuity (Figure 8). The reflection configuration suggest slump or debris deposits. This facies is distributed in the east flank of the basin in the footwall of the bounding fault, which creates this high-energy depositional regime. Banuwati Shale Member In Banuwati Shale interval, the “H” and “A” type are the same both seismically and lithologically, therefore this interval is only divided into three seismic facies: “M”, “D”, and “H/A” type. The “H/A” type is bounded by conformable surface at bottom and toplapping surface at the top with some erosional features especially in the basin flank area. Reflection configuration is mostly parallel with divergent indication from Hariet location in basinward direction, high amplitude, and high continuity (Figure 5 and 6). The reflection configuration suggest uniform rate of sedimentation and small change laterally in the basin center. This facies is spreadout covers most of Asri basin with some erosional indication specifically at west flank of Asri. More erosional features at bottom and conformable surface at the top bound the “M” type facies. Subparallel reflection configuration with moderate amplitude and moderate continuity (Figure 7) characterized this facies type. This facies is specifically found in the southwest area. Erosional features at top and bottom bound “D” type facies. Its reflection configuration is chaotic, low amplitude, and low continuity (Figure 8). The “D” type is spreadout along the bounding fault of the eastern part of Asri. This facies has similar character with the “D” in the Banuwati Coarse Clastic member suggest periodic sedimentation of slump and debris deposits from the high relief in the eastern part.

WELL DESCRIPTION Banuwati Coarse Clastic Member This formation lies beneath the granitic basement as early fill sediment when the basin started to

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form. From FMI, mudlog and log analysis of Hariet-2 well (Figure 4), which located in Western part of basin depocenter, the lithology consists of gravel conglomeratic on the lower zone. This lower zone comprised entirely a series of stacked, thin (1-2 ft), erosive, probably channelized unit with occasional matrix support conglomerate. From each individual unit (8-10 ft) shows a broad fining upward sequence comprising an erosional lower bounding surface, gravel/coarse sand lag, and unstratified gravelly sand or matrix-supported conglomerate. The erosional also shows scours and load cast features indicating high-energy flows (Figure 9). This matrix supported conglomerate indicates possibility of this lithology is being deposited in minor debris flow with contain high mud during floods event or within a confined channel system. Overall, this lower section is interpreted as stream-flow deposited in braided fluvial system or in braided fan. The FMI interpretation also indicates NW and N source of deposition (Figure 9). In the upper part, Banuwati Coarse Clastic in Hariet-2 well is consist of fine to medium grain to coarse grain sand with common scouring, erosional bounding surfaces. Finer grain usually common in the upper part of this section, with very well sorted and shows regular planar bedding in FMI images (Figure 9). These sand are likely to develop as channel fills in braided stream or in distal braided fan system with channel switching (Figure 10). The sediment is also suggesting NW trend of sediment source. In Darlene-1 well on eastern side of the area (Figure 3 & 10), the lithology is consisting of very coarse, immature grains (granite, basalt, quartz, feldspar, kaolinite and mafic mineral). From mudlog data the typical of conglomerates is clear, grey, green to dark green, white hard, friable, and subrounded to angular. The matrix is fine to medium sand. This conglomerate is very tight and has no porosity and permeability. From mud log analysis, the shale in Darlene-1 also can be divided into two parts; the reddish part in the lower section and the gray and dark brown shale matrix in the upper section. This shale matrix is interpreted related to sub aerial environment in the lower part and become sub aqueous in the upper part. All the section in

Darlene-1 is interpreted as alluvial fan facies, with immature conglomeratic facies, mafic mineral from basement source and the immature lithic fragments supports the interpretation as alluvial fan deposits (Figure 10) Stratigraphically, this facies is equivalent with Banuwati Coarse Clastic, and interpreted as alluvial fan deposited created by big normal fault in the eastern boundary of this basin. Anastasia-1 is located about 3 km NW of Darlene-1 shows different characters. The Banuwati Coarse Clastic interval consists of fining upward sandstone sequence and shale intercalation. Clean mature sandstone, with subarkose composition, medium to coarse grain, subangular to sub rounded moderate to well sorted and some organic matter dispersed in laminae. The shale is laminated, with some microcrystalline siderite. Wireline log shows some fining upward sequence and followed by coarsening upward sequence below the Banuwati Shale interval. This interval is interpreted as fluvial channel system with some abandonment facies and interfluves in the upper part (Figure 10). Mega-1, which drilled on the SW of the basin depocenter, penetrates 198 feet of Banuwati Clastic interval. The lithology consists of intercalation of shale and sandstone with some coals section. The sandstone is very argillaceous, silty and abundant of muddy matrix (Figure 10). Log analysis suggest that Mega-1 is mainly consists of coarsening upward sequence with thin (4ft) fining upward sequences. The depositional environment is interpreted as a part floodplain deposits and to the upper part is considered as abandonment and marsh and swamp deposits. Banuwati Shale Member After the deposition of Banuwati Coarse Clastic member, Banuwati Shale is deposited on top of this member. In Hariet-2 well, The Banuwati Shale has 310 feet thick with black, bedded organic rich, has TOC ranging from 3%-5% with HI up to 400 (Sukanto et al., 1998). The kerogen is 80% of amorphous algae, 5-20% vitrinite and 5% inertinite. The Pediastrum sp. Suggested freshwater environment and the recovery of M. Howardii and Echritiporites sp. In core section

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indicates Oligocene age. This Banuwati Shale is found almost in entire area (except in Darlene-1 well where the facies are conglomeratic facies, Figure 12). Some coarsening upward sequences, high resistivity, and low density characterizes this black, organic rich shale. Palynology and well log analysis suggested that this section is deposited under sediment starved, anoxic deep lake environment, which progressively shallower through time into shallow lake and possibly fluvio-lacustrine when reach lower Zelda member. Banuwati Coarse Clastic in Mega-1 well shows different characteristics than in Hariet-2. The Banuwati Shales have less TOC contents and has gray to reddish brown shale, suggesting some oxic condition. Paleosols possibly develop in the succession. This shale is interpreted as part of interdistributary bay or interfluves to shallow lacustrine environment and gradually to fluviatile environment on the upper part.

DEPOSITIONAL ENVIRONMENT Early fill sedimentation of Asri Basin can be distinguished into two parts incorporating seismic facies analysis and well log analysis: early fill coarse clastic depositional system and low energy fine clastic system. In the early basin forming, the accommodation space still less than accommodation space, therefore coarse clastic is developed around basin area. FMI analysis suggests main source of depositional environment comes from north, with minor source come from local depositional high in western and eastern area (Figure 11). The depositional environments vary from braided river system in the middle part of the basin, the abandonment facies and floodplain in western part and some alluvial plain in between the braided river system. In the Eastern area, the sub-aerial alluvial fans also develop mainly from the early form of the big normal fault system. Some minor fan (medial-distal) is also developing in the western and northern area (Figure 11, purple color), which is coming from local topographic highs at that time.

The fine clastic system is developing in this area when the accommodation space is bigger than the rate of sediment supply that related to basin form accelerating of its subsidence rate. The depositional environment is become less of coarse clastic, sediment-starved and become deeper relating to the underlying sediment. This transgressive event creates deep lake in the central of the basin, and possible shore lakes, floodplain and interdistributary bays in the western area. Alluvial fan (sub-aqueous) possibly occurs in the eastern area adjacent to the normal fault system (Figure 12). This oil prone finer grain clastic sediments provides the best and the only known type 1 source rock for this basin, and also can be act as seal for the reservoir below.

DISCUSSION

The seismic generated facies was generally constructed independent. However, since well log data support the interpretation, we incorporate the interpretation directly informs of the facies map. The “M” and “D” facies in the Banuwati Coarse Clastic interval are more unique both from seismic and the well-log interpretation. The “A” and “H” is more uniform in the seismic but gives more interpretation in the well-log interpretation due to its vertical resolution. Since Hariet-2 gives more fan type rather than channel system compared to Anastasia-1, The “H” type is mostly represent the fan part of the whole system in this area. This is does not mean that braided channel system in figure 11 is whole comes from “A” type but mostly from the parallel-sub parallel, moderate-high amplitude and moderate continuity type of the “H” and “A” type (Figure 5-6). The Banuwati Shale seismic interval gives more uniform seismic feature due to uniform in lithology. Both of seismic feature and correlative lithology is spread out cover all the basin area. The basin filling succession of early basin fill is function of variable mainly on tectonic/rift, climates, erosion/sedimentation, and some volcanism (Cohen, 1990). In the Asri basin, the forming of early fill sediments is start from

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alluvial/fluvial setting. This sediment is possibly related to basin forming, or in pre-rift or still in sag basin (Guritno, 2005). The depositional pattern of the facies clearly represents at least two distinct facies unit, which is early alluvial-fluvial stage and shallow-deep lacustrine stage. The early alluvial-fluvial stage in this basin is affected mostly by sedimentation process. The sedimentation is generally in high depositional energy regime, which would create braided system rather than meandering rivers system. However, some ponds or small shallow lake possibly has been deposited in the basin center since the well-log and seismic data is limited. The shallow lacustrine stage is recognized in the Anastasia-1. Hariet-2 has no shallow lacustrine facies indication, means some erosional has occurred along the west flank during this stage. This can be happening because of some tectonic process in the western flank where the shallow lake possibly eroded out by alluvial fan. When the tectonic process of basin forming becomes more active, accommodation space is created bigger than relative sediment supply, and transgression event is occurred. The basin become deeper and Banuwati Shale member deposited relatively spread out and more widely than Banuwati Coarse Clastic member. The facies can be recognized as shallow lake in the margin of the basin depocenter and deep lake around middle of basin depocenter.

CONCLUSION

There are four seismic facies related to Banuwati Coarse Clastic member in Eocene-Oligocene age in the Asri Basin. These seismic facies can be correlated with geological depositional environment, which is alluvial fan, fluvial-braided system, fan lobe, and floodplain/abandonment facies. When the accommodation space is greater than sediment supply, which probably cause by activation of rift initiation or basin sag/subsidence process, the Banuwati Shale member is deposited as transgression facies in lacustrine system.

ACKNOWLEDGMENT

We thank Nusatriyo Guritno for his advice, support and discussion, CNOOC management, BPMIGAS, and CNOOC Partners for data permission, CNOOC peer for their discussion, and CNOOC drafting personnel for preparing some figures.

REFERENCES Aldrich, Jeffrey B., Gary P.R., Susandhi R., Martin A.S., 1995, Paleogene basin architecture of the Sunda and Asri basins and associated non-marine sequence stratigraphy, Proceedings of the International Symposium on Sequence Stratigraphy in SE Asia, IPA. Brown, Jr. L. F., and W. L. Fisher, 1977, Seismic stratigraphic interpretation of depositional systems: examples from Brazilian rift and pull-apart basins, in Charles E. Payton, ed., Seismic stratigraphy-application to hydrocarbon exploration: AAPG Memoir 26, p. 213-248. Butterworth, Peter J., Christopher D. Atkinson, 1993, Syn-rift deposits of the northwest java basin: Fluvial sandstone reservoirs and lacustrine source rocks, IPA Clastic Core Workshop. Carter, David Charles, 2003, 3D seismic geomorphology: Insights into fluvial reservoir deposition and performance, Widuri field, Java Sea, AAPG Bulletin, v.87, No 6, PP. 909-934. Cohen, A. S., 1990, Tectono-stratigraphic model for sedimentation in Katz, B. J.ed., Lake Tanganyika, Africa in Lacustrine Basin exploration; case studies and modern analogs, AAPG Memoar 50, 1990 Gardner M., Jim M.B., Donna S.A., Jonathan S., Nunuk F., Istanto, 1999, Sequence Stratigraphy and Play Concepts: Gita Member of Talang Akar Formation, Asri and Sunda Basins, Technical Report. Guritno, N., 2005, Sedimentasi awal cekungan Asri dan struktur yang berkembang pada sedimen tersebut, Master Thesis, ITB. Lambiase, J. J., 1990, A Model for tectonic control of lacustrine stratigraphic sequences in

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continental rift basin in Katz, B. J. ed. 1990,, Lake Tanganyika, Africa in Lacustrine Basin exploration; case studies and modern analogs, AAPG Memoir 50. Mitchum, Jr. R. M., P. R. Vail, and J. B. Sangree, 1977, Seismic stratigraphy and global changes of sea level, Part 6: Stratigraphic interpretation of seismic reflection pattern in depositional sequences, in Charles E. Payton, ed., Seismic stratigraphy application to hydrocarbon exploration: AAPG Memoir 26, p. 117-133. Sangree, J. B., and J. M. Widmier, 1977, Seismic stratigraphy and global changes of sea level, Part 9: seismic interpretation of clastic depositional facies, in Charles E. Payton, ed., Seismic

stratigraphy-application to hydrocarbon exploration: AAPG Memoir 26, p. 165-184. Sukanto, J., Nunuk F., J.B. Aldrich, G.P. Rinehart, J. Mitchell, 1998, Petroleum Systems of the Asri Basin, Java Sea, Indonesia, Proceedings 26th IPA Annual Convention. Wight, A., Friestad, H., Anderson, I., Wicaksono, P. Remington, C.H., 1997, Exploration History of the Offshore Southeast Sumatra PSC, Java Sea, Indonesia, in Petroleum Geology of Southeast Asia, Fraser, Matthews, and Murphy (eds.), Geol. Soc. Sp. Pub. No. 126, 121-142. Zhu, X., Qi J., Zhong D., Yang Q., Zhang Q., 2005, Tectonic and Sedimentary Research on the Sunda and Asri Basins, Technical Reaport.

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FIGURE 1: Asri basin Location map

FIGURE 2: Stratigraphic column of Asri Basin

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FIGURE 3: Seismic section W-E in Asri Basin, showing the depocenter. Banuwati Shale is shown in dark green area, and Banuwati Coarse Clastic Member in light green area. Note that all the early basin fill

facies are pinch out to the basement high in the west, where in the east in changing facies and terminated in the big normal fault.

FIGURE 4: Hariet-2 well type log

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FIGURE 5: "H" type facies. Darkgreen, lightgreen and red horizon are Banuwati Shale member, Banuwati Coarse Clastic and basement respectively. Banuwati Coarse Clastic reflection configuration mostly parallel-subparallel (a) and divergent at basinward direction (b), moderate-high amplitude, moderate-high continuity. Purple color in Figure 11 mostly represented by the parallel, high amplitude, high continuity reflection configuration whereas the green is combination with the "A" type (Fig.6) Banuwati Shale member reflection configuration mostly parallel, high amplitude, high continuity.

FIGURE 6: "A" type facies. Horizon is the same as previous figure. The Banuwati Coarse Clastic member reflection configuration has subparallel-contorted, moderate-high amplitude, moderate continuity. Banuwati Shale member is the same as in the "H" type (Figure 5).

FIGURE 7: "M" type facies. Horizon is the same as previous figures. The Banuwati Coarse Clastic member shows more erosional at the top. The Banuwati Shale reflection configuration is chaotic, low amplitude and low continuity.

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FIGURE 8: "D" type facies. Horizon is the same as previous figures. The Banuwati Coarse Clastic member bounded by erosional surfaces with chaotic, low amplitude, low continuity reflection configuration. The Banuwati Shale member shows the same reflection configuration as the underlying layer.

FIGURE 9: FMI (Formation Micro-Imager) of Hariet-2 Well, showing gravelly-conglomerate facies with erosion surface on the lower part of Banuwati Coarse Clastic Member (right side). On the left side, The FMI shows some minor erosion surface, thin planar laminae, and scattered mudclast, which found in

Upper part of Banuwati Coarse Clastic Member.

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FIGURE 10: Schematic regional cross section from Mega-1, to Darlene-1 (relatively W-E), showing different facies change in Banuwati Coarse Clastic Member, green is interpreted as floodplain and abandonment facies, gray as main fluvial system (braided), yellow as an alluvial fan system.

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FIGURE 11 Facies map interpretation of Banuwati Coarse Clastic Member, shows the possibly direction of depositional source, the depositional environment and related position for the basin. This interpretation

based on seismic facies analysis, well data and well log analysis.

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FIGURE 12: Facies map interpretation of Banuwati Shale Member, shows the possibly direction of depositional source, the depositional environment and related position for the basin. This interpretation

based on seismic facies analysis, well data and well log analysis.