Stratigraphy Seismic and sedimentation Development of Middle Baong Sand , Aru Field , North Sumatera Basin

This paper defines the stratigraphic sequence focused on Middle Baong Sand. The analyses aim to understand the sedimentation pattern regarding to sequence stratigraphy model including its lateral and vertical succession based on seismic and well data. The study can be used in ranking the prospect for new oil field. Based on the analyses in 39 seismic sections and 2 wireline log, the area are consist of three depositional sequences, namely sequence I (consist of HST I) Sequence II (consist of TST II and LST II), and Sequence III (consist of TST III). Baong Formation are deposited when the sea level are rising regionally at Middle Miocene (N7-N15) makes the sediment deposited in deep water environment. while Middle Baong Sand are deposited in the minor falling stage placed at N13 (Middle of Middle Miocene). In this episode, there is a change in depositional setting from bathyal to middle neritic. Clastic origin of this deposits were interpreted came from South-South West direction or from Bukit Barisan where at that time is started to uplift.


Introduction
Popular geophysical method now days in exploration is seismic reflection because it could gave valuable information to geologist about detail of subsurface condition, such as stratigraphy and the geological structure.The information extracted from seismic data further used as as tool for hydrocarbon prospecting at interesting area.Through an understanding of sequence stratigraphy, Seismic data can be used to give us the information about important geological factors which control patterns of stratigraphy layer.
Considering many prospect in North Sumatra that have not been explored, it was very interesting to accomplish stratigraphic sequence analysis in order to obtain new oil and gas reserves, particularly in the area of Aru (Fig 1).One of the main reservoir in the area is Middle Miocene Baong formation that deposited in deep water environment dominated by shally facies.At the time of the deposition of this formation occurs transgressive phase resulting the coarse sediments known as the Middle Baong Formation.Seismic Interpretation This interpretation is used to determination of horizon boundary distribution in 2D and system tract horizons by analyze the seismic internal configuration.

Regional Geology
Main characteristic of North Sumatera Basin is a series of North South fault patterd that formed high and low structure (horst and graben or half graben (Fig 2).The structural trend is controlled by the basement main configuration that influenced by north south tectonic activity.
The Early Tertiary sediment at North Sumatera Basin was known as a product syn-rift deposition at Early Oligocene Early Miocene (  Baong Formasion was deposited coincide with regional sea level rise.It shown with global sea level rising at 15,5 million years ago (N8-N9) and supported by depositional enviroment changing from paralic to bathyal which related to major subsidence.

Seismic Interpretation
39 Seismic section is used in this research (Fig 4).Section interpretation is specially on Middle Baong Sand Depositional, marking by green horizon and the bottom is marking by red horizon (Fig 5).There are four seismic facies on Middle Baong Sand that can be observed in seismic section.They are identified based on its internal seismic configuration and termination of the waveflet (fig 7).
a. Facies I.The lowest part of Middle Baong sand.Characterized by downlap termination and sigmoid progradation.This facies can be found in every trajectory of the seismic with almost similar thickness.Based on termination and internal configuration that present in this facies, it can be identified as highstand system tract sediment.b.Facies II is the second layer of the middle Baong sand.Characterized by interal sub paralel cofiguration.This facies was not developed in every trajectory of the seismic, lenses can be identified in seismic cross-section, lies from south to north south.c.Facies III is the third layer of this middle Baong sandstone.Characterized by downlap termination.This facies developing almost on all trajectory of the seismic.Lenses can be identified by the seimic cross section.This facies lies from north to north east, which lenses lies from south to south west of the resarch area.Based on termination and internal configuration that identified in this facies, it can be interpreted as low stand system tract sediment.d.Facies IV.The upper part of middle Baong sand.Characterized by internal subparalel.This facies can be found in every trajectory of the seismic.Based on termination and internal configuration that identified in this facies, it can be interpreted as transgresive system tract sediment.Based on seismic data analysis, research area consists of three sedimentation sequences, which are :Sequence I that consist of Highstand System Track (HST)-1, Sekuen II, consist of Transgressive Sytem Track (TST)-II, and Sekuen III that consits of Low stand system track (LST)-III and trasngressive system tract (TST)-III (fig 8).The lateral continuity of each of the sytem track were relatively similar, spreading almost all over the research area, because of the relative similar location and the time of sedimentation.The pattern of spreading affected by the geometry of sedimentary base which is the upper part of lower Baong Shale.In general, the highland lies in the south south west area, while the lowland area lies in north-north east area.This condition affect the thickness of system track which tend to thicker to north east filling the lower part.
The middle Baong sandstone in the research area is a member of Baong Formation, in lithostratigraphic dominated by deep sea shale facies.While in the middle sandstone Baong contain sandstone intercalated with shale.This caused by changing of water depth from bathyal to middle neritic caused by tectonic activity and drop of sea level present at the time of deposition (Situmoreang et., al., 1994).This condition affected the acomodation space and sediment supply.
Regionally, Sandstone of Baong Formation in midlle part exist on the two part of basin, but the sedimentation not reach to basement basin (deposenter) from North Sumatera Aru Basin due to minimun sedimen acomodation.
Regionally, previous research prove that in the early Midlle Miocene (N7-N13) base part of Baong sedimen resource is from Malaka shelf in North part.On that time, Malaka shelf and Ashan arc been the mainly resource of sedimen in North Sumatera Basin.High structural is located in the south / southwestern area while lower structural has north / north-east direction.Tectonic activity in the area is compressional tectonic.

Fig 9 .
Fig 9. Schematic Configuration of Structure in Research Area in Middle Miocene (N13 N16), without scale Sedimentation of the basin is started by Transgresive Sequence of Pre-Baong such as Parapat Formation, Bampo and Belumai.This sequence is deposited continuously until Middle Miocene when this area is covered by sea until the deposition of Baong shale in base part.Sedimentation material resource predicted from East direction of basin, that is from Malaka Shelf which dominated by fine grained sediment.In Late of Middle Miocene, occurred uplift of Bukit Barisan and influence the depositional environment at that time.Sea levels begin to shrink and change bathyal environment be top and middle neritic.At that time there was a change of origin deposition material that originally came from the east be from the west (Fig 10) (Bukit

Fig 10 .CONCLUTIONS 1 .
Fig 10.Sedimentation history of Middle Baong Sand Deposition of Middle Baong Sand end with the quiesence of tectonic activity.High elevation caused by tectonic activity previously eroded and resulted in the seismic unconformity above Middle Baong Sand.Sea level becomes bathyal again, and the resulting depsits was also changed back into shale.The upper Baong Shale start to deposits.CONCLUTIONS 1.There are four electrical facies can be found in the region, namely: facies 1 characterized by fining upward pattern logs, facies 2 which is characterized by a blocky log patterns, facies 3 is characterized by upward coarsening patterns and facies 4 characterized by a blocky pattern