East Natuna Basin

11.1 INTRODUCTION

The East Natuna Basin is located to the west of Kalimantan Island (Figure 1). It is filled with clastic Tertiary sediments. Large deelo!ments of limestones ("uild#u! or !latform) are also !resent within the $!!er %iocene and &liocene. E'!loration wells hae demonstrated the !resence of large olumes of gas "oth on Indonesia and %alaysian side.

11.2 REGIONAL GEOLOGY 11.2.1 TECTONIC SETTING The East Natuna Basin is "ounded at the west "y Natuna rch to the east it o!ens to *arawa+ Basin. The south !art is "ounded "y *unda *helf and the northern !art "y ,ietnam Basin. -utchison (1/) diided the East Natuna Basin into *o+ang *u" Basin and North East Natuna Basin. They are se!arated "y the &aus#0anai 0idge a feature !arallel to the eastern side of Natunarch which also forms the northeastern "oundary of *o+ang *u" Basin. To the east "oth "asins o!en to *arawa+ Basin. In a Late retaceousEarly Eocene reconstruction for the area 2hite 3 2ing (14/) concluded that the East Natuna Basin was !art of a large fore#arc "asin e'tending from offshore ,ietnam ,ietnam across Natuna waters to *arawa+. $nli+e in the 2est Natuna Basin the main structural trend in the East Natuna Basin is NN2# **E5 only in the northern !art *2#NE trending structures e'ist (2ongsosanti+o 3 2iro6udo 1/7). This *2#NE trending structures are e'tensional faults and half gra"ens similar to the ones found in 2est Natuna Basin "ut the rift magnitude is generally less than the ones in the 2est Natuna Basin. In the East Natuna Basin Basin all the structures structures are e'tensional there seems to "e no com!ressional tectonic !hase !hase ha!!ened ha!!ened in this area. 2hite 3 2ing (14/) stated stated that the structu structural ral trend seems to follow follow the underlying "asement fa"ric which is formed "y the eastward migrating su"duction 8one (Figure 9).

11.2.2 STRATIGRAPHY The stratigra!hic framewor+ for the East Natuna Basin does not a!!ear much in !u"lished literature5 the one used in this !a!er is a com!ilation of seeral !u"lished and un!u"lished wor+s. The oldest sediment found in East Natuna Basin is of Late :ligocene to Early %iocene age (&ertamina 3 BEII& 1/;) (Figure <). This sediment com!rises a lower sandstone unit followed "y a shale unit. The sandstone is the e=uialent of  $!!er >a"us Formation in the 2est Natuna Basin. This de!osit is inter!reted to "e of alluial !lain and delta !lain origin. The shale that oerlies it is the e=uialent of the Barat Formation of the 2est Natuna Basin and consists of greenish and grey marine shales. This se=uence re!resents the ma'imum transgression of the area. These se=uences hae not "een !enetrated "y any well in the *o+ang *u" Basin. The sediments oerlying this Barat shale are the sandstones of Lower rang Formation which is formed in the regressie !hase terminating the transgression. The regressie !hase was immediately followed "y a transgression. Both the regressie and transgressie transgressie sandstone se=uence forms the Lower rang sandstones. The transgression ended "y the forming of Lower rang shale that consists of grey and "rown marine shales. In the south

forming of the Lower rang shale then followed "y the of *o+ang sandstones. These formations are de!osited in Early %iocene through %iddle %iocene time. In the north !art oerlying conforma"ly the Lower rang shales is the Terum"u Formation. &u!illi (14<) diided the formation into a lower mem"er and an u!!er mem"er. The lower mem"er is termed the !latform mem"er and is characteri8ed "y stratified wac+estone !ac+stone and "oundstones. The u!!er  mem"er is termed the reef mem"er which consists of highly fossiliferous diagenetic dolomite. The "oundary "etween the lower mem"er and the u!!er mem"er is unconforma"le in as some !laces. This unconformity mar+ed the end of the regressie !hase. The u!!er mem"er of this Terum"u Formation is formed of many reefal "uild#u!s with marine shale and marls filling the de!ressions "etween the "uild#u!s. ?uring the %iddle %iocene to Late %iocene forming of the Terum"u Formation in the North the southern area undergone transgressie !eriod later recorded as the u!!errang *hale (&ertamina 3 BEII& 1/;). This shale was followed "y $!!errang sandstone. The sandstone e'tends also in the northern !art oerlying the Terum"u Formation. The youngest formation %uda oerlies unconforma"ly all the oldest sediment and formed as a transgressie sedimentary succession.

11.2.3 DEPOSITIONAL SETTING In East Natuna Basin when the su"duction along the line from *2 Borneo to offshore ,ietnam "ecame inactie during :ligocene the !reious fore#arc "asin sediment de!osition was followed "y the de!osition of shallow and dee! waterfacies 2hite 3 2ing 14/). The oldest sediments in this "asin are the Late :ligocene. These sediments are the result of the rifting !hase which also affected the East Natuna Basin es!ecially the northern !art forming the *2#NE half gra"ens. ?uring Early to %iddle %iocene the *outh hina *ea s!reading continued and caused the forming of the NN2#**E structures following the "asement wea+ 8one !atterns. In %id :ligocene to Early %iocene a !eriod of tectonic =uiescence a !ost rifting !hase ha!!ened (>inger et al. 1<). The de!osition resulted in the Keras and $!!er >a"us Formations. Barat Formation was de!osited after them is inter!reted to "e included in "oth !ost#rift and syn#inersion megase=uence. The "oundary "etween !ost#rift and syninersion megase=uence is mar+ed "y an onla! surface to lower !art of  Barat seen on the u!!er !art ofthe Barat Formation. om!ressional !hase during Early %iocene to %id %iocene these stresses to East Natuna Basin is not too significant to form any com!ressional structures which only caused regional u!lifting. ?uring Early %iocene to Late %iocene seeral cycles of sedimentation in the *outh *o+ang *u""asin resulted in the  rang For mation and the *o+ang *andstones. In The NE Nat una Basin t he formation s formed the rang and Terum"u Formations. The &eriod of u!lift was ended "y a general unconformity in Late %iocene which formed the "oundary "etween the syn#inersion and the !ost#inersion !roducts. The last cycle of sedimentation in East Natuna Basin formed the e=uialent of the %uda Formation.

11.3 PETROLEUM SYSTEM 11 3.1 Source Rock The :ligocene section is thinner less than <@@@ ft e'ce!t in some local gra"ens in the northeast and to

the east towards the central !art of the "asin. *ource roc+ maturation in thin :ligocene sediments would re=uire much thic+er oer"urden. %ature :ligocene sediments are "elieed to "e !resent in %iocene gra"ens and in the east and southeast !arts of the "asin where oerlying sediments are sufficiently thic+. %aturation occurred during &liocene time. The %iocene section consists of sandstone and shale inter#"edded with some coals de!osited in deltaic or !aralic enironment. >eochemical analysis indicate that the section contain fair to good source roc+ !otential for "oth oil and gas. The coal "eds contain u! to 9;A wa'y sa!ro!els which are +nown as oil source +erogen. %ature %iocene e'ists in the eastern and southeastern !arts of the "asin. The &lio#&leistocene sediments are !ro"a"ly still immature although su"sidence and ra!id sedimentation occurred in the eastern !arts of the "asin.

11.3.2 Reservoir Rock The :ligocene section contains sandstones with fair to good reseroir =ualities. The sand# shale ratio in the :ligocene and %iocene sections generally decreases eastward. They are !ossi"ly shale out 6ust to the east of the "asin. Both :ligocene and %iocene sandstones constitute a main reseroir o"6ectie for this area. &etro!hysical data are rare the >a"us Formation atweIlN#1C has !orosities of u! to 1/A and !ermea"ilities of << m? within the u!!ermost !art of the unit. The rang Formation at well *o+ang#1 contains 7@A fine grained net sand with an aerage !orosity of 9;A and !ermea"ility 9D@ m?. Beside the clastic reseroirs the %id %iocene to Lower &liocene car"onate "uild#u!s !roides e'cellent !otential reseroirs for the East Natuna "asin. In general seeral de!ositional facies can "e distinguished in "oth formations ranging from fore reef and reef slo!e in the east to reef "ac+ reef  and finally lagoonal facies in the west. The "est reseroirs are e'!ected to coincide with the sand and reef deried sediments within a "ac+ reef setting. &orosities measured in core in #1 and Bursa#1 range from 1@#9/A with !ermea"ilities u! to @@m?.

11.3.3 Tr!s The East Natuna tra!s are generally related to normal faults and growth faults old "asement highs or dia!iric structures. ar"onate "uild#u!s also contri"ute im!ortant tra!s for this "asin.

11." H#$rocr%o& P'# $!!er %iocene (&liocene) reefal "uild#u!s on tilted "loc+s This !lay has "een e'tensiely e'!lored and com!rises car"onate "uild#u!s sealed "y "athyal shales and charged "y :ligocene#Early %iocene source roc+s from the :ligocene gra"en andor *arawa+ Basin (Figure 7). Build#u!s deelo!ed on the crests and ridges of old tilted fault "loc+s that reemerged when there was a "rief mild reactiation. 0eefal "uild#u!s are concentrated at the "ase and the to! of the car"onate interal associated with high# stands of the Late %io#&liocene transgressie !hase Terum"u and %uda car"onates. harge for "oth oil and gas is inter!reted to "e deried from the dee! *arawa+ "asin to the east. Tests of the hydrocar"on "earing interals indicate good reseroir deelo!ment in these "uild#u!s with !ac+stone and wac+stone the most common lithologies. :ligocene%iocene sands in tilted fault "loc+s and across dra!ed intergra"en highs. This !lay com!rises clastic reseroir sealed "y intraformational shales in fault#di! closures and across dra!ed intergra"en highs "oth charged from down#di! :ligocene and Early %iocene source roc+s (Figure 7). Lac+ of sealing shales in the unconformity oerlying $!!er rang and Terum"u limestone is regarded as the main reason for the negatie tests of this !lay.

0eferences *hell om!anies 19 &etroleum >eology of the Bursa B Bloc+ East Natuna rea 0e!ort for  &ertamina. Esso E& Natuna Inc. 3 &ertamina 19 *ourcce roc+ &otential In the oint *tudy rea of East Natuna Basin *iuth ina *ea Indonesia.