Petroleum Geology of Cepu Area, Central Java, Indonesia Northern part of Java area (East Java in this case) is a stale ac! arc asin" #his condition resulted thic! sedimentary deposit $hich had good hydrocaron pot ential" #ectonic #ectonic development occured in this area, mar!ed y uplifting, faulting and folding as in present condition" %ig"& %ig"& Pro'le of Nothern part of East Java asin II"&" egional Geomorphology of Cepu Area II"&"&" *endeng +one *endeng +one is moutain range located in the north side of Nga$i su-one, orien ted to $esteast direction" #his mountain range consist of deep marine sediment $hich had een faulted and folded intensively forming anticlinorium" North margi n is ordered y andulatung .epresion, then south margin is ordered y volcan ic mountain (/olo +one)" #his mountain lies from f rom 0ngaran 1ountain on the $est si de to Nga$i and 1o2o!erto Area" est of this mountain is still can e trac!ed un til susurface on 1adura strait" %ig" II"& Phisiography of Central and East Java (emmelen) *endeng 1ountain on $est part s $ide is aout 34 !m, then ecome narro$ to the eas t $ith length appro5imately appro5imately 674 (de Genevreye 8 /amuel, &9:6)" Characteristic of *endeng +one is line of hills $ith lo$ elevation, $avy hill morphology elevated on 74644 m" #his $esteast lineament re;ects some folding and faulting trendi ng $esteast" %olding and thrusting intensity in this mountain have ig intensit y in $est part and ecome $ea! in east part" #hrust fault ma!e the unit oundary to e structural oundary" oundary" #he occurence of fault and fold ecause of compressi ve force yield fractures, faults and $ea! -ones trending southeastnorth$est, so uth$estnortheast and northsouth" II"&"6" andulatung +one andulatung .epression is physiographic depresion due to tectonic activity lies et$een *endeng +one and emang +one, formed on Pleictocene" #his depresion ha s $esteast trend" #he narro$est part of this area is llocated on Cepu Area, the n ecome $ide to east direction as far as sedimentation distance of 1adure strai t and until no$ sedimentation in this one is still occur" andulatung .epresion generally can e classi'ed into synclinal valley morphology unit consist of Ce pu Area and o2onegara" enga$an /olo river ;o$ in this area, form meandering m eandering s ystem" 1ost of andulatung Area is 'lled y 'ne grain clastic sediment such a s clay, silt from
" emang +one emang -one is high hi gh elevated area separated y andulatung Area and paralel to *endeng +one" #his area consist of anticlinorium trending $esteast due to tect onuc activity happen on late #ertiary" #his foleded mountain has enough high ele vation, average is less than 744 m" /ome of those anticlines are young anticline s mountain and have not een eroded intensively" intensively" #hose e5ogenic activitiy ma!e t he mountain loo! li!e ac! of hill" /tructurally, emang +one is fault -one loc ated et$een caronate /helf on north of Java (Java /ea) and deep asin located on south of emang (*endeng asin)"
e formed on the top" /tratigraphy of *endeng +one can e divided into 9 %ormation from oldest to youn gest" #he formations are @
%ig" II"6"&"& egional stratigraphy of *endeng +one (arsono, &9B>) a" Pelang %ormation Pelang %ormation is the oldest formation located in 1andala *endeng Area" memers from the oldest to the youngest @ anyuurip 1emer #his memer consists of interedded clayey marl, marl, clay, tuDaceous calcareo us sandstone, and tuDaceous sandstone" #hic! of deposit reach 6:4 m and this d eposit is deposited on N&4N&7 (1iddle 1iocene)" /entul 1emer #his memer consists of lithology $hich has same characteristic $ith anyuurip 1 emer" #he diDerence is pointed to the occurence of ed $hich contain thic!er t uDaceous material " #otal thic!ness of this deposit is aout 744 meter and aged N&F (Early 0pper 1iocene)" *ere!
#his formation conformale on .anar one"
II"6"6" 1andala emang /tratigraphy
emang stratigraphy are consist of some formation elo$ from the oldest to the youngest
%ig" II"6"6"& /tratigraphic coloumn of East Java asin (1ud2iono and Pireno, 644 &) II"6"6"&" Ngimang %ormation #his formation consists of shale $ith silt interedding, 'ne sandst one, limestone and coal" .epositional environment of this formation is on delta system, lacustrine and shallo$ marine on Eocene until " #uan %ormation #his formation consists of claystone eds $ith limestone interedding" #here is facies change south$ard in $hich lithology $ill grade i nto shale and claystone facies (/oe2ono, &9B& on andoo! of Petroleum Geology 6 44F)" .epositional Environment is deepmiddle neritic" II"6"6"3" #a$un %ormation #his formation consists of silty shale $ith limestone in teredding" ?n top deposit, sandstone facies $ith little clay inside dominate" < ocally, there is spotted limestone found" #op of #a$un %ormation is also called as Ngrayong 1emer" .epositional Environment of this formation is open marine un til shallo$ marine in the top formation" #his formation is formed on 1iddle 1ioc ene (N9N&>) (ahard2o 8 Hiyono, &99>, on andoo! of Petroleum Geology, 644F)" II"6"6"7" Ngrayong 1emer
#his memer is also called as 0pper ?ritoiden*ala! y #r ooster (&9>:)" an emmelen (&939) named this emmer as 0pper emang eds " Ngrayon g /andstone name has een introduced y rou$er (&97:)" e proposed local type of this sandstone ased on uart- sandstone found in Ngrayong illage, Jatirogo, i n $hich that memer are composed of sandstone mainly $ith intercalation of coal and sandy clay" arsono (&9B>), descrie Ngrayong as memer of #a$un %or mation, consists of oritoid limestone and shale in lo$er part" In the upper par t, there are sandstone $ith limestone intercalation and lignite" #his unit i s ag ed on 1iddle 1iocene, N9N&6" .epositional environment of this unti is ;uvial o r sumarine ased on outcrop found in northside of illage (Jatirogo, #a$un)" ?n south side of village, this depositional environment of formation change into m arine environment" 1arine deposit of this formation on Ngampel Area sho$ shallo$ ing up$ard pattern from shoreface to each" #his unit al so sho$ hiatus on mouth of Java /ea" Ngrayong 0nit is main reservoir of Cepu ?il'eld, ut there is shal e occurence in the south and east of this 'eld" #otal thisc!ness of this unit i s varied, average is more than >44 m" II"6"6"F" ulu %ormation #his formation consists of edded clastic limestone and sandy limestone" N&3)" II"6"6":" Honocolo %ormation #his formation consist of yello$ro$n marl, contain gla uconite" #here are interedding of calcarenite and claystone" .epositional envi ronment of this formation according to Pur$ati (&9B:, on andoo! of Petroleum G eology, 644F) is deep neritic until middle athyal and formed on 1iddle 1iocene 0pper 1iocene (N&3N&F)" II"6"6"B" :" #his formation had een classi'ed into memer of ) didn t continue his oservation aout the occurence of unconformity et$een
II">" egional Geology /tructure of Cepu Area East Java asin generally are formed y main primary structure from south to nor th" *endeng +one and 1adura strait lie in $esteast direction, especially are ch aracteri-ed y fold, fault and thrust one" /outhern part of emang +one and an dulatung are
characteri-ed y disharmonic fold type" eside that, this -one is characteri-ed y dome structure associated $ith fault structure as li!e as anti
cline in Ngimang" Northen part of emang +one and 1adura are characteri-ed y uplifted and eroded anticlinorium structure on PlioPleistocene" #his structure is associated $ith stri!e slip fault system trending northeastsouth$est" #he fa ult system continue until /outh *alimantan Area" #here are 6 primary geological structure in East Java asin@ a" Northeastsouth$est structural pattern, also called y 1eratus type" " Hesteast structural pattern, also called y Java type" %ig" II">"& egional structural pattern in Indonesia (/atyana, 6447)
*endeng +one on Early 1iocene is active tectoni-ed area" In tectonic regional f r ame$or!, *endeng +one is included on the part of East Java asin" #his asin has occured e5tensional tectonic regime on Paleogene and result many normal faults in highlo$ morphology form" ?n Neogene, East Java asin $as compressed y compr essional regime and caused reactivation on normal faults" and yield many thrust faults" #hose faults cut Neogene sediment and resulted thrust faults" #hese thru st faults can e trac!ed in $est part of *endeng area" ?n east part of *endeng a rea, there are many anticlines $ith plunged a5is to the east" %old and thrust in tensity $ill e decreased to the east, then dissapeared on the south" #he phenom enon e5plain that there are only anticlines in north part $hich can e trac!ed u ntil /uraaya" #hese fold path$ay verge directly $ith volcanic arc and only sepa rated y Nga$i Alluvium"
PE#?
I"&" /ource roc! /ource roc! is roc! $hich have potential to create and e5pel hidrocaron in enou gh uantity" #here are 3 formation $hich act as source roc!, they are @ Ngimang %ormation, * u2ung %ormation, Ngrayong formation and #uan %ormation" Generally, they have sh ale $hich can create !erogene" eased on maturity, all of them had een mature t hermally" #hree formation aove had een eing mature on Paleogene, e5cept #uan %ormation $hich formed on 1iocene age" Ngimang %ormation has good source roc!, prooved y occurence of good #?C (&L3L ) on shale and caronaceous limestone also 34L for coal)" /ource roc! on this fo rmation is classi'ed into eDective source roc! ecause this source has supply economic hydrocaron (97L oil and gas) in NE Java asin" *u2ung %ormation has classi'ed into possile source roc! due to no identi'cati on y geochemical method to some factor (#?C, o, and !erogen type)" #uan %ormation has een classi'ed into eDective source roc! due to high conte nt of #?C (4,7>6,> L)"
accumulate hidrocaron" ?n Nort East Java asin, there are > formation act as reservoir roc!, they are *u2ung formation, Ngrayon g formation and 1undu %ormation" *u2ung formation is divided into > part, *u2ung I, II and II" *u2ung I consist o f shelfal euivalent to deep $ater Prupuh limestone of the East Java M 1adura a sin" It has the est reservoir characteristic of North East Java asin" *u2ung I I consist of limestone form one of main target in central deep and provide reser
voir" 44 m" Porosity is ranged et$een 6B36L $hile permeai lity is ranged 7794> m. $ith thic!ness is aout 474 m" I">" /eal /eal roc! is roc! $hich act as seal, usually impermeale and has 'ne grain li!e marl, shale or clay so hidrocaron accumulated under this seal cannot migrate o r lea! to the other place" Primary seal in North East Java asin is thic! shale facies of #uan %ormation" #uan shale provides primary top seal to the underlyi ng *u2ung and Ngrayong reservoir" /hale of the formation generally drapes over t he *u2ung formation (reef) and eDectively capped any trapped hydrocaron elo$ it" I"3" #rap #rap is geological feature $hich can accumulate hidrocaron after they have migr ated" Generally, traps are divided into 6 varian, sratigraphic and structural tr ap" 1ost trap in North East Java asin is structural trap, mainly is anticline $ ith simple variation" #he other type are faults and stratigraphic traps" %aults trap $ill give some ris!, due to proper timing of forming and occurence of seal ed" Anticline trap formed on PlioPleistocene tectonic phase compression thrus tfolding" Anticlines $hich have north$est a5is direction have ecome the most e Dective hidrocaron trap" /tratigraphy trap on this area is oriented to onlap f orming from shale related to reefal limestone" ?n some other 'eld, there are so me stratigraphic trap li!e patch reef comple5es $ith some structural control $hi ch proaly initiated reefal gro$th" Generally, traps in this asin are cut y t hrust fault rending northeastsouth$est and this faults act as oil$ater oundar y" I"7" Proper #iming of 1igration Proper time of migration is time in $hich hidrocaron migrate from source roc! i nto reservoir roc! so the hidrocaron $ill e accumulated into right trap and no lea!ed occur" ased on tectonic history, there are 6 phase of tectonic period" %irts is 1iddle 1iocene tectonic phase (after deposition of Ngrayong formation) , second is Plio Pleistocene" /ource roc! from Ngimang formation had een matur e on late 1iocene" ased on this fact, 'rst migration occured on this period" # hem, second migration occured after 'rst phase PlioPleistocene tectonic" ?n th at time, oil from *u2ung formation has een mature" 1igrasi hidro!aron atau per pindahan hidro!aron pada Ce!ungan Ja$a #imur 0tara dapat ter2adi secara lateral maupun secara verti!al" Generally, lateral migration in this asin $ill oriente d to north direction ecause of some thic!ening of sediment north$ard, same $ith sedimentation direction" ?I<%IE<. 1ore than 67 oil 'eld have een
discovered and development in Cepu area until / uraaya" ut most of them have een leaved" No$ only 7 oil 'eld (*a$engan,
gest oil 'eld in North East Java asin" Cumulative production in this 'eld can achieve &74 million cuic meters at &994" 0pper #a$on formation, Ngrayong sands tone, and ottom Honocolo formation are layers that produced hydrocaron" No$, o il production use pump system ecause the formation pressure decrease" Pertamina doesn t use E? method (Enhanced ?il ecovery)" ?il in this area is collected and dispersed y $ater" %igure III">"7"& ?il .erric!, primary recovery y oil pump #his oservation site still under the head of Pertamina operational at *a$engan oil 'eld" In this location, oil inta!e is used y modern method (land shore)" # here are 6 type of tool that is used to oil inta!e $ithin the earth" #he 'rst tool is mechanic type" #his type is old tool, so the depth otained is n t high and the result of oil inta!e isn t optimal" #he second tool is hydraulic ma chine type" #he technology of this tool ne$er than the 'rst tool, so the depth otained is high" #his tool capale of reaching depths of :44 meters" #his tool also ale to display data recording (result of oil inta!e) digitally"
