Fasies intrerpretation

Facies types were interpreted on the basis of

integrated evaluation of petrophysical responses in

combination with biofacies analysis and core

descriptions, based on existing facics models of the

modern Mahakam Delta (Allen, et al., 1976; Allen

and Mercier, 1994). The Mahakam Delta is a lobate,

fluvial-tidal dominated system characterized by sand

deposition in fluvial and tidal channels in the delta

plain, and associated distributary channel and

channel-mouth bar deposits in the delta front. Coal

deposits characterize Miocene lower tidal delta plain

facies, in contrast to the rarity of proto-coals (peat) in

the modem delta. Wave energy in the modern

en\.ironment is minor and appears to have been the

same during deposition o f middle and late Miocene

paleo-Mahakam deltaic sediments. Benthic f o r m

biofacies contributed to subdivision of these

depositional environments into several delta plain,

delta front. and open shelf en\konments.

Reefal limestones formed 011 the fringes of the

modern and ancient delta systems in areas where the

delta was inactive Limestones are thin (10-20 ft

thich) 111 clear-water areas close to active delta lobes,

but arc locally ~ e r tyh ich (hundreds of feet) in areas

separated from actn e deltaic sedimentation for long

periods of tiinc (e g . near shelf margins and in faulted

or folded areas \vhere paleo-relief provided more

fa1 orable settings)

Artinya:

Jenisfasiesdiinterpretasikanberdasarkanevaluasi menyeluruh atastanggapanpetrofisikadikombinasi dengananalisisbiofaciesdanintideskripsi,berdasarkan modelfacicsada dariyang modernDelta Mahakam(Allen, etal, 1976;. AllendanMercier, 1994). Delta Mahakamadalahsebuahlobate,sistem dominasifluvial-pasang surut yang ditandai denganpasirpengendapan disaluranfluvialdanpasang surutdi deltapolos, dan terkaitdistributary channeldandepositobarsaluran-mulut didepandelta. batu baradepositociriMiosenrendahpasang surutdeltaplainfasies, berbeda dengan kelangkaanproto-bara

(gambut) dideltamodem. Energi gelombangdimodernen\. ironmentadalahringan dantampaknyatelah menjadisama selamapengendapantengah dan akhirMiosenpaleo-delta Mahakamsedimen. Bentikformbiofacieskontribusi terhadappembagianinilingkungan pengendapanmenjadi beberapadeltaplain,deltadepan. danrak terbukaen\konments.

BatugampingReefalterbentuk011pinggiransistemdeltamodern dan kunodi daerah di manadeltaadalahBatugampingaktiftipis(10-20 ftThich) 111daerah yang jelas-air dekatlobusdeltaaktif,tapibusurlokal~erTYHich(ratusan meter) di daerahdipisahkan dariactnedeltasedimentasilamaperiodetiinc(misalnya. dekatmarginrakdanmenyalahkanatau daerahdilipat\vherepaleo-bantuan yang diberikan lebihFA1pengaturanorable)

Organic geochemistry

Basic organic geochemical screening analysis (TOC.VR. HI) and proprietary typing analyrses were done oncuttings. core samples. and fluids from numerouswells for determining thermal inaturih and as a basisfor identifying potential sourcc I'acies (primarilyin thetidal delta plain setting.

Stratigraphic andgeochemical data. and bottomhole temperature inwells provided the data Icr thermal model studiesIntegrated geochemicai analysis and basin modelingproduced petroleum system models which characterizesource rocks. kitchen areas timing of generation,

and migration pathways

Analisisgeokimiaorganikskriningdasar(TOC.VR. HI) dananalyrsesmengetikproprietarydilakukan padastek. sampel inti. dancairan dariberbagaisumuruntuk menentukaninaturihtermal dansebagai dasaruntukmengidentifikasi potensiI'aciessourcc(terutama dipasang surutdeltapengaturanpolos.stratigrafidanDatageokimia. dan bawahsuhulubang disumurdisediakandataICRstudi modeltermalGeochemicaianalisis terpadudan pemodelancekunganmenghasilkan modelsistem petroleumyang mencirikanSumberbatuan. dapur daerahwaktu pembentukan,dan jalurmigrasi

Late Middle Miocene and younger strata

Sedimentation in the lower kutei basin has been dominated by a delta system similar in type andlocation to the modern Mahakam Delta since at leastthe late middle Miocene (sequences above X8, Fig.2). Since that time, the principal river transportsystem has stayed in more or less the same location asthe present-day Mahakam River, which is currentlyincised across anticlines of the Samarindaanticlinorium. This part of the stratigraphic successionin the Lower Kutei Basin is characterized by thick,aggrggradational facies units, punctuated byprogradational or backstepping (flooding/transgression)units. Delta front sands are a common reservoir type,Yenning thick, stacked succcssions in fields like

Attaka (Unocal; Fig. 1): Tunu, and Peciko (TotalIndonesie).

Within stacked successions, systems ofbifurcating distributaries resulted in development ofseparate lobes that both complicate the depositionalpatterns and make possible thc presence of localizedlateral pinchouts with stratigraphic trap potential.Delta plain source facics form similar verticallystacked successions. Facies generally become moredistal toward the north. south, and east.

Sedimentasi di bawah Kutai basin telah didominasi oleh sistem delta serupa dalam jenis danlokasi ke Delta Mahakam yang modern setidaknya sejakMiosen tengah akhir (urutan atas X8, Gambar.2). Sejak saat itu, transportasi sungai utamaSistem telah tinggal di kurang lebih lokasi yang sama denganSungai Mahakam saat ini, yang saat inimenorehkan seluruh anticlines dari Samarindaanticlinorium. Ini bagian dari suksesi stratigrafidi Bawah Kutai Basin dicirikan oleh tebal,fasies aggradational unit, diselingi olehprogradational atau backstepping (banjir / pelanggaran)unit. Pasir depan delta adalah jenis reservoir umum,Yenning tebal, succcssions ditumpuk di bidang-bidang sepertiAttaka (Unocal, Gambar 1.): Tunu, dan Peciko (TotalIndonesie).Dalam suksesi ditumpuk, sistembifurcating distributaries mengakibatkan pengembanganlobus terpisah bahwa kedua menyulitkan pengendapanpola dan membuat kemungkinan adanya THC dari lokal

pinchouts lateral dengan potensi perangkap stratigrafi.Delta facics sumber polos membentuk vertikal miripditumpuk suksesi. Fasies umumnya menjadi lebihdistal menuju utara. selatan, dan timur

Alternation of aggrading, prograding. and

bachstepping deltaic staching patterns IS probabl) due

to generally high but variable rates of basin

subsidence Strong aggradational stacking was due

to somewhat balanced but 01 erall high rates of

subsidence and sediment supply Global-custatic

changes of sea lciel almost certainl) also influenced

Eacies c\ clicih . but probable) not to the degree of local

basin subsidence

Bergantinyaaggrading, prograding. danbachsteppingpolastachingdeltaISprobabl) karenauntuk tingkatumumnyatinggi tetapivariabelcekungansubsidenceaggradationalKuatsusunadalah karenauntuk agakseimbang tetapi01eralltingkat tinggisubsidensi dansedimenpasokan global-custaticperubahanlautlcielhampircertainl) juga dipengaruhiEaciesc\clicih. tapikemungkinan) tidakdengan

tingkatlokalcekunganamblesan

A regular succession of middle and late Miocene (-I 6

to 8 Ma; XIO-XI. Fig. 3) shelf edges, commonly

associated with carbonate buildups, was mapped.

Beginning near the present-day shoreline, these shifted

eastward to the present-dayr shelf edge as Miocene

delta complexes prograded across the Lower Kutei

basin. Since 8 Ma. deltas have migrated

inconsistently. across the previously formed delta

platform in response to late miocene and plio-pleistonce glacial-eustatic changes in sea level and to local fault activity

Sebuahsuksesiregulertengah dan akhirMiosen(I-68Ma, XIO-XI. Gambar. 3) tepirak, umumnyaterkait denganpenumpukankarbonat, dipetakan.Dimulaidi dekatgaris pantaisaat ini,inibergesertimurke raktepimasaDayrsebagaiMiosenkompleksdeltaprogradeddiBawahKutaibasin. Sejak8Ma. deltatelahbermigrasitidak konsisten. dideltaterbentuk sebelumnyaPlatformdalam menanggapiMiosen AkhirdanPlio-pleistonce perubahanglasial-eustatic dipermukaan lautdanaktifitas patahanlokal

Deltaic sediments older than late middle Miocene

(between X15 and X8 sequence horizons, Fig. 2) have

been penetrated by onshore wells (Vico and Total

Indonesie PSC blocks) and in Unocal's southern

blocks. Younger post-late-middle Miocene Mahakam

Deltaics (younger than X8) thin southward in

association with a shift to more distal facies. These

strata were truncated near the present shoreline during

late Neogene uplift and development of anticlines in

the Samarinda anticlinorium. Unocal's southern

blocks are dominated structurally by the Adang fault

systemn, which was most active from the latest early

Miocene to late itliocenc. Three major delta systems

are recognized in this area: the early Miocene Maruat

system (X15-X13), and the middle Miocene Yakin

and Sepinggan systems (XI 3-X8). The depocenters

of these deltaic systems shifted progressively to the

east-northeast (Fig. 2)

Sedimendeltatua dariakhirMiosentengah(antara X15danX8urutancakrawala, Gambar. 2) memilikiditembus olehsumuronshore(Vico dan TotalIndonesiePSCblok) danUnocalselatanblok. Mudapasca-akhir-tengah MiosenMahakamDeltaics(lebih muda dari X8) tipisselatandiasosiasi denganpergeseranfasiesdistallebih. inistratadipotongdekatpantaihadir selamaakhirmengangkatNeogendan pengembangananticlinesdianticlinoriumSamarinda. UnocalselatanblokdidominasistrukturalolehkesalahanAdangsystemn, yangpaling aktifdariawalterbaruMiosenakhiritliocenc. Tigasistemdeltautamadiakuidi daerah ini: awalMiosenMaruatsistem(X15-X13), dantengahMioceneYakindan sistemSepinggan(XI 3-X8). Thedepocentersdari sistemdeltabergesersemakinketimur-timur laut(Gambar2)

Maruat deltaics consist of interbedded sandstone,

shale, coal, and limestone deposited mostly in delta

plain and delta front environments. Fluvial and

distributary channels in the interval are narrow and

appear to have been deposited as a consequence of

rapidly shifting river channels. Delta progradation

ended abruptly with deposition of the locally thick

Maruat Limestone (marked by X13 horizon). The

basal boundary of the limestone may be a regional

disconfonnity and is interpreted as a combined

flooding surface and sequence boundary. Variations

in thickness of the Maruat are partly associated with

irregular paleobathytnetry caused b), early movement

of normal faults within the Adang System. Normal

fault movements accelerated during deposition of the

overlying Yakin Formation deltaics. The iowermost

part of thc Yakin is characterized by a prodelta shale

unit that downlaps onto the Maruat Limestone (X 13

surface).

Maruatdeltaicsterdiri daribatu pasirinterbedded,serpih, batubara, dan batu kapurdisimpanterutama dideltapolos dandeltalingkungandepan. fluvialdansalurandistributarydalam intervalsempit dantampaknya telahdisimpansebagai konsekuensi daricepat bergesersaluransungai. Deltaprogradation

berakhir tiba-tibadenganpengendapansecara lokaltebalMaruatKapur(ditandai dengan X13horizon). itubatasbasalbatu kapurmungkindaerahdisconfonnitydandianggap sebagaigabunganbanjirpermukaan danbatas sekuen. variasidalamketebalanMaruattersebutsebagianterkait denganpaleobathytnetrytidak teraturmenyebabkanb), gerakanawalkesalahannormal dalamSistemAdang. normalgerakankesalahandipercepatselamapengendapanatasnyaYakindeltaicsFormasi. Theiowermostbagian dariTHCYakinditandai denganprodeltashaleunit yangdownlapskeMaruatKapur(X 13permukaan).

The Yahin Forinatton {X13-X?2) consists of severat

thick coarsening-upward delta lobes that form both

aggradationai and progradational stacking patterns

The Yakin is divided into an upper and iower series

of sandy delta front deposits characterized by

1 verticaily stacked distributary channel sandstone units

!ndividuaI distributary lobes are separated by prodelta

sha!es deposited during flooding events possibly related to lobe shifting. a thicker shale in the middle of the yakin represents a

more significant flooding event possibly associated with custatic sea level rise.

TheYahinForinatton{X13-X? 2) terdiri dariseveratteballobusdeltapengkasaran-atas yang membentukbaikaggradationaidan polasusunprogradationalTheYakindibagimenjadi sebuah serialatas daniowerberpasirdeltadepositodepanditandai dengan1verticailyditumpuksaluranunitbatupasirdistributary!ndividuaIdistributarylobusdipisahkan olehprodeltasha! esdiendapkan selamaperistiwa banjirkemungkinan berhubungan denganlobuspergeseran. serpihtebalditengahYakinmerupakan acarabanjirlebih signifikankemungkinan terkait dengancustatickenaikan permukaan air laut

The Yakin thickens abruptly across some of the faults

in the Adang fault zone, demonstrating

syndepositional activity of the faults.

Yakin deltaics are sharply overlain by a thick,

aggradational succession of coaly deltaic sediments in

the X12-XS interval (Sepinggan Deltaics), which were

deposited in tidal delta plain environments. The

boundary between the Sepinggan and Yakin deltaics

(X12) is interpreted as a regional sequence boundary.

The Sepinggan Deltaics unit contains abundant coal,

and is the most significant hydrocarbon source in

Unocal southern blocks. It extends from the

Paternoster platform northward at least to the Handil

field area. Hydrocarbon accumulations in reservoir

sands within the succession (e.g., at Sepinggan,

Mutiara, and Handil fields) may have been

self-sourced. The Sepinggan deltaics grade upward

into distal deltaics and reefal (?) liinestones that are

laterally equivalent to the main Mahakam Deltaics to

the north.

TheYakinmengentaltiba-tibadibeberapa kesalahandi zonasesarAdang, menunjukkanKegiatansyndepositionaldarikesalahan.Yakindeltaicsyangtajamditindih olehtebal,suksesiaggradationalsedimenhitam legamdeltadiintervalX12-XS (Sepinggan Deltaics), yangdisimpan dalampasang surutdeltalingkunganpolos. itubatas antaraSepinggandanYakindeltaics(X12) diartikansebagai batasurutanregional.TheSepingganUnitDeltaicsmengandungbatubaramelimpah,

dan merupakan sumberhidrokarbonyang palingsignifikan dalamUnocalblokselatan.Ia meluas dariPlatformPaternosterutarasetidaknyaHandilyangdaerahlapangan.Akumulasihidrokarbondalam reservoirpasirdalamsuksesi(misalnya, diSepinggan,Mutiara,dan bidangHandil) mungkin telahself-bersumber. TheSepinggandeltaicskelasatasdeltaicskedistal danreefal(?) liinestonesyanglateralsetara denganDeltaicsMahakamutamautara.