AIIuvial Fan Facies and Their Distribution in the Lower Talang Acar Formation,

2007

Vo l. 4 NO.2

Petroleum Science

AIIuvial Fan Facies and Their Distribution in the Lower Talang Acar Formation, Northeast Betara Oilfield, Indonesia Xie Chuanli , Ma Haofan , Liang Honggang , Li Dongmei , Qi Xiuli and Xian Benzhong fχéy

Laboratory 01Petro/ωm Engineering under Ministry 01 Education, China University 01 Petroleum, Beijing 102249, China)

Abstract: Th is paper studies the alluvial fan facies characteristics and distribution in the Lower Talang Acar Formation in the Northeast Betara Oilfield. The conglomerate sedimentary characteristics and its distribution were studied based on core data, logging interpretation and seismic property analysis. Th e research indicated that alluvial fan deposits of Bed F in the Lower Talang Akar Formation were characterized by co缸se granularity, poor sorting and low quality. Sand-bodies accumulate longitudinally, and inter-layers are poorly developed, extending locally in the transverse orientation. Typical logging response of alluvial fan system is summarized, and conglomerate is characterized by low gamma-ray (GR) intensity, low resistance , high density and low value between DLD and LLD , which reflects poor physical reservoir properties , coarse rocks and relatively high density. Conglomerate is developed mainly in the northeast and middle-south ofthe studied area. The upper part ofbed F is found in a small area in the middle-south, while the lower part ofbed F is developed in a relatively large area in the middle-south. Key words: NOrtheast Betara Field , Lower Talang Acar Formation, alluvial fan facies , logging responses , conglomerate

1. Introduction Northeast Betara (N EB) Field is located in the Jabung Bl ock in South Sumatra. It is the middle host

of the Betara Complex and is immediately to the west of and adjacent to the Betara Deep hydrocarbon kitchen (Fig. 1) (Santa Fe Energy Resources Jabung Ltd. , 1999).

Strait

Fig. 1 Jabung Block and N. E. Betara Field location map (Santa Fe Energy Resources Jabung Ltd. , 1999) The NEB structure , seismically defined as an anomalous faulted anticline , was found on June 16 , 1995 by Well NEB-1 that encountered 266 feet of Lower Ta1ang Akar Formation with 117 feet of net hydrocarbon-bearing sand in eight individual sandbodies (Santa Fe Energy Resources Jabung Ltd. ,

1999). Three of the sandbodies were tested at a combined rate of 18.22 MMCFID (million cubic feet per day) and 432 BCPD (b arrels condensate per day). Well NEB-2 , drilled in March , 1996 , encountered 42 feet of net gas pay in two sandbodies of Lower Ta1ang Akar Formation (Sa1am, et al. , 1996). In the following

Vol. 4 NO.2 Al luvial Fan Facies and Their Distribution in the Lower Talang Acar Fonnation, Northeast Betara Oilfield, Indonesia years after the discovery of this Talang Akar accumulation, many delineation and development wells were dri11ed and successfully resulted in gas and oil producers. Up to date , fifty two oil and gas wells have been dri11ed. Twenty wells among these existing wells have been put on production since J anua可 2001. Sixteen wells(NEB-7, 8, 9, 16, 17, 18, 19, 20, 21 , 25, 27, 28, 29, 32, 33, 34) are being put on production (Lu, et al. , 2004). The NEB field is a condensate gas reservoir, and the COz content in the gas varies great1y. Accordingly, the field is generally divided into three regions: Region 1 in the east, CO z content about 52%; Region 2 in the middle and south, CO z content about 27%; and Region 3 in the west, CO z content about 16%. The m句 or reservoirs in the Lower Talang Akar Formation are finecoarse grained conglomerate and have higher net sand/gross interval ratio, compared to the timeequivalent interval in adjacent Ripah and NB fields to the west and north , where this interval is a major producing section (Tangkalalo , et a l., 1997). From the results of production and previous studies , there are some challenges in NEB field in studying strata distribution, sandstones prediction, fluids variances and reservoir simulation (Ambrose , et al. , 1997). So an integrated study was carried out based on all the data by combining geophysics, lithology, sedimentology and sequence stratigraphy as well as reservoir description.

2. Geological setting Northeast Betara is located in the Jabung Block, South Sumatra Basin near the transition to the Central Sumatra Basin, Indonesia. The Jabung Block is si阳ated on the island of Sumatra along the Sunda Island Arc where the ocean crust of the Indian Ocean subducted northward under the Sundaland Craton (Durlofsky, 1992). Extensional back-arc stresses along this Sumatra Basin extended onto the adjacent Sunda Shelf to the north. These rift basins are fundamental to the presentday prolific hydrocarbon system in Sumatra in that they accumulated thick syn-rift restricted organic-rich lacustrine sediments that form the primary hydrocarbon source rocks (Wang , et al. , 1997). Compressive tectonic movement began in the Middle Miocene and accelerated in the Pliocene, inverting many of these earlier basins and graben, and creating a variety of structural trap configurations called "Sunda Fold" types which are characterized by the high-angle reverse fau1ting of reactivated older normal fau1ts. The majority of the oi1 and gas fields in Central Sumatra, South Sumatra and Sunda Basins occur in

19

these structural settings (Zhang, et al. , 1997). Hydrocarbon sources for oil and gas charging of Betara Complex could have come from the Betara Deep , a large half-graben that lies adjacent to and to the east of the Northeast Betara discoveries (Cimolai , et al. , 1993). Coals and organic-rich sediments within the Lower Talang Akar and Lahat formations in the deepest portion of Betara Deep , the proven hydrocarbon generation area, are the hydrocarbon source rocks for these fields. The seal for the Lower Talang Akar reservoir is the regional transgressive shale of the Upper Talang Akar, as well as locally, intra-formational shale of the Lower Talang Akar. The sedimentary succession in the South Sumatra Basin comprises a single transgressive cyc1e, which commenced from Late Eocene to Early Oligocene with deposition of the syn-rift transgressive alluvial , fluviode1taic locally lacustrine and marginal marine facies of Lahat and Talang Akar Formations. These sediments progressively filled the subsiding half-graben and eventually covering the basement highs (Simlote, et al. , 1985). Marine conditions were eventually established during the continuing transgression sequence with deposition of an open marine facies of the Gumai Formation, which consists of marine shale, clay-stone, marls and fine-grained sandstones (Fig. 2). Initial uplift of the Sunda Shield to the east in the Middle Miocene marked the end of the Early Tertiary transgression sequence , and the beginning of the regression sequence that continues to the present day. The Middle Miocene uplift and compression caused inversion of the previous depocenters as well as further uplift of basement highs. The compression also continues to the present day and has resu1ted in many of the hydrocarbon traps found in the Jabung Block as well as through South Sumatra. The regressive distal de1ta front to marginal marine fluvio-de 1taic facies of the Gumai and Air Benakat Formations were deposited as a resu1t of increased sediment load from the Sunda landmass to the northeast. This regressive cyc1e was periodically interrupted by subt1e transgressive events primari1y comprising distal de1ta front shale, distributary mouth bar sands, de1ta bar sands , channel sands and interdistributary shale (Fig. 2). A rapid increase in compressive tectonic movement in the Late Miocene accelerated sediment influx from the emergent areas. The regressive lower

20

2007

Petroleum Science

which may be a continuation of the regressive fluviodeltaic Muara Enim Formation (Fig.2) (Santa Fe Energy South

Resources Jabung Ltd. , 1999). No r1 h

Lithofaci出

Hvdrocarbonl Seismic elemen ll. I horizon

rcctononic events

lI pper sandy

member Lower member

工E口口二乙

Syn.ri 白

allllvial to lluvio. lac lIstrine

Rcd

Pre.collision paSS1Vc margm Fig.

2

^ccrction of continental 仕agments

Generalized s位atigraphy and tectonic evolution ofthe Jabung Block (Santa Fe Energy Resources Jabung Ltd. , 1999)

3. Alluvial fan facies characteristics and distribution Terrestrial sedimentation occurred during the Eocene age. This filled in the half rift valley and denudation region locally. As well , tufIaceous sandstone , conglomerate, breccia and clay-stone filled in grabens and low-lying topography by alluvial , fluvial , lacustrine sedimentation during the rift valley period. Transgression began in part of the study area during the

Late Eocene , and extended largely from Late Oligocene to Miocene continuously. Petroclastic rocks on the basement formed overlying deposits , and carbonate rocks were developed on tableland, as well as on highs of the fault blocks. During sea level fall , carbonate rocks experienced weathering and dissolution , then formed secondary holes in ancient highs. Sea floor fans (consisting of petroclastic rocks) were developed in deep water. The most widespread transgression occurred during the Middle Miocene and shale as the cap bed of

Vo 1.4 No.2 Al luvial Fan Facies and Their Distribution in the Lower Talang Acar Fonnation, Northeast Betara Oilfield, Indonesia Gumai spread widely in the study area. Then elevation and extrusive volcanism began , shallow sea deposits and terrestrial clay-stone were developed (Fig3). Extrusion happened in the northwest direction in the

area 企om Pliocene to Pleistocene , and terrestrial sediments were developed. Subsequently, frequently volcanic activity was found everywhere in the South Sumatra Basin.

Middle depression

Western rise

21

Eastern slope

E

0.5 Aυ

l

的1.5

'" ~

2.0 2.5

3.0 3.5

l!!mI

~

@o

[Ajill

Upper Talang Akar Forrnation Lower Talang Akar Forrnation Gumai Fo盯nation Air Benakai Formation

[Mj5jJ Muara-Ka出 i

Formation

Fig. 3 Seismic interpretation section in the Jabung Block There are 6 sets of reservoir beds 企om bottom to top: 1) Basement reservoir beds: Mesozoic uplife , ancient highs , as well as Eocene cranny or weathered granite and quartzite , which are good reservoir beds; 2) Lahat reservoir beds: Tuffaceous sandstone , conglomerate and breccia deposited in faulted or low-lying regions , which belonged to lake , brackish lake sedimentarγ system; 3) Talang Akar reservoir beds: Sandstone , sandy conglomerate , siltstone , and shale developed in Lower Talang Akar Formation, including most1 y delta and river depositional environment system. It is one of most important sandstone reservoir beds in the study area, and experienced depositional periods from t1uviallacustrine to early sea transgression; 4) Batu Raja carbonate rock reservoir beds: Tableland carbonate rocks widely distributed, 18-68 meters thick, and associated carbonate swell and organic reefs , 36-110 meters thick; 5) Gumai reservoir beds: Fine sandstone and siltstone distributed along basin margin; 6) Air Benakat reservoir beds: Formed in the environment of regression, marine facies sandstone increasing gradually upwards. The shallow sea and de1ta sandstones are good reservoir beds. Most of conglomerate d叩osits in bed F at the bottom of the Lower Talang Akar Formation are basal conglomerates d叩osited on the Pre-Tertiary unconforrnity, and belonging to a terrestrial environment of alluvial fan or braided river origin. Alluvial fan deposits characteristics and their planar distribution are discussed as follows. 3.1 Lithology characteristics Bed F in the Lower Talang Akar Formation ,

generally speaking, belong to alluvial fan to t1uvial sedimentary environments , in which alluvial fan deposits are characterized by coarse granularity, poor sorting and low reservoir quality. Sand bodies accumulate longitudinally, and inter-layers are poorly developed, extending locally in the transverse orientation (Fig. 4). 1) Grains characteristics Poorly sorted sub-round coarse and very coarse sands with small pebbles up to 1 cm are loosely packed in light brown clay (Fig. 5). Qua血， granite and gneissic composite quartz; fragments of well sorted metaquartzite are more abundant than in other samples; Chert 企agments with quartz veins and pale brown claystone fragments with radiolarians are also found here. 2) Pores & cements Rounded pebbles of granite composite quartz (below) and metaquartzite (above) and a few of smaller quartz grains t1 0at in pale brown clay matrix , along with siderite crystals. This is probably paleosol (Fig. 6). Ductile rock fragments compressed. Single crystals of siderite with round margins t1 0ated in matrix. 3) Porosity and permeability Due to poor sorting and abundant rock debris , bottom conglomerate in Lower Talang Akar FOIτnation has no good property. Core experiment result of well NEB-7 indicate change of rock porosity from 2.8 to 12.8 % and air permeability from O.Ol x lO- 3μm2 to 253xl0- 3μm2 with the difference of sorting (Fig. 6).

22

2007

Petroleum Science

Physical property

1.og curves LDL ，在mlcc

GRI , API

ßIIC‘ μs/ft 80

Fig.4

100

10002000 }OOO

Core photo

POR, %

K. mD 1

0.1

02

0 .3

Bottom alluvial fan and its logging response and physical property in the Lower Talang Akar Formation in Well NEB-12

t 当

,

'

8

a , x25 ,

0

07

11

I~

11

1.1

I

"

; ， 1 将|叨 lqji?12:l 飞 1 1 '1

‘,1>

polar

b, x25 , X polar

飞

c , x25 , Fig. 5

11

4

气í(

polar

9

Iω1!

iZ

t 飞"

，气}。

d, x25 , X polar

Lithology characteristics of alluvial fans in bed F ofthe Lower Talang Akar Formation in NEB Oilfield (Well NEB-12 , 5554.7ft, TVD) Note喝:

Top: partic1 es and their roundness; Bottom: matrix with floating quar!z grains and siderite

23

Vo 1.4No.2 Alluvial Fan Facies andηleÎr Distribution in 也e Lower Talang Acar Formation, Northeast Betara Oilfield, Indonesia

slate basement. Each type of basement has its typical logging response (Fig. 7). MSFL. n' l1l I DT， μS /O .305111 2001140 40

Strata

EUEhdN ∞

』-gL ESEU

• DST3 .DST7 • DST6

Fig.6 Conglomerate structure and its prope町 characteristic in Well NEB-7 Notes: Le ft: 5341 .5 ft , rock debris/pebble located in channel bottom (difIerent 击。mbedE)，甲 =12.8% ， K，..=253 x \O，3阳n'; Right: 54 \0 .8宜， v町ypo明'iy sorted erosive structure in alluvial d叩osits (top ofbed F)，申 =2.8% ， K，..=O.0Ixl0 阳、 Very poorly sorted alluvial plain sh四t wash deposits (interval E-water zone)，申=

F 、J

m·T

3

2.8% ， K田.=0.01 X \0，3阳n'

D

Carbonate Cranite D Sandstone Notcs: NU l11 bcr on thc Ic ft of scction is d~pth. 111

3.2 Logging response characteristic

Due to scarce core materials in the studied area, logging information is the most important data to study depositional environment and reservoir beds distribution. In order to distinguish bottom conglomerate in the Lower Talang Akar Formation 仕om the basement, firstly, logging responses of different types are summarized and the basement can be c1assified into sandstone basement, granite basement and carbonate rock basement as well as NEB唰43

A\HJ1;~~;11'1

1/;1; ,

,

NEB俑 l

Fig.7 Pre-Tertiary basement reservoir beds in the South Sumatra basin (Yu, et al. , 2005) On the above basis , bed F in the Lower Talang Akar Formation trace correlation was carried out in the transverse orientation to confirm distribution of alluvial fan conglomerate in bed F. In the research, typical logging response of alluvial fan is summarized (Fig.8) , N 仨 B-34

NEB-32

.'\J! 寸;

才 J

F:li

世

J} 、

j:

iji){)

Fig.8 Alluvial fan logging response characteristics of conglomerate in the Lower Talang Ak ar Formation in NEB Oilfield

Petroleum Science

24

and conglomerate is characterized by low GR , low resistance , high density and low value between DLD and LLD , which reflects poor physical reservoir properties , coarse lithology and relatively high density. 3.3 Distribution of conglomerate On the basis of logging r，臼ponse characteristics and seismic property analysis, logging interpretation data were used ωclassify conglomerate stages. There are 4 stages of conglomerate recognized in bed F. Conglomerates pile up on each other, and the boundary is unclear. There are only 2 stages of conglomerate in some other wells. Clay-stones were developed between conglomerate deposits , which were studied by statistics ofthickness of different stages of alluvial fans , and the map of conglomerate distribution W出 completβd in the study area. Fig. 9 indicates that conglomerate was developed in the northeast and middle-south of the study area. The 60-70m (upper part of bed F) conglomerate was found in a small area in the middle-south. The 70-80m (lower part of bed F) conglomerate was developed in a relatively large area in the middle-south. Logging curve

-

21 贝贝

2007

analysis indicates that logging response of conglomerate has clear reflection in the northeast of the study area. There are no useful logging cu凹es to recognize conglomerate on the basement in the middle-south of the study area, because of the lack of useful GR curves which may have been caused by logging technique limitations. Therefore , a combination of seismic prope此y and logging data was used to interpret conglomerates of F bed in 出e area. Only basic interpretation could be carried out, due to 出e limitation of seismic information resolution. The conglomerates of bed F could be classified into two pa口s: upper part and lower part.ηle upper part of bed F is in a limited area, but the lower part is relatively widespread. (Fig. 9 and Fig. 10). Detailed interpretation of alluvial fan conglomerates was carried out in the northeast area based on logging curves. Single conglomerate 也ickness is from 25 to 30 丘， and each of them distributes fairly alike. It is manifest that the flow direction was 企om southeast to northwest during sedimentary stages of Fl and F2 , and the flow direction W臼企om southwest to northeast during sedimentary stages of F3 and F4, which could be recognized by contour of sandstone thickness (Figs. 11-14)

-!(){\()I' ，

Lcg~nd

曰~ h， nllne

队， 11

Sundstonc pn己 dicted by sCÎ i>n1 ic

[a 0、 cr !ilY

linc

写 andSlonc

prl!di t: tcd by logging dala

Fig. 9 Distribution of alluvial fan conglomerate ofupper part (F l+F2) ofbed F in the Lower Talang Akar Fo口nation ， NEB Oilfield

Vol.4 NO.2

Al luvial Fan Facies and Their Distribution in the Lower Talang Acar Fonnation, Northeast Betara Oilfield, Indonesia

L o.' g o.' !hJ

l2'J日]万「 holi 川、 '...11

Sand i-I (llk'predl o.'， cd

()、 erlω1111 ...

飞i:md!>kmè

predirlcd

h，、、 k嗯μ II1 gdat"

Fig. lO

Dis位ibution

ofalluvial fan conglomerate oflower part (F3+F4) ofbed F in Lower Talang Akar Fonnation , NEB Oilfield

民日\.，240

'lI' A.:辛。~ A

M缸

‘ NloB P'.C? I" ,

吐白惜一

\"…飞

飞鸟

A户丁 n.~nO

+.J\ E飞Hd 《町旧" n Nl 剖" v ⑥

ONLB IOJ

Nf l1.1