Control Effect of Faults on Oil-Gas Contribution in a Block of

Indonesia

Wenqi Zhang

, Chunlei Li

and Zhenhua Bai

Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083,China.

Email:zhang_wenqi@petrochina.com.cn

Keywords: Drape anticline, fault, distribution of oil and gas

Abstract: Block A in Indonesia is a faulted anticline structure limited by reverse thrust faults on the east and west

boundaries. The tectonic evolution mainly experienced rift period, rift-depression period, depression period

and compression inversion period. Traps formed during the Pliocene-Pleistocene crustal inversion period,

matching well with large-scale hydrocarbon generation and expulsion. The NNW strike faults, NNE strike

faults and NEE strike faults mainly developed in the block. The NNW strike boundary fault s are reverse

thrust faults developed on the basis of the normal faults of the previous graben boundary and controlled the

formation of structural trap of Block A. During the large-scale period of hydrocarbon generation and

expulsion, the boundary inversion thrust faults acted as a channel for vertical migration of oil and gas.

Hydrocarbons from the Lower Lahat Formation and the Talang Akar Formation were communicated to the

upper reservoirs, and the oil and gas were blocked laterally. The NNE trending faults were controlled by

NNW strike faults, and mainly developed in the rift-depression period and lateral sealing for oil and gas.

The vertical and horizontal distribution of oil and gas in Block A makes the structure as the main controlling

factor for oil and gas distribution in Block A.

1 OVERVIEW

Block A is located at the junction of South Sumatra

basin and Central Sumatra basin in Indonesia. The

Betara Co mp lex structure in the middle o f Jabung

block is the main gas field of Betara Co mplex

structure, with abundant hydrocarbon resources

(Xue et al., 2005). The tectonic evolution of Block

A main ly e xperienced four stages, including the

mid-Eocene to Oligocene rifting stage, the late

Oligocene to the early Miocene rift-depression

transition stage, the early Miocene to the end

Miocene depression stage, the early Eocene to the

present compression stage. The Lahat Format ion

deposited in the late Eocene rifting period, the

Talang Akar Formation deposited in the Oligocene

rift-depression transitional period, the Batu Raja

Formation, the mid-Gumai Formation, the late A ir

Benakat Formation and the MuaraEnim Format ion

deposited in the Miocene period, and the Kasai

Formation deposited during the Pliocene-Pleistocene

compression inversion period (Rashid et al., 1998;

Suseno et al., 1992). The sedimentary sequence of

the Lahat Format ion lacustrine mudstone and the

lower Talang Aka transitional facies sedimentary

sequence are the majo r hydrocarbon source strata.

The reservoirs are clastic sands tone of Lower Talang

Akar Formation, clastic sandstone of Upper Talang

Akar Format ion, sandstone of Gumai Format ion and

carbonate of Batu Raja Formation, and the Lower

Talang Akar sandstone is the main pay format ion of

Block A.

2 STRUCTURAL FEATURES

2.1 Stratigraphic Features

The Lahat Formation and the Talang A kar formation

was mainly filled sediments during the semi-graben

rift, overly ing the Pre-Tertiary basement. The

lacustrine mudstone of Lahat was mainly filled in

the semi-g raben. The lower Talang A kar Format ion

mainly deposited alluvial-braided river-delta facies

of the transitional facies, and was mainly composed

300

Zhang, W., Li, C. and Bai, Z.

Control Effect of Faults on Oil-Gas Contribution in a Block of Indonesia.

In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 300-303

ISBN: 978-989-758-342-1

of sandstone, siltstone and gray, lime-green

mudstone, shale and thin coal seam. The Upper

Talang Akar Formation transited into river-delta and

marginal sea sediments and was main ly co mposed

of interbedded shale, siltstone, a small amount of

sandstone and several thin layers of limestone. Batu

Raja deposited on broad carbonate platforms, the

Gu mai Format ion was fine sandstone and siltstone,

and the Air Benakat Format ion was marine

sandstone deposited in a regressive environment

(Xue et al., 2005; Rashid et al., 1998; Suseno et al.,

1992; Lu et al., 2004; Yuan et al., 2012).

Before the tectonic inversion of the Pliocene-

Ple istocene, the tectonic formation in area A was the

high-draped anticline structure draped the former

Tertiary buried hill. During the Pliocene-Pleistocene

period, the cross-sectional tendency of the half-

graben boundary faults was reversed due to the

extrusion effect, and the t wo boundary faults of

semi-graben became reverse faults. At the same time,

the strata flexural deformat ion occurred and formed

saddle-shaped anticlines. Under the control of the

boundary faults, the tectonic strike was consistent

with the strike of the boundary faults. At the high

structural site, the drape anticline was divided by the

NNE faults and the NEE faults to formed several

different types of small anticline traps and fault

blocks. Figure 1 shows the structure of Talang Akar

of Block A.

Figure 1: TVDSS map of Talang Akar of block A.

2.2 Fault Characteristics

Block A is faults developed, the faults were formed

in d ifferent periods with different sizes and different

types of fault system. According to the development

of fau lts and the features of fau lts, three main fault

systems were developed: NNW strike fault system,

NNE strike fault system and NEE strike fau lt system.

The NNW strike fault system is boundary faults

of Block A, ma inly reversal thrust faults developed

fro m the rifting stage to the depression stage and

later on the normal faults of the graben boundary

activated by the inversion period, and controlled the

Block A structure. The NNE t rending faults are

controlled by NNW strike faults and are main ly syn-

sedimentary norma l faults developed from the

depositional period of Talang Akar to Gu mai

sedimentary period. These faults did not activate

during tectonic inversion. The NEE strike faults are

mainly small fau lts, and mainly developed at the end

of the sedimentary period of the Talang A kar an d

had no controlling effect on the sedimentation. They

are non-sedimentary faults and main ly played stress-

regulating roles.

On the plane, three sets of fault systems combine to

form multiple small anticlines and fault blocks. In

the profile, the faults are straight and the fault

displacements are relatively s mall. The fault

complexes are opposite or relative, and a few faults

are Y-shaped.

3 FAULT CONTROL ON OIL

AND GAS DISTRIBUTION

Faults in Block A have an important control over the

distribution of oil and gas. Inversed faults of the

east-west boundary not only control the format ion

and distribution of traps, but also channeled for

transport and distribution during hydrocarbon

migration.

3.1 The Faults Control the Trap

Formation and the Trap Formed

Period Had Good Matching with

Large-Scale Hydrocarbon

Expulsion Period

As mentioned above, the NNW strike and NNE

strike faults controlled the formation of structural

traps of block A, and formed different trap types.

However, the formation of oil and gas reservoirs is a

variety of geological elements within the

petroliferous basin. The geological processes are

matched in time and space of hydrocarbon expulsion

period (Lu et al., 2004). Therefore, the traps formed

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fault

depth co ntour

Control Effect of Faults on Oil-Gas Contribution in a Block of Indonesia

301

earlier than or synchronized with the hydrocarbon

expulsion period a re the necessary conditions for the

formation of oil and gas reservoirs in Block A.

In Block A, the rift-sag boundary faults

developed from mid -Eocene continuously to the end

of Miocene. In the basin of Early Pliocene, faults

affected by the ext rusion, the strata overlying the

pre-Tertiary basement e xperienced tectonic

deformation and formed basement intrusive

extrusion anticlines during the Pliocene-Pleistocene.

With the continued extrusion, the structural

amp litude gradually increased. From the end of the

Ple istocene to the present, it entered the relatively

quiescent period of tectonic activ ity. W ith the

increase of burial depth, the anticline remains intact.

The Lower Talang Akar and Lahat strata in the

Tertiary a re the main source rocks, and oil

generation began in the late Miocene. The organic

matter in the Pliocene source rocks entered the

mature stage and began to enter hydrocarbon mass -

generation and e xpulsion periods (Yuan et al., 2012).

With the formation traps, oil and gas entered these

traps and formed reservoirs. As traps formed time

matcheed well with a large nu mber of hydrocarbon

expulsion stages and the post-tectonic structures

were well preserved, it p rovided favorable

conditions for the preservation of

hydrocarbons .Figure 2 shows the cross section of

Block A.

Figure 2: Cross section of block A.

3.2 Reversed Boundary Inversion

Thrusts Were Important Vertical

Transport Channel for

Hydrocarbon Migration

The reverse rift thrust faults developed fro m the

Eocene at the rift-semi-graben border run through

the strata from the lo wer Tertiary Lahat and the

lower Talang Akart source rock to the Upper Air

Benakat, and the reverse thrust reverse faults

became the most important vertical channel of

hydrocarbon mig ration, especially during the

Pliocene-Pleistocene period of hydrocarbon mass -

generation and expulsion. During these period the

reverse rift thrust faults were the reactivation of the

boundary faults and the vertical transport of

hydrocarbons to the Upper Talang Akar, Bata Raja

and Gumai reservoir.

3.3 NNE Faults Controlled the Local

Accumulation of Oil and Gas

The NNE fault developed mainly fro m the Talang

Akar to Air Benakat depositional stages and formed

different types of local traps with the boundary and

NEE faults. On the one hand, part of the faults in the

Lower Talang Akar source rock directly

communicate with the source rocks and reservoirs.

On the other hand, the oil and gas migrated fro m the

lower part to the upper part through the boundary

inversion thrust faults and further through the NNE

strike faults in the horizontal direction, and finally

formed different oil-gas contact and water-oil

contact hydrocarbon reservoirs.

3.4 Control of Fault Lateral Sealing on

Hydrocarbon Accumulation

The mechanis m of fault docking is that the two

layers of the two sides of the fault contact with each

other due to the relative movement of hanging wall

and footwall. At the same time, the mudstone

undergoes plastic deformat ion when the format ion

slides; the fault is blocked by mudstone, so that the

sandstone on both sides of the fault is in contact

with the cross-section mud ca ke, and form a lateral

seal (Fu et al., 1998; Yield ing et al., 1997; Allan,

1989; Lv and Fu, 2002; Zou et al., 1992; ith, 1996).

The boundary faults acted as a compressive reverse

faults and the strata on both sides of the faults differ

wide ly in their permeability, especially under the

action of the Miocene extrusion tectonic movement.

The fault surface was in a co mpressive stress state

and the fault has obvious sealing in the lateral

direction, so that oil and gas are ma inly enriched in

the upwelling trap of the faults. The vertical slide

distance of the NNE strike is generally 10-50m.

Both sandstones are basically ju xtaposed with the

mudstone facies of the plate. At the same time,

because the sandstone and mudstone are interbedded

thinly in this area, the mudstone was subjected to

shear during the sliding and was squeeze into the

Air Benakat

Gumai

Bata Raja

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1500

1300

1100

900

700

Fault

Basement

Upper Talang Akar

Lower Talang Akar

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fault plane to form a continuous fault mud that

conducted to the lateral fault closure, and controlled

local oil and gas accumulation. The NEE strike

faults are relatively s mall and the vertical slid ing

distance is 2-10m. Both sides of sandstones are

ju xtaposed and the smaller sliding distance made it

difficult to form a continuous cross -section mud

cake on the fault planes, and played a lateral

migration channel role of hydrocarbon . Figure 3

show the reservoir section of Lower Talang Akar of

Block A.

Figure 3: A reservoir section of lower Talang Akar of

block A.

4 CONCLUSIONS

Based on the analysis of the controlling role of

Block A on hydrocarbon accumulation, it shows that

the faults played an important role in controlling the

accumulation and distribution of oil and gas in

Block A. The reverse thrusts controlled the

formation of tectonic traps in Block A and were

important channels for the vertical migration of oil

and gas, and controlled the distribution of oil and

gas throughout the Block A Formation. The NNE

faults are secondary faults and controlled the

formation of local traps. They were also the

important channels for oil and gas redistribution.

Faults sealed the oil and gas sideways and formed

different types of oil and gas reservoirs with

different oil-gas-water contacts.

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