hydrocarbon columns of oil and gas fields in the …

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Number 46 – November 2020 Page 51 of 74

HYDROCARBON COLUMNS OF OIL AND GAS FIELDS IN THE

SOUTH SUMATRA BASIN

Veridaus Napitupulu1, Miftahul Jannah1, Michael Silaen1, Herman Darman1,2

1 Indogeo Social Enterprise, Indonesia 2 Currently with Petronas Carigali, Kuala Lumpur, Malaysia

*Correspondence author: [email protected]

INTRODUCTION

The South Sumatra Basin is a prolific basin with a long history of oil and gas exploration that started

in early 1900s. It is located in the southern part of

Sumatra Island, Indonesia (Figure 1), where more

than 300 oil and gas fields have been discovered.

The basin contains multiple hydrocarbon reservoir horizons, from Pre-tertiary Basement to Miocene

age. The kitchen areas also have various depths,

with different levels of maturity. This petroleum

system variation is reflected by the field size

distribution and the heights of the hydrocarbon

columns.

Many authors have discussed the petroleum

system of this basin (e.g. Argakoesoemah & Kamal,

2004; Bishop, 2000; Carrillat et al., 2013; Clure,

2005; Ginger & Fielding, 2005) and they are useful

for providing regional geological perspectives. The oil and gas fields in South Sumatra are usually

discussed independently by the operators (e.g.

Carrillat et al., 2013; Chalik et al., 2004; Lu et al.,

2005; Suta & Utomo, 2006). Different seismic

vintages, density and quality make seismic

correlations across the basin challenging. Apart from that, the data restriction is another hurdle in

making correlation of the fields. Therefore,

statistical studies of the fields and play level

understanding become challenging.

This study utilizes the Indonesian Petroleum Association Atlas of South Sumatra oil and gas

fields, which was published in 1990 (Courteney, et

al.,1990). Several new fields, especially in fractured

basement reservoirs, were added, based on recent

publications. Additional information from the IHSE

fields database was used to support the study. These data allow a statistical study on hydrocarbon

column heights in the South Sumatra Basin. The

result of this study could explain more about the

charge and seal elements in the basin.

REGIONAL GEOLOGICAL SETTING

The South Sumatra Basin is a back-arc basin at

the southern margin of Sundaland and located in

the south of Sumatra Island (Figure 1). The basin was formed by three major tectonic phases: 1) An

extension stage during late Paleocene to Early

Miocene, forming north-south trending grabens

that were filled with Eocene to Early Miocene

deposits; 2) A normal faulting phase from the Early

Miocene to the early Pliocene with NW-SE

orientation; and 3) Basement-involved

compression, basin inversion, and the reversal of normal faults in the Pliocene to Recent, forming the

anticlines that formed major hydrocarbon traps in

the area. Many of the normal faults that formed the

depositional basins in South Sumatra have been

reactivated and some have been inverted during the Miocene to Plio-Pleistocene compression and

basin inversion. The subduction of the Indian

Oceanic Plate beneath the Asian Continental Plate

beneath Sumatra Island plays a major role in the

later structural development.

The basement of the South Sumatra basin is comprised of Pre-Cenozoic metamorphic and

igneous rocks (Figure 2). Granitic intrusions were

observed in places. The main Tertiary sediment

depocenter is located in the middle of the basin as

shown in Figures 1 and 2 and it is called the Palembang Sub-basin. Paleogene sediments

include syn-rift sediments called Lahat and Lemat

Formations, which were deposited in this sub-

basin. At late syn-rift stage, the Oligocene fluvio-

deltaic Talang Akar Formation was deposited with

some coals as the source rocks and was distributed over a wider area. The Baturaja Formation

carbonates developed locally in the Early Miocene,

mainly on structural paleo-highs. This reservoir

level is covered by the shale dominated Gumai

Formation, which is known as a regional seal in the basin. The Air Benakat fluvio-deltaic –shallow

marine clastic reservoirs developed in the Middle

Miocene and the system continued to the Late

Miocene, the so called Muara Enim Formation.

This clastic system has reservoirs with intra

formational seals. The Pliocene Kasai Formation is

the youngest seal horizon in the system (Figure 3)

HYDROCARBON COLUMN DATA

There are 60 fields with hydrocarbon column data

in this study. Although they are only about 20% of

the total number of oil and gas fields discovered in

the South Sumatra Basin, these fields are spread

across the basin and provide good coverage for the

study. The majority of the data was extracted from the Oil and Gas Fields Atlas of South Sumatra, a

1990 publication from the Indonesian Petroleum

Association compiled by Courteney et al. Additional



information of several fields was added to this

database, including Dayung Field (Suyoto &

Bethancourt, 2010), Sumpal Field (Chalik et al.,

2004), Kaji-Semoga Field (Atkinson & Hutapea.,

2006), Makmur Field (Lu et al., 2005) and Betara

Northeast Field (Suta & Utomo, 2006),

From the structural map the crest, oil-gas contact,

oil-water contact, and gas-water contact were

extracted. Figure 4 shows the definition of these

terms in a structural petroleum trap model. Most

of the available structure maps do not specify the

crest, which is the maximum culmination of the

structure. These numbers were estimated from

contour maps available in published papers. The

coverage of the maps is usually restricted to the

field area and unfortunately the spill points are not always observable from those maps. Therefore, it is

impossible to understand the maximum storage

capacity of the structure (Figure 4).

Figure 1. Sediment thickness map (Isochore) of the South Sumatra Basin and hydrocarbon fill of selected fields. Dark grey polygons are fields. Black dash lines are structural lineation. Hydrocarbon column and isochore contours

are in meters. A-A’ section is shown in Figure 3.



The height of a hydrocarbon column is important

information, as it reflects the charge and seal elements of the petroleum system. The oil and gas

charge depends on the source rock type, quantity,

maturity, timing and migration factors. For the seal

element, the column height also depends on the

sealing capacity of the cap rock. In general, a long hydrocarbon column indicates a strong top seal

and effective charge capacity. Shorter columns may

indicate possible leakage through the seal and/or

a limited hydrocarbon charge to the trap. If spill

points are known, it will tell more about the charge

capacity. Hydrocarbon columns that extend from

the crest to the spill point, also known as full to spill, indicate a strong charge from the kitchen and

maximum holding capacity of the cap rock.

HYDROCARBON COLUMNS PER PLAY LEVEL

The oldest reservoir in the South Sumatra Basin is

the fractured Pre-tertiary basement. There are 4

fields included in this study and they contain long

Figure 2. Stratigraphy of South Sumatra basin (after Ginger & Fielding, 2005) with hydrocarbon occurrences indicated as bubbles. Red bubbles indicate gas, and green bubbles indicate oil occurrences. Bubble sizes reflect the

relative sized of oil and gas discoveries.



gas columns (Figures 2 & 5). This is a relatively new

play in the basin, with the first commercial

discovery in the early 1990’s, but it contributes

significant volumes of gas. The Sumpal Field with

an 800 m gas column is the longest column in the

basin. The field was discovered in 1994 and is currently operated by ConocoPhillips. Some

experts mentioned that the gas contacts in the

basement play may vary in depth (Woodroof and Li,

2020; pers. communication), but for the fields in

South Sumatra, this phenomenon cannot be

demonstrated in our limited data base.

The Late Oligocene to earliest Miocene Talang Akar

Formation is a fluvio-deltaic sequence which is a

significant reservoir horizon and oil and gas play in

the basin (Figure 2 & 6). It is represented by 30

fields in this study. The Pendopo Field has the longest oil column in this play (450 m), followed by

Gunung Kemala (330 m), which contains a mixture

of oil and gas. These fields were discovered before

the Second World War; more precisely in 1922 and

1938, respectively. The shorter columns of this

reservoir group are oil dominated and the longer

columns tend to have more gas caps (Figure 6).

The carbonate reservoir of the Baturaja Formation,

which formed in the Early Miocene (Figure 2) is

another significant oil and gas play in the South

Sumatra Basin. It is represented by 14 fields with hydrocarbon columns. Similar to the Talang Akar

Formation, the longer columns contain more gas.

The Musi Field has the longest column with 232 m

of gas and 16 m of oil. The crest of Musi Field is

about 2500 m below the surface (Figure 7A).

Fourteen (14) fields with hydrocarbons represent

the Air Benakat play, deposited in fluvio-deltaic to

shallow marine settings during the Middle

Miocene. This play has more oil, and the Bajubang

2 Field has the longest column, 175 m, followed by

Bajubang with 155 m of hydrocarbon column. Both

fields have 40 m of gas column.

Figure 3. Regional geologic cross section of the South Sumatra basin (after Hennings et al., 2012). The stratigraphic units are:

1. 6. Upper Miocene-Pliocene sandstone, shale, coal - volcanic Kasai Formation 2. 5. Middle-Upper Miocene sandstone and shale - Air Benakat and Muara Enim Formations

3. Lower Miocene organic shale and rare sandstone –Gumai Formation 4. Lower Miocene platform and reefal carbonate - Baturaja Formation

5. 2. Paleogene/Neogene granite wash and clastic sequences - Lemat and Talang Akar Formations 6. Pre-tertiary crystalline and metamorphic basement.

Figure 4. A model cross section showing the petroleum trap elements, like the crest and hydrocarbon contacts (after Railback, 2011).


Number 46 – November 2020 5 of 74

The Muara Enim

play, which is Late

Miocene in age (Figure 2), is the

youngest prolific oil-

gas interval,

represented by 4

fields in this study. The Grissik gas field

holds a gas column of

234 m. Although the

play is relatively

young and shallow,

the seal still has the capacity to hold

hundreds of meters

of hydrocarbon

columns (Figure 7C).

The Grissik gas column is an

anomaly which is

most likely caused by

local geological

situation, such as a

higher geothermal

gradient.

CHARGE AND TRAP

GEOGRAPHIC DISTRIBUTION

The geographic distribution of the hydrocarbons is

shown in Figure 1. The basement gas reservoirs

with long columns (e.g., Suban, Dayung and

Sumpal) are clustered together in the Central

Palembang Sub-basin, which is believed to be the kitchen area of the gas. The kitchen could be

deeper than 5000 m. The source rocks within this

sub-basin are buried deep into the gas window.

The fractured basement fields have an effective

seal, which is probably the Paleogene shale as

they can hold a few

hundred meters of gas

column. The seal is

relatively old and deeply

buried as well. This situation may allow them

to seal long hydrocarbon

columns.

The fields in the southern

part of the South Sumatra Basin are

clustered around the

Muara Enim Deep. These

fields have shorter

hydrocarbon columns

and generally have more oil. The Muara Enim Deep

is less than 5000 m. This

area is better for maintaining oil generation and

preservation conditions.

Another cluster of fields occurs in the Tamiang High near the eastern margin of the South Sumatra

basin. These fields have shorter hydrocarbon

columns, and more oil has been discovered in this

area. The oil in this area originates from the

southeast extension of the Central Palembang Basin. This basin is shallower compared to the

kitchen area of the fractured basement fields and

produces more oil.

Several fields were discovered in the north of the

South Sumatra Basin, in a sub-basin called Jambi

Sub-basin (Figure 1). The Betara Northeast field has the longest gas column in this area. South of

Betara Northeast are mainly oil fields with gas

caps. The Jambi Sub-basin is shallower than

Central Palembang Sub-basin, and most of the

source rocks remain in the oil generation window.

THE RELATIONSHIP BETWEEN HYDROCARBON COLUMN AND AERIAL SIZE

Our data suggest that, in general, structures with

larger aerial size tend to have longer hydrocarbon

columns. This is understandable, as small structures tend to have small closure heights

between the depths of the crest and spill point and

therefore can only host small hydrocarbon

columns.

Large structures generally have larger closure heights and therefore have a greater chance for

large hydrocarbon column heights. However, these

structures are not always filled to spill, due to other

factors, like insufficient hydrocarbon charge

volume, leakage through faults, or limited top seal

capacity.

Figure 5. Gas columns of 4 fields in Basement reservoirs in the South

Sumatra Basin.

Figure 6. Hydrocarbon columns of fields with Talang Akar clastic reservoirs. Red column is gas and green column is oil.



This relationship between aerial size and column

height is shown in Figure 8, where the cluster plot

is divided into 4 classes. The basement play stands out as represented by Sumpal, Dayung and

Sumpal and is grouped into Class 1. This class has

the longest hydrocarbon columns, all filled with

gas, and generally have larger aerial sizes. The

Sumpal Field has a gas column of 800 m, the longest column recorded in the South Sumatra

Basin.

The second class in terms of column vs aerial size

in this study includes the Betara Northeast,

Pendopo and Gunung Kemala fields (Figure 8). The

fields in this class are in Talang Akar Formation clastic reservoirs. The Betara Northeast Field has

the largest aerial size in the basin (126 km2) and

holds 213 m of gas column. Pendopo and Gunung

Kemala also have significant aerial sizes and

hydrocarbon columns.

Class 3 in Figure 8 are dominated by fields with Baturaja limestone reservoirs. This class

comprises the Kaji-Semoga and Musi Fields. Both

fields deviate from the general trend. The Kaji-

Semoga Field covers a relatively large area (about

53 km2), but only 77 m of hydrocarbon column, with both oil and gas. This is caused by the fact

that the Kaji Semoga is partly a stratigraphic trap.

The Musi Field has the opposite relationship, it has

a long hydrocarbon column (248 m) but occupies a

relatively small area. This is because the trap is a

reefal build-up, a trap type that tends to be

relatively tall for the aerial size.

The remaining fields with smaller aerial sizes and

shorter hydrocarbon columns are grouped into



Class 4 (Figure 8). These fields have less than 200

m of hydrocarbon columns and have an aerial size

of less than 40 km2.

THE RELATIONSHIP BETWEEN HYDROCARBON COLUMN AND CREST DEPTH

The data collected in this study suggest a poor correlation between depth of burial and

hydrocarbon column heights. Basement reservoirs

are the oldest reservoirs in age, but they are not the

deepest in the basin. The basement reservoirs observed in this study (Figure 9) range from 1250

m deep (Dayung Field) to 2100 m (Suban Field).

The reservoirs which are comprised a mixture of

igneous and metamorphic rocks contain gas

columns up to 800 m (Sumpal Field). However,

Teras Timur Laut has only 30 m of gas column.

Figure 8. Scatter plot of HC Column Height vs Field Aerial Size.

Figure 9. Scatter plot of HC Column Height vs Crest depth



There are many Talang Akar and Baturaja

reservoirs which are located at the same depth

range as the large basement reservoirs. The Pendopo Field with the Talang Akar reservoir and

the Musi Field with the Baturaja reservoirs, for

example, are discovered in greater depths than the

basement reservoirs in this study.

Middle Miocene Air Benakat reservoirs form a cluster at shallower depths. Their crests are

located throughout a wide range of depths. The

deepest crest is almost 2000 m (Lagan Field) to 95

m (Betung Jambi). The Air Benakat crests cluster

at around 500 m depth (Figure 9).

Only a few of the Upper Miocene Muara Enim reservoirs are included in this study (Figure 9).

Their crests are located about 500 to 1300 meters

below the surface. The Grissik Field has a 234 m

gas column, and its crest is only 500 m below the

surface. Its character stands out in the scatter plot

in Figure 9.

Overall, the reservoirs which are less than 1000 m

deep have more oil columns (Figure 10). Some

studies have indicated that gas cap may have leaked, resulting in columns which contain more

oil. The permeability of the shallow cap rocks may

allow the gas to escape, but not the oil.

The fields which are located below 1000 m tend to

have longer gas columns (Figure 10). This indicates a more compacted seal, but it may also indicate

more gas charge from a deeper kitchen. The

Pendopo Field, which has an oil column of 450 m

at a depth of 2500 m is anomalous, which requires

further explanations.

DISCUSSION AND RECOMMENDATION

Due to limited data, the number of reservoirs per

field is not well known. It is known that some fields have multiple pay zones in different reservoir levels

but not all are reported. Therefore, comparing the

hydrocarbon columns and the total field size

volumes could not be done in this study.



Comprehensive field studies and reports are

required to make this comparison.

As kitchen data is usually difficult to obtain due to

great depths, hydrocarbon columns are a good

source of information about the kitchen. In this

study, it is seen that deep kitchens supply more gas, whilst shallower kitchens supply more oil to

the nearby fields. Basin modelling studies are

needed to support this observation.

To improve the understanding of kitchens and

hydrocarbon field columns, more data is required

for the study. Ideally, it should be coupled with regional seismic interpretations and detailed field

studies. There is more data available after the IPA

Field Atlas (Courteney et al., 1990) was published,

which should be included in this study. It is

recommended that the government release more data for studies, which certainly will improve the

understanding of present observations and

hopefully lead to new discoveries.

CONCLUSION

This hydrocarbon column study, with limited data

availability, suggests some

general trends. Local

factors, like variations in

hydrocarbon charge and leakage, can cause

deviations from the general

patterns.

The basement play can

hold much longer gas

columns than younger plays. The distribution of

these fields is also very

localized, which is

somewhere close to the

Palembang Deep Sub-

basin.

Fields in shallower basins

are more likely to be

charged with oil, as shown

by the fields around the

Jambi Sub-basin and Muara Enim Deep. The oil columns in the South

Sumatra basin are generally shorter compared to

the gas columns.

In general, the younger and shallower reservoirs

will most likely have cap rocks with a lower sealing capacity. Therefore, the younger and shallower

plays have shorter columns compared to the

basement play. In certain conditions, there are

anomalies due to the local geological conditions.

Hopefully, this study can stimulate more detailed

studies, with more data and incorporating additional parameters that may affect hydrocarbon

column heights, and also determine possible

differences in column heights between gas-

dominated versus oil-dominated accumulations.

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