HYDROCARBON COLUMNS OF OIL AND GAS FIELDS IN THE SOUTH SUMATRA BASIN

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, 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 and they are useful for providing regional geological perspectives. The oil and gas fields in South Sumatra are usually discussed independently by the operators. 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. 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.


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 subbasin.At late syn-rift stage, the Oligocene fluviodeltaic 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).

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 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).

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.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.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.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 Subbasin.
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 gasdominated versus oil-dominated accumulations.

Figure 1 .
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.

Figure 2 .
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.

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 4 .
Figure 4.A model cross section showing the petroleum trap elements, like the crest and hydrocarbon contacts (after Railback, 2011).
more oil.The Muara Enim Deep is less than 5000 m.This area is better for maintaining oil generation and preservation conditions.

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

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

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