Overpressure Characteristic in the Langkat Field, North Sumatra

Basin, Indonesia

Hazmanu Hermawan Yosandian 1, Hengki Irawan

1, Binti Wasik Atul Ulum

1, Irawan Youdha

Tribuana 1 and Patra Embara

1

1 Geological Engineering Department, Institut Teknologi Bandung

Abstract. The North Sumatra Basin is an overpressured and productive basin in Indonesia. Top

overpressure in the basin is found in Lower Keutapang Formation, just above Baong Formation. In The Aru

and Kuala Simpang Field, Baong Fomation is shale dominated and forming mud volcano, which is a strong

evidence of overpressure occurence in the basin. In the Langkat Field, top overpressure is also detected in

Lower Keutapang and Baong Formation, meanwhile, its distribution and generating mechanism related to

geological environment are still unknown. The methods used in this study are analyzing drilling parameters

and wireline logs with additional data from geochemical analysis which are vitrinite reflectance (Ro), Tmax,

and Total Organic Carbon (TOC). After analyzing drilling parameters and wireline logs, top overpressure in

the research area detected in Lower Keutapang and Baong Formation. Then,the pressure versus depth profiles

show that overpressure condition decrease until Middle Baong Sandstone and increase again until becoming

normal hydrostatic condition in Belumai Formation. So, it can be concluded that overpressure condition in

the research area characterized by shale prone formation (Upper and Lower Baong Formation), rather than

sand prone formation (Middle Baong Sandstone and Belumai Formation). Results reveal that the

overpressure generating mechanisms at the research area are compaction-related (loading) and fluid

expanding-related (unloading). The loading mechanism is caused by rapid sedimentation rate of the basin and

the unloading mechanism is caused by clay diagenesis, hydrocarbon generation, and vertical transfer.

Therefore, this research conclude that overpressure is related to geological enviroment and history at the

research area.

Keywords: Overpressure, Baong formation, compaction.

1. Introduction

The North Sumatra Basin is a well-known overpressured basin in Indonesia [1]. One of Pertamina’s

fields in the basin, the Langkat Field, has overpressure condition detected in Lower Keutapang and Baong

Formation [2]. However, the overpressure characteristics of the Langkat Field, which are the distribution and

generating mechanisms related to its geological environment, are still unknown.

This research uses four wells data at the Langkat Field which will be analyzed to find the characteristics

of overpressure (Figure 1). It is highly important to know the characteristics because overpressure give heavy

consequences in oil and gas industry. For example, overpressure drives hydrodinamics movement of oil and

gas. Meanwhile, it will cause many financial and safety problems while drilling, such as kick and blow out if

no or less accurate prediction is done. Therefore, overpressure characteristic and prediction are essential

before drilling projects can be started at the research area.

2. Method

This research uses pressure measurements, mudweights, drilling events, and wireline logs to identify

pore pressure trend at each wells. Those data are presented as pressure-depth and logs-depth profile. Then,

the profiles will be analized to determine overpressure trend and its connection to the geological

Corresponding author. Tel.: + 6285695007434, +6281280778436

E-mail address: patraembara@windowslive.com

2014 3rd

International Conference on Geological and Environmental Sciences

IPCBEE vol. 73 (2014) © (2014) IACSIT Press, Singapore

DOI: 10.7763/IPCBEE. 2014. V73. 9

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environment.

Fig. 1: Research Area

Fig. 2: (a) Sand-rich and shale-rich intervals discrimination (Katahara, 2006). (b) Wireline log responses to loading

mechanism (Ramdhan, et al., 2011). (c) Wireline log responses to unloading mechanism (Ramdhan, et al., 2011). (d)

Dutta crossplot (Dutta, 2002)

Mudweight could be an indicator for the detection of overpressure marked by the increament of

mudweight but not all increament of mudweight is caused by the increasing of pore pressure. The increasing

of mudweight can be caused by geomechanical problem

Drilling event i.e kick, loss, blow out, pull/drag, hole fill, connection gas, etc, may be caused by

overpressure. Another data that we use to anlyze overpressure in this research is wireline log data. The

wireline log data i.e sonic, bulk density, neutron porosity and resistivity log are often used for analyzing the

overpressure [3]. After that, data processing will be conducted to discriminate between sand-rich and shale-

rich value from wireline logs. The discrimination use crossplot density (RHOB) against the difference

between neutron porosity and the porosity derived from density logs (PHIN-PHID) (Figure 2a) [4]. The 41

responses of wireline log data will show two basic types of overpressure generating mechanisms, which are

loading and unloading. Loading mechanism response will show constant trend of logs, caused by retention

rate of pore fluid matching compaction rate of rocks (Figure 2b) [5]. Meanwhile, unloading mechanism will

show opposite direction of log response from normal trend because the pore fluid expand faster than

compaction rate (Figure 2c). Another method to analyze the mechanism is using crossplot between sonic and

bulk density log, known as dutta crossplot (Figure 2d) [6]. Geochemical data also will be used to check

hydrocarbon generation’s role.

3. Occurence and Distribution

At typical well Tanjung Putus Barat-1 (TPB-1) well, top overpressure is detected in Upper Baong

Formation at a depth of 1450 m (Figure 3a). Although overpressure condition occurs until the well reached

Lower Baong Formation, sonic log shows a minor decrease of the trend while reaching Middle Baong

Sandstone (MBS). That response might be caused by sand-rich intervals in MBS. Then, sonic log begin

coming back to normal trend when the well reach Belumai Formation, which consist of sandstone, siltstone,

and shale. The mudweight value might be overbalanced to anticipate high pore pressure, therefore, it does

not show pressure decline. The trend keep going to normal, as it begins to reach dolomite and limestone of

basement.

Fig. 3: Pore pressure profile of Tanjung Putus Barat-1. (b) Wireline log responses of Tanjung Putus Barat -1. (c) Dutta

crossplot of Tanjung Putus Barat-1. (d) Source rock maturity of Tanjung Putus Barat-1

Sonic log and pressure profile of typical well Gohor Lama-1 (GHL-1) well shows top overpressure

detected in Keutapang Formation at a depth of 900 m (Figure 4a). The Keutapang Formation consists of

sandstone and siltstone interbedded with claystone and shale intercalations.

4. Generating Mechanism

Wireline log profiles show clear unloading responses at typical well TPB-1 (Figure 3b). Two

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mechanisms possibly generate overpressure at the well are clay mineral transformation and hydrocarbon

generation. Dutta Crossplot shows smectite transformation at 1200 – 1800 m (Figure 3c). The crossplot is

confirmed by 73,58 ⁰C/km of geothermal gradient, with 30 ⁰C of surface temperature asumption, therefore,

smectite can transform to illite at 1200 m. The maturity data (vitrinite reflectance and Tmax) show the

mature source rock is detected at 2016 m (Figure 3d). Therefore, the overpressure generating mechanism at

TPB-1 is unloading, which are clay mineral transformation and hydrocarbon generation, which assisst

generating pressure at 2016 m.

At GHL-1, log responses show loading response which is constant trend in sonic and resistivity log.

Meanwhile, porosity and density log show minor unloading trend (Figure 4b). For solving the uncertainty,

Dutta crossplot is made and shows no mineral transformation (Figure 4c). No indicator of unloading

mechanism, so we may conclude GHL-1 has overpressure generated by loading mechanism.

5. Conclusion

At the Langkat field, overpressure occurence and distribution is asscociated with shale lithology. The

top overpressure is detected by wireline logs at shale dominated Upper Baong Formation (TPB-1).

Generating mechanism analysis show that overpressure at the research area are fluid expanding-related

(unloading) which is caused by clay diagenesis i.e smectite to illite and hidrocarbon generation. Another

overpressure mechanism is loading mechanism.

Fig. 4: (a) Pore pressure profile of Gohor Lama-1. (b) Wireline log responses of Gohor Lama-1. (c) Dutta crossplot of

Gohor Lama-1.

6. Acknowledgment

Authors thank to INOV (Indonesia Overpressure Study) ITB and Pertamina E&P for the opportunity to

study overpressure in The North Sumatra Basin.

7. References

[1] Aziz, A. and Bolt, L.H. 1984. Occurrence and Detection of Abnormal Pressures From Geological and Drilling

Data, North Sumatra Basin. Proceedings of Indonesian Petroleum Association, 13th

Annual Convention, 195-220.

[2] Manik, P. and Soedaljo, P. A. 1984. Prediction of abnormal pressure based on seismic data: a case study of

exploratory well drilling in pertamina UEP I and UEP II work areas. Proceedings of Indonesian Petroleum

Association, 13th

Annual Convention, 461-505.

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[3] Mitchell , M. And Mouchet, J.P., 1989. Abnormal Pressure While Drilling . Editions Technip

[4] Katahara, K. 2006.Overpressure and shale properties: stress unloading or smectite-illite transformation ? In:

Expanded Abstracts, 76th SEG Annual Meeting, New Orleans, 1–6 October, 1520-1524

[5] Ramdhan, A.M., Goulty, N.R., and Hutasoit, L.M., 2011, The Challenge of Pore Pressure Prediction in

Indonesia’s Warm Neogene Basins, Proceedings of Indonesian Petroleum Association, 35th Annual Convention,

IPA11-G-141

[6] Dutta, N.C., 2002. Deepwater geohazard prediction using prestack inversion of large offset P-wave data and model.

The Leading Edge, 193-198

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