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Chapter 1: Background of the Project
1.1 INTRODUCTION TO THE PROJECT
The Dent Peninsula has become the area of interest to numerous
geoscientists, particularly to the oil and gas sectors since the Central Sabah subbasin has been proven to contain oil and gas. Previous reports demonstrated an
active petroleum system in this basin (presently known as SB 305 Block) with
significant of oil and gas discoveries (Leong and Azlina, 1999). Chan (2008)
reported an active petroleum system within the Sulu Sea-East Sabah Basin region
that could contribute significantly to hydrocarbons generation and accumulation.
Haile and Wong (1965) found gas seepages and mud volcanoes in the onshore,
which could give a good indicator for an active petroleum system.
As the onshore area became the area of interest for the numerous
geoscientists in oil and gas, the study that focused on the onshore Dent Group
sediments is attempted to answer some of the question raised. The onshore Dent
Group sediments form part of the sedimentary sequence within the Sandakan SubBasin which largely extent in the southeastern portion of the Sulu Sea off eastern
Sabah. In the offshore, the Dent Group is currently being investigated for its
hydrocarbon potential. The available data on probable source rocks and their
quality is quite sketchy and limited. Therefore, the present study has been
undertaken to evaluate the petroleum potential, particularly for the source rock
quality assessments. The source rocks evaluation was carried out by geochemical
and petrological techniques, integrated with 2D basin modelling.
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Chapter 1: Background of the Project
1.2 OBJECTIVES OF THE STUDY
The objectives of this study are as listed below.
i. To reconstruct depositional environment of the source rocks.
ii. To determine the amount, type, and thermal maturity of organic matter.
iii. To evaluate the hydrocarbon generating potential of the source rocks based
on organic geochemical and petrological analyses.
iv. To perform oil to source rock correlation
v. To estimate the timing of optimum maturity for hydrocarbon generation based
on burial history and thermal maturity modelling.
vi. To evaluate the processes of hydrocarbon generation, migration, and
accumulation using 2D basin models.
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Chapter 1: Background of the Project
1.3 LOCATION OF THE STUDY AREA
The study area, onshore Dent Peninsula, is located in the eastern part of
Sabah (Figure 1.1-a). The study area is bounded by latitude from 40 55’ N to 50 30’
N and longitude 1180 30’E to 1190 15’E. It is situated between two passive margin
of the Sulu Sea and the Celebes Sea. Based on the structural provinces, the Dent
Peninsula is classified as Northeast Sabah basin (Leong and Azlina, 1999). The
Northeast Sabah basin later was divided into two sub-basins which are the Central
Sabah sub-basin and the Sandakan sub-basin. The Dent Group rocks investigated
are into the Sandakan sub-basin, in SB306 Block area (Figure 1.1-b). The active
exploration areas and proven wells that produced significant amount of
hydrocarbons are within SB 305 Block, which is located in the Central Sabah subbasin. In the SB 306 Block, there is only one exploration well (Sebahat-1), drilled in
1987 that penetrated on top of the carbonate and produced dry gas (Petronas,
1988).
Figure 1.2 shows the geological map of the Dent Peninsula, redrawn after
Haile and Wong (1965). The map shows outcrop locations, distributed mainly in the
Dent Group sediments. The Segama Group is assigned as basement and
underburden rocks.
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Chapter 1: Background of the Project
a
N 10 0
Sulu Sea
Dent
Peninsula
Celebes Sea
N50
b
Borneo
E 110 o
E 115 o
E 120 o
Figure 1.1. Maps show the location of the study area. (a) Satellite image shows the Borneo Island, and highlighted are the
location of the study area (in the red rectangle). (b) Simplified geological map of Sabah shows the outline of Neogene basins
and their sub-basin and provinces (after Leong and Azlina, 1999).
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Chapter 1: Background of the Project
N
Explanation
Togopi Formation
Upper Ganduman Formation
Lower Ganduman Formation
Sebahat Formation
Tungku Formation
Libung Formation
Ayer Melange
Track
Main Road
River
Outcrop
Location
Felda Wilayah
Sahabat
Tungku
Figure 1.2. The study area, Dent Peninsula, with outcrops localities (geological map after Haile and Wong, 1965).
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Chapter 1: Background of the Project
1.4 SUMMARY OF PREVIOUS WORK
Previous authors have carried out geological studies within the study area.
Hutchinson (1989) and Hall (1996) outline the regional tectonic evolution of South
East Asia. Locally, geological studies in Sabah area are summarized by Rangin
(1989), Tongkul (1991), Noad (1998 and 2003), Leong (1999), Leong and Azlina
(1999), Hutchinson (1989 and 2005), Balaguru and Nicholas (2003), and Balaguru
(2006a and 2006b). Heat flow related to Celebes Sea and Sulu Sea are described
by Rangin et, al. (1990), Rangin and Silver (1990 and 1991) based on Oceanic
Drilling Programme (ODP) results.
1.4.1 Geological Works
An early geological study in the Dent Peninsula areas was carried out by
Haile and Wong (1965), who produced a geological map. They divided the Dent
Peninsula into three Groups, including Dent Group (youngest), Segama Group,
and Kinabatangan Group (oldest) and later divided the Dent Group into three
Formations (Togopi Formation, Ganduman Formation, and Sebahat Formation,
from youngest to oldest). Subsequently, the Ganduman Formation was sub-divided
into lower and upper parts. The Lower Ganduman Formation, known as Maruap
member, was distinguished from the Upper Ganduman Formation by its greater
sand content. Stratigraphically, Sebahat Formation is the base of the Dent Group,
followed by Lower Ganduman Formation, Upper Ganduman Formation and Togopi
Formation as the most top of the formations. In the offshore, there are two distinct
basins closed to the Dent Peninsula, North-East Sabah Basin and Central Sabah
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Chapter 1: Background of the Project
Basin (Leong and Azlina, 1999). The Sandakan sub-basin, which has been
allocated as a part of North-East Sabah Basin, is correlatable to the study area.
According to Ismail Che Mat Zin (1994), deltaic deposition began in the late Middle
Miocene with the aggrading shale-rich Sebahat Formation, accompanied by distal
reef growth sometimes on subsiding volcanic highs. He also reported that the Dent
Graben in the offshore was starved of sediments until the late Miocene that was
marked by an eastward prograding sequence of Sebahat Formation.
1.4.2 Geochemical Studies
Numerous people have carried out the geochemical studies of both offshore
and onshore part of both North-East Sabah Basin and Central Sabah Basin. In the
onshore, Abdul Jalil (1993) and Abdul Jalil and Azlina (1995) concluded that the
Dent Group and the Segama Group exhibited poor to negligible hydrocarbon
source rock potential with Type III/IV kerogen predominant. Chan (2008) predicted
the source rock organofacies consist of coastal plain dispersed organic matter
based on the deltaic sediments, similar to Type D/E Organofacies (Pepper and
Corvi, 1995) of the Mahakam Delta or Baram Delta. Mohd Razali and Mohd Fauzi
(2002) reported the Sebahat Formation contains fair to very good organic carbon
richness as indicated by the TOC values ranges from 0.91 to 2.05 % wt. Poor to
very good organic carbon of the Ganduman Formation with TOC values ranging
between 0.30 and 10.62 wt.% and have poor to fair hydrocarbon generating
potential, indicated by S2 values ranging from 0.12 to 4.44 mgHC/g rock. In the
offshore, the Dent Group contains only poor to fair organic carbon content.
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Chapter 1: Background of the Project
The source rock of Ganduman Formation is immature as indicate by virtinite
reflectance value less than 0.40 % VR (Khalid Ali, 2003). Leong and Azlina (1999)
reported that the onshore Dent Group sediments are still immature for
hydrocarbons generation as indicated by 0.25-0.40% VRo. However, they
cautioned that the measurements could be influenced by weathered samples.
Khalid Ali (2004) showed that the Ganduman sediments are immature based on
biomarkers and vitrinite reflectance data. According to Petronas (1988), the liquid
and gaseous hydrocarbons encountered in the Sandakan Sub-Basin are believed
to have been sourced from shales within the Miocene Sebahat Formation.
However, no direct correlation between oil to source has been studied.
1.4.3. Basin modeling studies
Basin modelling studies have mainly been done for the offshore area,
particularly for the Central Sabah Sub-basin and Sabah Basin (Leong and Azlina,
1999). Azlina Anuar (1994) indicates there are three major kitchen areas in the
Northern part of the Sabah Basin. A recent 2D modelling study of the offshore
Sulu-Celebes Sea by Chan (2008) indicates that the source rock interval is the
Sebahat Formation with Libung and Tungku Formation as a possible source rock
bearing intervals. Charge modeling was constrained due to lack of geochemical
data. The time of hydrocarbons expulsion was estimated to begin in early Miocene,
with significant gas accumulation and minor oil reported.
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Chapter 1: Background of the Project
1.4.4. Heat Flow Studies
The favorable present day heat flow with values ranges from 35 mWm2 to
64 mWm2 is considered the heat source that activated the source rocks (Chan,
2008). However, heat flow within the Sulu Sea and Celebes Sea is variable. In the
Sulu Sea, heat flow value spans an unusually wide range from 25 mWm2 to 184
mWm2 (Anderson, 1980). Present day heat flow measurements derived during the
Ocean Drilling Project Leg 124 show variable values of 25 mWm2 to 198mWm2 in
southeast Sulu Sea Basin and 48 mWm2 to 70 mWm2 in the northwest Sulu Sea
Basin (Hinz and Block, 1990). The high values, indicating thermal anomalies, are
located close to the deformation front of the Sulu Negros trench (Rangin and
Silver, 1990). Rangin and Silver (1990) also reported that the average heat flow in
the Celebes Sea basin is more consistent (66 mWm2) with an Eocene age,
although this is not tightly constrained. The average geothermal gradient in Dent
Peninsula is 35 oC/km but a lower measurement of 21 oC/km was observed in
Sebahat-1, possibly due to blanketing of thick shale of Sebahat Formation (Leong
and Azlina, 1999). A similar problem has been reported by Hutchinson (2005)
shows low geothermal gradient of average 21 oC/km in Sulu Sea.
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Chapter 1: Background of the Project
1.5 PROBLEM STATEMENT
Based on summary of previous work, it must source rock evaluation has
been done in the Central Sabah sub-basin, particularly in the offshore area. There
has been no direct study or correlation of the source rock in the adjacent offshore
Sandakan sub-basin. As stated above, the Dent Group sediment is largely extent
to the offshore areas. Therefore, this research will provide valuable input to solve
certain problems that were addressed such as the distribution and depositional
condition of the source rock, source rock quality and the processes of petroleum
system.
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