SUBSURFACE ANALYSIS OF SUNDALAND BASINS: SOURCE ROCKS, STRUCTURAL TRENDS AND THE DISTRIBUTION OF OIL FIELDS

Transcription

1 SUBSURFACE ANALYSIS OF SUNDALAND BASINS: SOURCE ROCKS, STRUCTURAL TRENDS AND THE DISTRIBUTION OF OIL FIELDS A THESIS SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE MASTER OF SCIENCE BY SWARDHUNI PETHE DR. RICHARD H. FLUEGEMAN- ADVISOR BALL STATE UNIVERSITY MUNCIE, INDIANA DECEMBER, 2013

2 Table of Contents List of Figures...2 Acknowledgements...4 Introduction...5 Geology.8 Basement..9 Tertiary Basins...10 Talang Akar Formation.. 11 Air Benakat Formation..12 Methods...15 Outline, Data Collection 15 Data Processing..16 Results.18 South Sumatra Basin..18 Sunda and Asri Basins...30 Ardjuna (NW Java) Basin..38 Discussion...48 Conclusion..56 Appendix I...57 Appendix II.61 Appendix III 69 References

3 List of Figures Figure 1 Satellite image of the study area with the wells from Google Earth... 7 Figure 2 A simple geological map of Sumatra depicting the position of back-arc basins... 9 Figure 3 Structure of the basins and deposition with respect to the deformation of the basement Figure 4 A typical example of the stratigraphic sequence in South Sumatra Basin Figure 5 Comparison between the stratigraphic sequences in the Sumatra and Java Basins Figure 6 2D Structural contour map of the basement (top)- South Sumatra Basin Figure 7 3D Structural model of the top of the basement- South Sumatra Basin Figure 8 2D Structural contour map of the Talang Akar Formation (top)- South Sumatra Basin Figure 9 3D Structural model of the top of the Talang Akar Formation- South Sumatra Basin Figure 10 2D Structural contour map of the Air Benakat Formation (top), South Sumatra Basin Figure 11 3D Structural model of the top of the Air Benakat Formation- South Sumatra Basin Figure 12 2D Isopach map- between the top of the Talang Akar Formation (upper surface) to the top of the basement (lower surface)- South Sumatra Basin Figure 13 Location map for cross-sections 1 & 2, South Sumatra Basin- South Sumatra Basin Figure 14 Cross section 1- A-A, N-S, South Sumatra Basin Figure 15 Cross section 2- B-B, SW-NE, South Sumatra Basin Figure 16 Structural contour map of the basement (top) showing the distances between the wells and the grabens- South Sumatra Basin Figure 17 Structural contour map of the basement (top) - Sunda/Asri Basins Figure 18 3D Structural model of the top of the basement- Sunda/Asri Basins Figure 19 Location map for the cross-sections 1& 2- Sunda/Asri Basins Figure 20 Cross section 1- A-A, General structural trend of the Sunda/ Asri Basins from SW to NE, Sunda/Asri Basins Figure 21 Cross section 2- B-B, SW-NE, Sunda/Asri Basins Figure 22 Structural contour map of the basement (top) showing the distances between the wells and the grabens- Sunda/Asri Basins Figure 23 2D Structural contour map of the basement (top), Ardjuna Basin Figure 24 3D Structural model of the top of the basement- Ardjuna Basin Figure 25 2D Structural contour map of the Air Benakat Formation (top)- Ardjuna Basin Figure 26 Location map for cross-sections 1& 2- Ardjuna Basin Figure 27 Cross section 1- A-A, W-E, Ardjuna Basin Figure 28 Cross section 2- B-B, N-S, Ardjuna Basin Figure 29 Structural contour map of the basement (top) showing the distances between the wells and the grabens- Ardjuna Basin Figure 30 Example of wrench faults from Los Angeles Basin Figure 31 Contour map of the basement from Central Sumatra showing NW-SE trending wrench faults 51 Figure 32 Location map of the South Sumatra Basin showing wrench faults Figure 33 Index map of the NW Java showing the distribution of the wells in the area Figure 34 2D Isopach map of the Air Benakat Formation - South Sumatra Basin Figure 35 2D Structural contour map of the top of the Talang Akar Formation- Sunda/Asri Basins

4 Figure 36 2D Structural contour map of the top of the Air Benakat Formation- Sunda/Asri Basins Figure 37 2D Structural contour map of the top of the Talang Akar Formation- Sunda/Asri Basins Figure 38 3D Structural model of the top of the Talang Akar Formation- Sunda/Asri Basins Figure 39 3D Structural model of the top of the Air Benakat Formation- Sunda/Asri Basins Figure 40 Location map for cross-section A-A', Sunda/Asri Basins Figure 41 Cross-section 3 A-A', SW-NE, Sunda/Asri Basins Figure 42 3D Structural model of the top of the Talang Akar Formation- Ardjuna Basin Figure 43 3D Structural model of the top of the Air Benakat Formation- Ardjuna Basin Figure 44 Location map for cross section 3 A-A', Ardjuna Basin Figure 45 Cross section 3- A-A, N-S, Ardjuna Basin

5 Acknowledgements I would like to express my gratitude to my committee chair, Dr. Richard H. Fluegeman for his constant support and guidance throughout my academic career at Ball State. I would also like to thank my committee members Dr. Jeffry Grigsby and Dr. Kirsten Nicholson for their support and valuable suggestions on my thesis. I am very grateful to Dr. William Ade, for providing me the funding to work on this project and also for his valuable insights. I want to thank my family and my friend Aneesha Balakrishnan, without whose inspiration, this work would not have been successful. 4

6 Introduction Sumatra is the largest island in Indonesia. On a global scale it ranks 21 st (as of 2011) in oil production. Most of the Indonesia s oil is produced from the oil fields of Sumatra. Sumatra is a part of the Sunda shelf which has been surrounded by the intensely tectonic margins. As a result, a large number of structural basins have formed on the shelf. These basins act as very good traps for the oil accumulation. Depending upon the exact location of the basin, source rock and reservoir rock formations may vary although they are equivalent in age. In this study the focus is on the Talang Akar Formation, which is the source rock in our study area and the Air Benakat Formation, which acts as a reservoir rock. The purpose of this study is to verify the Ade observation (Ade, W., pers. comm.) about the oil fields and structural basins of Sumatra. According to the observation, 95% of all commercial oil fields in the region occur within 17 km of seismically mappable mature sedimentary rocks in the producing basins. Geophysical data available from this region is being used for the analysis of Sumatra basins. This data is part of the geophysical database of the South East Asia- Pacific region, donated by L. Bogue Hunt; commonly known as the LBH Database. It contains physical, lithological, and paleontological logs, seismic sections, structural maps and detailed geological reports of many exploration sites. Sumatra s oil is sourced from its mature Tertiary back-arc basins. Owing to the subduction zone on the west of Sumatra, the Sunda shelf underwent extension and rifting, which resulted in the formation of these basins. North, Central and South Sumatra basins are among the major on-shore basins. According to Koesoemadinata (Koesoemadinata, 1969) the deposits of the initial transgressive sequence have yielded large quantities of oil in Sumatra, especially in South Sumatra. Talang Akar Formation is an important source rock from this sequence. As he has stated, the earliest wells were drilled in the Air Benakat Formation- part of the regressive sequence- which is a great reservoir for the oil. Among the important off- shore basins are the Sunda and Asri basins. NW Java basin area is spread both on and off the shore of Java. All of these basins have numerous sub- basins or individual smaller oil fields. 5

7 In this study of the Sumatra basins, well log data from the LBH database were used. In order to assess the basin structures and the distribution of source rock, 2D structural maps of three different units were created; namely, the Talang Akar and Air Benakat Formations and the basement. Isopach maps provided the variation in the thickness of the deposition. Most of the data used here is from the explorations undertaken before Thus the status of the wells indicated on the maps may have changed. However, the attempt is made to focus on the areas of known history of production. The validity of the given Ade observation is tested in this study. It is hoped, that the conclusions of this study will help determine the extent of the potential area for exploration, surrounding the producing grabens of the Sundaland basins. 6

8 Figure 1 Satellite image of the study area with the wells from Google Earth 7

9 Geology The geology of Sumatra is very complex. It is a good example of subduction-related structural features. Subduction of the Indian-Australian plate under the Sunda plate and the subsequent tectonics governed the formation of the rift basins of Sumatra and Java. Sediments are deposited in the back-arc basins adjacent to the stable Sunda Shelf (Koesoemadinata, 1969). North, Central and South Sumatra are the three major oil producing basins in Sumatra. Sunda, Asri and NW Java are also very important for their petroleum production. Although the stratigraphic sequence and depths of the basins vary over this entire region, the general geology is constant. As a result of the extension of the Sunda shelf, related to the subduction in the west, back-arc basins were formed between the Mobile belt and the stable Sunda craton (Clure, 2005). Rifting and subsidence generated many extensive and regional faults. This process resulted in the formation of grabens that were filled with deposits during the Tertiary (Fig. 2). Folding of strata at the end of the Tertiary may have further changed the structural complexity of the basins (Koesoemadinata, 1969). On maturation these basins became an important site of petroleum generation in Indonesia. The source rock, Talang Akar Formation, is of late Oligocene to early Miocene age. Whereas the reservoir forming Air Benakat Formation was deposited during mid to late Miocene. 8

10 Figure 2 A simple geological map of Sumatra depicting the position of back-arc basins (Barber, Crow, & Milsom, 2005) Basement Basins in Sumatra and Java are mainly Tertiary, lined by the pre-tertiary crystalline basement rocks. The basement is comprised of mostly igneous and metamorphic rocks and occasionally sedimentary rocks toward eastern Indonesia, as observed from the well logs. Rifting that resulted in the depositional basins took place during Tertiary. Basement rocks were highly deformed into horst and graben structures. Major rifts were formed trending NE-SW on the Sunda shelf (Clure, 2005) 9

11 Tertiary Basins The rifting phase began in early Tertiary and continued until late Oligocene-early Miocene. During this period, structural grabens were formed and filled by the deposition of fluvio-deltaic sediments (Doust & Noble, 2008). The major basins contain transgressive and regressive facies that provide the source rocks and the reservoir rocks respectively (Fig. 3). For the purpose of this project, two formations were studied in particular; Talang Akar and Air Benakat Formations. These are very important formations in Sumatra and Java. It is a petroleum system where the Talang Akar Formation is the source rock and Air Benakat Formation is the reservoir. Since there are numerous basins in this region that are separated from each other, different formations or petroleum systems may exist, where the source rocks and the reservoir rocks belong to a different stratigraphic sequence. Therefore to simplify the data, such formations were grouped with the Talang Akar Formation or the Air Benakat Formation, based on their equivalent age. 10

12 Figure 3 Structure of the basins and deposition with respect to the deformation of the basement (Barber & Crow, 2005) Talang Akar Formation (TAF) Talang Akar was deposited as a transgressive sequence during late Oligocene and very early Miocene. It overlies the basement rocks. In some basins Talang Akar may be underlain by other formations; such as lacustrine deposits of Lemat and Lahat Formations in South Sumatra and Jatibarang Volcanics in Jatibarang basin of NW Java. Talang Akar is comprised of fluvio-deltaic fine shales and silts as well as sands. Some of the sands deposited in near-shore marine environment are fairly porous and permeable and contain oil in the stratigraphic and fault-related traps (Doust & Noble, Petroleum systems of Indonesia, 2008). Thickness varies from the basin to basin, but this formation can be as thick as 7000 feet ( m) in the deepest parts of the basin. Intermittent coal seams are also found, which may act as a source of hydrocarbons. Lower unit of the Talang Akar Formation is known to produce good quality oil in more quantities than that of the upper unit (Koesoemadinata, 1969). The shales of Menggala and Pematang Formations from Central Sumatra were included as equivalents to the Talang Akar Formation. 11

13 Figure 4 A typical example of the stratigraphic sequence in South Sumatra Basin (Clure, 2005) Air Benakat Formation (ABF) The Air Benakat Formation is a thick sequence of sands deposited in a regressive system. It is comprised of several units of marine sandstones from deep to shallow marine environment. Air Benakat was deposited during middle to late Miocene, when the rifting phase was over. Since it was deposited as part of the regressive marine sequence, the shallower deposits of Air Benakat have broader distribution. It is the main reservoir rock in the South Sumatra basin. Its average porosity is about 25% (standard range for a sandstone is 5-25%) which makes it an excellent reservoir rock (Bishop, 2001). In parts of South Sumatra, the lower Palembang Formation is equivalent to the Air Benakat (Fig. 5). In NW Java Basins, the Main and Massive intervals of the Cibulakan Formation are equivalents of the Air Benakat Formation (Fig. 5). The Main and Massive are thick sand deposits as thick as 3000 feet (914.4 m). They 12

14 are comprised of the lowstand, transgressive and highstand intervals. Since the shallow marine sediments tend to be coarse, they have better porosity. The lowstand sands have an average porosity of 27% and have a very good reservoir quality compared to the transgressive and highstand sands (Purantoro, Butterworth, Kaldi, & Atkinson, 1994). The Cibulakan Formation is deposited across Ardjuna and Jatibarang sub-basins of the NW Java. 13

15 Figure 5 Comparison between the stratigraphic sequences in the Sumatra and Java Basins (Doust & Noble, 2008) 14

16 Methods Outline A vast amount of data is available to us in the form of well logs and seismic sections from various oil fields of Indonesia. They provide us the basement depths and the depths to the rock formations of our interest. This stratigraphic data is combined with the geographical locations to produce the stratigraphic and structural maps. Profiles for the individual wells were created in the geophysical software called RockWorks. It allows us to manage stratigraphical data, the production status of the wells and the location. After determining the UTM coordinates and project dimensions in the software, structural and isopach maps for the entire study area were created. In addition to the maps, several cross sections and 3D structural models were created to understand the basin structure in a better way. The maps, and the cross sections, are useful to analyze the position of producing oil fields with respect to the structural features. The wells with good show of oil were chosen for drawing the cross sections. Data collection The geophysical data on South East Asia is available to the Ball State University s Department of Geological Sciences, as the LBH database. It includes topographic maps and subsurface profiles, obtained from seismic, resistivity, gamma ray methods etc., as well as detailed geological reports of several oil fields. The main focus of this study is on south Sumatra and northwest Java, Indonesia. Therefore the data representing the oil fields in Sumatra were chosen specifically. This data set has well logs based on electrical resistivity, gamma ray, spontaneous potential and lithology. These logs are important as they contain the location of the well and the depths of the specific geologic formations. In order to utilize this data and make meaningful interpretations, it is necessary to create extensive geological maps. RockWorks is a powerful software package that allows such data processing and can handle large databases. A database was compiled in an Excel spreadsheet, including the information such as well location, elevation, total depth. The depths to the tops and bottoms of both Formations (Talang 15

17 Akar and Air Benakat) and the depth to the top of the basement were entered manually after the spreadsheet was imported into the software. Appropriate symbols were chosen to indicate the status of the wells in terms of production. Some of the once-producing wells drilled in 70 s and 80 s may not be in production today, but they are in the potentially productive oil fields; hence important. Structure of the basement is important in order to determine the basin boundaries; therefore dry holes were equally important in order to obtain the depth of the basement. Formation tops in all the well logs were not marked. If they were missing, they were determined by comparing those logs with other logs or seismic sections from the same field or basin. Depending on the area or the basin, formation names may vary. In that case, their equivalents from the local stratigraphic sequence were used. Data Processing It is important to use the same units for all the maps and figures. Elevation and depths were maintained in feet. The geographical coordinates found on the well logs are in the form of degrees, minutes and seconds. They were converted into the decimal degree (DD.ddddd) using software called Garmin. This is the acceptable format for RockWorks. All the latitude figures for the wells in the southern hemisphere were given a negative value. A spreadsheet containing well name, location, elevation, and total depth, was imported into RockWorks and each borehole entry was attributed with the formation depths and the well s production status using appropriate symbols. Once all the data were entered into the software, the next step was to convert all the latitude and longitude figures from decimal format into the UTM (meter) system. It was done by a simple inbuilt menu option. Since most of the wells in were located in the south of the equator; UTM zone 48S (S for the southern hemisphere) was assigned to the database. Computing the project dimensions was the last step before creating various types of maps. Simple menu functions allowed the creation of a variety of maps, such as the structural maps of basement, Talang Akar and Air Benakat Formations. These maps are based on the given depth measurement of the individual wells. 2D isopach maps are particularly useful when we need to evaluate the thickness of a Formation. These isopach maps were produced for the Talang Akar and Air Benakat Formations. Since 16

18 the Talang Akar overlies the basement in most case; the top of the basement was used as a lower surface to indicate the thickness in the isopach maps of the Talang Akar Formation. This is a good way to observe the sedimentary deposition with respect to the basement structure. Every map contains 1) the well locations that are denoted by their status symbol, and 2) appropriate legend. Vertical profiles or cross sections were also produced to better understand the basement structure. Contour interval (CI) for the structural contour maps and the isopach maps is 500 feet. The goal of this study is to find out if the oil fields are situated within a 17 km distance from the grabens; therefore, only the producing wells and the wells with a good show of oil were treated as potential fields. The wells with an unknown status were not considered while measuring the distances, since the information pertaining to the show of oil was unavailable. To measure the distances between the wells and the grabens, structural contour maps of the basement were used. Based on the density of the contours, margins of grabens were determined for every basin. Using Microsoft Excel, percentage and the average number of wells situated within a 17 km margin were calculated. Table 1 Well status symbols Symbol Status Unknown Dry hole Oil show Gas show Oil & gas show Oil well Gas well Oil & gas well Suspended oil well 17

19 Results The maps described in the results are very large scale maps. The individual clusters of the wells are actually several smaller oil fields or basins. Fault lines are included in the structural maps and 3D models. The whole study area has been divided into three separate basin areas; South Sumatra, Sunda/Asri and NW Java. For the well status symbols used in the maps and the cross sections refer to Table 1 in the Appendix III. More maps, 3D structural models and cross sections are given in Appendices I and II. South Sumatra Basin Figure 6 is a structural map of the basement in South Sumatra Basin area. The depth of the basement based on the data points ranges from 1000 (304.8 m) to 10,000 feet (3048 m) below sea level and more commonly from 3000 to 7000 feet ( m) below sea level. Most of the producing wells are situated in the deeper parts of the basin. In the upper half of the map there is a group of wrench faults. The South Sumatra Basin is deeper towards its south end. 18

20 Fault; arrow points the downthrow side Strike slip Figure 6 2D Structural contour map of the basement (top), CI= 500ft (152.4m) 19

21 Figure 7 gives a better idea of the basement structure. Comparison of Fig. 6 to this 3D model shows that the producing wells are situated in the grabens or on their flanks. Towards the south, the basement is as deep as 10,000 feet (3048 m). Depth (feet) Distance (meters) Figure 7 3D Structural model of the top of the basement 20

22 The structure of the Talang Akar Formation in Fig. 8 is consistent with the structure of the basement. Location of the deep troughs and highs matches in both the maps (Fig. 6 & 8). Comparing these two maps, the thickness of Talang Akar appears to be approximately 1000 to 5000 feet ( m). Study of the isopach maps will confirm this observation. Fault; arrow points the downthrow Strike slip Figure 8 2D Structural contour map of the Talang Akar Formation (top), CI= 500ft (152.4m) 21

23 Based on the 3D model of the Talang Akar Formation (Fig. 9), general trend of the grabens seems similar to that in Fig. 7. The deep graben in the south, as seen in the structural model of the basement (Fig. 7), appears shallower in Fig. 9; suggesting that it was filled heavily with sediments. Depth (feet) Distance (meters) Figure 9 3D Structural model of the top of the Talang Akar Formation 22

24 The structure of the Air Benakat Formation in Fig. 10 is somewhat similar to that of the Talang Akar Formation in Fig. 8; especially in the lower half of the map. However, Air Benakat does not directly overlie the Talang Akar Formation. Fault; arrow points the downthrow Strike slip Figure 10 2D Structural contour map of the Air Benakat Formation (top), CI= 500ft (152.4m) 23

25 In Fig. 11 depth to the top of the Air Benakat Formation varies by 5000ft (1524m). Depth (feet) Distance (meters) Figure 11 3D Structural model of the top of the Air Benakat Formation 24

26 This map (Fig. 12) has been created using top of the Talang Akar Formation as the upper surface and basement as the lower surface. In most of the logs that represent the wells in this map, Talang Akar lies directly above the basement. The area represented by purple indicates the thickness of 0 to 1000 feet ( m). These are in fact the deep grabens. Thickness in feet Figure 12 2D Isopach map- between the top of the Talang Akar Formation (upper surface) to the top of the basement (lower surface), CI= 500ft (152.4m) 25

27 Figure 13 is the Location map of the wells in South Sumatra Basin displaying the section lines A-A and B-B. A A Figure 13 Location map for cross-sections A-A and B-B, South Sumatra Basin, Distances in meters 26

28 Cross section A-A passes through three producing wells from different oil fields. The wells on both ends are situated on the flanks of grabens (Fig. 6); however, the one in the middle is situated in the graben. The isopach map in Fig. 12 clearly shows this middle part of the area where the Talang Akar Formation is thicker. All cross sections in this study exhibit the interpolated surfaces of the Talang Akar Formation, Air Benakat Formation and the basement. For more detailed stratigraphic column, refer to Fig. 5. Figure 14 Cross section A-A, SW-NE displaying the structural trends in Talang Akar Formation (TAF), Air Benakat Formation (ABF) and the basement. 27

29 In Fig. 15, note that the producing well in the center of the cross section B-B (Fig. 15) is situated where the Talang Akar Formation is shallower than the surrounding area. Also, the well on the right end of the section is a producing well, situated further inside the graben. This elevated structure of the Talang Akar Formation is clearly visible in the 3D model (Fig. 9) towards east. Figure 15 Cross section B-B, N-S displaying the structural trends in Talang Akar Formation (TAF), Air Benakat Formation (ABF) and the basement. Producing wells situated on the flanks of the grabens as well as in the middle of the graben. 28

30 In Fig. 16, the contour line of feet ( m) was chosen as the graben margin, based on the contour density and basin slope (Fig. 7). The average distance between the oil fields and the nearest grabens is km. Out of 36 potential oil fields only 28, i.e % of the fields lie within the 17 km distance from the grabens. Fault; arrow points the downthrow Strike slip Figure 16 Structural contour map of the basement (top) showing the distances between the wells and the grabens, CI= 500ft (152.4m) 29

31 Sunda and Asri Basins There are two basins in the following map (Fig. 17). The one on the west is the Sunda basin; while Asri basin is in the NE part of the map. They are controlled by major faults (Doust & Noble, 2008). The producing wells in the Sunda basin clearly lie in the vicinity of the fault-bound grabens. 30

32 Fault; arrow points the downthrow side Figure 17 Structural contour map of the basement (top) - Sunda/Asri Basins, CI= 500ft (152.4m) 31

33 The overall shape of the Sunda and Asri basins is visible in the 3D model in Fig. 18. They are separated by a ridge, trending approximately north-south. Depth (feet) Distance (meters) Figure 18 3D Structural model of the top of the basement- Sunda/Asri Basins 32

34 Figure 19 is the Location map of the wells in Sunda and Asri Basins displaying the section lines C-C and D-D. C D D C Figure 19 Location map for the cross-sections C-C and D-D - Sunda/Asri Basins, Distances in meters 33

35 Cross section C-C in Fig. 20 shows the general structural trend of the basins in southwest-northeast direction. From left to right, the line C-C crosses the Sunda and Asri basins. Both basins are very close to each other and have a series of grabens of varying depths. For more detailed stratigraphic column, refer to Fig. 5. Cross section C-C C C Figure 20 Cross section C-C, displaying the structural trends in Talang Akar Formation (TAF), Air Benakat Formation (ABF) and the basement. General structural trend of the Sunda/ Asri Basins from SW to NE, This is a modeled cross section created by incorporating the depths of the closest wells. 34

36 The cross section in Fig. 21 shows that the producing wells are situated on the basement high or flanks of the adjacent grabens. This is also the area where most of the producing wells are located according to the structural map of the basement in Fig. 16. Cross section D-D D D Figure 21 Cross section D-D, SW-NE displaying the structural trends in Talang Akar Formation (TAF), Air Benakat Formation (ABF) and the basement. Location of the wells on the horsts is noticeable. 35

37 In Fig. 22, the distances from the nearest graben were measured for several wells on a structural map of the basement, using a measurement function in the RockWorks. On observing the density of the contours, the contour line of feet (-1524 m) was determined to be the margin of the grabens. There are total 23 potential oil fields in this map. Three of them lie outside the grabens, but within a 17 km margin. The distance of the oil fields from the grabens ranges from 8 to 12.6 km; averaging km. 36

38 Fault; arrow points the downthrow side Figure 22 Structural contour map of the basement (top) showing the distances between the wells and the grabens, CI= 500ft (152.4m) 37

39 Ardjuna (NW Java) Basin Ardjuna Basin (Fig. 23) is located in the west of the NW Java Basin area. Wells in this basin are some of the deepest and highly producing in Indonesia. Fig. 23 shows three of the major faults that separate the grabens. Depth to the basement ranges from 3000 (914.4 m) to 9000 feet ( m) below the sea level. 2D structural map (Fig.23) of the Ardjuna Basin shows that there is a series of grabens trending N-S or NE-SW and varying in the depth. It continues towards Jatibarang Basin in the east, where more fault bound grabens are situated. From the distribution of the producing wells, the oil fields in this basin occupy the area between these large grabens (Fig. 23). 38

40 Figure 23 2D Structural contour map of the basement (top), CI= 500ft (152.4m). Note the three faults in the center of the map. Fault; arrow points the downthrow side 39

41 The number of producing wells increases from blue to green area on the structural map in Fig. 23. As per the 3D model (Fig.24), depth of the basin decreases in the same area. This observation is very similar to that of the Sunda and Asri basins; where the most producing wells are situated on the flanks of the grabens. Depth (feet) Distance (meters) Figure 24 3D Structural model of the top of the basement- Ardjuna Basin 40

42 In Fig. 25, depth to the top of the Air Benakat Formation ranges from 1200 ( m) to 4800 feet ( m) below sea level; although most of the production appears to be in the area (marked in green), where the depth is 2400 ( m) feet ( m) below sea level. 41

43 Figure 25 2D Structural contour map of the Air Benakat Formation (top), CI= 500ft (152.4m) Fault; arrow points the downthrow side 42

44 Figure 26 is the location map of the wells in Ardjuna (NW Java) Basin displaying the section lines for the cross sections E-E and F-F F E E F Figure 26 Location map for cross-sections E-E and F-F - Ardjuna Basin, Distances in meters In Fig. 27, there is a gradual change in the depth of the basement and it becomes shallower from west to east. The alignment of the three faults (Fig. 22), marked by the change in the depth is apparent in the cross section (Fig. 27). 43

45 Cross section E-E E E Figure 27 Cross section E-E, W-E displaying the structural trends in Talang Akar Formation (TAF), Air Benakat Formation (ABF) and the basement. The wells are located between the faults. 44

46 Figure 28 is displays the structure of the Ardjuna basin in north-south direction. The drop in the basin depth is consistent with that in the structural map of the basement (Fig. 23) and the 3D model (Fig. 24). The well in the middle has a good show of oil and it is situated where the basement is higher than the areas immediately next to it. Cross section F-F Figure 28 Cross section F-F, N-S displaying the structural trends in Talang Akar Formation (TAF), Air Benakat Formation (ABF) and the basement. The producing well in the middle of the section is situated on the flank of the graben, as seen in the structural contour map of the basement (Fig. 23). On the left of section F-F, ABF is interpolated by the software and crosses the TAF; however, it is absent in that location and has an onlap. 45

47 In Fig. 29, the contour line of feet (-1524 m), seen as a break between yellow and green colors, was established as a margin for the grabens. Wells PSI Z, PSI PZ, Dempo 1, Rinjani and Soputan are not part of the Ardjuna basin; however, they were included to provide a better perspective of the area. The distance between the producing wells and the graben ranges from 0.5 to 20.6 km, with an average of km. Out of 47 potential oil fields, 43, i.e. 92% fields are situated within 17 km from the grabens. 46

48 Figure 29 Structural contour map of the basement showing the distances between the wells and the grabens, CI= 500ft (152.4m) Fault; arrow points the downthrow side 47

49 Discussion As evident from the maps, the wells are concentrated inside or around the basins; especially the ones that have production potential. Basins on the Sunda Shelf are roughly NE-SW oriented and are fault bound. Folding of the Sunda Shelf on the east of the Subduction zone has generated numerous small and large faults that are parallel to the strike of folding. Concentration of the data points in parts of the maps makes the basins appear merged. To resolve this problem, the maps were broken down in several separate regions; focusing on a small slice of the longitudinal area at a time. The first section comprised of the South Sumatra Basin (Fig. 6). The second section contains the Sunda and Asri Basins (Fig. 17) while the third section includes the NW Java Basin area (Fig. 23). Results obtained this way contain more defined basin boundaries and more pronounced structural features. Structural basins of Sumatra and Java are fault basins. Most of the producing basins of this region lie northeast of the mobile belt. Important producing wells of Sumatra and Java are in the central and south Sumatra, Sunda, Asri and Northeast Java basins. The Talang Akar Formation is a major source of petroleum in the basins of Sumatra. These fine shales are deep seated fluvio-deltaic sediments that were deposited during the rifting phase (mid Eocene- early Miocene) of the Sunda craton (Doust & Noble, Petroleum systems of Indonesia, 2008). In central Sumatra they are replaced by the Brown Shale Member of the Pematang Formation. In the basins of NW Java, the reservoir rocks equivalent to the Air Benakat Formation are the Main/ Massive sands of the Cibulakan Formation. When the rifting phase was over, the deep grabens were heavily filled in by marine sediments (Purantoro, Butterworth, Kaldi, & Atkinson, 1994). In off-shore Java basins on the shelf, a thick marine sequence of the Air Benakat/ Cibulakan Formations acts as a very good reservoir rock. 48

50 South Sumatra Basin Although wells with a good show of oil are situated in the deeper parts of the South Sumatra Basin; several producing wells are situated on the flanks of the grabens and on the horsts (Fig. 6 &15). Correlation of these wells with other wells in the same or adjoining fields shows that the sequence of deposition and age of the formations are equal. Therefore, it is evident that the faulting or uplifting occurred after the sediments were deposited in the respective basins. This observation can be supported by the fact that the Talang Akar Formation was deposited during the postrift phase when the region was undergoing tectonic movements and structural changes (Doust & Noble, 2008). A paper by T. P. Harding, Petroleum Traps Associated with Wrench Faults (Harding, 1974) talks about the importance of wrench faults in petroleum trap formation. In a region influenced by high tectonic activity, intense folding and faulting produces very complex structures such as wrench faults. The complexity of these structures is due to their intersecting strike-slip, normal and reverse faults. Due to this, the rock strata are offset and the structural traps are formed. Comparing the folds, faults and the basin structures in Sumatra to this model, the wrench faults appear to be a common feature in the basins of Sumatra. Maps of various oil fields in central and south Sumatra indicate the position of oil fields dispersed in between the wrench faults. These faults are situated on the eastern side of the geanticlinal belt that runs along the length of Sumatra. This folding of the pre-tertiary and Tertiary rocks and the subsidence of basins could be part of the developmental history of these wrench faults. Fig. 30 is an image from the literature (Harding, 1974) that represents a typical wrench fault system from the Los Angeles Basin. The fault structures in this image are similar to those from the structural map (Fig. 31) from the database and location map of the South Sumatra Basin (Fig. 32). This correlation provides a clue to the possible location for the exploration. 49

51 Figure 30 Example of wrench faults from Los Angeles Basin (Harding, 1974). Notice the intersecting faults. 50

52 Figure 31 Contour map of the basement from Central Sumatra showing NW-SE trending wrench faults from LBH database. 51

53 Depth in feet Fault; arrow points the downthrow Strike slip Figure 32 Location map of the South Sumatra Basin showing wrench faults Another paper (Ryacudu, Djaafar, & Gutomo, 1992) states a similar observation about the North Sumatra Basin. The wrench faults in this basin provide a path of migration for oil. These papers and the study of the structural maps from the LBH database, suggest that the wrench faults form good structural traps. Therefore, more emphasis should be given on identifying the wrench faults during future exploration. The deposits of the Talang Akar Formation lie deep into the grabens directly above the basement rocks; except when it is underlain by Lemat/ Lahat Formations. Therefore, in the isopach map of the Talang Akar Formation (Fig. 12), areas of less thickness represent the grabens. Oil fields often coincide with these areas. In the structural map of Air Benakat Formation the producing oil fields fall into an area where 52

54 Air Benakat Formation is shallow, i.e. approximately to feet ( m). Shallower depth makes it easy to access the reservoir due to which the earliest production came from the shallower wells (Koesoemadinata, 1969). Sunda and Asri Basins Figure 17 is a contour map of the basement from the Sunda and Asri Basins. The two basins are bound by faults on their eastern flank. The map and the 3D model (Fig. 18) of the basement give a clear understanding of its structure. The Sunda and Asri basins are situated in a wedge between Sumatra and Java islands. As of 1997 the oil reserves of these basins were expected to be 1500 MMbo (Doust & Noble, 2008). There are closely spaced grabens up to 7000 feet ( m) deep, clearly seen in the 3D model (Fig.18). Based on the well data, the source rock- reservoir system in these basins appears to be Talang Akar- Air Benakat/ Batu Raja. Batu Raja is a reef formation that acts as a good reservoir at many sites in Sumatra- Java. Cross sections (Fig. 20 & 21) constructed through Sunda/ Asri neatly display the location of wells in relation to the structure of the grabens. Most of the producing wells are located on the flanks, in shallow areas of the grabens. This observation is consistent with previous research in the Sunda and Asri basin area (Doust & Noble, 2008). The average distance between the wells and the producing grabens is km; which is within the 17 km margin described in the Ade observation. Northwest Java Basin Area- Ardjuna Basin Another example of a typical structural basin on the Sunda Shelf is the Northwest Java Basin area. The LBH database contains a large amount of well data from this area. Ardjuna and Jatibarang are the major basins in the northwest Java area. Oil fields in the western part of this area belong to the Ardjuna Basin (Doust & Noble, 2008). The source rock in Ardjuna Basin is Talang Akar. In the NW Java area, Talang Akar Formation is underlain by Jatibarang volcanics (Atmadja & Noeradi, 2005). Well logs show that in some of the sections, the Talang Akar Formation contains layers of coal which is an important source of 53

55 hydrocarbons as found in south Sumatra basin area (Davis, Noon, & Harrington, 2007). Although Air Benakat Formation is not present in this basin area, its equivalent Main/ Massive sands act as reservoirs. These are thick sands deposited during the marine regression. Lowstand intervals of the Main sands have good porosity and permeability which make them good reservoir rocks (Purantoro, Butterworth, Kaldi, & Atkinson, 1994). Well logs indicate a good show of oil as well as gas in the Main and Massive sands. These sands can be as thick as three thousand feet. The clusters of wells occur between the N-S and NE- SW trending series of faults. Jatibarang basin is slightly east of the Ardjuna basin. These sand units also occur in the Jatibarang basin of NW Java. Here they are part of the Cibulakan Formation and given as the upper Cibulakan Member in the well logs. As these sands were deposited toward the end of regression, they are thinner toward the basin walls. This affects the resulting structural and isopach maps where the grabens do not appear prominent. This error was compensated by manually constructing the fault lines based on the available maps from the literature and database (Doust & Noble, 2008). Figure 29 is a structural map of the basement for the Ardjuna basin. The segments were drawn to measure the distance between the wells and the margins of the nearest grabens. As mentioned in the results, the average of this distance is km. It is less than 17 km and thus, concurrent with the Ade observation. The actual number of oil fields and producing wells is greater than the number of wells shown in the structural map (Fig. 29). That could have lowered the percentage of wells occurring within the 17 km margin. Figure 33 is an index map of the northwest Java, showing the distribution of wells in the area. A large number of those wells are producing wells. Hence, the actual percentage of producing fields within 17 km from the grabens could be equal to or greater than 95, for the Ardjuna Basin. 54

56 Figure 33 Index map of NW Java showing the distribution of the wells in the area. 55

57 Conclusion In the South Sumatra Basin, 77.78% of the oil fields are located within 17 km of the producing grabens; with an average distance of km. In the Sunda and Asri Basins, all of the oil fields occur within 17 km distance from the producing grabens; with an average distance of km. In the Ardjuna Basin area of the northwest Java, 92% of the oil fields lie within 17 km of the producing grabens; with an average distance of km. The results of this study indicate that the Ade observation, 95% of all commercial oil fields in the Sumatra region occur within 17 km of seismically mappable structural grabens in the producing basins is true for the Sunda/Asri Basins. Given the high percentage of data being in the hypothesized distance range, further study of the individual oil fields and recent data may yield similar results for the Ardjuna Basin. The well data used for the South Sumatra Basin in this study is not uniformly distributed. Hence, definite conclusions as to what factors control the regional structures, cannot be drawn. Therefore, it is necessary to study the individual oil fields using seismic surveys and well data in order to establish a relationship between the structural features and the oil production. The structural maps and cross sections indicate that the flanks of the grabens or the horst region may prove to be the potential sites for future exploration. Comparison between the obtained maps and those from the literature suggests that the wrench faults provide good oil traps and may be the potential sites for the exploration as well. 56

58 Appendix I: Structural and isopach maps South Sumatra Basin Thickness in feet Figure 34 2D Isopach map of the Air Benakat Formation, CI= 500ft (152.4m) 57

59 Sunda/ Asri Basins Fault; arrow points the downthrow side Figure 35 2D Structural contour map of the Talang Akar Formation (top), CI= 500ft (152.4m) 58

60 Fault; arrow points the downthrow side Figure 36 2D Structural contour map of the Air Benakat Formation (top), CI= 500ft (152.4m) 59

61 Ardjuna (NW Java) Basin Figure 37 2D Structural contour map of the Talang Akar Formation (top), CI= 500ft (152.4m) Fault; arrow points the downthrow side 60

62 Appendix II: 3D models and cross sections Sunda and Asri Basins: 3D Structural models Depth (feet) Distance (meters) Figure 38 3D Structural model of the top of the Talang Akar Formation- Sunda/Asri Basins 61

63 Depth (feet) Distance (meters) Figure 39 3D Structural model of the top of the Air Benakat Formation- Sunda/Asri Basins 62

64 Sunda and Asri Basins: Cross Sections Figure 39 is the location map of Sunda/Asri Basins displaying the section line for cross section 3 Figure 40 Location map for cross-section A-A'- Sunda/Asri Basins, Distances in meters 63

65 Cross section G-G G G Figure 41 Cross-section 3 G-G, SW-NE displaying the structural trends in Talang Akar Formation (TAF), Air Benakat Formation (ABF) and the basement. 64

66 Ardjuna (NW Java) Basin: 3D structural models Depth (feet) Distance (meters) Figure 42 3D Structural model of the top of the Talang Akar Formation- Ardjuna Basin 65

67 Depth (feet) Distance (meters) Figure 43 3D Structural model of the top of the Air Benakat Formation- Ardjuna Basin 66

68 Ardjuna (NW Java) Basin: Cross sections H H Figure 44 Location map for cross section H-H, Ardjuna Basin, Distances in meters 67

69 Cross section H-H H H Figure 45 Cross section H-H, N-S displaying the structural trends in Talang Akar Formation (TAF), Air Benakat Formation (ABF) and the basement. 68

70 Appendix III: Tables Tables 2, 3 and 4 show the measured distances between the producing wells and the grabens, for the South Sumatra, Sunda/Asri and Ardjuna Basins respectively. Table 2 Distance measurement table for the South Sumatra Basin Distance in km Rimbabat Bulian Bentajan Bentajan Bentajan Bentajan North Kluang Jemakur Kerang Kerang Kerang Tabuan Tabuan Selatan Average Table 3 Distance measurement table for the Sunda and Asri Basins Distance in km Lestari Nurbani PSI ZUD Average

71 Table 4 Distance measurement table for the Ardjuna (NW Java) Basin Distance in km PSI FT PSI FT PSI OV PSI OY PSI XW PSI XW PSI X PSI GG PSI WG PSI P PSI P PSI P PSI P PSI ML PSI MV Average Table 5 Well log data Bore Longitude Latitude Elevation Total Depth Abab Asri Astari Bakung Bakung(stanvac) Banuwati Bentajan Bentajan Bentajan Bentajan Bentajan Bentu Berlian Besai Betara Budiarti Budiarti Buka

72 Bulian Bunga Candi(Tjandi) Capang cecilia cicih Cikarang Cilamaya Timur Cimalaya Utara Cinta Ciwaringin Cory Darmi Dedeh Dempo Dewi selatan Djambu Elly Emi 1e Enny Erna Esi Fanny Farah Farah Flora Gaby Gajah Gayatri Gede Gita Gita A-7ST GN Grissik Hariet Harimau Harimau Harimau Harimau 3A Harimau Harimau Harimau

73 Harimau Hatty Hera Herawati Ibul Ibul 2A Ida Ina Indah Intan Ira Irma Jambu Jangga Janti Jemakur Judy Kapas Strat Karlina Kartika Kartini Kartini Kartini Kartini Kartini Kartini Utara Katomas Kejumat Kemala Kerang Kerang Kerang Ketaling Kijang Kitty KMM Krisna Krisna KRK Kukui Laksmi Lastri Lematang