January 1, 2019 | Author: Sementara Tidak Aktif | Category: Shale, Shale Gas, Porosity, Clay Minerals, Minerals
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The shale gas resource in Indonesia is still being assessed. The Head of Geological Agency for Energy and Mineral Resources Ministry, R. Sukhyar, in September 2011, estimated the Indonesian shale gas resource to reach 570 trillion cubic feet (tcf) based on their latest studies (Petromindo, 2011). This paper includes a discussion of the shale gas potential in Western Indonesia and Eastern Indonesia and makes a comparison with shale gas potential in the U.S. The aim of this paper is to encourage shale gas development in Indonesia. The author used a desktop study and an X-ray diffraction study as input for this paper. From Sumatra to Papua, Indonesia has many oil  producing basins. These basins have thick shales with high total organic content (TOC) and offer attractive opportunities as unconventional gas for the future. To better estimate the future Indonesian shale gas  potential more detailed petrographic studies and mapping will be required. INTRODUCTION

Indonesia has many basins that have potential for shale gas production. This paper analyzes the properties of shale required for economic shale gas development. To help appreciate this topic, it is important to understand what is unique about the evaluation of shale gas potential. The underlying control factors are the physical properties of the shale and its geological age. This paper includes comparisons  between shale gas properties in the U.S. and Indonesia. Therein, I consider the shale qualities and the effectiveness of hydraulic stimulation of shale gas reservoirs. *

INPEX Corporation

This paper also gives an idea for the shale gas  potential in Indonesia by considering mineral composition of shale and thus predicts the proper completion techniques for future shale gas  production. METHODS

The objective of this study is to determine the shale gas potential in Indonesia using a literature study and sub-surface cores. The author collected  published literature from various journals, papers, and existing X-ray diffraction data. These were compared with a collection of sub-surface core data and shale reservoir properties. The comparison includes the mineralogy and lithology of shale gas fields that have been produced in the U.S. DISCUSSION AND RESULTS Shale Gas Property

Shale is a class name for all fine-grained argillaceous sediments, including mud, clay, and mudstone. Shale, as defined based on the grain size, can be defined to be clay and silt; clay < 4 microns and silt 4 to 62 microns. From actual examples of producing shale gas fields in U.S., we know certain shale gas/oil criteria such as Vitrinite Reflectance (Ro) and Total Organic Content (TOC) values must be greater than 1%, Hydrogen Index (HI) values are >100, and shale thicknesses are greater than 75 ft. These must be  present for successful success ful shale s hale gas ventures. Shale gas system includes adsorbed gas and pore gas. Also, in shale gas projects, kerogen is a critical factor that affects the quality of the shale gas. The quality of the shale gas can be assessed by the mineral framework and pore system. The mineral composition of shale includes quartz, feldspar, clay minerals, calcite, dolomite, siderite, pyrite,  phosphate and organic matter. Reservoir porosity greatly affects the hydrocarbon capacity, where porosity in shale gas reservoirs are greatly affected by clay mineral content. A group of

clay minerals to be considered is the Smectite Group. The Smectite group is comprised of (Ca, Na, H) (Al, Mg, Fe, Zn) 2 (Si, Al) 4O10 (OH) 2-xH2O. This clay mineral group contains an H 2O element that reduces the shale porosity. Smectite will swell during slick water fracs and thus contributes to poor reservoir quality by reducing effective porosity. Another clay mineral that causes concern is illite. While high illite percentage is a benefit to hydraulic stimulation, because illite is highly brittle, it may cause stuck pipe. Quartz also is very influential to shale gas  productivity; for as with illite, quartz provides  brittleness within the shale gas reservoir. The more quartz content, the more effective will be the hydraulic stimulation but if, the quartz content is small the shale will be more plastic and thus will be less effective to hydraulic simulation. Another common factor, as seen in producing shale gas fields in the United States, is the age of the shale reservoir which primarily ranges in age from Paleozoic to Mesozoic (Figures 1- 2). In those old basins, the clay minerals are usually trioctahedral smectite, Mg-rich chlorite, interstratified chlorite/smectite and corrensite. Diagenesis of Smectite to chlorite depicts a trend toward diminished swelling and increased stability (Chang et al., 1986; Kook Son-Byeong et al., 2001). While in younger basins, clay minerals are dioctahedral and make mechanical instability. U.S. vs. Indonesia Shale Gas Mineralogy and Lithology

Mississippian Barnett Shale is one example of a successful shale gas project with shale gas resources ~25-252 Tcf (Schlumberger, 2005). Barnett Shale generally contains less than one-third clay minerals, according to Bowker (2002), these clays are dominantly illite with minor smectite  percentage. Illite in the Barnett Shale reservoir interval is approximately 27% with minor Smectite  percentage (Bowker, 2003). The Barnett formation in the Northern Fort Worth Basin, Texas (Figure 2), is a siliceous mudstone (Papazis, 2005). The thickness of the Barnett Shale interval is approximately 300-500 ft. TOC of the Barnett Shale ranges from 3-13% wt (Montgomery et al., 2005), with Ro> 1.4% (Jarvie et al., 2007).

The percentage of quartz in the Barnett Shale reservoir interval is 45% (Bowker, 2003a) (Figure 3). Therefore the operation of hydraulic stimulation is very good and creates multi-planar fractures. Most of the shale gas plays in the U.S., such as the Fayetteville, Haynesville, Marcellus and Woodford have high silica content, with average above 40% (Tudor, Pickering, Holt, 2010). If we summarize and analyze the geological age of shale gas plays in the U.S., an average of U.S. shale gas geological age is Mesozoic-Paleozoic (Tudor, Pickering, Holt, 2010). One could initially consider the potential of shale gas in Indonesia in the West. Both the South Sumatra Basin and Central Sumatra Basin have shale gas potential. In the South Sumatra Basin, one  potential shale gas reservoir would be the Gumai Shale with Ro approximately 0-0.5%, TOC 0.340.39% wt (Pertamina 2011), with an average thickness of 100-300 ft. Smectite percentage in Gumai Shale is around 5% and illite-smectite approximately > 10%. Potential Yield (PY) is 0.530.81 mg HC/g (Hermiyanto and Sudini, 2009). Based on these criteria Gumai Shale could likely  produce gas. Since the Gumai Shale is Late Oligocene – Early Miocene in age, drilling and completion operations will require competent technology because of the possibilities of tight holes or getting stuck during drilling or completions. In the North Sumatra Basin, the Middle Miocene Baong Shale has TOC> 1-2% wt, Ro> 1.3, with shale thickness > 30 m and high quartz content (PPPTMGB ,2011) (Figure 4). From the X-ray diffraction analysis, it is known that the Smectite content has ranges of 3-33% where the average is 14.3% and Illite content ranges from 4 to 18% with an average of 8% (PPPTMGB, 2011) In Eastern Indonesia, prospects for shale gas are somewhat different because the basins’ age in Eastern Indonesia are older than those in Western Indonesia. Shale in Jurassic interval in Masela Field , Bonaparte Basin is more accurately described as a claystone and siltstone. Nevertheless, the shale fulfills the criteria, with TOC> 2.0% wt, Ro> 0.6% (Corelab, 2003a) and shale thickness up to 90 ft. From the X-ray diffraction analysis, Illite-Smectite content ranges between 0-18%, with minor Smectite of approximately 0-9.8%, and a high value of Quartz of >30% (Corelab, 2003a) (Figure 5) .

From the X-ray diffraction analysis shows that the  percentage of smectite in mixed-layer illite/smectite in Jurassic shale decreases down-hole from 60-70% to 15-20% (Corelab, 2003b) (Figure 6) . The potential of shale gas in eastern Indonesia in the old basins with depth > 2000 m, the production stage will be easier because of minimum swelling clay and increasing stability in completion. CONCLUSION

In Western Indonesia, Gumai Shale and Baong Shale have potential for shale gas. One potential shale gas in Eastern Indonesia is the Jurassic Masela shale which is fulfills all criteria of shale gas. But Jurassic shale in Masela is very deep and lies offshore, so it would be difficult to develop. Yet Jurassic shale in Masela area can be analogue for other old shales in Eastern Indonesia. It will be necessary to conduct further shale studies in Eastern Indonesia to find the new shale gas  prospect, because of the high potential of shale gas/oil in old basins with minor illite/smectite  percentage and high quartz content (Corelab, 2003a, 2003b) ACKNOWLEDGMENTS

The author thanks the Technical Program Committee of IPA for selecting this paper to be  published, and for reviewing this paper. INPEX Corporation and INPEX Masela are acknowledged for supporting the author to conduct and publish this  paper. Toru Akutsu (INPEX), Henry Bandjarnahor (INPEX), BP MIGAS, MIGAS contributed significant literature, unpublished data, and discussions. REFERENCES CITED

Bowker, K.A., 2002, Recent developments of the Barnett Shale play, Fort Worth Basin, in D.E. Law and M.Wilson, eds., Innovative Gas Exploration Concepts Symposium: Rocky Mountain Association of Geologists and Petroleum Technology Transfer Council, October, Denver, Colorado, 16 p.

offshore basins: Clays and Clay Minerals, 34, 407423. Corelab., 2003a, Core Description, Petrographic, Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) Analyses of Core Samples from the Abadi-B Well, and XRD Analysis of cutting samples from Abadi-B Well, Timor Sea Eastern Indonesia, 2003. Corelab., 2003b, Petrographic, Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) Analyses on Twenty-Nine Samples from the Twenty-Nine Samples in the Abadi-A Well, Timor Sea Eastern Indonesia, 2003. Courtesy of Tudor, Pickering, Holt & Co. Energy Information Administration., 2010 Hermiyanto, M.H., N. Sudini., 2009, Organic  petrology and Rock Eval characteristics in selected surficial samples of the Tertiary Formation South Sumatra Basin: Indonesia Geological Journal, v.4  No.3, p.215 -227. Jarvie, D.M., R. J. Hill, T. E. Ruble, R. M.Pollastro., 2007, Unconventional shale-gas systems: The Mississippian Barnett Shale of northcentral Texas as one model for thermogenic shalegas assessment: AAPG Bulletin, v.91, p.475-499. Kook Son-Byeong., Yoshimura, T., Fukusawa, H., 2001, Diagenesis of dioctahedral and trioctahedral smectites from alternating beds in Miocene to Pleistocene rocks of the Niigata basin Japan: Clays and Clay Minerals,49, 333-346. Montgomery, S.L., D.M. Jarvie, K.A. Bowker, and R.M. Pollastro, 2005, Mississippian Barnett Shale, Fort Worth Basin, north-central Texas: Gas-shale  play with multi-trillion cubic foot potential: AAPG Bulletin, v.89, p.155-175. Papazis, P.K., 2005, Petrographic characterization of the Barnett Shale, Fort Worth Basin, Texas: Master’s thesis, University of Texas at Austin, Austin, Texas, 142 p., CD-ROM (SW0015), available from Bureau of Economic Geology, University of Texas at Austin.

Bowker, K.A., 2003, Recent development of the Barnett Shale play, Fort Worth Basin: West Texas Geological Society Bulletin, v.42, no.6, p.1-11.

Pertamina., 2011, Shale Gas Progress study at Lemigas presentation.

Chang, H.K., Mackenzie, F.T., Schoonmaker, J., 1986, Comparisons between the diagenesis of dioctahedral and trioctahedral smectite Brazilian

Petromindo , 2011, Sept , News Briefs PPPTMGB “LEMIGAS” at Lemigas presentation., 2011 Schlumberger., 2005, Shale gas white paper.

Figure 1 - U.S. vs Indonesia Geological Age

Figure 2 - Shale Gas Plays in U.S. (Energy Information Administration based on data from various published studies. Updated : March 10, 2010)

Figure 3 - Jarvie et al. (2007): Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale gas assessment- AAPG Bulletin 91 (4):475-499

Figure 4 -

Ternary diagram of Baong Shale, North Sumatra Basin (PPPTMGB “LEMIGAS” at Lemigas presentation on Nov 22, 2011)

Figure 5 - Ternary diagram of Abadi Claystone and Siltstone, Masela field (Corelab, 2003a)

Figure 6 - The percentage of smectite in mixed-layer illite/smectite Jurassic Shale vs Depth (Corelab, 2003b)

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