2D Resistivity Cross Section Interpretation of Shallow Hydrocarbon Reservoir in East Aceh, Indonesia

2D Resistivity Cross Section Interpretation of Shallow Hydrocarbon Reservoir in East Aceh, Indonesia Nurul Aflah Lecturer at Petroleum section, Department of mining Faculty of Engineering, Syiah Kuala University, Banda Aceh, Indonesia, e-mail: nurul_aflah@yahoo.com Kikis Muchlis Lecturer at Geophysics section, Department of mining Faculty of Engineering, Syiah Kuala University, Banda Aceh, Indonesia, e-mail: kikis.muchlis@gmail.com Syafrizal Post graduate student, Department of Physics, Syiah Kuala University, Banda Aceh, Indonesia, e-mail: iamsyafrizal88@gmail.com Sabrian Tri Anda Staff, Department of mining Faculty of Engineering, Syiah Kuala University, Banda Aceh, Indonesia, e-mail: sabrian.tri.anda@gmail.com ABSTRACT This study aims to interpret subsurface resistivity imaging of hydrocarbons trap and lithology. We conduct the Geo electrical method at one marginal oil field in the eastern part of Aceh, a rich gas and oil province in Indonesia. Using the results of this method is expected to give a resistivity 2D cross section images showing the depth and the lithology of the reservoir layer to provide recommendations on the prospects of oil and gas. The principle of the method is that the electric current is injected into the earth's natural through two electrodes, while the potential difference that occurs is measured by two electrodes potential. From the results of measurements, electric current and potential difference can be obtained by variation of the electrical resistivity of the layer below the measuring point. The resistivity variations may image the possibility of hydrocarbon trap. The result shows that the possibility of top formation of hydrocarbon at depth average of 60 meters below the surface and the lithology is sandstone with the resistivity range from 6 to 40 ohm meter. KEYWORDS: geoelectric, reservoir, lithology, Aceh INTRODUCTION Northern Sumatra Basin is a tertiary basin containing the potential of more than 5 billion barrels of oil equivalent (Doust, 2007). This basin covers almost 80% of the east coast of Aceh Province. After the Arun gas field development in 1977 with the commencement of gas liquefaction project, Aceh exploration activities on land were very rare due to the conflict between the Indonesian government and the Free Aceh Movement. Not only commercial - 1659 -

Vol. 21 [2016], Bund. 04 1660 activities in the exploration of natural resources but academic research activities were also affected. There are many oil wells that abandoned as a result of depletion or uneconomic of oil and gas in the area. Some of them are abandoned old wells that operated by the Dutch company in Perlak, East Aceh. However, most of oil and gas wells still contain potential reserve, and it is proved by illegal drilling of people living surrounding the area. Figure 1: Hydrocarbon Basin in Indonesia (Doust, 2007) GENERAL GEOLOGY IN THE AREA Based on the geological map of the study area in eastern Aceh district (Cameron et al, 1981) we can see that Sediment in Peureulak, East Idi, and Sungai Raya is dominated by Serulla consisting of interbedding sandstone and mudstone. Then followed by quarter formations (alluvial) consisting of sand, gravel and clay covering of the village Gedung janeng,, Bukit Pala, Alulhok, alue putih, Paya Mauligo, Tanah Rata, and Paya Uneu. Next, the formation QTjr (Julu Rayeu) consisting of alternating sandstone, claystone and mudstone containing lignite covers an area Peudawa and Peureulak Barat. Areas that have a potential of oil and gas are in Rantau Panjang, Beurandang and Sungai raya. Moreover, there is also marginal oil field located in Area Rantau Panjang that crossed by a long fault from northwest to southeast of Peureulak. The map also shows several fault trends and major anticline structures where the hydrocarbon is may be trapped.

Vol. 21 [2016], Bund. 04 1661 Figure 2: Geology map of East Aceh

Vol. 21 [2016], Bund. 04 1662 STRATIGRAPHY IN THE AREA The stratigraphy in Northern Sumatra Basin was identified by Cameron, 1981 and Muhadiono in 1976. Formations that are shown on the geology map in the area are: Baong Formation This is mainly composed of carbon dominant dark gray shale, rich in forams, showing a shallow marine environment. The maximum thickness measured reaches more than 700feet. Keutapang Formation This formation consists of fluvial sandstone sea neritic in deep and fine clastic as a result of the lifting of the Bukit Barisan to the east Sunda plate. Mulhadiono (1976) identified four depositional environment Keutapang sandstone formations at the bottom of the Arun area; shallow water, Inter deltaic, delta, and estuary. Sereula Formation Seureula Formation consists of sand dominant interbedding shale and mudstone. Seureula sandstone containing fragments of shell more than the formation Keutapang which are coarser. This formation was deposited in an environment outside neritik along Early Pliocene. Julu Rayeu Formation Julu Rayeu Formation consists of a combination of sand and shale is rich with volcanic material, Conglomerates and sometimes contains tuff. This formation stretches aligned on the formation of terrigenous Seureula and deposited in shallow marine environments. Thickness varies between 250-600 m. Nowadays, Julu Formation Rayeu still not be considered as a potential target. Figure 3: Lithostatigraphy of Northern Sumatra Basin (Mulhaldiono)

Vol. 21 [2016], Bund. 04 1663 RESISTIVITY ELECTRICAL METHOD In the geoelectric method, resistivity values obtaining from the measurement is defined as the apparent resistivity (ρa), the apparent resistivity defined by: ρρ aa = KK ΔΔVV II where K is a geometric factor, ΔV is a potential difference and I is the current strength. In fact, the earth is a medium layered with each layer having a different resistivity. Apparent resistivity is the resistivity of a fictitious homogeneous medium equivalent to medium layered. Note the picture below. Earth surface Earth surface ρ 1 ρ 2 Actual resistivity ρ a Apparent resistivity Figure 4: Layers of the actual resistivity (ρ 1 ρ 2 ) and apparent resistivity (ρ a ). In Figure 4 the apparent resistivity can be converted into actual resistivity, for example in terms of layered medium consisting of two layers having different resistivity (ρ 1 and ρ 2 ). In geoelectric measurements, the medium is considered as a homogeneous medium one layer that has a resistivity value is apparent resistivity (ρa). Conductance pseudo layer is equal to the number of conductance of each layer is ρa = ρ 1 + ρ 2 (Indriana, RD, and Danusaputro, H., 2006) WENNER-SCHLUMBERGER ARRAY The Wenner- Schlumberger array can be defined as a merger of mapping and sounding techniques. The result causing the changed value of geometric factor k: kk = ππ nn(nn + 1)aa

Vol. 21 [2016], Bund. 04 1664 Figure 5: Illustration of measurement datum points using the Wenner-Schlumberger array (Syafrizal, 2013) RES2Dinv is software that uses to determine the pseudo-section of resistivity beneath the surface. RES2DINV is a computer program that will automatically determine a twodimensional (2-D) resistivity model for the subsurface that use the data obtained from electrical imaging surveys (Griffiths and Barker 1993). Since it is a Windows based program, all Windows compatible graphics cards and printers are automatically supported. METHODOLOGY Data acquisition use resistivity meter Supersting R8 and Wenner-Schlumberger array that used roll-along technique (Fadhli (a), 2015). The acquisition had been done using two lines with each spread is 330 m and space within the electrodes is 6 m. The data is then processed to give 2D resistivity images that show geological subsurface (Milsom, 2003). After the results imaged, the layer which contains oil and gas could be determined and interpreted. RESULTS AND DISCUSSION Data acquisition were done in 2 line with 330 m on different location. a) PA PA is the cross-section from northwest to southeast which coordinate is; 4 48'13.12"N 97 47'45.13"E and 4 48'16.82"N 97 47'55.12"E.

Vol. 21 [2016], Bund. 04 1665 Figure 6: 2D Cross-section with resistivity value of PA line Figure 6 shows that a couple of minor fault from resistivity pseudo-section from the PA line. The fault can be the boundary of different lithology. This boundary imaged at line of 90 m, 162 m, and 180 m. Generally, PA line has imaged 4 layers. The first layer was top soil mixed by clay with resistivity 10-100 Ωm at 5 m depth. The second layers contain clay-sand with the resistivity 6-10 Ωm at 35 m depth. Furthermore, muddy sand with resistivity 6 Ωm, appeared at the third layer by range of 10 to 68 m depth. In the last layer, at 60 m depth, is the layer with the resistivity value of 8 to 40 Ωm, and this might be alluvium sediments (Fadhli (b), 2015).We interpreted it as the sand layer, and it is predicted as shallow hydrocarbon reservoir. b) PB PB is the cross-section from west to east which coordinate is; 4 48'58.49"N 97 47'33.58"E and 4 48'57.93"N97 47'44.10"E. Figure 7: 2D Cross-section with resistivity value of PB line The fault imaged at the distance between 60 and 78 m, 10 m depth in PB line. The minor faults is shown at the distance 120 to 138m. The shallowest depth on PB consist of sand-clay with various depth of 20 m from the distance 0 138 m at the line. In addition, at the distance between 138 and 330m, sand-clay lithology is found at the depth of 9 m.

Vol. 21 [2016], Bund. 04 1666 Furthermore, the muddy sand was obtained at 9-68 m depth with resistivity under 6 Ωm, in other hand, the sand layer was predicted as shallow hydrocarbon reservoir founded at depth of 60 m and deeper. Resistivity value that obtained from both of lines (PA and PB) show low value of resistivity. It is clearly that the resistivity ranges from 2-10 Ωm might be the water barrier in the reservoir (Syukri et al, 2015) CONCLUSION Based on the interpretation of the resistivity values in 2D cross section, the top of shallow reservoir located at a depth of 60 meters with sand rock lithology. These resistivity heterogeneities might be the results of water fill the pores (Hong-Jing, 2014). From stratigraphic information, the possibility of formation reservoir between Seureula (lower seureula) and ketapang (top ketapang). The possibility hydrocarbon trap that allows is anticline, a combination of faults and anticline (fault related structures) and lenses (lens-shaped reservoir). REFERENCES 1. Cameron, N.R., Clarke, M.C.G., Almss, D.T., Aspden, J.A. & Djunuddin, A. (1980) The geological evolution of northern Sumatra. lndonesia Petroleum Association, Proceedings of the 9th Annual Convention, Jakarta, 1980, 9, 149-187 2. Cameron NR, Bennett JD, Mc C. Bridge D, Djunuddin A, Ghazali SA, Harahap H, Jeffrery DH, Kartawa W, Keats W, Rocks NMS, Whandoyo R, Ngabito H, Thompson SJ. (1983, Peta Geologi Lembar Langsa, Sumatra, PPPG, Bandung 3. Doust., H, Noble., R.A. (2007) Petroleum Systems of Indonesia., Marine and Petroleum Geology 25, 103-129 4. Fadhli., Z., Saad., R., Noordiana., M.M., Azwin., N, Bery., A.A, 2015 Mapping Subsurface Karst Formation Using 2-D Electrical Resistivity Imaging (2-DERI), Electronic Journal of Geotechnical Engineering, vol.20, pp:349-358. 5. Fadhli., Z., Saad., R., Noordiana.,M.M., Syukri., M, Azwin., N, Bery., A.A, 2015, Determination of Limestone and Overburden Resistivity Values for Tropical Region Using 2 D Electrical Resistivity Imaging (2-DERI) Method, Electronic Journal of Geotechnical Engineering, vol 20, pp:1447-1456. 6. Griffiths, D.H. and R.D. Barker. 1993. Two-Dimensional Resistivity Imaging and Modeling In areas of Complex Geology. Journal of Applied Geophysics. 29:211-226 7. Hong-Jing., J, Shun-Qun, L., Lin., L, 2014 The Relationship between the Electrical Resistivity and Saturation of Unsaturated Soil, Electronic Journal of Geotechnical Engineering, vol.19, pp:3739-3746. 8. Indriana, R. D. & H. Danusaputro. 2006 Uji Nilai Tahanan Jenis Polutan Air Laut Dengan Metode Ohmik Dan Geolistrik Tahanan Jenis Skala Laboratorium. Berkala Fisika, 9(3):145-149. 9. Milsom, J., 2003 Field Geophysics The Geological Field Guide Series Third Edition, John Wiley and Sons Ltd, England.

Vol. 21 [2016], Bund. 04 1667 10. Mulhadiono, Pramu Hartoyo & Soedaljo, P.A. (1978) The middle Baong sandstone unit as one of the most prospective units in the Arun area, North Sumatra. Indonesian Petroleum Association, Proceedings 7th annual convention, Jakarta, 1978, 107-132. 11. Syafrizal, 2013 Interpretasi Struktur Bawah Permukaan Kawasan Karst Laweung menggunakan Metode Resistivitas 2D, FMIPA, Universitas Syiah Kuala, Banda Aceh. 12. Syukri., M, Fadhli., Z., Saad., R, 2014 The Investigation of Hot Spring Flow Using Resistivity Method at Geothermal Field Ie-Seu um, Aceh Indonesia, Electronic Journal of Geotechnical Engineering, vol.19, pp:2419-2427. 2016 ejge