MAGNETOTELLURIC RESISTIVITY IMAGING OVER THE KAWENGAN OIL FIELD AND BANYUASIN PROSPECT, NORTH EAST JAVA BASIN

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1 MAGNETOTELLURIC RESISTIVITY IMAGING OVER THE KAWENGAN OIL FIELD AND BANYUASIN PROSPECT, NORTH EAST JAVA BASIN Hendra Grandis 1, Djedi S. Widarto 2, Edi P. Utomo 2, Waluyo 3, and Fred Hehuwat 2 2 Department of Geophysics and Meteorology ITB, Jl. Ganesha 10, Bandung Research Center for Geotechnology LIPI, Kompleks LIPI Sangkuriang, Bandung Directorate of Upstream, Pertamina, Gedung Kwarnas Pramuka, Jakarta ABSTRACT A magnetotelluric (MT) survey has been carried over the Kawengan field and the Banyuasin prospect to assess the potential of the deep reservoir in the Kujung carbonate and to clarify an indication of a shale diapiric mass in the core of the Kawengan anticlinal structure. The MT response data in terms of apparent resistivity and phase difference were collected during the fieldwork consisting of two MT survey lines and a line across perpendicularly to the Kawengan field and the Banyuasin prospect, respectively. One- and two-dimensional inversion methods were used to model subsurface resistivity structures beneath these three survey lines. Based on 1-D and 2-D resistivity models, we conclude that there are no indications of the occurrence of a low resistivity shale diapiric mass in the subsurface of the core of the Kawengan structure. There is also no indication of thickening of the low resistivity unit interpreted to represent the Tuban Formation shales in the crestal part of the structure. The existence of carbonate build-ups along the Banyuasin line as indicated from seismic data, seems to be confirmed by the results of this MT survey. The build-ups possibly grew from a platform at a depth of around 3000 meters rising to meters below surface. The existence of a large carbonate build-up seems to be indicated located to the NW of the MT survey line. This build-up seems to be separated from the adjacent carbonates by a fault. INTRODUCTION MT measurements along two traverses at Kawengan field and one traverse at Banyuasin prospect (both in North East Java Basin) were carried out in November-December The purpose of the study is to assess the possibility of the existence of Kujung carbonate reservoirs below the present producing shallow clastic reservoirs in the Kawengan field. As for Banyuasin prospect, the MT survey is intended to assist to clarify the ambiguities of the seismic data, i.e. indication of apparent carbonate buildups which was of poor quality for the deeper parts. GEOLOGICAL GEOPHYSICAL FRAMEWORK The Kawengan Oilfield is located in the onshore part of the North East Java Basin, NW of the town of Cepu. The Kawengan field represents a WNW-ESE trending asymmetric anticline, bounded to the South by a Northdipping thrust. It is cut by a number of faults into several segments, including: Dandangilo, Wonocolo, Kawengan shallow, Kawengan deep, Wonosari, Ngudal and Kidangan (Soetantri et al., 1973). The production started in the end of 19 th century and up till today which comes mainly from shallow Neogene clastic reservoirs in the Wonocolo and Ngrayong Formations. However, since the early 1970-ies, hydrocarbon exploration in the offshore parts of the North East Java Basin targeted the Lower Tertiary carbonates of the Kujung (and Pre-Kujung) Formation, in which a number of discoveries were made. From the late 1980-ies and early 1990-ies on, onshore exploration began focusing on the Kujung carbonates, and it resulted in a number of discoveries (Ardhana, 1993).

2 In its effort to boost production of its old fields, PERTAMINA began to consider the possibility of the existence of Kujung carbonate reservoirs below the present producing shallow clastic reservoirs in the Kawengan field. Some seismic lines across the Kawengan structure show indications of the presence of Kujung carbonates, and an ambiguity which might be the result of either a pull-down effect within the carbonates, or it might be the result of a shale bulge or diapiric structure of the shales of the Tuban Formation, resulting in a lowering of the top of the carbonate reflector due to the thickening of the shale section. To solve the ambiguity, the Geotechnology Research Center of the Indonesian Institute of Sciences (Puslit Geoteknologi LIPI) proposed the use of the magnetotelluric (MT) technology. As the MT method records the resistivity of the subsurface rocks, bearing in mind the large resistivity difference between carbonates and shales, it should be (theoretically) possible to delineate the carbonate-shale boundary (the Tuban Formation Kujung Formation boundary) at depth. MAGNETOTELLURIC (MT) SURVEY Basically, MT is a geophysical method that measures naturally occurring, time-varying magnetic and electric fields. From these measurements we can derive resistivity estimates of the subsurface, from the very near surface to up to 100 kilometers depth or more. The disturbance of the earth s magnetic field by solar wind and electrical storms create timevarying electro-magnetic (EM) waves. These primary fields induce secondary electric and magnetic fields in the conductive Earth. The transient variation of the EM fields recorded at the surface of the Earth is therefore diagnostic to the subsurface electrical properties (conductivity or resistivity). Low frequencies travel further into the earth than high frequencies (skin effect). MT measurements are conducted with coils / magnetometers that record magnetic fields, while the electric fields are measured with dipoles (wires connected to porous pots. The data are connected to a sensor box/amplifier where filtering and amplification of the signals take place. Digitally recorded data are then transformed from time-domain to frequency domain, from which apparent resistivity and phase vs. frequency is derived (MT sounding data). Two SSW-NNE trending MT traverses were chosen approximately perpendicular to the strike of the Kawengan field. The MT traverses MT-01 and MT-02 were designed to coincide with existing seismic traverses, KWG-05 and KWG-07 respectively. This would enable correlation of the seismic data with the results of the MT survey. Along traverse MT-01 measurements were conducted at 30 stations, the distance between stations was 300 meters on the northern and southern limbs of the structure and 250 meters for ten MT stations measured over the crestal part of the Kawengan structure. As the total length of the MT-01 traverse was 8,150 meters, it exceeded the length of the seismic line by 2,400 meters, which was distributed evenly between the SW and NE ends of the seismic line (each occupied by four MT stations). Along MT-02 traverse only 23 stations were surveyed. For MT survey at the Banyuasin prospect, the single traverse, i.e. MT-03 comprises 37 stations. The interval between the stations were 300 m above the predicted carbonate build-ups and 510 m in the intervening areas. The Banyuasin survey took place during adverse weather conditions with much rain and thunderstorms, which caused a lot of natural noise. RESULTS A 2-D MT inverse modeling was applied to all traverses. The inversion method is similar to the well-known Occam s inversion developed by degroot-hedlin & Constable (1990) and Uchida (1993). The inversion algorithm is based on the smoothness-constrained inversion which results in smooth 2-D resistivity model of the subsurface. Figure 1 shows the 2-D resistivity section for MT-01 traverse (from SSW to NNE), in which the depth of the section is limited to 3500 meters. The upper part of the section is dominated by low resistivity rocks (less than 10 Ohm-m). The abrupt changes of thickness of this upper layer are clearly visible especially at

3 the southern and northern flanks of the crestal part. These might indicate fault-boundaries. The lower part of the section shows a slightly more resistive layer. More resistive rocks are found only at the northern flank of the crest and at the north end of the profile. Discrimination into smaller/thinner units of differing resistivity is not possible on the 2-D MT model, as the model tends to average out resistivity values around the transition/ boundary between units of differing resistivity. The same problem with 2-D MT model (i.e. impossible to discriminate into smaller/thinner units of differing resistivity) is also encountered in MT-02 traverse (Figure 2). Therefore, we used 1-D models inferred from each MT sounding data at the various MT stations. For each MT traverse, the correlation of models between MT stations were then performed. Figure 3 depicts the correlated resistivity units along with the geological interpretation for MT-01 and MT-02 traverses. The boundary of the resistivity unit might not correlate with the formation boundary. However, from 1-D MT models of MT-01 and MT-02 traverses especially from the crestal part, the following stratigraphy of resistivity units could then be recognized, starting from top to bottom: i. Surface Unit, of varying resistivity (intermediate to high) comprising alluvium and other Quaternary sediments. ii. Intermediate Resistivity Unit, comprises Tertiary sediments younger than the Ngrayong Formation, including sediments of the Wonocolo, Ledok and Mundu Formations. iii. Upper High Resistivity Unit, mainly comprising quartz sandstones of the Ngrayong Formation and possible calcarenites of the Bulu Formation/ Platen Complex. iv. Low Resistivity Unit, predominantly clayey sediments of the Tuban (Tawun) Formation. v. Lower High Resistivity Unit, carbonates of the Kujung Formation and metamorphic rocks and igneous intrusives comprising the Basement Complex. As with geological/structural interpretaion, differences of depth of occurrence of resistivity units might indicate the existence of a fault. However, the nature of the fault is a matter of interpretation, whether one is dealing with a normal or reversed/thrust fault. As the Kawengan structure is usually assumed to represent a pop-up compressional structure in the Kening Trough, located at the western end of the Miocene inversion zone of the North East Java Basin, the faults identified along the traverses have been interpreted as representing thrust faults. In view of the ambiguities inherent of the MT method, they might as well represent normal faults. Like has been the case with the results of the Kawengan MT survey, the results of MT measurements at Banyuasin prospect (MT-03 traverse) were processed in a 2-D and 1-D model depicted in Figure 4 and Figure 5 respectively. The models show the occurrence of a high resistivity unit underlying low resistivity sediments at shallower depth. The high resistivity unit being interpreted as representing carbonates of the Kujung Formation. These carbonates show indications of two culminations (build-ups). The geologic interpretation (Figure 5) shows an undulating boundary between the Tuban Formation clays and the carbonates of the Kujung Formation, occurring between 1700 and 3000 meters in the central part of the section. This might be interpreted as a limestone platform at a depth of 3000 meters on which carbonate build-ups occur reaching to 1700 meters below surface. To the NW end of the section another carbonate build-up might occur. CONCLUDING REMARKS A magnetotelluric (MT) survey has been conducted over the Kawengan (Field) structure and the Banyuasin prospect, to assess the potential of the deep reservoir in the Kujung carbonate. MT results did not indicate the existence of a low resistivity diapiric mass (thickening of the low resistivity unit interpreted to represent the Tuban Formation shales) in the subsurface of the core of the Kawengan structure. Furthermore, the lowermost resistivity unit identified was a high resistivity unit with true resistivities in excess of 500 Ohm-m. This unit has been interpreted as representing Kujung Formation carbonates and basement rocks comprising igneous intrusives and metamorphic rocks. The top of the Kujung carbonate occurred at depths of around 3000

4 meters at the northern and southern flanks of the Kawengan structure, rising to depths of around 2000 meters in the core of the anticline. The existence of carbonate build-ups along the Banyuasin Traverse as indicated from seismic data, seems to be confirmed by the results of the MT survey along the traverse. The build-ups possibly grew from a platform at a depth of around 3000 meters rising to meters below surface. The existence of a large carbonate build-up seems to be indicated located to the NW of the MT survey traverse. This build-up seems to be separated from the adjacent carbonates by a fault. degroot-hedlin, C., and Constable, S., 1990, Occam's inversion to generate smooth, twodimensional models from magnetotelluric data, Geophysics, 55, Soetantri, B., L. Samuel, G.A.S. Nayoan, 1973, The Geology of the Oilfields in North East Java, Proc. 2 nd Ann. Conv. Ind. Petr. Assoc., Uchida, T., 1993, Smooth 2-D inversion of magnetotelluric data based on statistical criterion ABIC, J. Geomag. Geoelectr., 45, REFERENCES Ardhana, W., 1993, A depositional model for the Early Middle Miocene Ngrayong Formation and its implications for exploration in the East Java Basin, Proc. Ind. Petr.Assoc., 22 nd Ann. Conv., Figure 1. Resistivity section obtained from 2-D MT data inversion for MT-01 traverse (Kawengan field).

5 Figure 2. Resistivity section obtained from 2-D MT data inversion for MT-02 traverse (Kawengan field). Figure 3. Resistivity section and geological interpretation from 1-D MT modeling for MT-01 and MT-02 traverse (Kawengan field).

6 Figure 4. Resistivity section obtained from 2-D MT data inversion for MT-03 traverse (Banyuasin prospect). Figure 5. Resistivity section and geological interpretation from 1-D MT modeling of MT-03 traverse (Banyuasin prospect).