IN WESTERN AUSTRALIA - Department of Mines and Petroleum

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26 PWA April Edition - Magnetotelluric Surveys Empire decided to contact the company responsible for carrying out the survey and contracted DMT, to carry out a new MT survey starting in November 2000 to evaluate a number of other prospects that had been identified from the re-interpretation of the seismic data. The prospects identified included small four-way dip closures on the Rough Range anticline, (the largest named Brooke); a large fault dependent closure at the northern end of the anticline updip of Lefroy Hill 1 (Tess); a fault dependent closure to the east of the Rough Range fault (Jennifer); and updip of Parrot Hill 1 (Elysia). The survey was carried out between 10 November and 4 December 2000 by DMT technologies under the supervision of Empire Oil & Gas. Field operations went very smoothly with very few hitches and overall productivity was on average 30% higher than expected, which enabled additional programme to be recorded. After the first two days recording, Empire undertook the acquisition programme leaving the DMT operator, Bob Mecionis, to concentrate on the analysis of the data. The main advantage of this method of operation to Empire was the flexibility to be able to change the programme as preliminary results were obtained. The ability to be able to record data on any particular day was subject to the vagaries of solar weather. A total of 165 points were recorded – approximately 60 more than was originally planned. The Rough Range area is well suited to the MT method due to the sharp contrast between the saline Birdrong reservoir and the overlying shales and also due to the ideal ground conditions. DMT were provided with log data from the Rough Range 1A well. Analysis of all other points was done without prior knowledge of any existing interpretation or of the results of the other wells recorded. The initial test programme consisted of a line across the Rough Range field including the Rough Range 1A well plus points recorded at the following wells – Rough Range 6, Rough Range 10, Rough Range 11, Central Rough Range 1, Rough Range 2 and Rough Range 7. Interpretation of the line over the field confirmed the existing interpretation within expected margins of error for both structure and fluid content. For the additional well points the Z-scan plots accurately identified the top of the Birdrong to an accuracy of +/- 3 m and identified fluid content and column heights with complete accuracy. The author can personally confirm that these results were obtained in strict ‘blind test’ conditions. In addition, the top reservoir was an unambiguous pick on all the analysed plots, thin zones within the Windalia Radiolarite typically gave a weak oil responses followed by a bland medium to high electromagnetic (EM) impedance response through the Muderong Shale followed by a sharp kick to a low EM impedance zone corresponding to the Birdrong Sandstone, which gave either a strong water response sometimes preceded by a few metres of an oil response. It was concluded that the method appeared to be working to better than expectations and that the remainder of the survey should proceed as planned and possibly expanded. The main results of the remainder of the survey, were that the Brooke, Tess, Elysia and Jennifer prospects all yielded encouraging results in terms of picked depths and predicted oil columns. Recordings over a number of other minor leads and prospects (e.g. a lead updip of Roberts Hill 1) all gave negative results. A number of other wells were analysed and these mostly gave consistently good correlations although the oil column of 3-6 m (from logs) in Parrot Hill was not identified and there was a depth error of 12 m at Lefroy Hill 1. The updip Parrot Hill prospect, Elysia, was affected by steep dip and it was found to be necessary to hand migrate the results using shallow dips derived from the seismic data to obtain a more meaningful map. It was subsequently decided to test the most promising prospect, Brooke, identified from the MT survey. This prospect was relatively small in size but predicted to be about twice as large as the Rough Range field. It was about 400 m from Rough Range 7 which had good oil shows but was poorly controlled by seismic and what seismic there was, was affected by a strong velocity field. Unfortunately, Brooke 1 came in low to prognosis and was dry. It is interpreted that the well was just on the wrong side of a normal fault running perpendicular to the Rough Range fault and sub-parallel to the nearby 2D Drilling operations at Rough Range (image courtesy of Empire Oil and Gas)
seismic line and that both the nearby MT points and the 2D seismic line were imaging the upthrown side of the fault. A fault was interpreted from the MT results but was placed about 100 m to the north of the well location. Steep shallow dips are believed to be responsible for the data not being imaged in the correct place. There was no seismic line running in this direction and the shallow data from the MT survey was not initially analysed. At the same time as the well Brooke 1 was being drilled, the third MT survey to be acquired in the Carnarvon Basin, the Sandalwood MT survey, was being acquired in the adjacent permits EP359 and EP412. This survey was conducted in June 2001 and comprised 88 recordings, many of which were in very remote and difficult locations. A large number of leads and prospects were tested. The most encouraging results came from a sizeable prospect in EP359 on the east side of the Learmonth fault adjacent to Learmonth 2 and opposite Trealla 1. An additional 5 wells were analysed in this survey and all had depth accuracies of less than 5 m with the exception of Trealla 1, which was out by 18 m and this was attributed to unusual ground conditions. It was next decided to test the potentially largest prospect, Tess. This prospect had previously been identified from the seismic data and was located on a seismic line, so it was not purely a test of the MT method. As this well was being drilled, depths were on prognosis until near the base of the Muderong Shale, a thrust fault was intersected and a repeat Muderong Shale section was encountered. The location on the seismic line appeared to be well back from the main fault and the fault intersected was essentially subhorizontal and almost impossible to see on the seismic line. Empire did try to reprocess this line but the tapes were unreadable. However, it would probably have required 3D seismic in order to image the fault in question. Again, the MT data that supported the seismic interpretation were probably imaging data from the other side of a fault. The Perth Basin Empire acquired a small MT survey comprising 13 points in April 2001. This consisted of a number of points in the Gingin area, the Bullsbrook 1 well and a number of points over the Eclipse prospect. The results of this survey gave an excellent correlation between the log data at Bullsbrook 1 and the MT plot, the correlation at the Gingin wells was not as good although some of the gas zones at Gingin 1 were correctly identified. The survey also predicted depths at Eclipse close to that predicted from the seismic data and also predicted gas saturation in the main target reservoir. Eclipse was defined by a 2D seismic grid and exhibited a well-defined AVO anomaly, which appeared to closely match the mapped structure. Eclipse 1 was drilled in 2003 with disappointing results, although there was a 3 m oil column in the upper sand. The AVO anomaly was probably due to very clean, high porosity sand. Depth predictions from both the seismic and MT data were quite accurate. Summary and Conclusions Although to date, the MT method has not resulted in a commercial discovery in WA, the vast majority of exploration wells based on seismic data have also failed this test. However, the AMT technique has given prediction. The major advantages of the MT method are: 1. The ability to record data in areas with poor or uninterpretable seismic signal, e.g. areas with surface basalt or weathered near surface PWA April Edition - Magnetotelluric Surveys 27 carbonates. 2. The ability to more precisely predict depths where the seismic data is affected by changes in velocity caused by thick layers of carbonates or salt. 3. The ability to record data in environmentally sensitive areas with restricted or no vehicular access. 4. It provides information about electrical rock properties in addition to acoustic rock properties. Disadvantages of the MT method are: 1. In areas of complex stratigraphy it may be difficult to accurately correlate events. However, recording more closely spaced points may solve this problem. 2. It can only be used in dry surface conditions – although it can be used over shallow salt water or on the seabed. 3. Recording is not possible during unfavourable periods of solar activity or in culturally noisy areas (i.e. close to radio transmitters, power lines, etc.). 4. Some depth shifting occurs due to diurnal and seasonal changes in the earth’s magnetic field – recording frequent calibration points can substantially reduce these inaccuracies. Worldwide, both conventional MT and AMT continue to be used in oil and gas exploration. The most recent developments include the recording of seabed surveys for sub-salt and sub-basalt imaging. Conventional MT has been used successfully in Papua New Guinea to image beneath overthrust carbonates. DoIR image courtesy of Paul Cartwright
Page 1: APRIL 2004 PETROLEUM IN WESTERN AUS
Page 4 and 5: 2 PWA April Edition - International
Page 6 and 7: 4 PWA April Edition - Minister’s
Page 8 and 9: 6 PWA April Edition - Director’s
Page 10 and 11: 8 PWA April Edition - 2003 Review s
Page 12 and 13: 10 PWA April Edition - 2003 Review
Page 14 and 15: 12 In 2002, a major remote sensing
Page 16 and 17: 14 Two wildcat wells in WA-286-P (M
Page 18 and 19: 16 bearing. No significant hydrocar
Page 20 and 21: 18 The oil is a good quality 49.2°
Page 22 and 23: 20 PWA April Edition - 2003 Review
Page 24 and 25: 22 PWA April Edition - Resources Br
Page 26 and 27: 24 PWA April Edition - Magnetotellu
Page 30 and 31: 28 PWA April Edition - State Acreag
Page 32: 30 PWA April Edition - State Acreag
Page 35 and 36: the contractors they select have th
Page 37 and 38: y introducing appropriate organisms
Page 39 and 40: International Risk Consultancy It w
Page 41 and 42: Table 1. Reserves as at 31 December
Page 43 and 44: Gingin 0 48,545 3,169 Gipsy 315,750
Page 45 and 46: KEY: Classification 2D: 2D Reflecti
Page 47 and 48: PWA April Edition - Petroleum Wells
Page 49 and 50: Wandoo Petroleum Pty Ltd * Apache N
Page 51 and 52: PETROLEUM (SUBMERGED LANDS) ACT, 19
Page 53 and 54: WA-15-R Active H 11 19 Apr 2006 Tex
Page 55 and 56: * ChevronTexaco Australia Pty Ltd T
Page 57 and 58: Norwest Energy NL Roc Oil (WA) Pty
Page 59: EXECUTIVE Director General Jim Lime

IN WESTERN AUSTRALIA - Department of Mines and Petroleum

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