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24 PWA April Edition - Magnetotelluric Surveys Peter Kirk, Petroleum Geophysicist Peter Kirk Geophysical Consultancy Pty Ltd Of all the geophysical techniques used in petroleum exploration in Australia over the last 50 years or more, the magnetotelluric or MT method has been used the least frequently and is probably the least familiar method to explorationists. There have, in fact, been five MT surveys conducted in WA, one conventional MT survey and four audio frequency MT (AMT) surveys, all of which have been conducted onshore. Although to date the technique has not resulted in any major discoveries, it has produced some interesting results and in the case of one survey it accurately predicted the results of two unsuccessful wells. It deserves to be used more frequently, particularly in areas where the seismic technique has significant problems (mainly due to shallow limestone) or where seismic acquisition is restricted due to environmental problems. The Magnetotelluric Method Magnetotellurics (MT) is a division of geophysics which studies the earth’s naturally occurring electromagnetic field and the telluric (from the Greek Tellus meaning earth) currents caused by fluctuations therein. Many researchers say that this source is “many orders of magnitude greater than the strengths of fields that can be generated with man-made sources on the surface of the earth”. The primary energy source is naturally occurring electromagnetic waves that are confined to the space between the ionosphere and the earth’s surface, these circumnavigate the globe and are consequently known as ‘spherics’. The frequencies of these waves cover a spectrum from 10 -3 Hz to 10 4 Hz (about 22 octaves). The low to mid frequencies are caused by the interaction of the natural solar wind with the earth’s magnetic field, whilst the higher frequencies are generally Magnetotelluric Surveys for Petroleum Exploration in Western Australia attributed to distant lightning strikes. Nearby lightning strikes, powerful man-made transmitters and highly irregular solar activity due to solar flares all produce levels of unwanted noise that prevent the recording of useful signal and also introduce spurious ‘static’ delays or depth shifts. Constant monitoring of sunspot activity or ‘space weather’ is necessary but this is greatly facilitated by the availability of reliable online data compiled by solar observatories, including the one at Learmonth in WA. Since the bulk of the useful energy is generated by the interaction with the solar wind, it is normally only possible to record during daylight hours. The primary energy source induces telluric currents just below the earth’s surface in large sheets, preferentially through conducting layers such as brine filled sedimentary rocks and certain mineral deposits. These currents flow more slowly through resistive layers such as dense limestone, volcanics such as basalt, tight non-porous rocks and evaporites including salt (although salt in solution is highly conductive, solid salt is highly resistive). The currents can be readily measured as a result of the horizontal potential gradients and the horizontal and vertical magnetic gradients that they produce at the surface. A modern recording instrument normally incorporates two electrical antennae horizontally at right angles and three magnetic coiled antennae horizontally and vertically at right angles. These signals may be recorded separately or the magnetic and electrical signal may be combined and recorded in stereo on digital magnetic tape. A high sampling rate is required for the higher frequency components and this is provided by modern DAT recorders, principally used by the music industry. The depth of penetration of the induced currents within the earth depends upon the frequency of the primary source with lower frequencies necessary to induce currents at greater depths. In order to measure currents kilometres below the surface, we need frequencies with periods of several minutes. Typically the length of each individual recording is 20 minutes for petroleum exploration. The digitally recorded signals, which contain information for all depths, are demodulated by analogue or digital computer to final form for analysis. The recorded signal contains frequency-phase vs. amplitude information relating to the incoming field at the surface, the decaying earth carrier field, and the modulation resulting from the earth’s resistivity reflection coefficients. The former two fields are extracted using least square methods. Only the earth’s resistivity profile remains as a function of frequencies. The depth of investigation is a result of the frequency of the data and the resistivity, and this is approximately described by the well known ‘skin depth equation’ - skin depth (m) = 500 p/f. Simplification of the “skin depth” equation is used to convert the profiles to depth. The result of this process is a series of electric and magnetic reflection coefficients. These are then combined to form the apparent resistivity series defined by Z = E/H as a function of depth. In addition phase values with depth are also obtained. The depths derived from the skin depth equation can then be corrected at a calibration well within the survey area. However, changes in the overburden composition, irregular variations in the earth’s magnetic field and accelerated solar activity can affect the depth accuracy. This can occur from dayto-day or from one survey area to another. These inaccuracies can be reduced by recording at a known calibration point at least every day and sometimes continuously throughout the day. Sophisticated inversion algorithms can also be used to produce 2D and even 3D plots of apparent resistivity using all 5 recorded signals at each
station. However, for conventional MT recording these models generally lack the resolution needed for accurate prospect delineation. In addition to the primary frequencies induced in the telluric currents, higher order harmonic energy is also generated. The audio frequency magnetotelluric (AMT) technique varies from conventional MT by attempting to analyse the higher frequency harmonics of the recognised lower frequency carrier waves that propagate within the earth. As with conventional MT, the final output is a pseudoresistivity curve but with greater vertical resolution. These plots, when compared to wireline data from wellbores, often show a good correlation with a self potential (SP) log. In addition, the phase component within low resistivity (i.e. porous) zones may be further analysed for phase distortions considered typical of hydrocarbon pore saturation. Because the frequencies of the data are in the audio frequency range, the analogue signal can be listened to by an experienced operator in the same manner as used by a trained sonar operator and then categorised as being strongly or weakly typical of water, gas or oil. Computer software to try to perform this task digitally has been written but it is currently not as good. The scientific validity of this technique is not theoretically proven but even where these ‘shows’ are not definitive they are often useful in correlating responses from one station to another. The Canning Basin The first MT survey for petroleum exploration in WA was conducted in the central Canning Basin for Elf Aquitaine in 1968. It was a fairly extensive conventional MT survey, which provided useful information about the basin structure and sediment thickness in the area of the survey. The second survey in the Canning Basin was an AMT survey conducted by Digital Magneto-telluric Technologies (DMT) for Kingsway Resources 2001 to help evaluate the Sally May prospect (formerly known as Cetus) in 2003. This prospect is a large Ordovician sub-salt play with 4-way dip closure previously identified from both a seismic grid and from an aeromagnetic survey. The MT survey comprised approximately 20 recordings over the prospect plus calibration recordings at two offset wells; Looma 1, which discovered oil at the main prospective reservoir levels and Fruitcake 1, which is the closest well to the prospect. The survey produced results that closely matched data from the wells and provided information on the depth structure of the prospect, likely reservoir quality, thickness and fluid fill. Looma 1 encountered oil in six zones, 3 zones in the Nita Formation carbonate section and 3 in the deeper Acacia Formation sandstone. All zones correctly were predicted from analysis of the MT data without detailed prior knowledge. Looma 1 did not flow oil to surface due to very poor permeability in the reservoir sections. Fruitcake 1 was an unsuccessful test of a shallow (post-salt) play type. The crest of the structure identified from two perpendicular lines of MT recordings more closely matched the result predicted from the aeromagnetic survey than that previously interpreted from the seismic survey. The seismic data interpretation was severely affected by velocity variations due to varying salt thickness and also infilled eroded channels at the top of the salt. The MT data predicts a 45 m oil column at the structurally highest point recorded. The Sally May prospect is likely to be drilled in 2004. The Carnarvon Basin Three separate MT surveys were carried out in this basin, mainly focussed on the Rough Range and Giralia anticlines. In 1999-2000, Empire Oil & Gas carried out technical work to appraise the Rough Range oilfield, culminating in the drilling of Rough Range 1B and Central Rough Range 1. The Rough Range anticline was formed by Miocene aged compressional reactivation of the Rough Range fault (a major Jurassic fault). A small oil accumulation exists within the Early Cretaceous Birdrong Sandstone at the crest of the anticline. Since the accumulation is far from being filled to spill, it is likely to have remigrated from a nearby accumulation following the Miocene compression. The anticline is easily observable at the surface that comprises weathered Miocene aged Trealla Limestone. The recent weathering of the Trealla carbonates has resulted in rapid near-surface velocity variations, which badly affect the seismic data recorded over the anticline. PWA April Edition - Magnetotelluric Surveys 25 A Brief History of the Magnetotelluric Method 1939 First documented experiments with MT reported by Schlumberger in France. 1953 Cagniard discovered that the ratio of E/H as a function of frequency could yield a plot of resistivity with depth. This was also discovered independently by Russian researchers. 1960’s MT becomes main technique for evaluating new basins in Europe and North Africa. Extensive use of MT in Siberia results in the discovery of many giant oilfields. 1968 First MT survey in Australia carried out by Elf in the Canning Basin. 1980’s Development of AMT technique. Improvements in instrumentation. Used for prospect evaluation as well as regional studies. Used in many in-house research groups for companies like Shell, Chevron, Amoco, Arco, Sohio, etc. 1990’s First use of MT on seafloor to evaluate sub-salt and sub-basalt plays. Further improvements in instrumentation result in very lightweight portable systems and better sampling accuracy. Constant monitoring of solar activity possible. GPS surveying. Empire commissioned additional work to try to address this problem including the use of pre-stack depth migration (PSDM). Unfortunately, Central Rough Range 1 came in low to prognosis and proved to be on the edge of the field. A post mortem of the well results concluded that the PSDM method could not determine seismic velocities with sufficient accuracy to define the small accumulation present. Following the drilling of Central Rough Range 1, the Rough Range field was now surrounded by unsuccessful wells – Rough Range 4, 5, 6, 10, 11 and Central Rough Range 1. It was now possible to determine a very accurate velocity field using well data alone. This was done and a revised map of the field produced. Immediately following this, the results of a magnetotelluric survey carried out in 1987 were discovered. This survey was carried out for Nomeco who were one of the participants in the Ampolex led joint venture. The MT survey was recorded and analysed following the acquisition of the 1986 seismic survey but prior to the drilling of Rough Range 11. The map of the top Birdrong Sandstone then obtained from the survey matched exactly with that derived from the seismic data corrected with the velocity field derived post Central Rough Range 1 (totally independently). In addition, the MT survey would have accurately predicted the results of Rough Range 11 and Central Rough Range 1 to within 2 m. A close examination of the results of the survey also showed that the top of the Birdrong Sandstone could be picked unambiguously with a high degree of accuracy. In addition, the method allowed prediction of the fluid content of the reservoir and column heights predicted over the field area also proved to be accurate.
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
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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°
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Page 24 and 25: 22 PWA April Edition - Resources Br
Page 28 and 29: 26 PWA April Edition - Magnetotellu
Page 30 and 31: 28 PWA April Edition - State Acreag
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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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