The term "petroleum" originates from the combination of two medieval Latin words petra meaning "rock" and oleum meaning "oil". The search for petroleum or hydrocarbon pre-dates modern civilisation predicated by ancient Sumeria. The Sumerians would use a sticky black liquid called bitumen or asphalt to attach ﬂint arrowheads to shafts for hunting. This liquid would become the ﬁrst petroleum product ever used by the human race(Bilkadi,1984). Uses of hydrocarbon were limited up until the 1850’s when the distillation of kerosene from artiﬁcially produced petroleum resulted in cheaper and superior method to illuminate candles and lamps and eventually replaced whale and rosin oil (Fleming, 1967). The birth of the gasoline powered combustion engines in the 1880’s heralded the start of the petroleum revolution, a world economy based on a cheap high density energy source which brought with it prosperity and an insatiable desire for petroleum products.
With the controversial peak oil crisis looming and the number of world class discoveries decreasing, the demand for oil and gas never been so great (Hall and Day, 2009)... Thank you shale oil and numerous geopolitical factors. With large, easy to produce ﬁelds diminishing, exploration companies have to remain on the technological cutting edge to discover new oil and gas ﬁelds both in on-land and deeper marine environments. The increased drilling costs and exploration risk has brought new challenges for detecting hydrocarbon in deep water settings. Transition into deep water settings has forced geoscientists to be more certain of the presence and position of hydrocarbon especially in deep water settings where the costs to drill exploratory wells can reach in excess of 100 million dollars (Kulkarni, 2005). Since the 1920’s seismic methods have traditionally detected acoustic properties to reduce exploration risk, however with increasing costs, new geophysical methods must be employed to detect density, magnetic and electrical rock properties to be certain of future discoveries (Coraggio et al., 2012; Yang et al., 2011 and Jain et al., 2008). The 21st century obviously brings with it unique challenges, but with new energy technologies, improved hydrocarbon remote sensing techniques and unconventional hydrocarbon sources the threat of peak oil can be overcome.
Electromagnetic (EM) methods can be divided into active and passive categories. There has been a transition from passive source EM such as the magneto-telluric (MT) method towards active source EM such as the controlled source electromagnetic (CSEM) method.
The MCSEM method goes by many other names. Some of these are CSEM imaging (CSEMi) and sea bed logging (SBL). MCSEM detects electrical resistivity contrasts (Kong, 2002). Marine CSEM (i.e., MCSEM or mCSEM) often targets thin electrically resistive hydrocarbon reservoirs in conductive surroundings (e.g., ocean and sub-ocean sediments).
MCSEM was originally developed for deep water studies of oceanic lithosphere in the 20th century (Cox, 1981). The method expanded into hydrocarbon exploration, beginning with the ﬁrst survey carried out on an oilﬁeld in oﬀshore Angola (Ellingsrud et al., 2002). The MCSEM method has been advanced by the eﬀorts of university and industry researchers such as Scripps Institution of Oceanography, The University of Toronto, Cambridge and Southampton University over the past two decades. MCSEM and MT have been linked in their development (Constable, 2010). Early MCSEM research combined both natural and active EM sources to image resistivity variations beneath the sea ﬂoor (Constable, 2010), while MT identiﬁes resistivity variations on large scale structures at basin level. Most modern MCSEM are performed to ascertain nearsurface shape resistivity variation, in particular the detection of electrically thin resistive bodies, although they are still useful for deeper oceanographic studies (Peace, 2005).
Seismic methods are an industry standard hydrocarbon exploration technique. Combinations of seismic and non-seismic methods have been utiltized to reduce exploration risk in recent times. Methods such as gravity, magnetism, well logging and more recently MCSEM have been incorporated into hydrocarbon exploration (i.e., Coraggio et al., 2012; Yang et al., 2011 and Jain et al., 2008).
MCSEM is useful as a supplementary non-seismic method as it detects resistivity rather than acoustic properties. MacGregor (2006) demonstrates that a strong relationship between the MCSEM electromagnetic response and fluid saturation percentage exists. Phillips (2007) states hydrocarbon reservoir saturation can be characterised ahead of drilling. Hydrocarbon saturation can be characterised because the electromagnetic field amplitude increases proportionally with hydrocarbon saturation. MacGregor (2006) highlights that amplitude and velocity analysis can only accurately detect hydrocarbon saturation between 0 and 10% (see Figure 1). MCSEM otherwise has the ability to detect hydrocarbon saturation due its resistivity variation. Typical MCSEM targets include petroleum, natural gas and gas hydrates.
Targets which are thin, or underneath thick tabular salt/basalt targets, or deep and with low resistivity comparable to the surrounding geology (see Table 1) will limit MCSEM’s effectiveness. MCSEM is useful in deep water turbidites and deltas, over stacked reservoir sequences or where there are large resistivity contrasts such as in shallow gas hydrates (MacGregor, 2006). Geological environments such as reservoirs at the edges of salt diapirs and in carbonates can be explored by the MCSEM method. MCSEM surveys have become an integral part of deep ocean petroleum exploration and appraisal (Gribenko and Zhdanov, 2007) and future use for monitoring purposes may be possible as investigated by Noel et al. (2010), Noel et al. (2011) and Liang et al. (2012).
|Works Well||Deep water turbidites
Deep water deltas
Stacked reservoir sequences
Under shallow gas or gas hydrates
|Feasible||Flanks of salt diapirs
Shallow water areas
|Needs Research||Beneath thick tabular salt/basalt
Thin deep, low resistivity targets)
Table 1: The practicality of the MCSEM method in different geological environments. MCSEM works in a variety of geological environments however it may not work in some geological settings.
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