SOES6002: Modelling in Environmental and Earth System Science
Assignment 1
Semester 2, 2004
Geophysical Modelling
Deadline: Thursday 11th March, 14:00
An international energy company is exploring an area of continental margin where it
hopes to find some substantial hydrocarbon deposits. From analysis of existing
seismic surveys, together with some knowledge of sediment histories and basin
thermal histories, there are some grounds to be optimistic. The company has
identified a geological structure that constitutes a trap – but using conventional
geophysics, there is no way of telling whether the pore spaces in the structure
contain oil, water, or gas, or some combination. The oil company is concerned by the
enormous costs of drilling a well to confirm the hydrocarbon or otherwise. It has had
its fingers burnt a couple of times recently due to bad risks, and is wavering.
The Energy Ministry of the coastal state is keen to see this new area of shelf opened
up and developed. It does not want the oil company to walk away. So the Ministry
persuades the oil company, with the help of a bit of subsidy, to carry out a controlled
source EM study of the promising structure, to determine whether or not there are
any significant hydrocarbons. The consultancy company that you work for has been
commissioned jointly by the energy company and the coastal state to come up with a
preliminary design for the EM survey. The job lands on your desk. Your assignment
is to write a short report, illustrated with some hard numbers and graphs, to assess:
- whether an EM survey can confirm or rule out the presence of
hydrocarbons within the reservoir?
- What the broad design parameters should be for the survey: i.e. what
frequencies will provide useful information, what transmitter orientation(s) should be
used, and what range of source-receiver offsets should be used?
- What key indicators in the data should be used for the assessment of the
results?
The potential reservoir structure is 950 m below the sea bed. The water depth is
1250 m. Typical resistivities in oil-bearing reservoirs of this type are in the range 75
ohm-m to 225 ohm-m. The reservoir layer has a vertical thickness of 85 m. The
sediments above the reservoir are expected to have a resistivity of typically 1.5 ohmm, and those below should have similar properties.
The final complication is that substantial thicknesses (400 m or more) of evaporates
dating from the opening of the ocean basin are known to occur in some locations
along this margin. The evaporites, if present, would lie at about 1400 m below the
sea floor, and would have resistivities in the range 150 to 300 ohm-m. Could the
proposed EM survey distinguish between hydrocarbons in the reservoir layer, and
evaporite below it? If so, how?
To do this, you will need to use the 1-D forward modelling code for seafloor CSEM
sounding, ‘1dcsemnda’; and import the resulting files into excel in order to plot the
graphs. Instructions and practice will be given during the three computer lab classes
on Monday, Tuesday and Wednesday afternoons.
(Young and Cox 1981; Chave and Cox 1982; Sinha, Patel et al. 1990; Sinha,
Constable et al. 1998; MacGregor, Sinha et al. 1999; Sinha 1999; MacGregor, Sinha
et al. 2001; Eidesmo, Ellingsrud et al. 2002; Ellingsrud, Sinha et al. 2002)
References
Chave, A. D. and C. S. Cox (1982). “Controlled Electromagnetic Sources for
Measuring Electrical Conductivity Beneath the Oceans 1. Forward Problem and
Model Study.” Journal of Geophysical Research 87(B7): 5327-5338.
Eidesmo, T., S. Ellingsrud, et al. (2002). “Sea Bed Logging (SBL), a new method for
remote and direct identification of hydrocarbon filled layers in deepwater areas.” First
Break 20: 144-152.
Ellingsrud, S., M. C. Sinha, et al. (2002). “Remote Sensing of hydrocarbon layers by
Sea Bed Logging (SBL): results from a cruise offshore Angola.” The Leading Edge.
MacGregor, L., M. Sinha, et al. (2001). “Electrical Resistivity Structure of the Valu Fa
Ridge, Lau Basin, from marine controlled source electromagnetic sounding.”
Geophys. J. Int. 146: 217-236.
MacGregor, L. M., M. C. Sinha, et al. (1999). Use of marine controlled source
electromagnetic sounding for sub-basalt exploration. EAGE 61st Conference.
Sinha, M. C. (1999). Marine controlled-source EM sounding: practical considerations
for survey design. The LITHOS Annual Scientific Report. S. C. Singh, M. C. Sinha
and P. J. Barton. Cambridge, University of Cambridge: 95-101.
Sinha, M. C., S. C. Constable, et al. (1998). “Magmatic processes at slow spreading
ridges: implications of the RAMESSES experiment at 5745'N on the Mid-Atlantic
Ridge.” Geophys. J. Int. 135: 731-745.
Sinha, M. C., P. D. Patel, et al. (1990). “An active source electromagnetic sounding
system for marine use.” Mar. Geophys. Res. 12: 59-68.
Young, P. D. and C. S. Cox (1981). “Electromagnetic Active Source Sounding Near
the East Pacific Rise.” Geophysical Research Letters 8(10): 1043-1046.