The starting point of the study was the need in Danish onshore areas for more knowledge on alternative methods that could be used for hydrocarbon exploration, as an alternative to seismic investigations.
The starting point of the study was the need in Danish onshore areas for more knowledge
on alternative methods that could be used for hydrocarbon exploration, as an alternative to seismic investigations. DONG E&P A/S and Nordsøfonden approached GEUS, suggesting a study of seven different methods. The Danish Energy Agency was interested in the subject and suggested that an actual test of a method be carried out as a part of the project.
Review of selected non-seismic methods for onshore hydrocarbon exploration in Denmark. ALTKUL Project Report Part 1
Magnetotelluric investigation of the Tønder area, Denmark. ALTKUL Project Report Part 2
The seven methods considered are listed below with the most important remarks and conclusions:
Method 1: Surface geochemistry
This is almost an established field in the oil and gas industry, and a number of
techniques - related to actual content of selected hydrocarbons or to the microbial
effect of them - are used for different purposes, including exploration. There are
many methods used to observe hydrocarbons at the surface. Although there are
gaps in our understanding of how they actually migrated to the surface from
deeply buried accumulations of hydrocarbons, there is agreement that this happens
continuously. Often but not always their presence at surface is indicative of
hydrocarbons at depth vertically below the micro seepage anomalies, whereas
there is greater uncertainty in the case of macro seepage. The method has been
used in Denmark, a few examples are described. The methods have a place in
greenfield exploration in the Danish region; care should be taken in the design of
the survey layout and multi-parameter solutions are recommended.
A review of geochemical surveying for hydrocarbons in Denmark 1972-2002 is
enclosed as an appendix.
Method 2: Gravimetric modelling
The key to a proper use of gravity data in exploration is the integration with other
types of geophysical data. Traditionally this has involved joint modeling of gravity
and seismic data. Examples involving integration with electromagnetic and magnetic
data are also reported.
The existing gravity data set from onshore Denmark is of high quality. Surveying
with airborne systems used extensively offshore is not going to improve this data
Method 3: Magnetotellurics (MT, AMT and ZTEM)
The MT method is based on induction of time varying current in the subsurface
from natural sources in the ionosphere, and involves measurements of the electromagnetic
field variations at the surface. The method has undergone significant
improvements with respect to acquisition of high quality data and subsequent 2D
and 3D modelling of the data. Artificial sources may be utilised to improve the
signal to noise ratio for high frequencies.
The advantage of the MT method is the large depth range covered and the simplicity
of the field work, if no artificial sources are involved. The disadvantage of
the method in relation to hydrocarbon exploration is that thin resistive layers such
as hydrocarbon reservoirs or small high resistive structures are transparent. Basically
the MT method is capable of mapping conductive structures, and MT has
therefore been utilised in a search for conductive zones associated with pyritization
above or in the surrounding of a hydrocarbon reservoir. Frequencies in the
audio range (AMT) is utilised in shallow investigations. The ZTEM method is a recent
airborne implementation of the AMT method and builds on an older AFMAG
system that never became widely used. The various hydrocarbon plays in Denmark
are concerned with deep targets and high degree of pyritization is therefore
also expected to be at large depth (>
1 km). A required depth of investigation
deeper than 1 km excludes the audio frequency range. Nevertheless, knowledge
of near surface conductivity variations are valuable in a modelling of MT data.
The dense data coverage obtained with airborne systems (ZTEM or any controlled
source airborne method) may improve the interpretations.
Recently interpretations of induced polarization in MT data have been reported
from field studies in China. The induced polarization is linked to the occurrence of
pyritization associated with hydrocarbon reservoirs. High quality data are required
in order to be able to map induced polarization effects. A case study from China
with known occurrences of gas reservoirs showed a good correlation between interpreted
induced polarization and reservoirs.
Although MT data are unable to map resistive structures in any detail, the method
has been used for structural interpretations of salt structures, where the contrast
in resistivity between salt and surrounding clastic sediments or carbonates is
high. MT has also been used for sub-salt/sub-basalt interpretations.
Method 4: High-Moment Electromagnetics (HMEM)
Controlled source electromagnetic methods have during the last decade received
considerable attention in offshore hydrocarbon exploration. Several case studies
are available that demonstrate the possibility of mapping resistive hydrocarbon
reservoirs. The key to this ability of mapping (thin) resistive structures offshore is
the use of a galvanic source, whereby low frequent alternating currents are injected
into the seabed from high moment electromagnetic (HMEM) dipoles. Onshore
applications of controlled source methods have been reported, but HMEM
methods have not been used extensively. The Long Offset Transient Electromagnetic(LOTEM) system developed at the University of Cologne has been in
use for about three decades and the system was commercialised by KMS Tech8
nologies Inc. This LOTEM system is, however, no longer available for routine
turnkey projects onshore. Another system referred as MTEM (multi transient EM)
was promoted fairly recently for onshore applications, but the development of this
system is now entirely related to offshore work. A focussed source system(FSEM) has been developed recently at the Institute of Innovative Methods of
Geophysics, Moscow, Russia, and the advantage of this system compared to
conventional HMEM system has been described. Land application for FSEM is
mentioned as a possibility, but no onshore case studies are reported. The FSEM
method builds on a long tradition of using electromagnetic methods for hydrocarbon
exploration in Russia and in the Soviet Union. This experience has also inspired
a significant amount of work with onshore electromagnetic methods in
The workload involved in HMEM measurements is fairly high and this has impacted
the applicability of the systems developed. Cultural electromagnetic disturbances
have impacted the applicability of the methods, but a recent application
of HMEM to a CO2 injection test site in the Ketzin area near Potzdam showed that
it is possible to obtain data of high quality in this high-noise area of Germany. The
data may, however, be distorted by coupling to cables and pipelines, if these are
present in a survey area.
The workload involved in HMEM prevents to some extent commercial application
of these methods as a cost-effective exploration tool. De-risking in the evaluation
of hydrocarbon prospects should be considered.
Method 5: High-Powered Spectral Induced Polarization (HPSIP)
Case studies from China with application of high-powered spectral induced polarization(HPSIP) report good correlation between hydrocarbon occurrences and
induced polarization anomalies. The spectral induced polarization (SIP) method,
or alternatively the complex resistivity (CR) method, was developed for mineral
exploration in the early 1950'ies. In particular, the methods respond to occurrences
of pyrite and this is what qualifies the method as a candidate in onshore
hydrocarbon exploration. Several case studies in relation to hydrocarbon exploration
from the United States are reported by Zonge - a major provider of EM instrumentation.
They also provide statistics showing a good correlation between IP
anomalies and occurrences of hydrocarbon. Investigations of IP are also referred
in relation to the focused source EM technique referred in method 4; i.e. mapping
of resistors as well as polarization. The most recent developments in China involve
co-located measurements of spectral/frequency domain induced polarization
and time-domain IP.
The workload involved in HPSIP measurements is similar to HMEM and thereby
fairly high. This has therefore also impacted on the amount of applications reported.
Cultural electromagnetic disturbances have also a negative effect on the
applicability of HPSIP and IP in general.
Method 6: Electron Para-magnetic Resonance (EPR)
Application of electron para-magnetic resonance was included among the evaluated
methods because it was promoted to us as a tool by TST Technology Inc.
We conclude that the claims by TST Technology Inc. are not valid.
Method 7: Airborne Transient Pulse Surveys
The airborne transient pulse survey system advertised by Pinemont Technologies
Inc. is essentially airborne AMT. In contrast to the ZTEM system, this system
measures the time varying electric field. The frequency range is only suitable for
Getting a test of one of the methods based on electromagnetic theory organised caused
some difficulties. An experiment with a galvanic controlled source was considered to be the
optimum choice. However, based on various contacts and failed attempts to organise a
test, a contract was entered into with Uppsala University for some initial tests of the MT
method. The test was carried out in August 2012 and is reported in a report (
ALTKUL Project Report Part 2