significantly smaller compared to the seismic reflection method.
For analysis of basins, petroleum systems, plays
and prospects the individual HC companies have
their own proprietary HC assessment methodology.
The work flows are based on their experience, knowledge, data access, and method performance statistics.
Today, seismic data and well data are the dominating
data type. Here, we discuss the potential that electromagnetic data has for adding new and valuable information to the HC evaluation process. Offshore EM
data has been available for some years, but it takes
time to get enough experience to include a new data
type in the evaluation process.
20.8.1 Basin Scale Analysis
A sedimentary basin can have one or several working
petroleum systems, and a system can contain one or
several petroleum plays. In a working petroleum system, source rock, reservoir rock and overburden rock
must be present. In addition trap formation, hydrocarbon migration and accumulation must occur. Finally,
the timing of events must also be correct.
In the case study from West of Shetland, seismic
imaging of the potential sedimentary strata is hampered by the basalts covering the area. The inverted
EM data shows a conductive unit below the resistive
basaltic complex. This unit is interpreted as a sedimentary basin, and EM data can in this way improve the
HC evaluation at basin scale. Gross depth, shape, size
and electrical properties of the basin can be interpreted
from the EM results.
If source rocks are present in the region, the
interpreted EM data can give a first indication of
potential for maturation and large-scale HC migration
patterns. If information about heat flow and temperature gradients exist, theoretical oil and gas windows
can be suggested for the interpreted basin.
The approximate thickness and shape of the resistive basaltic complex is also mapped. The base of the
basalts is deepest in the centre of the basin and
shallows gradually towards the basin margins. In this
case, if the basaltic layers work as vertical fluid
barriers, potential hydrocarbons could migrate
towards the margins of the basin. The Rosebank discovery is situated in such a position (Fig. 20.28). Here,
HC is found both in traditional sandstone reservoirs
and in reservoirs with volcanic influence. Although the
HC may have migrated from the Shetland Basin east
of the basement high, these are all positive indications
that the basin below the basalts could have a working
petroleum system.
20.8.2 General Prospect Identification
To map out lateral variations in the electrical
properties, 3D CSEM data is needed. Collecting both
inline and broadside data enable measuring vertical
and horizontal resistivity. When several EM
anomalies are identified, from the resistivity measurement alone, it is difficult to know what is causing the
various anomalies, and additional information is
needed. In combination with seismic data, CSEM
data can be used for prospect identification. Seismic
is the superior tool for identification of structural traps,
but to identify stratigraphic traps is a much more
difficult task, and CSEM data can here give crucial
additional information. In such a work flow CSEM is
used as a general prospect mapping tool and not as a
DHI tool. The mapped anomalies can have several
interpretations other than subsurface hydrocarbons.
These can be stratigraphic and lithological variations,
structural topography, intrusives, extrusives, salt,
carbonates or evaporites. When CSEM data is used
in this way for prospect and lead identification within a
play, in the end, the prospect must also be mapped on
3D seismic data to become a drillable prospect.
20.8.3 DHI Analysis
Since CSEM was introduced to HC exploration it has
mainly been used as a direct hydrocarbon indicator.
The method can then potentially influence the prospect
ranking process. When it is used in this way it should
always be used in combination with all available geophysical and geological data. In the cases described
above, except for the West of Shetland case, the
method has been used in this way.
One way to use the results from a CSEM DHI
analysis in the risking process, is for modification of
the original geological chance of success for the
prospect.
542
S.E. Johansen and P.T. Gabrielsen
For analysis of basins, petroleum systems, plays
and prospects the individual HC companies have
their own proprietary HC assessment methodology.
The work flows are based on their experience, knowledge, data access, and method performance statistics.
Today, seismic data and well data are the dominating
data type. Here, we discuss the potential that electromagnetic data has for adding new and valuable information to the HC evaluation process. Offshore EM
data has been available for some years, but it takes
time to get enough experience to include a new data
type in the evaluation process.
20.8.1 Basin Scale Analysis
A sedimentary basin can have one or several working
petroleum systems, and a system can contain one or
several petroleum plays. In a working petroleum system, source rock, reservoir rock and overburden rock
must be present. In addition trap formation, hydrocarbon migration and accumulation must occur. Finally,
the timing of events must also be correct.
In the case study from West of Shetland, seismic
imaging of the potential sedimentary strata is hampered by the basalts covering the area. The inverted
EM data shows a conductive unit below the resistive
basaltic complex. This unit is interpreted as a sedimentary basin, and EM data can in this way improve the
HC evaluation at basin scale. Gross depth, shape, size
and electrical properties of the basin can be interpreted
from the EM results.
If source rocks are present in the region, the
interpreted EM data can give a first indication of
potential for maturation and large-scale HC migration
patterns. If information about heat flow and temperature gradients exist, theoretical oil and gas windows
can be suggested for the interpreted basin.
The approximate thickness and shape of the resistive basaltic complex is also mapped. The base of the
basalts is deepest in the centre of the basin and
shallows gradually towards the basin margins. In this
case, if the basaltic layers work as vertical fluid
barriers, potential hydrocarbons could migrate
towards the margins of the basin. The Rosebank discovery is situated in such a position (Fig. 20.28). Here,
HC is found both in traditional sandstone reservoirs
and in reservoirs with volcanic influence. Although the
HC may have migrated from the Shetland Basin east
of the basement high, these are all positive indications
that the basin below the basalts could have a working
petroleum system.
20.8.2 General Prospect Identification
To map out lateral variations in the electrical
properties, 3D CSEM data is needed. Collecting both
inline and broadside data enable measuring vertical
and horizontal resistivity. When several EM
anomalies are identified, from the resistivity measurement alone, it is difficult to know what is causing the
various anomalies, and additional information is
needed. In combination with seismic data, CSEM
data can be used for prospect identification. Seismic
is the superior tool for identification of structural traps,
but to identify stratigraphic traps is a much more
difficult task, and CSEM data can here give crucial
additional information. In such a work flow CSEM is
used as a general prospect mapping tool and not as a
DHI tool. The mapped anomalies can have several
interpretations other than subsurface hydrocarbons.
These can be stratigraphic and lithological variations,
structural topography, intrusives, extrusives, salt,
carbonates or evaporites. When CSEM data is used
in this way for prospect and lead identification within a
play, in the end, the prospect must also be mapped on
3D seismic data to become a drillable prospect.
20.8.3 DHI Analysis
Since CSEM was introduced to HC exploration it has
mainly been used as a direct hydrocarbon indicator.
The method can then potentially influence the prospect
ranking process. When it is used in this way it should
always be used in combination with all available geophysical and geological data. In the cases described
above, except for the West of Shetland case, the
method has been used in this way.
One way to use the results from a CSEM DHI
analysis in the risking process, is for modification of
the original geological chance of success for the
prospect.
542
S.E. Johansen and P.T. Gabrielsen