Exploration Insights February 2020 | Page 26

Exploration Handbook | 27
26 | Halliburton Landmark
© 2020 Halliburton
Where are my
depocenters for the
key play elements?
Will the play work?
What are the risks?
Are there any traps?
Are they on the
migration pathway?
What is the likely
STOIIP?
Figure 2> Workflow highlighting the decision gates within the exploration and production lifecycle, where subsurface contour maps are critical inputs.
The critical importance of subsurface contour maps in the decision-making process is highlighted in
Figure 2.
GENERATING A SUBSURFACE CONTOUR MAP
Information on subsurface features is derived in its raw form by conducting subsurface studies or
surveys. This can include gravity surveys, magnetic or electromagnetic surveys, or, more commonly,
seismic imaging. Contour generation from seismic volumes requires a user to interpret or pick an event
or subsurface feature along its full extent, by analyzing subsurface reflections and their amplitudes,
while identifying and interpreting major faults to help establish the structural style. This process is
repeated for each horizon of interest across a survey. From a full set of seismic section interpretations,
values of equal time or depth can be tied together to produce one or more contour maps.
Increasingly, though, contour maps are being supplemented by additional subsurface data, such as
well formation tops, to create secondary contour maps from 3D models or frameworks. These models
provide a digital representation of the subsurface and can comprise several surfaces or maps for
different rock formations. Modeling the subsurface in this manner allows geoscientists to maintain an
evergreen depiction of the unseen structures and geology. In the following section we will discuss how
to generate these secondary contour maps from 3D frameworks.
Map Creation Using Modeling Software
The methodology used to develop a comprehensive subsurface model is reliant upon the integration of
numerous geological and geophysical datasets. While there are several ways to generate a subsurface
map, depending on the software platform being used, the overall methodology is similar. The steps
involved in this process are summarized below.
Project Initiation
Maps are generally created in areas of exploration interest, and the selected time interval or surface to
be mapped is determined by the presence of prospective source rock and reservoir horizons.
Data Collection
Initial data collection involves conducting a focused data search for
the time interval and area of interest. These data may be proprietary
and include pre-processed datasets, or the data may have come
from published and publicly available sources, where the data has to
be carefully extracted from images (with appropriate permissions,
as required). Captured data types include structural cross-sections,
well data, contour maps, and seismic data (Figure 3).
Data Digitization and Interpretation
All of the gathered data need to conform to particular standards
in order for this information to be consumed within any software
platform. Commonly, a geoscientist will first spatially reference the
data, and convert them into a collection of points. Each point will
have a unique longitude (x-coordinate) and latitude (y-coordinate), as
well as a value of depth or time (z-value).
Model Generation
Following import of these standardized data, the geoscientist can
begin building the subsurface model, using the software’s bespoke
modeling engine or application. This process of modeling permits
one or more surfaces, or maps, to be generated simultaneously.
Surface specific data are then sequentially added to the application,
where the user can apply a series of geological rules and algorithms,
as well as defining an appropriate cell size or gridding resolution.
If the raw data are in two-way travel time, but the user wishes to
build a depth model, velocity models can be constructed to perform
the conversion. These velocity models describe the lateral and
vertical changes in seismic velocity; however, to create these, time-
depth data or an estimation of velocities based on the rock type
present is first required.
“Increasingly, contour maps
are being supplemented
by additional subsurface
data, such as well formation
tops, to create secondary
contour maps from 3D
models or frameworks.
These models provide a
digital representation of
the subsurface and can
comprise several surfaces
or maps for different rock
formations.”