Exploration Insights September 2019 | Page 24

24 | Halliburton Landmark
A
Passive Margin
Basin
Fold and Thrust Belt
Rift
Basin
Intracratonic
Sag Basin
© 2019 Halliburton
© 2019 Halliburton
B
Figure 3 > The tectonic regime has a direct impact on the structural framework of the basin and, consequently, affects
where play elements might be deposited and preserved.
The positioning of the section transect must also be considered. Is the section representative
of the basin as a whole or is it placed to intersect certain features, such as a rift basin that has
syn-rift source rock potential or a structural feature that provides potential trap geometries?
In most cases, it is advisable to cover the deepest stratigraphy (to capture the widest
stratigraphic interval) and focus the section where data are available to validate and constrain
insight.
Structural Framework
At the most basic level, a play cross section summarizes the tectono-stratigraphic evolution
of the study area and helps to define the type of basin (extensional, compressional, or
transtensional) and, therefore, what structural style is expected by the geologist. For example,
a transect through a sag basin is structurally very different from a passive margin or foreland
basin (Figure 3), with implications for the stratigraphic response and prospectivity alike (Gluyas
and Swarbrick, 2004).
It is important to note whether the seismic dataset is represented in two-way travel time
or has been converted into depth. The representation of units in two-way travel time is not
directly proportional to true stratigraphic thickness due to seismic velocity changes. For
example, velocities tend to increase with depth due to compaction, so deeply buried beds
may appear thinner than their true thicknesses. Seismic velocity changes through the section
may also create false geometries, such as the pull up effect seen below salt caused by its
low velocity. If the line has been depth-converted, it is important to consider what model was
applied, in order to test the validity of the depth information provided.
Integration of tectonic element maps, surface geology maps, depth maps, isopach maps,
and 3D depth data also constrains the structural framework. Wells and well tops can be
used, but bear in mind that wells are preferentially drilled on highs and, therefore, may not
be representative of deeper stratigraphy. In frontier basins, where data are sparse, a basic
structural framework can be gained from an understanding of the plate-scale evolution of the
region (Figure 4).
Tectono-stratigraphic phases need to be identified, in order to begin to understand the fill
of any basin. These phases will vary depending on the tectonic regime. In a rift-basin, they
can broadly be grouped as pre-rift, syn-rift, and post-rift elements; and if relevant, any other
main tectonic phases can also be identified, such as inversion. They can be defined from
geometries identified on seismic and in cross sections, and typically have a predictable
stratigraphic response.
Infilling the Stratigraphy
Filling in the stratigraphy of the section is largely accomplished using well and outcrop
data. These provide specific stratigraphic data, such as the depths to particular horizons
Foreland Basin
Foreland
Basin
Exploration Handbook | 25
© 2019 Halliburton
Continental sediments Fine-grained deep-
water siliciclastics (marine) Sabkha
evaporites Coarse-grained shallow-
water siliciclastics Fine-grained siliciclastics
(lacustrine) Salina/saltern
evaporites Volcanics
Coarse-grained deep-
water siliciclastics Shallow-water
carbonates Deep-water
evaporites Basement
Fine-grained shallow-
water siliciclastics (marine) Deep-water
carbonates Biosiliceous
ooze/chert Oceanic crust
Organic-rich
sediments
Figure 4 > Different tectonic evolutions produce different basin types, in terms of their
structure and stratigraphic fill. The difference is clear, looking at: A) a fold and thrust
belt, where thick-skinned and thin-skinned thrust packages affect the distribution of
stratigraphy; compared with B) a tectonically simple foreland basin.
or play elements, thicknesses of units, and lithofacies. Data to
infill the remaining tectono-stratigraphic packages are derived
from chronostratigraphic charts, GDE maps, and stratigraphic
information in text and facies maps. The principles of sequence
stratigraphy, (described in the April edition of the Exploration
Handbook), can be employed to make predictions of the likely
stratigraphic response to base level change away from areas of
data constraint. The Neftex Insights portfolio includes a proprietary
sequence stratigraphic model that allows predictions to be made
at a high degree of temporal resolution.
In the absence of well data, we can use our understanding of the
tectonic regimes to give insight into the stratigraphic fill associated
with each tectonic phase. Certain tectonic regimes give rise to
predictable stratigraphic responses that result in the deposition of
certain petroleum system elements. For example, syn-rift tectonics
and facies vary from those deposited once a post-rift tectonic
regime is established. Syn-rift facies are often characterized at
their commencement by continental and lacustrine sediments.
If rifting continues and a passive margin is established, shallow-
marine facies will be deposited (Figure 5). The Neftex Sequence
Stratigraphic Model can be overlain on top of the major tectono-
stratigraphic phases to give a higher degree of temporal resolution
when considering different events.
Global and regional events can also be used to influence the
interpreted stratigraphy. For example, could oceanic anoxic events
have resulted in organic enrichment in the area, and when were
“At the most basic level,
a play cross section
summarizes the tectono-
stratigraphic evolution of
the study area and helps to
define the type of basin”