Seismic Sequence Stratigraphy and Hydrocarbon Potential of the Permo- Triassic Karoo Group Southwestern Ogaden Basin Ethiopia
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Date
2021-03-19
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Addis Ababa University
Abstract
Ogaden Basin is the largest sedimentary basin in Ethiopia covering an area of
approximately 350,000 sq. km and having an estimated sediment thickness of over 7 km
in the deepest part. The Karoo sequence which lies at the base of the sedimentary section
is a Permian - Triassic terrigenous clastics deposited in a tensional to trans-tensional
tectonic setting. The sequence thickness reaches 3000 m and contains proven gas reserve
in excess of 6 TCF. Previous studies of the Karoo sequence focused primarily on the
sedimentology and lithostratigraphy of the section. This study presents the first
integrated sequence stratigraphic analysis of the Karoo section using seismic and well
data and in addition investigates the hydrocarbon potential of the sequence.
The first phase of the work involved seismic data reprocessing. Seismic data quality in the
study area is generally inadequate for sequence stratigraphic analysis. Hence, in the
current study, seismic lines with a total length 296 kms were reprocessed to improve
reflectivity and continuity, especially in the Karoo section. Analysis of shot gathers
showed that proper removal of source related groundroll and amplitude burst noises was
imperative to recover reflections. The trace edit and spike and burst edit tools were used
to effectively remove noise bursts and the groundroll noise was attenuated using linear
Radon transform. Surface consistent amplitude correction, predictive deconvolution and
velocity analysis looped with residual statics are among the other processing steps
applied to the data. Stacking was done using the second pass velocity field which was also
used in post stack migration of lines. The result of reprocessing, compared with the
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previous processing, showed improvements mainly in recovering reflectivity in the Karoo
section as well as in continuity of reflections.
The reprocessed seismic lines, two additional seismic sections and well data were used to
define four second order sequences and at least four higher order sequences in the
uppermost sequence. The transgressive systems tract and the overlying highstand
systems tract separated by a maximum flooding surface constitute the bulk of these
sequences. The dominant depositional systems in systems tracts include shallow and
deep lacustrine, fluvio-deltaic and possibly sublacustrine fans and turbidites. Our study
suggests that the major control in creation of accommodation space is related to wrench
tectonics which gave rise to narrow pull apart sub-basins and their surrounding steep
flanks. These structures are oriented NE-SW to E-W. The study also suggests a number of
potential hydrocarbon plays associated with the identified systems tracts.
At the last phase of the work, 1D and 2D basin and petroleum system modeling was
carried out to analyze the hydrocarbon potential of the Karoo sequence based on well
data and the new sequence stratigraphic interpretation. The modeling process simulates
the basin’s thermal, hydrocarbon generation and accumulation history. The models,
calibrated with Vitrinite reflectance (Ro) and temperature data, show that heat flow
trends are similar throughout the study area with the exception of the western basin
margin area which has consistently higher values. 2D modeling result shows peak
transformation in sequence two, three and bottom of sequence four source rocks was
reached during upper Permian, middle Triassic and middle Jurassic respectively and that
at present all potential source rocks have reached complete transformation. Simulation
results also show potential reservoir targets are dominantly gas charged in the deeper
parts whereas shallower targets are filled with high GOR oil.
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Keywords
Seismic Sequence, Stratigraphy, Hydrocarbon, Potential, Permo Triassic Karoo, Group Southwestern, Ogaden Basin Ethiopia