TITLE:
Evaluation of Source Rock Potential for Hydrocarbon Generation in Shallow Offshore, Lamu Basin, Kenya
AUTHORS:
Dennis Ombati, Githiri John, Maurice K’Orowe
KEYWORDS:
Geochemical Analysis, Petroleum System Modeling, Rock-Eval Pyrolysis, Kerogen Typing, Vitrinite Reflectance, and Transformation Ratio
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.11 No.5,
May
24,
2023
ABSTRACT: The ever-increasing demand for oil and gas has driven its exploration in
rather extreme conditions. In Lamu offshore, which is hitherto underexplored,
most of the wells already drilled turned out dry save for a few wells with
hydrocarbon shows despite the promising reservoir properties and related
geological structures. This, therefore, necessitated a source rock evaluation
study in the area to ascertain the presence and potential of the source rock by
integrating the geochemical data analysis and petroleum system modeling. The
shallow Lamu offshore source rock quantity, quality, and maturity have been
estimated through the determination of the total organic carbon (TOC) average
values, Kerogen typing, and Rock-Eval pyrolysis measurements respectively. Geochemical data for Kubwa-1, Mbawa-1,
Pomboo-1, and Simba-1 were evaluated for determining the source rock
potential for hydrocarbon generation. Petroleum system modeling was applied in
evaluating geological conditions necessary for a successful charge within a
software that integrated geochemical and petrophysical characterization of the
sedimentary formations in conjunction with boundary conditions that include
basal heat flow, sediment-water interface temperature, and Paleo-water depth.
The average TOC of 0.89 wt % in the study area suggests a fair organic richness
which seems higher in the late cretaceous (0.98 wt %) than in the Paleocene
(0.81 wt %). Vitrinite reflectance and Tmax values in the study area
indicate the possible presence of both mature and immature source rocks. Type
III Kerogen was the most dominant Kerogen type, and gas shows are the most
frequent hydrocarbon encountered in the Lamu Basin with a few cases registering
type II/III and type II. The charge properties (i.e. Temperature,
transformation ratio, and Vitrinite reflectance) over geologic time at each of
the wells have been estimated and their spatial variation mapped as seen from
the burial history and depth curves overlaid with temperature, transformation
ratio, and Vitrinite reflectance respectively. From the upper cretaceous
maturity maps, the results seem to favor near coastal regions where average TOC
is about 1.4 wt %, Vitrinite reflectance is more than 0.5%, transformation
ratio is more than 10%, and temperatures range from 80°C to 160°C. The results postulate the absence of a definitive effective source rock
with a likelihood of having cases of potential and possible source rocks.
Moreover, greater uncertainty rests on the source rock’s
presence and viability tending toward the deep offshore. Geochemical analysis
and petroleum system modeling for hydrocarbon source rock evaluation improved
the understanding of the occurrence of the possible and potential source rocks
and processes necessary for hydrocarbon generation.