الفهرس 
Chapter 2: GEOLOGICAL SETTINGOnly 14 pages are availabe for public view
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Abstract
This study was carried out to evaluate the source rock and to determine the
organic richness of the studied unit, as well as, the effects of structural and stratigraphic
implications on the subsurface inferences, in order to conduct more activities at area.
The area of study is located in East Bahariya area which lies at the northeastern part of
the Western Desert, Egypt. It is bounded by latitudes 29° 45´ - 29° 48´ N and longitudes
29° 24´ -29° 30´ E which belong to Qarun Petroleum Company and represents the
development leases of Yomna and “X” oil fields.
Four wells (Yomna-1X, Yomna-7a, Yomna-22 & Yomna-25) were used in this
work. The data used in the source rock evaluation of ARF Member were mud logs,
geochemical analysis and wire line logs in addition to twenty 3-D seismic sections. The
geochemical results included (TOC wt.%, Rock-Eval pyrolysis, kerogen composition
and vitrinite measurements), and the available wire line logs used in determining the
organic richness of the fore-mentioned rock unit included resistivity logs, porosity logs
(Sonic, Neutron and Density) and gamma-ray logs.
A geological review of the study area including stratigraphy, structure and
tectonic implications that operated in East of Abu Gharadig Basin, northern Western
Desert is exhibited. The stratigraphic succession was defined from the drilled wells in
the area, which exhibited rock units ranging in age from the Jurassic Khattatba
Formation, which unconformably overlain the Pre-Cambrian Basement complex, to the
Miocene Moghra Formation. The constructed isochore maps for the study area revealed
a general thickening in the southwestern part toward “X” oil field, especially in
Khoman Formation due to the growth thickness on the major WNW-ESE fault. ARF
Member showed a widespread correlatable rock unit all over the study area. The
description of the rock composition, from the mud logs and wireline logs, suggested
that, it was formed mainly of limestone with locally some shale streaks and organically
rich with free gases. It represented a maximum flooding surface (MFS) and the
suggested depositional environment was slightly outer shelf marine environment.
The seismic interpretation of the faults and horizons was conducted on twenty
3D seismic sections, and some of these horizons were mapped which included Abu
Roash “G”, “D” Members and Khoman Formation. The interpreted seismic sections
and structure maps showed that, the area was affected by many faults which trending
(WNW-ESE, NW-SE and E-W). Also, they showed a right lateral strike movement in
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the late Cretaceous time where the WNW oriented deep-seated fault was rejuvenated
by this movement which could be due to a regional shear couple in the north Western
Desert.
Detailed source rock analyses were carried out on representative cutting
samples collected from three wells in the study area. The geochemical results revealed
that, the ARF Member is a source rock, with good to very good potential to generate
mainly oil and is considered as a strongly oil-prone source rock (Type I- II Kerogen)
for most of the studied area. Also, the kerogen microscopy investigation showed
predominant of oil-prone sapropelic materials associated with little humic debris
(unstructured lipids 85-100% and vitrinite up to 10%). The top of the oil window
(defined by 0.6% Ro) appeared to be encountered close to the top of the ARF Member
at approximately 8100 feet. The unstructured lipids TAI values appeared to be in
agreement with the Ro% values and suggested a slightly lower maturity than that
derived from the Ro% values.
Moreover, a geochemical analysis was carried out on one gas sample and one
crude oil obtained from Yomna-22 well within the study area which recovered from the
Upper Cretaceous ARE Member at (2662 – 2677 & 2713 - 2717) ft. The oil sample
from Yomna-22 well has 24.1o
API. The whole oil gas chromatograms showed a normal
crude oil with an n-alkane distribution pattern in the range of iC4 to n- C35+. In addition
to, the oil sample suggested to be generated at moderately thermal maturity close to
0.6% Ro and an expulsion temperature of " ~ "125o C. Besides that, the oil was generated
from a predominantly marine organic source rock which deposited under mixed
environmental conditions.
The discussions of geochemical characterization, Biomarker characteristics of oil and
source extract shows oil–source rock correlations as the following:
1) The Eastern trough of the Abu Gharadig Basin is located in the Northern part of
the concession. This sub-basin comprises a Well-developed Jurassic shale section
with excellent source rocks in the Khatatba Formation which deposited in a
continental to inner-middle shelf environments. Several intervals within Khatatba
have oil and gas prone source rocks (Kerogen type’s Ⅱ & Ⅲ) also the late
cretaceous Abu Roash “F”, carbonate on inner-middle neritic environment has
outstanding oil source characterization (Kerogen type’s Ⅱ).
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2) The Development of Yomna field structure on level Abu Roash is thought to be
pre-oil migration that took place after Khoman deposition. This evidenced by the
angular unconformity between the Abu Roash and the overlying Khoman beds,
where the almost flat lying Khoman layers are on-lapping onto the underlying,
relatively steeply dipping, Abu Roash “A” Surface. This in-turn suggests that the
Mubarak structure has been formed after Abu Roash but before Khoman, i.e.
before oil migration.
3) The source rock extracts possess a degree of maturation level similar to that of oil
samples. The extracts of the ARF derived from mixed marine inputs with a limited
terrestrial contribution and Khatatba formations are seen to be derived from mixed
organic sources in which terrestrial dominates marine sources, and deposited in
transitional environments under less anoxic conditions.
4) The time of oil generation from Khatatba sources started most likely during the
middle to late santonian (Abu Roash deposition) or about 90 million years ago.
However expulsion did not commence until 75 million years ago during
Campanian (Base Khoman Deposition) and stopped expelling oil 34 million years
ago during the Miocene. Thus timing of oil migration relative to the structure trap
formation.
5) Generation and expulsion of gas from the Khatatba continue to the present day.
6) The late cretaceous Abu Roash “F”, carbonate on inner-middle neritic
environment has outstanding oil source characterization (Kerogen type’s Ⅱ). Oil
generation and expulsion from the Abu Roash “F” carbonate started during the
Masstrichtian (about 80 million years ago) or 10 million years after the lower Safa
generation and expulsion
7) ARE oil sample are slightly more mature than the extracts which deposited in
transitional environments with mixed organic source input.
8) The gross composition and biomarker analyses of oil and source rock extracts
support the indigenous mixed source of the ARE oil which are related to different
sources including the ARF, Khatatba formations.
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Thermal and geochemical modeling is one of the most widely used techniques
used to predict the validity of an area for future and upcoming oil and gas discoveries.
The integration between the available geochemical data of the sedimentary units led to
the construction of basin models to evaluate the burial and thermal histories of the
sedimentary sequence. In addition, determination of the favorable zones for the
generation of hydrocarbon from ARF source rock was defined based on the deposition
and the thickness of the overburden rocks. Moreover the timing of the hydrocarbon
generation and expulsion by using the 1-D Petro-Mod software was accomplished.
The thermal modeling showed that, the oil window in the study area was started during
the Late Cretaceous at about 30 Ma and the end of hydrocarbon generation not reached
up-till now. The maturity distribution throughout the study area reflected the presence
of immature ARF sediments occupied the northern and southern parts of the map in X1X well area, whereas the analysis is affected by the unstructured lipids Also, the
hydrocarbon transformation of Abu Roash “F” Member X-1X show that the generated
hydrocarbon from the early mature source rock reached about 0.25 Mtons Kg/m2 and
2.73 & 1.68 Mtons for Yomna-1X U.Safa & L.Safa respectively, and the migration
pathways were assumed.
Only if sufficient hydrocarbons are generated in the source rock, primary
migration (i.e. expulsion of hydrocarbon from the source rock) will occur whether
diffusion or pressure-driver mechanisms operate. As might be expected, there is a lag
between the onset of petroleum generation and expulsion because a minimum degree
of petroleum saturation within the source rock is required. However, petroleum can
only migrate from a coarse-grained rock into a fine-grained rock if the capillary
potential is overcome (or if fractures are present). where the sediment sorting is poor
or the mudstones are particularly silty, only a very small capillary barrier may need to
be overcome before petroleum can migrate from coarse-grained into fine-grained rocks.
A small petroleum accumulation may build up in each layer of coarse-grained rock, and
then the seal may be breached, allowing further flow.
The modeled wells together with the depth structure contour map at the top of
ARF Member, and the permeability were used as the input parameters to locate the
possible pathways for hydrocarbon transportation (migration pathways) from the
expulsion area (kitchen) to the possible hydrocarbon accumulation areas which located
at the shallower depths. On the other hand, some of the petroleum system in the study
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area such as reservoir, source rock with its maturation stages and the proposed
migration.
Moreover, by integrating the subsurface information, the results of source rock
evaluation and the basin modeling of the studied area, the present study can stated that,
the ARF Member, Khatatab and AEB formation is rich with organic matter and both
could be considered as an effective source rock for the oil presented in Yomna 7a & 22
wells. Detecting the migration pathway of the hydrocarbon from source areas to
Reservoir in the study areas will open new rooms of prospect new locations of shallow
reservoir.
from the previous discussion it should be clear that petroleum flow is
predominantly vertical in fine-grained, low-permeability mudstones and laterally updip in coarse-grained, higher permeability rocks. The present study confirm the
importance of tectonic evolution of the far field and field scale areas in hydrocarbon
migration, which it should be considered in any forthcoming prospect generation.
It is recommended to test the selected proposed oil accumulations along the
paths of oil migration to enhance the productivity of this field & adjacent fields which
will add more new reservoir prospects. Accordingly we have a good opportunity to drill
more development wells to enhancement the productivity of the study area.