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Abu Gharadig Basin is one of the most prolific basins in the northern part of the Egyptian Western Desert. It attracted the attention of geologists and geophysicists for oil and gas investigation and exploration. This is due to the huge subsurface sedimentary section exists in the basin which include considerable reservoir rocks. The study area; Amana Field is located in the western part of East Bahariya Concession. It is bounded by latitudes 29º 30’ 15” N and 29º 33’ 51” N and by longitudes 29º 23’ 40” E and 29º 29’ 15” E. East Bahariya Concession lies in the northeastern portion of the Western Desert of Egypt in south of the northeast Abu Gharadig Basin and west of Qarun Oil Field. East Bahariya Concession is located between latitudes 29° 20’ N and 29° 40’ N and between longitudes 29° 20’ E and 29° 40’ E. It covers gross surface area of 74,000 acres. It is located southwest of Cairo along the Wahat (Bahariya) Road (around 170 kilometers south west of Cairo).
East Bahariya Concession, in which the study area is located, is valuable for all petroleum companies due to the availability of petroleum system elements. Where, the primary source rock is interpreted to be the Jurassic Khatatba Shales, with possible contribution from Late Cretaceous Abu Roash “F” Carbonates. The main objectives are the
Bahariya and Abu Roash “G” Sandstones. The shale and carbonate sections of Abu Roash Formation and Upper Bahariya Formation, would provide a top and lateral seals for the main objectives.
The main purpose of this study is to evaluate Amana Field by studying subsurface geologic structural features and hydrocarbon trapping of the area using 3D seismic reflection cube through picking and identification of the major structure elements and the horizon packages on seismic sections including interested reservoir markers; Abu Roash “G” Member and Bahariya Formation. Then available well logging analysis is conducted to evaluate the petrophysical parameters of the studied intervals to evaluate the interested formation and reservoir characteristics. It has been carried to determine the volume of shale, effective porosity, lithology identification (sand and carbonate) and fluid saturations (water and hydrocarbon) of Abu Roash “G” Member and Bahariya Formation. The study is subdivided into the following chapters
Chapter one: discussed location of the study area, objectives of the present study, available data for the study, methodology and exploration history of East Bahariya Concession.
Chapter two: debated the geologic setting of the central part of the north Western Desert, including regional view about surface geology, detailed discussion of subsurface stratigraphy and the regional stratigraphic column of the northern Western Desert, structural settings and tectonic framework of the northern Western Desert, where the study area is located, especially Abu Gharadig Basin with particular emphasis to the study area. It based on previously published literatures and the available subsurface geological data.
Chapter three: discussed 3D seismic data interpretation using 3D seismic cube of 58 km2 surface area. 3D seismic data interpretation was carried out using Petrel® 3D Seismic Interpretation Software to transform the physical responses displayed by the seismic lines into geologic information of interest and provide detailed information about the subsurface structural elements of the study area. The 3D seismic interpretation included horizon interpretation process and fault pattern interpretation.
The initial step in seismic data interpretation process is to tie geological horizons to seismic reflectors using synthetic seismogram to transform well data obtained from well logs in depth to their corresponding seismic events in two-way travel time. After seismic to well tie and obtaining the final formation tops, the seismic interpretation started using interpretation of an arbitrary line passing through the available four wells; (Amana_E_1), (Amana_1x), (Farasha_1x) and (Yamama_1x) to validate the time relation between the geological data derived from well logs and geologic horizons derived from seismic data. For horizon interpretation, the strong reflectors of Alamein Formation and Abu Roash “A” Member were interpreted to be used in correlation and fault sticks delineation over Early and Late Cretaceous ages to control the structural interpretation of this area, then the reflectors of Abu Roash “G” Member and Bahariya Formation were picked to make a package correlation for subsurface structure interpretation. Five horizon tops including Khoman Formation, Abu Roash “A” Member, Abu Roash “G” Member, Bahariya Formation and Alamein Formation were picked across the study area over a controlled seismic grid of in- lines and cross-lines and checked by arbitrary lines in all directions,
keeping in mind the overall geologic concept. The seismic markers are chosen carefully to be as near to the well picks as possible.
The fault pattern interpretation has been utilized through the seismic data in order to delineate the subsurface geological structural features affecting in this study area. Conventional and unconventional fault interpretation methods were used for fault interpretation process. Conventional fault interpretation method is based on change in reflector characters as discontinuities in reflection, then it has been followed by the automated seismic interpretation method or seismic coherence attribute. The use of unconventional methods could lead to achieve significant results and reduce the drilling risk.
3D seismic coherence (Variance) attribute has been created in the study area as an automated interpretation to confirm the structural features (fault elements) that appeared through conventional fault interpretation and to detect other new features as well. It is based on the similarity of adjacent seismic traces which can be related to the continuity of geology. Therefore, imaging discontinuities and following faults dissecting the studied formation tops can be easily achieved.
After full horizons and faults interpretation of the 3D seismic cube, maps have been prepared on a base map which shows the locations of seismic lines, studied well locations and the concession boundaries of the study area. The fault polygons have been interpreted for each horizon according to the intersection of the fault stick and horizon, then they have been mapped reflecting the major structural trend in the study area. Three types of seismic maps; TWT structure maps, average velocity maps and depth structure maps have been constructed on the tops of the interested
horizons; Khoman, Abu Roash “A”, Abu Roash “G”, Bahariya and Alamein. Structure maps have been constructed to figure out the general geological settings and the structural features affecting the study area.
The depth structure maps constructed on the tops Khoman, Abu Roash “A”, Abu Roash “G”, Bahariya and Alamein surfaces are similar to time structure maps of these horizons except in a slight variation due to lateral change in velocity. TWT and depth structure maps reveal that the study area is dissected by three major normal faults (F1, F2 and F18) which divided the area into three fault blocks. F18 is died out at F1. The faults trend of the study area is divided into two groups. The first main group trends to the NW-SE direction. The second minor group trends to the E-W direction at Amana structure (it is a higher structural area at the northwestern part of the study field and bounded by wells (Amana_1X and Amana_E_1). The NW-SE trend is the dominant one. The NW-SE trend is dissected by the E-W trend. This means that the NW-SE trend is older than the E-W trend. This distortion and changing of faults trend are likely due to structural compression near the end of Cretaceous. Set of normal faults trending in the NW-SE direction formed horst and graben faults and divided the area into high and low structures. The deeper structural picture is much the same but some of faults that dissected the Upper Cretaceous section (Abu Roash “A”, Abu Roash “G” and Bahariya) do not extend into the Lower Cretaceous and Jurassic sections and died out at the top of the Lower Cretaceous Formations. However, there are some additional deep-seated faults appeared in Alamein Formation. TWT and depth structure maps constructed on top Khoman Formation reveals that Khoman Formation is affected by a smaller number of NW-SE normal faults, which indicates the above-mentioned
faulting structural pattern did not affect Khoman Formation. Hence, Khoman layers are deposited onto the underlying Abu Roash succession after the structural uplift occurred in the area. This in turn suggests that the structure has been formed post Abu Roash succession and pre- Khoman time deposition. The major fault traces in Khoman may be due to the rejuvenation followed to the major faults because of the overweight after Khoman Formation deposition. They reveal three-way dip closures structure traps of NW-SE normal faults that appeared on the tops of Abu Roash “G” Member and Bahariya Formation.
3D structural model in Amana Field has been built and deduced from 3D seismic data along the main potential reservoirs Abu Roash “G”, and Bahariya surfaces and also from the interpreted fault polygons for each surface.
Abu Roash “G” seismic time surface, Bahariya seismic time surface and set of the most effective faults sticks in the study area have been loaded into Petrel® 3D Seismic Interpretation Software (2013) to build the structure model using the corner point gridding technique. In building 3D structure model processes, four steps were carried out as follows: fault modeling, pillar gridding, make horizons and make zones. These processes should always be considered together and undergo several iterations to enhance the final structure model.
The fault model outlines the faults in the structure model. It was built by using fault sticks of the most effective faults and the interpreted fault polygons over the tops of Abu Roash “G” Member and Bahariya Formation to guide the 3D fault modelling process. It was built up converting the fault sticks exported from the interpreted surfaces into key pillars, followed by adjusting the key pillars to the top and base of the
Pillar gridding creates the grid from the fault model. Three skeletons in three dimensions were created as a result of the pillar gridding of the 3D structure model along the top, middle and bottom skeletons. These skeletons are the architecture of the structure model and will be used in building the horizons and zones.
Abu Roash “G” and Bahariya seismic surfaces were utilized to make horizons process to build the 3D structure model on the grid formats. Also, the geological formation tops obtained from the well data have been used as control points at the location of the wells.
The obtained 3D structure model confirms the interpreted structure from seismic data. It shows the regional uplift occurred in the study area and discloses the structural features of NW-SE mapped faults affect the study area.
Chapter four: Figured out the well logging analysis and formation evaluation of the productive zones to determine the petrophysical properties and the reservoirs characteristics for the main reservoir rocks; Abu Roash “G” Member and Bahariya Formation. The well log data analysis has been carried out using Tech Log® Schulmberger Software.
Pre-computation process has to be accomplished before starting computation of the petrophysical properties of each pay zone. The pre- computations include determination of formation temperature (FT), resistivity of water-bearing zones (Rw) in the area of study. All the previous computations must be determined as they affect greatly on the petrophysical properties of the reservoir rock.
Qualitative correlation has been constructed passing through the available wells in the study area to obtain comprehensive information
about the various encountered formations. This correlation shows the wide distribution of Bahariya Formation and Abu Roash “G” Member all over the study area.
Comprehensive quantitative formation evaluation system has been carried out for the available wells in the study area to determine the main petrophysical parameters for the encountered reservoir rocks; Bahariya and Abu Roash “G”. The petrophysical parameters of the reservoir rocks; porosity, shale volume, net pay thickness, water and hydrocarbon saturations have been calculated based on a number of equations and empirical formula. Through the quantitative well log analysis, the shale volume has been calculated firstly using gamma ray logs. GR logs read high values and have little character variation between sandstones and shales due to high content of radioactive minerals. Therefore, the shale volume has been determined using the results of the combination between neutron and density logs for (Amana_1X), (Amana_E_1) and (Yamama_1X) wells and using the results of the combination between sonic and density logs for (Farasha_1X) well. The effect of shale does not depend only in its amount; it also depends on its type (distribution). Therefore, the type of shale has been determined based on Thomas- Stieber plots (neutron porosity-bulk density plots) for Abu Roash “G” Member and Bahariya Formation, in the available wells; (Amana_1X), (Amana_E_1) and (Yamama_1X).
The effective and total porosities have been calculated using the combination of neutron and density logs for (Amana_1X), (Amana_E_1) and (Yamama_1X) wells, while they have been calculated using the density log reading for (Farasha_1X) well. Then the resulted porosity values have been corrected from the effect of shale volume.
The determination of the fluid saturations is very important to complete the deduced petrophysical parameters of reservoir rocks. The estimation of water saturation (Sw) will lead to determination of the volume of hydrocarbons (Sh), where the summation of theses fluids equals 1. Water saturation (Sw) in the uninvaded zone and water saturation in the flushed zone (Sxo) have been calculated using “Indonesia formula”.
Then, the cut-off parameters of porosity, shale volume and water
saturation (ϕeff, Vsh and Sw) have been applied for Abu Roash “G” and Bahariya reservoirs as a result of the quantitative interpretation. These
cut-off parameters have been applied to identify the pay zones through the analyzed units in the study wells. These have been derived from
actual production data and listed as effective porosity cut-off (ϕeff) ˃ 12
%, shale volume cut-off (Vsh) ˂ 30 %, water saturation cut-off (Sw) ˃
55% for Abu Roash “G” reservoir and water saturation cut-off (Sw) ˃ 65
% for Bahariya reservoir.
After applying this cut-off parameters, the pay zones have been identified through the analyzed units in the study wells. The results show interesting intervals within the above- mentioned rock units. Abu Roash “G” Member is further divided into three main units; Upper Abu Roash “G”, Middle Abu Roash “G” and Lower Abu Roash “G”. Upper Abu Roash “G” is non-reservoir, while Middle and Lower Abu Roash “G” are pay zones. Abu Roash “G” Member represents the main productive zones in the four study wells through Middle and Lower Abu Roash “G”, while Bahariya Formation represents non-potential reservoir due to its small value of net pay thickness in only one well in the study area (Yamama_1X).
The reservoir lithology can be determined by various methods through neutron-density cross plot and density-photoelectric factor cross plot in the study wells. Both of neutron-density cross plot and density- photoelectric factor cross plot disclose that Abu Roash “G” reservoir consists of consists of predominance of sandstone with some siltstone and shale. (clean to shaly sandstones) with some data points above the clean sandstone line representing the oil effect and reflect the potentiality of the study area, while Bahariya Formation consists of sandstone laminated with shales. It is non-potential reservoir and mainly has water inside its pores in the study area.
The lateral distribution of hydrocarbon occurrences has been explained through a number of iso-parametric maps. These maps show the effect of some important petrophysical parameters such as the shale volume, the effective porosity and the fluid saturations (hydrocarbon and water saturation) for Middle and Lower Abu Roash “G” and Bahariya reservoirs. These iso-parametric maps complete the picture of hydrocarbon potentiality and delineating the areas for hydrocarbon accumulation. These maps judge the lateral variation of these petrophysical parameters which may be due to the lateral facies heterogeneity, where facies change from place to place without consistent trend or due to the complex structure affecting the study area or both of them. Shale percentage maps, effective porosity maps, water saturation maps, hydrocarbon saturation maps and net pay thickness maps have been created for Middle and Lower Abu Roash “G” and Bahariya reservoirs
Chapter five: involved a detailed study of the petroleum geology and hydrocarbon potentiality of the study area through evaluation of
geology, geophysical studies and all other subsurface data for better understanding to the nature of the productive zones in Amana Field. Evaluation parameters of a prospect which are the representative elements of petroleum system include essentially thickness of the structure, the source type and the reservoir facies, the seals and the entrapment mechanism to form an effective trap.
In the study area, the oil occurrence in the Cretaceous reservoirs is believed to have been sourced from the dark shales of Jurassic Khatatba Formation with some contribution from Late Cretaceous Abu Roash “F” carbonates.
The proven oil-producing reservoirs are sands within the Cretaceous Formations (Abu Roash “G” Member and Bahariya Formation). Abu Roash “G” Member is oil producing reservoir in all the study area wells. Bahariya Formation is oil producing reservoir in (Yamama_1X) well only. The oil potentiality is found in Abu Roash “G” Member due to the smallest value of net pay in Bahariya Formation.
3D structure model zone process creates zones above, below or in between the resulting horizons. 3D structure model zones depend mainly on the petrophysical evaluation of the well logging datasets and well tops from well correlation for the main reservoirs for better estimation to the sub-layering system. The structure model has been divided into four zones. Abu Roash “G” Member has been divided into three zones; Upper Abu Roash “G”, Middle Abu Roash “G” and Lower Abu Roash “G”, while Bahariya Formation is one zone.
Complex structural traps had been created due to the intensive tectonic events and the variable depositional environments during Pre- Cretaceous times. They can be easily recognized by seismic
interpretation and structure maps in the form of three-way dip closures of NW-SE normal faults. Migration ascended from Jurassic faults to fill the Cretaceous traps. Migration occurred vertically or over short distances from the surrounding graben like depocenters towards the adjacent faulted uplift.
The hydrocarbon discoveries in the three-way dip closures proved excellent vertical sealing by a thick shale sequences intercalated with the potential reservoir intervals. Abu Roash Formation massive intervals of limestone and shales would provide good top and lateral seals for both Abu Roash “G” and Bahariya reservoirs. The lateral sealing in the study area is very active especially when the reservoir sand juxtaposes non- permeable beds. Also, it is a proof of the good seal along the fault plane which might be either due to metamorphism nature or shale smearing within the fault gauge.
The seismic investigation discloses the depositional and tectonic history of the sedimentary fill of the basin, allowing recognition and mapping of potential geological plays and their assessment for hydrocarbon exploration. The interpretation of 3D seismic cube, the 3D structure model and the petrophysical evaluation of the well logging datasets for the main reservoirs; Abu Roash “G” Member and Bahariya Formation, have been integrated together to detect the best prospect areas for further exploration and development work.
Three separate geological structural culminations are upgraded to firm up new prospects for drilling. Three prospects (A, B and C) are proposed for future exploration and development activities with respect to their seismic structural highs and suitability of petrophysical parameters in Middle Abu Roash “G”, Lower Abu Roash “G” and
Prospect “A” is a three-way dip closures of NW-SE normal fault. It is located in the northern part of the study area along the major fault F2. Prospect “B” is a three-way dip closures of NW-SE normal fault. Prospect “C” is a three-way dip closures of NW-SE normal fault. It is located in the southern part of the study area along the fault F14.
It can be concluded that the Amana Field is characterized by three- way dip closures structure traps of NW-SE normal faults that constitutes over horst, graben and step-like fault patterns with same trend.
The major set of the NW-SE trending normal faults affect the study area, is believed to be produced as a result of the Early Miocene deformation which rejuvenated the Early Cretaceous NW-SE normal faults. These NW-SE oriented faults were rejuvenated by normal slip and dissected the Lower Miocene Basaltic Flows at the base of Moghra Formation, some of these faults also reach the surface up to Apollonia Formation.
At the end of Cretaceous, some areas were re-uplifted and followed by erosion and a remarkable unconformity between Upper Cretaceous and Lower Tertiary boundary. This illustrates the high structure of the study area.
To sum up certain recommendations for the future upcoming work at East Bahariya Concession, the following points should be taken into consideration:
A firm foundation in the form of a reliable wire-line logs is required to allow the maximum rewards from well logging analysis and formation evaluation of the productive zones to be held and then the economic impact of the well logging analysis could be assessed. The
whole number of the available control wells in the study area is highly recommended to be increased to rise the subsurface control and decrease the uncertainties of the work results.
The results of the present work are suggested to be compared to other results using other methods to determine better inclusive conclusion. This evaluation is anticipated to be guidelines for any future exploration in this area.