ORIGINAL PAPER

Fracture reopening by micro-earthquakes, a mechanism for oilseepage in mildly active rifts: a case study from Gemsaoilfield, the southern Gulf of Suez rift, Egypt

Shawky Sakran1& Muhammad Nabih2

& Ahmed Henaish2& Abdelmohsen Ziko2

Received: 19 September 2015 /Accepted: 9 March 2016# Saudi Society for Geosciences 2016

Abstract Episodic hydrocarbon seepage is present at Gemsaoilfield, southern Gulf of Suez rift, Egypt. The occurrence ofthe oil at this area has been known for thousands of years. Thenature and origin of this seepage is the main target of thepresent study. Surface geological mapping substantiated bysubsurface mapping using 2-D seismic lines and well log dataare integrated. Moreover, the present day stress orientationand magnitudes are integrated with the mechanical propertiesof the reservoir rock in order to produce a coherent interpre-tation for the migration pathways and the main causes of thehydrocarbon seepage. The study revealed that the episodichydrocarbon seepage at mildly active rifts such as the southernSuez rift can be resulted from the migration of entrapped hy-drocarbons along the damage zone of slightly active faultswith continuous micro-earthquakes. The pressure releasedfrom the micro-earthquakes causes the reopening ofpreexisting fractures where hydrocarbon found their easypathway to the surface through highly fractured rocks andunconsolidated sediments. The produced migration pathwaymap proposes the possibility of an unexplored HammamFaraun Member reservoir at the study area. The original oilin place of the predicted reservoir is estimated to be more than22 MMBO which encourages the design makers for moreinvestigation of this reservoir to increase its certainty andputting it in the plan of the future investments.

Keywords Gemsa . Oil seepage . Hydrocarbonmigration .

Gulf of Suez rift . Fault valve . HammamFaraunMember .

Micro-earthquakes . Seismicity

Introduction

Natural oil seepage was the incentive for exploration drillingby the pioneers of the petroleum industry (Judd and Hovland2007). Oil seepage can, in selected geological settings, delin-eate subsurface petroleum accumulations and provide infor-mation on hydrocarbon charge type or oil quality. The study ofnatural oil seepage has proven to be a valuable aspect in pe-troleum prospectivity assessment and exploration. Now, re-searchers have established many facts about hydrocarbonseeps according to many case studies all over the world.Seeps rarely occur in tectonically inactive basins; seeps maybe an early warning for earthquakes; hydrocarbons cannotmove upwards in cap rock because of capillary resistance,there has to be an open fault, or a fracture, through whichthe hydrocarbons can move (Sibson 2000; Elkhoury et al.2006; Doglioni et al. 2014).

Since the beginning of the last century, the Gulf of Suez rifthas been a highly prospective hydrocarbon location and thefocus of much oil exploration. In the southern Gulf of Suezrift, two large seeps are found. The first lies at the southern endof Gebel El Zeit area, from which its name means (Mountainof oil). The second oil seepage lies at Gemsa oilfield (Figs. 1and 2), being the purpose of the present study. This seepagebecame more interesting when a company called BSocieteSoufricre de Mines de Jemsah et de Ranga^ found oil withwater during drilling for sulfur in 1860. Drilling for oil startedin November 1885, with the spudding of de Bay No. 1, using asteam-driven rig where oil was found at depth 33 m inFebruary 1886. Another two wells were drilled in the same

* Ahmed Henaishahmed_henaish@yahoo.com

1 Geology Department, Faculty of Science, Cairo University,Giza 12613, Egypt

2 Geology Department, Faculty of Science, Zagazig University,Zagazig, Sharkia 44519, Egypt

Arab J Geosci (2016) 9:404 DOI 10.1007/s12517-016-2433-7

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year. Well 2 found minor oil shows, while well 3 had no oil.Between 1886 and 1887, five more wells were drilled deeperthan the other wells; all these shallow wells encountered oil oroil shows. In 1908, The Egyptian Oil Trust began the devel-opment of the Gemsa oilfield which was discovered for thefirst time more than 20 years before (EGPC 1996). About80 % of the total crude production of Gemsa was obtainedfrom Hammam Faraun limestone. The annual production ratereached its maximum in 1914, where a rapid decline occurredtill it currently reached five barrels per day. The cumulativeproduction of the field reached approximately 1.5 MMBO(EGPC 1996). The present research provides an integratedgeological and geophysical study to afford a reasonable originfor the onshore active oil seep at the Gemsa area. The hydro-carbon potentiality of the field is discussed in the light of theresults reached.

Methodology

A systematic study was done by correlating the onshoreseeps with the geological characterization and faultingstyle in the subsurface. The study started with detailedfield investigation of the exposed rocks at the Gemsa area,with emphasis on the exposed damage zone of the seepagefault. This is followed by subsurface mapping of the fieldusing the available 2-D seismic lines and well data, con-structing of the structural model of the field, studying theseismicity of the area and surrounding, determining thepresent-day stress orientation and magnitudes, estimatingthe rock strength of the reservoir, and finally proposing amodel for the mechanism of oil seepage in the area.

Stratigraphy

The stratigraphy of the Gulf of Suez has been discussedby many workers. According to Said (1962 and 1990)and Darwish and El Azabi (1993), the stratigraphic suc-cession of the Gulf of Suez rift is generally character-ized by three main tectono-stratigraphic sequences rela-tive to the Miocene rifting events, as pre-rift sequence(including the Precambrian basement rocks and a sedi-mentary succession up to the Oligocene), syn-rift se-quence (Early-Middle Miocene successions), and post-rift sequence (Late Miocene to Recent successions).The first and second sequences include important hydro-carbon source and reservoir rocks while the third depo-sitional sequence is important because of its evaporiticseal (Fig. 3). The major sedimentary successions accu-mulated under different structural settings on thePrecambrian Basement Complex with distinct inter-and intraformational unconformities and hiatuses of dif-ferent magnitudes.

The stratigraphic setting of the Gemsa oil field issummarized in a composite section compiled fromGemsa-7, Gemsa-14, Gemsa-24, Gemsa-23, Gemsa-8,Gemsa-19, Gemsa-25, and G.O.R-1 wells (Fig. 4). Therock sequence penetrated in the Gemsa field consists ofa sedimentary cover comprising Miocene and Pliocene-Recent deposits , rest ing unconformably on thePrecambrian basement rocks where the Paleozoic andMesozoic sequences common in the Gulf of Suez riftare missing. The basement rocks are covered by acourse to medium-grained sandstone body (containingsome granite fragments) that varies in thickness from afew meters to about 50 m (EGPC 1996). The Miocenesediments overlying the basal sandstone could be differ-entiated into the Middle Miocene Gemsa Formationwith its lower limestone part attributed to HammamFaraun Member. This member is composed of hard,

�Fig. 1 Location map of the studied area and surface projections of majorfaults in the southern Gulf of Suez, (modified after Bosworth 1995). Theinset shows the tectonic setting, major rift blocks, and fault trends of theGulf of Suez rift, (after Moustafa 2002)

Fig. 2 Field photograph of the oilseepage at the Gemsa area

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porous, often cavernous, partly dolomitic limestone. Itranges in thickness from 90 to 200 m. A thick sectionof salt and anhydrite of the Late Miocene South Gharib

Formation overlies the Gemsa Formation and is overlainby the anhydrite and shale beds of the Zeit Formationoverlain in turn by the Pliocene-Recent sediments.

Fig. 3 Simplified stratigraphic section of the southern Gulf of Suez, modified after Schlumberger (1984 and 1995) and EGPC (1996)

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Fig. 4 Simplified key well of theGemsa oilfield

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Regional structural setting

Gemsa oilfield lays at the southernmost part of the Gulf ofSuez rift which is the northern segment of a Tertiary rift sys-tem that extends from northern Egypt through the Red Sea tothe Gulf of Aden. The Gulf of Suez rift is more than 300 kmlong, currently inactive Oligo-Miocene intra-continental riftbetween the Sinai microplate and the African plate(Garfunkel and Bartov 1977). It is divided into three halfgrabens separated by two major transfer zones, theZaafarana and Morgan transfer zones. The dip direction ofthe strata changes from SW to NE in the northern and centralhalf grabens and back to SW in the southern half graben(Moustafa 1976) (Fig. 1).

The regional structural architecture of the study area iscontrolled by five main fault systems, namely Gebel El ZeitBoundary fault (GZBF), Little Zeit Boundary Fault(LZBF), Ras El Ush Boundary Fault (RUBF), Ranim-Tawila Fault System (RTFS), and Gemsa BoundaryFault (GMBF), (Fig. 1).

The GZBF is represented by an eroded fault-scarp separat-ing the basement rocks of Gebel El Zeit from the offshore EastZeit basin and extends from Ras Dib area to the northern partof Sarg El Zeit where the NW-SE trending Little ZeitBoundary Fault (LZBF) lies to the west of the GZBF andextends to the southern end of the Little Zeit region. Theoffshore area of Gebel El Zeit which extends from the south-ern end of the Gebel El Zeit basement rocks to east of RanimIsland is called Ras El Ush trend which is delimited at itseastern border by the NNW-SSE trending Ras El UshBoundary Fault (RUBF). Gebel El Zeit continues in the sub-surface as a high block at Ranim Island and is most closelyrelated further south to the basement high west of Geisum andTawila Islands. The fault trend which extends from the eastRanim Island to the west of Geisum and Tawila Islands iscalled Ranim-Tawila Fault System (RTFS). To the west, theGemsa Boundary Fault (GMBF) extends from Ras El Bahararea to the eastern Border of Ras Gemsa (Fig. 1).

Five transfer zones are present in the study area andcharacterize the relation between the previous fault sys-tems. The first transfer zone (TZ-1) lies between thetwo segments of the GZBF where they have the samepolarity and have a left-stepped arrangement. The twosegments are linked through a WNW-ESE transfer faultforming a zigzag pattern. The second transfer zone (TZ-2) lies between the GZBF and the LZBF. The GZBFand the RUBF have the same polarity and are over-lapped forming the third transfer zone (TZ-3). TheLZBF and RTFS are linked through the NE-SWtrending LZ4 transfer fault which represents the fourthtransfer zone (TZ-4). The fifth transfer zone (TZ-5) liesbetween the RTFS and GMBF where they are linkedthrough a NNE-SSW trending transfer fault (Fig. 1).

Structural geology of Gemsa fault block

The Gemsa fault block is divided into Ras Gemsa and LittleGemsa (Fig. 5). The oil seepage appears at the eastern coast ofRas Gemsa. The outcrops at Ras Gemsa form two parallelNW-trending ridges with maximum elevation of 50 m. Thesurface geological data measurements indicate that dip of thewestern ridge is generally towards the southwest, ranging be-tween 15° and 30° in the Gemsa Formation. The eastern ridgeis dipping at about 15° towards the NE. The two ridges form aNW-oriented anticlinal structure. This anticline represents afault propagation fold related to the main coastal fault. Theoutcrop of Little Gemsa also forms a NW-trending ridge withmaximum elevation of 40 m. The exposed rocks in this areaare mainly represented by the syn-rift Gemsa Formationcapped by the Middle to Late Miocene evaporites andPliocene-Recent sediments. The limestone of the HammamFaraun Member is highly fractured (Fig. 6) and is character-ized by the presence of several caves and damage zones re-sulted from man-made activities due to using of explosives insulfur mines at the area (Fig. 7).

The rose diagram of 112measured fractures along the dam-age zone of the NE dipping coastal fault (Fig. 8) indicated thatthe fractures are mainly oriented NW-SE followed by NE- andE-E-oriented fractures. The fractures occur as oppositely dip-ping sets of conjugate fractures of mode II shear fractures.These fractures are dipping at about 60° towards NE andSW. A set of early vertical mode I extension fractures are alsorecognized. This fracture system represents the main cause ofthe high transmissibility of this fault. Impregnated bitumenoccurs at the surface which indicates the prolonged historyof the hydrocarbon seepage in the area.

Subsurface mapping was accomplished using a combina-tion of 2-D seismic lines and well log data. The seismic data ofthe Gemsa field are poor and contain seismic holes (no datacoverage) which make structural modeling of the field a chal-lenging task. To overcome the problem of poor-quality seis-mic data, all the available geological and geophysical datawere integrated, including formation tops, missing sections,regional structures, and analog models of the rift structuresand the outcrop structures of the western side of the Gulf ofSuez rift. These were used in addition to a series of problemsolving cross sections to build up a structural model from thewell data and seismic interpretation.

Three structural cross sections (1:1 scale) are constructedby integrating surface and subsurface data (Fig. 9). The seis-mic interpretation is performed in the time domain and depth-converted by using time-depth relationship from the nearbyoil fields. The details of seismic interpretation are out of scopeof the present paper. Three structural depth maps for topPrecambrian basement rocks (Fig. 10), base HammamFaraun Member (Fig. 11), and top Hammam FaraunMember (Fig. 12) are created.

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The structural model revealed that Ras Gemsa forms aNW-trending asymmetrical anticlinal fold resulted from drag-ging along the footwalls of the GF-3 and GF-4 NW-strikingnormal faults. The southwestern limb has a dip ranging from15° to 25° towards the SW, and its northeastern limb has a dipof approximately 25° towards the NE.

The Ras Gemsa Fault block is dissected by five clysmicfaults (GMBF, GF-2, GF-3, GF-4, and GF-5) and two crossfaults (GF-1 and GF-6). It is divided into five tilted faultblocks, namely FB-1, FB-2, FB-3, FB-4, and FB-5. The

Hammam Faraun Member of the FB-2 and FB-3 tilted faultblocks represent the main producing reservoirs of the GeneralPetroleum Company (GPC) at the Ras Gemsa fault block.

Seismicity of the area

The southern part of the Gulf of Suez is one of the mostseismically active areas in Egypt. The distribution of historicaland recent earthquakes in the Gulf of Suez showed that there is

Fig. 5 Geological map of the Gemsa area shows the locations of the oil seepage, 2-D seismic lines, wells, and the constructed cross sections of theGemsa oilfield

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a concentration of the activity along the southern end of theGulf of Suez near the junction with the Northern Red Sea(Maamoun et al. 1984; Kulhanek et al. 1992). The higher rateof activity in the southern end of the Gulf of Suez is mainlyrelated to the adjustments in motion at the triple junction be-tween the African plate, Arabian plate, and Sinai microplate(Ben-Menahem et al. 1976; Daggett et al. 1986).

In the present study, a homogeneous catalog for therecent seismic activity on the southern Gulf of Suez forthe period from the 1904 to 2013 was compiled from theNational Earthquake Information Center (NEIC), theInternational Seismological Center (ISC), and publisheddata of Badawy (2001), Badawy and Abdel-Fattah(2006), and Hussein et al. (2006) and General PetroleumCompany BGPC,^ internal report (2009) (Figs. 13 and 14).According to the analysis of the distribution of events, itwas noted that the study area was affected by a number ofearthquakes that have occurred with magnitudes rangingfrom 1 to 6.8 Mb.

The study of the magnitudes and focal depths of theseismological data from 2000 to 2011 for 1265 earth-quakes in the southern Gulf of Suez showed that 93 %

of the earthquakes range from 1 to 2 M (Fig. 15) andthe focal depths range from 0.5 to 35 km (Fig. 16); inother words, these earthquakes can be classified asmicro- to minor-shallow earthquakes.

Orientation and magnitude of present-day stresses

The fault plane solution (Fig. 14) indicated that the present-day stresses in the southern Gulf of Suez is extensional wherethe maximum stress is vertical, the maximum horizontal stressis oriented NW-SE, and the minimum horizontal stress is ori-ented NE-SW.

Mathematical parameters and equations have been used toconstrain the magnitude of the vertical stress and the mini-mum horizontal stress using the data extracted from theGemsa oilfield. Stress is calculated at a depth of 500 m whichis the depth of theMiocene fractured carbonate reservoir in thefield. The vertical stress (σv) is calculated to be approximately12.75 MPa using the following equation (Fjaer et al. 2008):

σv ¼ ρgh ð1Þ

Fig. 6 A N 20° E striking, 60°WNW dipping fracture affectingthe limestone of the GemsaFormation at the Gemsa area,looking towards SW

Fig. 7 Caves resulted from man-made activities in sulfur mines at the Gemsa area, looking towards the NNE

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where

ρ Overburden densityg Gravityh Depth to point

The effective vertical stress (σ’v) is calculated using thefollowing equation (Fjaer et al. 2008):

σ’v ¼ σv–Pore Pressure ð2Þ

The pore pressure is about 1.1 MPa as estimated bycorrecting the head pressure to the 500 m depth before thetime of a strong oil seepage episode in 2009 (MoustafaTaher BGPC^, personal communication 2013); hence, the ef-fective vertical stress is 11.65 MPa.

The effective minimum horizontal stress is calculated usingthe following equation (Fjaer et al. 2008):

σ’h ¼ υ=1−υð Þ σ’v ð3Þwhere

υ Poisson’s ratio

This equation is used assuming linear elasticity, zero lateralmovement, and that the elastic properties of the rock havebeen constant throughout the whole process of rock forma-tion. The Poisson’s ratio was considered to be 0.11 for thehighly fractured and cavernous limestones of the HammamFaraun Member reservoir of the Gemsa oilfield. The effectiveminimum horizontal stress is estimated to be 1.4 MPa.

Mechanism of oil seepage and migration pathways

The migration pathway model for Gemsa hydrocarbon seep-age is reached throw integrating the following observations:

& Oil discharges of the Gemsa area are episodic.& The pore pressure increases during an earthquake event as

in 2009 where it reaches 2.75 MPa (Moustafa TaherBGPC^, personal communication 2013) where large quan-tities of oil were discharged.

& The Gemsa oilfield experienced rapid decline in produc-tion since 1914 till it reaches five barrels per day in thepresent day. This gives the question about the source of thelarge quantities of oil that discharge episodically.

The resultant migration pathwaymap assumes the possibil-ity of oil migration from an undiscovered Hammam FaraunMemeber reservoir of the FB-4 fault block, as it is the nearestprobable reservoir to the oil seepage location.

This was assured by calculation of the normal stress (σN)and shear stress (σS) in order to calculate the slip tendency ofthe GF-2 fault. Slip tendency has been used successfully tocharacterize fault slip (Streit and Hillis 2004) and fault slipdirections (Lisle and Srivastava 2004; Collettini andTrippetta 2007). Slip tendency analysis is based on the pre-mise that the resolved shear and normal stresses on a surfaceare strong predictors of both the likelihood and direction ofslip on that surface (Morris et al. 1996). Formally, the sliptendency (Ts) of a surface is defined as the ratio of shear stress(σS) to normal stress (σN) on that surface (Morris et al. 1996):

Ts ¼ σS=σN ð4Þ

The form of the stress equations are written as follows:

σN ¼ σ1 þ σ3ð Þ=2½ � þ σ1−σ3ð Þ=2½ �cos2θ ð5Þ

σS ¼ σ1−σ3ð Þ=2½ �sin2θ ð6Þ

where

σ1 The greatest principal stress axisσ3 The minimum principal stress axisθ Plane orientation with respect to σ1

In illustrating the use of these equations, stress was calcu-lated at depth of 750 m (the proposed oil-water contact) on theGF-2 fault. The greatest principal stress (σ1 ≈ σ’v) is calculatedto be approximately 16.25 MPa using Equations 1 and 2. Theminimum principal stress (σ3 ≈ σ’h) is calculated to be approx-imately 2 MPa using Eq. 3. Using Eqs. 5 and 6, σN is calcu-lated to be 12.68 MPa and σS is calculated to be 6.17 MPawhere θ = +30°. Using Eq. 4, slip tendency (Ts) is calcu-lated to be 0.48, which is a low slip tendency value(Morris et al. 1996).

The proposed model suggests oil leakage through thereopening of fractures associated with GF-2 fault zone.The resulted value of slip tendency suggests that fault

Fig. 8 Rose diagram of the measured fractures affecting the limestonesof the Gemsa Formation at the Gemsa fault block

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reactivation will not take place during seismic activity;instead, the reopening of preexisting fractures that ac-companies seismic events is most favorable where thepore pressure is greater than the minimum horizontal

stress, and this creates an open space within the GF-2fault zone for oil to migrate. Because of the increase ofopen space, fluid pressure in the fault zone temporarilydrops relative to the surrounding rock. The resulting

Fig. 9 a An ENE-WSW crosssection (A-A’) passing throughGemsa-24 and GOR-1 wells. b ANNE-SSW cross section (B-B′)passing through Gemsa-21 andGemsa-19 wells. c A NW-SEcross section (C-C′) passingthrough Gemsa-14, Gemsa-24,Gemsa-19, Gemsa-8, and Gemsa-23 wells. See Fig. 1 for cross-section colors

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Fig. 10 Top basement structuraldepth map of the Gemsa oilfield

Fig. 11 Base Hammam FaraunMember structural depth map ofthe Gemsa oilfield

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Fig. 12 Top Hammam FaraunMember structural depth map ofthe Gemsa oilfield

Fig. 13 Seismicity data of thesouthern Gulf of Suez for theperiod from 1904 to 2013 (datacompiled after NationalEarthquake Information Center(NEIC); the InternationalSeismological Center (ISC);Badawy 2001; Badawy andAbdel-Fattah, 2006; Hussein et al.2006; General PetroleumCompany BGPC^ internal report2009)

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fluid-pressure gradient can drive oil into the fault zoneuntil a new equilibrium is established. Such fluid mo-tion is known as fault valve mechanism (Sibson 1992).Further, oil will leak through fractures till it reaches theHammam Faraun Member present in both FB-2 and FB-3 fault blocks, where part of oil will recharge these

blocks and the other will continue through the GF-2fault zone. In both cases, oil will escape to the surfaceas it was assured from Gemsa-24 well that the evapo-rites (top seal) are highly brecciated and cavernous, es-pecially at the first 300 m depth. That means that thetop seal at FB-2 and FB-3 fault blocks is very weak;thus, oil will escape through vugs and caves not onlythrough the GF-2 fault to its surface location (Figs. 17and 18).

The oil-in-place volumetric has been calculated forGemsa predicted reservoirs using the volumetric methodand well logging analysis (Levorsen 1967; Dake 1998).The oil in place is expressed mathematically as

OIP ¼ Approximated Rock volume*Φ av* 1−Swð Þ ð7Þwhere

Φ av Average porositySw Water saturation

The total acre foot of the reservoir, or rock volume is cal-culated using the more suitable and accurate approach (i.e., the

Fig. 14 Epicenters, fault planesolutions, and composite faultplane solutions of earthquakes inboth Shadwan Island and GubalIsland (compiled from Badawy2001; Badawy and Abdel-Fattah2006; Hussein et al. 2006)

Fig. 15 Histogram of the magnitudes of 1265 event in the periodbetween 2000 and 2011 at the southern Gulf of Suez

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Trapezoidal Rule). In such approach, the irregularities of thereservoir in both shape and thickness can be considered andconsequently to obtain a fair value of the product rock volume.Using Eq. 8 (Levorsen 1967), the approximated rock volumeof the Hammam Faraun Member reservoir is calculated to be101,688 acre ft.

Approximated Rock Volume ¼ h A1þ A2ð Þ=2 ð8Þwhere

h Formation thicknessA1 Area of the upper surfaceA2 Area of lower surface

According to Alsharhan and Salah (1994), the average po-rosity of the Hammam Faraun Member is 10–16 % at thesouthern Gulf of Suez rift. The average porosity (Φ av) thatis used in the present study is 10 %. Water saturation (Sw) isestimated to be 30 % and oil saturation (So) is estimated to be70 % from Gemsa-24 well log using Eqs. 9 and 10(Schlumberger 2009), respectively:

Sw ¼ Ro=Rtð Þ 1=n ð9Þ

Assuming no gas saturation:

So ¼ 1−Sw ð10Þ

Fig. 16 Histogram of the focaldepths of 1265 event in the periodbetween 2000 and 2011 at thesouthern Gulf of Suez

Fig. 17 An illustration of theeffect of seismic activity onpermeability. a No permeabilityvariation, b increase ofpermeability resulted from anearthquake event. The fluidexpulsion exceeds due to theincrease in fracture aperture orforming new fractures

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where

Ro Formation resistivity in case of 100 % water saturationRt True formation true resistivity

N Saturation exponent = 2

From the prior, the maximum storage capacity for the pre-dicted Hammam Faraun Member reservoir was estimated to

Fig. 18 Proposed setting for oilmigration at the Gemsa area. aMigration pathway maps for thetop Hammam Faraun Member. bCross-sections B-B′ showing thepredicted reservoir and migrationpathway through the GF-2 fault tothe surface location. c A GoogleEarth image showing the surfacelocation of the oil seepage relativeto the GF-2 fault

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be approximately 10,166 acre ft. The hydrocarbon pore vol-ume was estimated to be about 2853 acre ft. which means thatmore or less 22 MMBO of original oil-in-place is estimatedfor the predicted Hammam Faraun Member reservoir.

Discussion and Conclusions

The pore pressure and the minimum stress (σ3) are twointeracting factors which determine the time and space of frac-tures in rock bodies. Until now, it is very difficult to determinewhich of the two factors motivates the fracturing process. Porefluids can affect fracture strength through a direct pressureeffect as well as through chemical interactions with rock ma-trix. Mechanically, pore pressure acts to reduce the normalstress tensor component throughout the rock mass (Hubbertand Rubey 1959). Over-pressuring may lead to repeated epi-sodes in which the rock is hydraulically fractured (Hunt 1990;Powley 1990). There is a strong relationship between over-pressuring and large-scale crustal processes such as earth-quake cycles and the reduction of strength of mature faults(Nur and Walder 1990; Rice 1992; Byerlee 1993; Milleret al. 1996).

Integration of surface and subsurface geological and geo-physical data followed by studying seismicity and the presentday stress magnitudes and orientation is a strong tool to iden-tify the causative factors and the migration pathways of hy-drocarbon seeps at mildly active rift basins. The present studysuggests oil leakage through the reopening of fractures asso-ciated with the damage zone of extensional faults in the pres-ence of micro-earthquake activities. Although fault reactiva-tion will not take place during micro-seismic activities, thereopening of preexisting fractures is most favorable wherethe pore pressure is greater than the minimum horizontalstress, and this creates open space within the damage zonesof extensional faults. The increase of open space leads to adecrease of pore pressure in the fault zone relative to the sur-rounding rocks. The resulted fluid-pressure gradient can driveoil into the fault zone until a new equilibrium is established.

Acknowledgments We are grateful to the authorities of The EgyptianGeneral Petroleum Corporation and General Petroleum Company (GPC)for the permission to publish this paper. We appreciate the critical reviewof the manuscript by Prof. Nabih Abdelhady Alsayed, Faculty ofPetroleum and Mining Engineering, Suez University, and Prof. AdelRamadan Moustafa, Faculty of Science, Ain Shams University.

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