Petroleum Geoscience
The impact of the Messinian Salinity Crisis on the petroleum system of the Eastern Mediterranean: a critical assessment using 2D-petroleum system modelling --Manuscript Draft--
Manuscript Number: Article Type: Thematic set article Full Title: The impact of the Messinian Salinity Crisis on the petroleum system of the Eastern Mediterranean: a critical assessment using 2D-petroleum system modelling Short Title: Eastern Mediterranean Petroleum System Corresponding Author: Alastair Fraser, PhD Imperial College London London, UNITED KINGDOM Corresponding Author E-Mail: [email protected] Other Authors: Abdulaziz Al-Balushi Martin Neumaier Christopher Aiden-Lee Jackson Abstract: The offshore Levant Basin demonstrates one of the most phenomenal natural examples of a working petroleum system associated with a relatively rapid unloading and loading cycle caused by the the Messinian Salinity Crisis (MSC). In this study, 2D basin and petroleum systems modelling suggests that the geologically instantaneous water unloading of c. 2070 m and subsequent rapid salt deposition and refill impacts the subsurface pore pressure and temperature in the underlying sediments. The pressure drop is modelled to be instantaneous, whereas the impact on temperature is more of a transient response. This has important consequences for the shallow sub- Messinian biogenic petroleum system, which is assumed to have experienced fluid brecciation associated with massive fluid escape events. Deeper Oligo-Miocene sediments are far less affected, thus indicating a "preservation window" for biogenic gas accumulations, which hosts the recent discoveries (Tamar, Leviathan, Aphrodite). Hydrocarbon accumulations of a "bubble point oil" composition are modelled to have experienced cap expansion during the drawdown, with the pressure drop being the primary control. This study suggests that seal-limited traps are expected to have undergone a catastrophic seal failure whereas the impact of the MSC is modelled to be less destructive for size-limited and particularly charge-limited traps. Section/Category: Messinian Salinity Crisis Manuscript Classifications: Geochemistry; Petroleum geology Additional Information: Question Response Are there any conflicting interests, No financial or otherwise? Samples used for data or illustrations in Confirmed this article have been collected in a responsible manner
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1 The impact of the Messinian Salinity Crisis on the petroleum system of the Eastern
2 Mediterranean: a critical assessment using 2D-petroleum system modelling
3
4 Abdulaziz Nasser Al-Balushi1*, Martin Neumaier2, Alastair J. Fraser1 and Christopher A-L.
5 Jackson1
6
7 1Basins Research Group (BRG), Department of Earth Science & Engineering, Imperial
8 College London, Prince Consort Road, London, SW7 2BP, England, UK
9 2Aachen Technology Centre, Schlumberger, Ritterstrasse 23, 52072 Aachen, Germany
10
11 *Corresponding author: [email protected]
12
13 1. Abstract
14 The offshore Levant Basin demonstrates one of the most phenomenal natural examples of
15 a working petroleum system associated with a relatively rapid unloading and loading cycle
16 caused by the the Messinian Salinity Crisis (MSC). In this study, 2D basin and petroleum
17 systems modelling suggests that the geologically instantaneous water unloading of c. 2070
18 m and subsequent rapid salt deposition and refill impacts the subsurface pore pressure and
19 temperature in the underlying sediments. The pressure drop is modelled to be
20 instantaneous, whereas the impact on temperature is more of a transient response. This
21 has important consequences for the shallow sub-Messinian biogenic petroleum system,
22 which is assumed to have experienced fluid brecciation associated with massive fluid
23 escape events. Deeper Oligo-Miocene sediments are far less affected, thus indicating a
1
24 “preservation window” for biogenic gas accumulations, which hosts the recent discoveries
25 (Tamar, Leviathan, Aphrodite). Hydrocarbon accumulations of a “bubble point oil”
26 composition are modelled to have experienced cap expansion during the drawdown, with
27 the pressure drop being the primary control. This study suggests that seal-limited traps are
28 expected to have undergone a catastrophic seal failure whereas the impact of the MSC is
29 modelled to be less destructive for size-limited and particularly charge-limited traps. (200
30 words)
31
32 Key words: Messinian Salinity Crisis, petroleum system modelling, biogenic gas,
33 Tamar, pressure, temperature, phase change.
34
35 2. Introduction
36 Surface processes play a pivotal role in moving loads from one area to another. This
37 redistribution of surface mass causes the Earth’s surface to respond either by subsidence
38 (in the case of loading) or uplift (in the case of unloading), which may subsequently affect
39 subsurface pressure and temperature equilibrium conditions (Allen and Allen 2013). From
40 the perspective of a solid Earth, the nature of the surface load, whether it is water, ice,
41 sediments or rocks, is irrelevant; the only thing that matters is the weight.
42
43 One of the most phenomenal natural examples of relatively rapid unloading and loading of
44 the Earth’s crust occurred ca. 5.96 million years ago in the Mediterranean Sea, during an
45 event that is known as the Messinian Salinity Crisis (MSC). During the MSC, the
46 Mediterranean Sea experienced a geologically instantaneous drop in sea level in excess of
47 ca. 1000 m (Ryan, 1976a; Ryan & Cita, 1978; Bartol & Govers, 2009; Urgeles et al., 2011),
2
48 and the rapid deposition of widespread evaporitic sequences that are up to ca. 2000 m
49 thick in the deeper parts of the basin, typically in areas floored by oceanic crust (Hsü, 1972;
50 Meijer & Krijgsman, 2005). Since the discovery of these evaporitic sequences, numerous
51 studies have attempted to explain the palaeo-geographic setting of the basin and the
52 depositional environment that governed the MSC. Despite these efforts, the relationship
53 between geologically instantaneous water unloading and evaporite loading, and the impact
54 that these had on subsurface pressures, temperatures and the subsequent distribution of
55 hydrocarbons in the eastern Mediterranean, have not been well established. For example,
56 the instantaneous drop in sea level is expected to be associated with a drop in pressure,
57 potentially causing hydrocarbon phase change. The first attempt to address this impact was
58 presented by Fraser et al. (2011) with further supporting evidence from 3D seismic data
59 provided by Bertoni et al. (2013).
60
61 The relationship between the surface processes and subsequent changes in subsurface
62 pressure and temperature conditions is well-established in some basins that have
63 experienced significant periods of tectonically driven uplift (e.g. Hammerfest Basin, offshore
64 northern Norway;Rodrigues Duran et al. (2013)). In the Hammerfest Basin, despite the
65 existence of mature, oil-prone source rocks (Ohm et al., 2008), almost all discoveries are
66 predominantly gas with uneconomical volumes of oil (NPD, 2014). Extensive Cenozoic
67 uplift, ca. 1000-3000 m of erosion and, therefore, removal of a crustal load along the
68 western basin margin (Laberg et al., 2012), as well as the high-latitude Quaternary
69 glaciation (Cavanagh et al., 2006) are thought to have caused trap tilting and exhumation
70 (Doré et al., 2002), leakage and redistribution of hydrocarbons due to phase change, gas
71 expansion, and subsequent flushing of oil from reservoirs (Nyland et al., 1992). This has
72 ultimately led to predominance of gas over oil in the Hammerfest Basin and elsewhere on
3
73 the Barents Sea shelf. The presence of extensive gas clouds and amplitude anomalies, in
74 addition to the documentation of palaeo oil-water contacts, together suggest a dominantly
75 leaky system and show that the structures defining the Snøhvit and Askelad fields, which lie
76 in the Hammerfest Basin, were once filled with significantly larger volumes of hydrocarbons
77 (Linjordet & Grung Olsen, 1978). Even though the origin, order and timing of the unloading
78 and reloading events are somewhat different, this is considered as an appropriate analogue
79 for the effect of the MSC in the eastern Mediterranean.
80
81 Recent exploration drilling results in the eastern Mediterranean have shown that the
82 offshore Levant Basin is a very prolific gas province. Proven gas reserves totalling ca. 30
83 trillion cubic feet (TCF) have been estimated for the most recent, sub-Messianian salt
84 discoveries in the Tamar, Leviathan and Aphrodite gas discoveries (Needham et al., 2013).
85 However, further exploration activity would benefit from a clearer understanding of the role
86 that the MSC played in shaping the present distribution of oil and gas in the region.
87
88 The eastern Mediterranean Basin, which comprises the Levant Basin, Nile Cone, offshore
89 Sirt, offshore Western Desert and the Herodotus Basin, is located in the south-eastern
90 Mediterranean region, near the complex boundary between the African, Arabian and
91 Eurasian plates. This study focuses on the petroleum system evolution of the Levant Basin
92 during the Messinian, although the results and implications of this modelling are relevant to
93 other basins in the region (e.g. offshore Sirt, Libya and Western Desert, Egypt) which have
94 experienced similar MSC drawdown effects. The Levant Basin is bound by the Nile Cone to
95 the south, the Dead Sea Transform to the east, the Cyprus Arc and the Latakia Ridge to the
96 north, and the Eratosthenes Seamount to the west (Figure 1).
97
4
98 This contribution aims to improve the understanding of the influence of the MSC on the
99 proven biogenic and speculative thermogenic petroleum systems of the eastern
100 Mediterranean. To achieve this we: (i) use 2D basin modelling to reconstruct the
101 subsidence history of the Levant Basin and (ii) assess the impact of the instantaneous and
102 significant drop in sea level and rapid salt deposition on subsurface pressure and
103 temperature (PT) conditions during the Messinian; (iii) use 2D petroleum systems modelling
104 to test the impact of the PT changes on the biogenic source rocks and gas accumulations
105 as well as on hypothetical thermogenic oil accumulations.
5
106 3. Geological Setting
107 The present Levant Basin is a remnant of a larger, Neo-Tethyan oceanic basin that opened
108 between several fragments of the Pangaea supercontinent in the Early Mesozoic (Bein and
109 Gvirtzman 1977; Dewey et al. 1973; Garfunkel and Derin 1984; Robertson and Dixon
110 1984a). Since Senonian times, the basin started to close as a result of the collision
111 between the Afro-Arabian and Eurasian plates which resulted in the subduction of the
112 Tethyan oceanic crust under the Cyprus Arc in the Neogene (Garfunkel 1998; Woodside
113 1977). Convergence between the African and Eurasian plates is still ongoing and is defined
114 by the active subduction boundary of the Cyprus Arc and the Latakia Ridge (Ben-Avraham
115 et al. 2002) (Figure 1).
116
117 The structure of the eastern Mediterranean margin and the adjacent deep-marine basin
118 preserves the signature of the main tectonic events concomitant with the opening and
119 closing of the Neo-Tethyan ocean. These events can be summarized into the following
120 tectono-stratigraphic phases (Figure 2): (i) Middle-to-Late Jurassic continental rifting and
121 break-up; (ii) Early Cretaceous thermal subsidence and formation of a passive continental
122 margin; (iii) Late Cretaceous-to-Recent convergence and contraction; (iv) Early Oligocene
123 Gulf of Suez rifting and Middle Miocene Red Sea break-up and (v) Late Miocene (i.e. MSC)
124 and subsequent Pliocene-Pleistocene flooding. In the following sections we provide a brief
125 summary of the main tectonic phases and depositional events associated with each phase.
126 Such a summary is important because these tectono-stratigraphic events directly impacted
127 the facies distribution in the basin which underpins the permeability structure and thus the
128 fluid flow during hydrocarbon migration.
129
6
130 (i) Middle-to-Late Jurassic continental rifting and break-up
131 Break-up of the northern part of Gondwana into several microplates (e.g., Tauride,
132 Cimmeride and the Eratosthenes Seamount) occurred in the Latest Palaeozoic to Early
133 Mesozoic (Garfunkel 1998; Hirsch et al. 1995; Longacre et al. 2007; Robertson and Dixon
134 1984a; Stampfli et al. 1991). A NE-SW trending graben and horst system, which records
135 NW-SE extension, is recognized at deeper structural and stratigraphic levels of the central
136 Levantine Basin as seen in seismic reflection profiles (Ben-Avraham et al. 2006; Gardosh et
137 al. 2010). Jurassic rift-related structures have also been observed in the northern part of the
138 Levant Basin (Al-Balushi et al. 2012).
139
140 (ii) Early Cretaceous thermal subsidence and formation of passive continental
141 margin
142 During the Late Early Cretaceous-to-Eocene, the Levant Margin transitioned into a passive
143 margin and thermal subsidence occurred (Bein and Gvirtzman 1977; Gardosh et al. 2010).
144 A shallow marine shelf formed proximal to the present-day coastline, whereas deeper
145 marine conditions were present in the centre of the basin (Ben-Avraham et al. 2006). The
146 Early-to-mid Cretaceous was characterised by the rapid growth and aggradation of a
147 carbonate platform (Ben-Avraham et al. 2006). In the mid Cretaceous, depositional cycles
148 are characterised by the progradation of the carbonate slopes into the basin, followed by
149 extensive growth of carbonate platforms on the margin (Ben-Avraham et al. 2006).
150
151 (iii) Late Cretaceous-to-Recent convergence and contraction
152 The convergence of the African-Arabian plate and the Eurasian plate commenced during
153 the Late Cretaceous and resulted in the initiation of a northwards-dipping subduction zone
154 in the northern part of the Levant Basin (Ben-Avraham 1989; Robertson and Dixon 1984b).
7
155 As a result of this convergence, a series of shortening structures, such as the Syrian Arc
156 fold belt, which were developed throughout much of the Levant region and northern Egypt,
157 were superimposed on the older (Late Mesozoic) horst and graben system (Ben-Avraham
158 et al. 2006). Shortening continued in the Levant region throughout the Cenozoic and
159 resulted in a second phase of Syrian Arc folding in the Early Miocene (Gardosh and
160 Druckman 2006). The bulk of the productive traps discovered in the Western Desert of
161 Egypt are found in Syrian Arc-related structures (Dolson et al. 2001) and the major recent
162 gas discoveries in the Levant Basin have been found in large Syrian Arc-related inversion
163 structures of Early Miocene age (Hodgson 2012).
164
165 (iv) Early Oligocene Gulf of Suez rifting and Middle Miocene Red Sea break-up
166 The Suez rift, about 70 km wide and 500 km long, forms the north-western branch of the
167 Red Sea. The extension and rifting in the Gulf of Suez began in the Oligocene (Patton et al.
168 1994). However, the main rifting phase occurred during the Miocene (Patton et al. 1994;
169 Steckler et al. 1998) which was followed by the Red Sea break-up in the Middle Miocene
170 (Serravalian age) (Dolson et al. 2001). The tectonic opening of the Red Sea separated the
171 Arabian plate from the African plate and initiated a new strike-slip plate boundary known as
172 the Dead Sea Transform (DST), along the Levant Margin (Mart et al. 2005).
173
174 Uplift associated with the Red Sea rifting played a major role in modifying the river drainage
175 patterns across Africa and acted as a key drainage divide and clastic sediment source for
176 the Proto-Nile River System (Macgregor 2012). The rise of the rift shoulders and other
177 concomitant African swells caused systems, which originally drained to the west, to switch
178 northwards and form the current Nile drainage system (Macgregor 2012). This change in
179 drainage direction increased the sedimentation rates in the Nile Delta to be among the
8
180 highest experienced on the African margins. This is supported by the source-sink volume
181 analysis that shows half of this clastic input is sourced from rift shoulders where the erosion
182 rate is estimated to be in excess of 80 m Ma-1 (Macgregor 2012).
183
184 2D seismic isopach mapping by Macgregor (2012) shows that deep-water Oligocene-
185 Miocene sediments exist as far north as northern Levant and their distribution is controlled
186 by the pre-existing structures. A long run-off turbidite system extending hundreds of
187 kilometres basin-ward from the southern shelf edge, such as the present-day Niger or
188 Mississippi systems, seems to provide a good analogue for predicting deep-water Nile
189 reservoirs.
190
191 (v) Late Miocene MSC and Pliocene flooding
192 At the end of the Miocene (c. 5.96 Ma), the Straits of Gibraltar closed as a consequence of
193 the continued convergence of the African plate with the Eurasian plate. The subsequent
194 reduction in the oceanographic connection of the Mediterranean Sea with the Atlantic
195 Ocean led to a drastic palaeo-environmental change known as the MSC (Hsü et al. 1977;
196 Meijer and Krijgsman 2005). A shortage of saline water recharge and high evaporation
197 rates driven by the arid Mediterranean climate resulted in a dramatic drop in sea level,
198 increasing the seawater salinity and promoting deposition of thick (locally over 2 km)
199 evaporite sequences (Hsü 1972; Hsü et al. 1977; Ryan 2009; Ryan and Cita 1978). The
200 magnitude of the Messinian sea-level drop has been a matter of debate since the event
201 was first recognised by(Hsü 1972). It has been estimated as 1300 m (Ben-Gai et al. 2005;
202 Urgeles et al. 2011), 1500-1900 m (Ryan 1976; Steckler et al. 2003), 1730-2230 m (Bartol
203 and Govers 2009), and up to 3500 m (Ryan 1978). A new estimate of c. 2070 m, which has
204 been used in this modelling study, has been suggested based on evidence for low-stand
9
205 Messinian shorelines offshore from the Western Desert, Egypt (Al-Balushi et al. 2013).
206 During this significant and geologically rapid sea level fall, the Levant Margin underwent
207 extensive erosion, resulting in the formation of deeply incised submarine canyons
208 (Gvirtzman and Buchbinder 1978).
209
210 Pliocene (Zanclean) flooding marked the end of the MSC (c. 5.33 Ma) and re-establishment
211 of normal marine conditions in the Mediterranean basins (Hilgen et al. 2007; Hilgen and
212 Langereis 1993). The Pliocene-to-Recent succession unconformably overlies the Messinian
213 Salt and consists of mainly Nile-derived sediments that reach a thickness of up to 4 km over
214 the Nile Cone area (Gardosh and Druckman 2006; Segev et al. 2006). Along the Levant
215 Margin, however, the Pliocene-to-Recent succession is thinner (c. 0.5-1 km) and
216 characterised by mud-dominated sequences composed of claystone and siltstone (Gardosh
217 and Druckman 2006).
10
218 4. Dataset and Methodology
219 This study utilises a 2D regional seismic reflection dataset, composed of multiple surveys
220 acquired between 2000 and 2005. Data are pre-stack time-migrated and cover ca. 185,000
221 km2 of the eastern Mediterranean Basin, extending from northern Lebanon across the Nile
222 Delta and westward to the offshore Western Desert and Herodotus Basin (Figure 1). The
223 average spacing between lines is ca. 10 km. The dominant frequency of the seismic data in
224 the sallower section (< 5 sec two-way time, TWT) ranges from ca. 7.5 to 25 Hz, giving a
225 vertical resolution of ca. 25-80 m based on an average velocity of 2600 ms-1. In the deeper
226 section, however, the dominant frequency ranges from ca. 10 to 15 Hz, giving a vertical
227 resolution of ca. 80-125 m based on a higher average velocity of 5000 ms-1. The data have
228 a record length that ranges from ca. 8 sec TWT in the Herodotus Basin, deep-water Nile
229 and Eratosthenes Seamount, to as much as 12 sec TWT in the Levant Basin. In addition,
230 we have access to 20-ultra-deep lines (record length of 14-20 sec TWT) that image
231 crystalline basement; these lines cross the Nile Cone.
232
233 Five wells that penetrate down to ca. 4500 m below seabed were available for this study
234 and provided stratigraphic control on the age of the mapped seismic packages (Mango-1,
235 North Sinai 21-1, Marakia-1, Sidi Barrania-1 and Memphis-1; Figure 1). Mango-1
236 terminated in the Upper Jurassic, North Sinai 21-1, Marakia-1 and Sidi Barrania-1
237 terminated in the Lower Cretaceous, and Memphis-1 terminated in the Upper Cretaceous.
238 Very few wells have published temperature data (Dubille and Thomas, 2012).
239
240 A 220 km long, NW-trending seismic cross section from the Levant Basin was selected for
241 basin and petroleum systems modelling (Figure 1 and Figure 3). The image quality is very
11
242 good to excellent down to ca. 7 sec TWT, at which point image quality deteriorates. The
243 stratigraphy on either side of the section was constrained by well data from the Levant
244 Margin (Figure 3a; (Gardosh & Druckman, 2006a; Gardosh et al., 2010; Gardosh et al.,
245 2011) and Deep Sea Drilling Program (DSDP) wells from the Eratosthenes Seamount
246 (Robertson, 1998c). The central part of the section is based on extrapolated stratigraphy
247 from the margins. However, there is a degree of uncertainty introduced here due to the
248 absence of direct well control below the Turonian and uncertainties associated with
249 interpreting deep stratigraphy across the main basin-bounding faults, some of which have
250 large throws (Figure 3).
251
252 Notwithstanding these uncertainties, this cross section was selected because: (i) it
253 provides a relatively clear, cross-sectional image of the subsurface geology from the Levant
254 Margin to the Eratosthenes Seamount, imaging down to basement depths; this is critical for
255 allowing the construction of a robust geological model that underpins the petroleum
256 systems model; (ii) it ties with well data on the Eratosthenes Seamount and the Levant
257 Margin, which offer excellent constraints on the stratigraphy and lithology of the main
258 depositional units; and (iii) it passes close to the main gas discoveries in the basin, thus
259 allowing a better understanding of the petroleum systems context of these important
260 hydrocarbon accumulations (e.g. Tamar, Leviathan and Aphrodite).
261
262 The seismic profile was depth-converted using velocity data from nearby wells in the Levant
263 Basin (Gardosh and Druckman 2006). A detailed description for the sedimentary fill based
264 on wells and seismic facies data along the seismic profile is presented below. This
265 description outlines the predominant depositional environment for each stratigraphic unit
266 and the likely lithology to be encountered in each, which is the controlling parameter for the
12
267 permeability distribution in the basin and thus the modelled hydrocarbon migration and
268 overpressure build-up.
269
270 Basin and petroleum systems modelling was conducted in PetroModTM. The basin model
271 provided a modelled pressure and temperature (PT) development through geologic time,
272 including the high-resolution modelling of the MSC. Embedded in that PT framework, the
273 evolution of the petroleum systems was modelled (hydrocarbon generation, expulsion,
274 migration, accumulation and preservation). This permitted a PVT-controlled simulation of
275 the reaction of potential pre-Messinian hydrocarbon accumulations and the Oligocene-
276 Miocene biogenic petroleum system to pressure and temperature variations associated with
277 the MSC.
278
279 5. Seismic-stratigraphic description of the 2D model
280 Nine seismic horizons (H1-9) bounding eight seismic-stratigraphic units (SU1-8) were
281 interpreted across the study area (Figure 2 and Figure 3). These horizons, from oldest to
282 youngest, are: (i) H1 - Pre-Jurassic (top basement); (ii) H2 - Mid Jurassic; (iii) H3 - Late
283 Cretaceous (Turonian); (iv) H4 - Late Eocene; (v) H5 - Base Miocene; (vi) H6 - Mid
284 Miocene; (vii) H7 - Base Messinian; (viii) H8 - Top Messinian and (ix) H9 -Sea Bed. The
285 seismic-stratigraphic framework is defined by classic megasequence and sequence based
286 stratigraphy which defines a distinct set of internal seismic facies. Seismic data quality and
287 stratigraphic data from wells are good down to the top Cretaceous surface, thus our
288 interpretation is more certain in the overlying interval. However, interpretations at greater
289 depths within the section, are relatively poorly imaged on seismic data and lack well
290 penetrations, hence are more speculative in terms of absolute age and lithology.
13
291 (i) Deep basin structure
292 The deep basin structure is defined by H1 (top basement). This reflection is mapped across
293 the study area and defines a transition from a chaotic and largely reflection-free package
294 below to a package characterised by continuous, parallel-to-divergent, high-amplitude
295 reflections above (Figure 3b and Figure 3c). The seismic expression of the package below
296 H1 suggests it corresponds to crystalline basement and that H1 therefore defines the base
297 of the sedimentary succession. Along the Levant Margin, the basement is at ca. 6 sec TWT
298 and it deepens westward towards the centre of the basin to ca. 7.5 sec TWT. It then rises
299 north-westwards to ca. 5 sec TWT, immediately beneath the Eratosthenes Seamount
300 (Figure 3c).
301
302 A series of NE-trending horst and grabens, bound by broadly NE-SW-striking normal faults,
303 are developed along H1 (Figure 3c). These structures extend from the Eratosthenes
304 Seamount in the west to the Levant continental shelf in the east. A recent gravity inversion
305 study by Cowie and Kusznir (2012) suggests that the Levant continental crust underwent
306 extreme thinning from ca. 35 km onshore to ca.10 km in the centre of the basin offshore,
307 which is broadly in agreement with other publications (Ginzburg et al., 1979; Makris et al.,
308 1983; Ginzburg et al., 1994; Ben-Avraham et al., 2002).
309
310 (ii) Pre-Jurassic-to-Mid Jurassic (SU1)
311 SU1 is bound below and above by H1 to H2, respectively, and is developed across the
312 entire width of the Levant Basin. SU1 is characterized by high-amplitude, relatively
313 continuous, parallel to divergent reflections, locally changing to a less reflective or
314 reflection-free seismic facies on fault-bound, intra-basin structural highs.
14
315
316 The lower boundary of SU1 is the near-top of the acoustic basement. The upper boundary
317 is correlated with a distinct change in seismic character from a high-amplitude, continuous
318 reflection set below to discontinuous, low-amplitude, occasionally shingled, reflections
319 above. The stratigraphy and age of SU1 is not well established because of the lack of well
320 control.
321
322 Well data from the Levant Margin show that Permian sediments overlie the Precambrian
323 basement (Heletz Deep-l, Gevim-1). It is, therefore, suggested that SU1 is dominated by
324 Permian, Triassic and Lower Jurassic strata (Gardosh & Druckman, 2006a). Based on the
325 onshore lithology from well data and the offshore seismic character, this unit is interpreted
326 as continental to shallow-marine siliciclastic and carbonate deposits.
327 (iii) Mid Jurassic-to-Late Cretaceous (SU2) 328 SU2 is bound below and above by H2 and H3, respectively, and it is thickest at the eastern
329 margin of the basin and reduces in the central and western parts. The eastern margin area
330 is characterised by a relatively discontinuous, low-amplitude seismic reflection series,
331 whereas more continuous, high amplitude reflections dominate the deep part of the basin,
332 further offshore.
333
334 The lower boundary of SU2 is correlated with the Mid Jurassic unconformity. The upper
335 boundary is correlated to a distinct transition of seismic character between several high-
336 amplitude reflections at the top of the unit and the overlying lower-amplitude reflections
337 marked by chaotic and reflection-free zones.
338
15
339 SU2 is penetrated by several onshore and offshore wells in the eastern part of the Levant
340 Basin. An important feature identified in these wells is a pronounced facies change from
341 coarse-grained, shelf-type carbonate units east of the present-day coastline to finer-
342 grained, slope and basin-type carbonate and siliclastics further west (Bein & Gvirtzman,
343 1977; Flexer et al., 1986).
344 345 (iv) Late Cretaceous-to-Late Eocene (SU3) 346 SU3 is bound below and above by H3 and H4, respectively, and it attains its maximum
347 thickness in the central part of the Levant Basin and displays marked thinning towards the
348 eastern and western margins. SU3 is characterised by transparent to medium chaotic
349 reflections.
350 The lower boundary of SU3 is correlated with the Late Cretaceous. The upper boundary is
351 correlated with the top of a relatively continuous higher-amplitude reflection series. The
352 change is seismic character corresponds to an unconformity surface.
353
354 Well data indicate that SU3 is composed of chalk and marl of pelagic to hemipelagic, deep-
355 water origin (Almogi-Labin et al., 1993). Deep-water chalky and marly limestone units
356 intercalated by chert-dominated beds have been observed in the Terbol-1 and Adloun-1
357 coastal wells of Lebanon (Chekka Formation; Nader, 2011; Hawie et al., 2013). The
358 unconformity at the upper boundary of this unit is indicated in wells by a pronounced
359 lithological break and a biostratigraphic hiatus between Eocene carbonates and the
360 overlying Oligocene clastic deposits (Gardosh & Druckman, 2006a).
361
362 (v) Late Eocene-to-Base Miocene (SU4)
16
363 SU4 is bound below and above by H4 and H5, respectively, and attains its maximum
364 thickness in the centre of the basin and displays a minimum thickness over the
365 Eratosthenes Seamount. This seismic package is characterised by moderate-amplitude,
366 discontinuous reflections with some localised chaotic reflections over the pre-existing
367 structural highs.
368 The lower high amplitude boundary of SU4 is correlated with an unconformity representing
369 a transition from Upper Eocene shale and carbonate-dominated sequences to a more
370 clastic-rich sequence in the Oligocene (Gardosh & Druckman, 2006a; Lie et al., 2011). The
371 upper high-amplitude boundary separates Oligocene sediments from Lower Miocene deep-
372 water sediments (Lie et al., 2011).
373
374 This unit is likely to be dominated by distal turbidite sequences composed of alternating
375 sandstones, shales and deep-water carbonates (Lie et al., 2011) and delivered to the basin
376 by the Proto-Nile river system. Over the Eratosthenes Seamount, DSDP wells indicate that
377 Oligocene successions are dominated by chalk and are overlain by a carbonate succession
378 comprising clasts of micritic limestone with reworked planktonic foraminifera of Mid Eocene
379 and Oligocene age (Emeis et al., 1996a).
380 (vi) Base Miocene-to-Mid Miocene (SU5) 381 SU5 is bound below and above by H5 and H6, respectively and it shows more pronounced
382 variation in thickness than SU4. The seismic character of this unit varies from a reflection-
383 free zone very close to the Levant Margin to medium-amplitude sub-continuous layered
384 reflections further west. In the centre of the basin, seismic data exhibits high-amplitude
385 continuous reflections and some mounded reflections, interpreted as siliciclastic, deep-
17
386 water turbidite channels and basin-floor fans, with onlapping geometry upon the anticlines
387 (Gardosh & Druckman, 2006a).
388 The lower boundary is the base Oligocene-Miocene unconformity. The upper boundary
389 corresponds to the Mid Miocene. Along the eastern margin, thick intervals of coarse-
390 grained sandstone and conglomerate were encountered in wells (e.g. Hof Ashdod-1)
391 (Gardosh & Druckman, 2006a). In the offshore, SU5 is composed of pelagic siltstone, deep-
392 water siliclastics and shale (Druckman et al., 1994; Gill et al., 1995). On the western side of
393 the section, over the Eratosthenes Seamount, DSDP results at sites 965 and 966 revealed
394 that shallow-water carbonate deposition was dominant during this time (Robertson, 1998c).
395
396 This unit represents the most prolific reservoir interval in the Levant Basin containing giant
397 gas fields (Aphrodite, Leviathan and Tamar) in the Lower Miocene high quality sands
398 trapped in the Syrian arc compressional structures (Durham, 2013; Needham et al., 2013).
399 (vii) Mid Miocene-to-Late Miocene (SU6) 400 SU6 is bound below and above by H6 and H7, respectively and shows symmetrical thinning
401 on both sides of the basin. This seismic package is characterized by low-amplitude, partly
402 discontinuous reflections, some mounded reflections and reflection-free zones.
403 The upper boundary of the unit, the base Messinian, is represented by a marked transition
404 in seismic facies between the low-amplitude, partly continuous reflection character of SU6
405 below to the chaotic and typically reflection-free seismic package of the Messinian salt
406 (SU7) above. As with SU5, this unit is composed mainly of pelagic siltstone, deep-water
407 siliclastics and shale (Druckman et al., 1994; Gill et al., 1995).
408 (viii) Messinian (SU7)
18
409 SU7 is bound below and above by H7 and H8, respectively. This unit generally displays a
410 chaotic seismic character with some locally bright internal reflectors that are interpreted to
411 represent re-deposited shelf deposits and different evaporite lithologies (Gardosh &
412 Druckman, 2006a; Lie et al., 2011).
413 The lower boundary of SU7 corresponds to the base Messinian and the upper boundary
414 corresponds to the top Messinian. SU7 is a distinct seismic package identified throughout
415 the basin, composed of a thick evaporitic series of Late Miocene age (Hsü, 1972; Neev,
416 1975; Ryan, 1978,2009). In the Levant Basin this unit is about 1-2 km thick (0.5-1 s TWT,
417 assuming a velocity of 4000 m/sec through salt), extending consistently across most of the
418 basin except close to the eastern margin and Eratosthenes Seamount where it pinches out.
419
420 No Messinian salt has been encountered by DSDP wells over the Eratosthenes Seamount.
421 Shallow-water carbonates (of Burdigalian age) at sites 965 and 966 have been reported by
422 Robertson (1998b) to contain extensive solution porosity. This is attributed to flushing by
423 meteoric waters during the MSC suggesting that the Eratosthenes Seamount was exposed
424 sub-aerially (Robertson, 1998c).
425
426 (ix) Pliocene to Pleistocene (SU8) 427 SU8 is bound below and above by H8 and H9, respectively. Throughout the basin, SU8 is
428 characterized by continuous, high to low amplitude reflections. At the eastern Levant
429 Margin, where the unit attains its maximum thickness, it is dominated by prograding
430 sigmoidal reflections.
431 The lower boundary of SU8 is a composite unconformity. In the basin centre, it corresponds
432 to top Messinian salt whereas on the Levant Margin, where the Messinian salt is absent, it
19
433 correlates with the Messinian Unconformity (Gardosh & Druckman, 2006a). The upper
434 boundary of SU8 corresponds to the sea-bed. Well data (e.g. Yam West-1; Figure 1) show
435 that the Plio-Pleistocene section, termed the Yafo Formation, is a mud-dominated unit
436 composed primarily of claystone and siltstone (Gvirtzman & Buchbinder, 1978).
437
438 6. Basin and Petroleum Systems Modelling
439 Basin and petroleum systems modelling simulates the evolution of a sedimentary basin
440 through time as it fills with sediments that may eventually generate or contain hydrocarbons
441 (Hantschel and Kauerauf, 2009). Building such a model requires going through two steps;
442 (i) constructing the basin model; and (ii) adding the petroleum systems components. In the
443 first step, the present day basin structure and stratigraphy is defined based on observations
444 from seismic and well data, often with the help of outcrop and analogues from similar
445 basins. The structural history and the evolution of heat flow and surface temperatures
446 through time then need to be defined. The main outcomes of this step are modelled
447 physical properties such as rock stress, pore pressure, effective stress, porosity reduction
448 by mechanical compaction, heat flow, temperature and burial history. In the second step,
449 the petroleum systems model is appended to the basin model by adding source rock
450 properties to simulate hydrocarbon generation, expulsion, migration, accumulation and
451 preservation. This simulation is fully controlled by the physical pressure and temperature
452 framework calculated in the first step. This is particularly important for the phase behaviour
453 of the modelled accumulations, which forms the main focus of this part of the study.
454
20
455 6.1 Basin Modelling
456 6.1.1 Structural evolution
457 Starting with the cross section as described above, the structural evolution of the basin
458 during pre-rift, syn-rift and post-rift times has been constrained (Figure 4), using the
459 backstripping technique (e.g., Hantschel and Kauerauf, 2009) The pre-rift basin geometry
460 is assumed to represent a broad and flat continental to shallow marine environment. This is
461 supported by the strata of the Triassic Ramon Group and the Lower Jurassic Arad Group
462 found in the southern Levant which are composed of limestone, dolomite, siliciclastic, and
463 evaporite successions (Goldberg & Friedman, 1974), suggesting that the southern Levant
464 region was located in a wide continental to shallow marine platform during this time
465 (Gardosh et al., 2011).
466
467 Syn-rift structures associated with Early-to-Mid Jurassic rifting event have been
468 reconstructed as a series of horst-grabens systems (Figure 4a). This was followed by post-
469 rift thermal subsidence, which gradually led to the development of deep-water conditions by
470 the Late Cretaceous (Figure 4b and Figure 4c). Evidence for deep-water conditions is
471 provided by pelagic carbonates that commonly contain chert of replacement origin, together
472 with localised organic-rich sediments recovered from hole (967) of the Ocean Drilling
473 Program (ODP) - Leg 160 drilled in the vicinity of Eratosthenes Seamount (Robertson,
474 1998c). In the centre of the basin (between the Eratosthenes Seamount and the Levant
475 Margin), the maximum depth was estimated to be ca. 1000 m during the Late Cretaceous
476 (Figure 4b) to Eocene (Figure 4c) as inferred from the planktonic foraminifers and
477 calcareous macrofossils present. However, carbonates growing on the intra-basinal highs
478 were apparently able to aggrade and keep pace with the thermal subsidence of the basin,
21
479 thus maintaining shallow water conditions compared to their basinal surroundings. Syrian
480 Arc folding which initiated in the Late Cretaceous was incorporated in these reconstructions
481 by developing a series of anticlines superimposing the intra-basinal highs.
482
483 During the Oligocene-to-Miocene (Figure 4d), rapid loading of deep-water turbidite sands,
484 transported by the proto-Nile River (Steinberg et al., 2011), accelerated the subsidence in
485 the centre of the basin. The Eratosthenes Seamount, however, was reconstructed to be a
486 prominent structural high during the Miocene. This is based on the Robertson (1998c)
487 interpretation that > 1000 m of vertical uplift is required to account for the shallow water
488 Miocene carbonates reported by Emeis et al. (1996b).
489
490 Basin reconstructions from the MSC to present are shown in Figure 4 e-f. The Eratosthenes
491 Seamount was exposed sub-aerially during the maximum drawdown (Figure 4e). At the end
492 of the salt deposition (5.60-5.55 Ma, Figure 4f) the water depth was assumed to close to 0
493 m, with an installation of a salt flat. At the end of the MSC (5.33 Ma, Figure 4g), the
494 subsidence in the basin accelerated due to the refilling of the basin by the Zanclean Flood
495 until reached its present aspect (Figure 4h), characterised by thick Plio-Pleistocene
496 sediments on the margin that thins basinward.
497
498 6.1.2 Modelling the MSC
499 An important part of the palaeo-geometrical evolution is the modelling of the water
500 drawdown during the MSC. As mentioned in the geological setting section (section 5.3), the
501 magnitude of the Messinian drawdown and the timing of the evaporite deposition are
502 disputed. In this model, we have applied a Messinian drawdown of ca. 2070 m. The drop in
503 sea level was assumed to have started at 5.96 Ma (Figure 5) based on high-resolution 22
504 astrochronology for the Pissouri Basin in Cyprus (Krijgsman et al., 2002). The duration of
505 the drawdown phase is assumed to be 360 kyr (ca. 5.96-5.60 Ma), and the subsequent
506 deposition of the bulk of the evaporites was assumed to have occurred within 45 kyr (5.60-
507 5.55 Ma) (Roveri et al., 2008a), resulting in a salt flat. The onset of Lago Mare deposition
508 has been modelled to be at 5.53 Ma based on Roveri et al. (2008b), accompanying a
509 progressive increase in water depth arbitrarily set to 1000m towards the beginning of the
510 Zanclean flooding (5.33 Ma), This event marks the abrupt filling of the basin with water and
511 the end of the MSC.
512
513 6.1.3 Thermal Boundary Conditions
514 The standard thermal boundary conditions used in basin modelling are the basal heat flow
515 and the sediment-water interface temperature (SWIT) through geologic time. The basal
516 heat flow in particular is poorly constrained and often introduces large uncertainties. This is
517 also the case in this study, with only limited access to direct thermal calibration data such
518 as corrected borehole temperatures and thermal maturity indicators such as vitrinite
519 reflectance.
520
521 The basal heat flow determines the amount of heat influx applied to the base of the model.
522 It is estimated through geological time based on the amount of lithospheric stretching, the
523 type and thickness of the crust, and other considerations such as deep mantle processes
524 (Allen & Allen, 2013). In our model, the heat flow history was estimated using the McKenzie
525 uniform stretching model (McKenzie, 1978). This model uses the subsidence, timing and
526 duration of the syn-rift (in our case, 176 to 145 Ma) and post-rift (145 Ma to present) phases
527 to estimate the stretching factor (and generate a palaeo-heat flow map.
23
528
529 The SWIT prediction through geological time was based on a model by Wygrala (1989),
530 which takes into account palaeo-latitude and associated palaeo-climate evolution, coupled
531 with the plate tectonic setting. The model then adjusts the SWIT for palaeo-water depth. For
532 the Levant Basin case, the predicted SWIT has been corrected further for specific
533 conditions (Figure 5) namely; (i) present-day elevated sea bed temperature of ca. 13.2 °C,
534 obtained from the Deep Sea Drilling Program (DSDP) down hole temperature measurement
535 at site 376, west of Cyprus (Figure 1) and (ii) the arid climate that characterised the
536 Messinian times. The SWIT is expected to have been much higher during the drawdown
537 than present due to the overall Messinian arid climate (Hsü, 1972; Ryan, 2009). A
538 Messinian lake floor temperature of > 35-45 °C was proposed by Hsu (1972b and 1984);
539 we therefore elected to use a Messinian SWIT of 40°C in our model.
24
540 6.1.4 Pressure and Temperature Evolution
541 The modelled present-day overpressure and isotherms is shown in Figure 6. Overpressure
542 is predicted in the Messinian Salt and the Eocene sediments due to their low permeability,
543 whereas the rest of the basin is normally pressured. Due to its very high thermal
544 conductivity, the salt also heavily perturbs the temperature, causing huge lateral variations
545 of the thermal gradient. An extraction at a pseudo-well location, roughly corresponding to
546 the lateral position of the Tamar field (Figure 6b), shows the modelled geothermal gradient.
547 The gradient in the post-salt section is > 40 ℃ km−1, < 10 ℃ km−1 within the salt and about
548 35 ℃ km−1 in the pre-salt Oligocene-Miocene section. The temperature at ca. 4800 m
549 burial depth, which is the approximate depth of the Tamar field reservoir, is ca. 94 ℃ with
550 an overlying salt thickness of ca. 1200 m. This corresponds to temperatures published data
551 for the Tamar field (Dubille and Thomas, 2012). Closer to the margin, temperatures at the
552 same burial depth are higher (Figure 6c) This shows the importance of salt thickness in
553 controlling the local thermal gradient and, therefore, in predicting reservoir temperature and
554 source rock maturation state.
555
556 A plot of pore pressure variations within the uppermost sub-Messinian sediments around
557 Messinian times (Figure 7b) shows that the modelled pore pressure had an instantaneous
558 response to MSC-related water unloading, the deposition of salt and the flooding. The
559 pressure profiles at any particular depth exhibit similar trends for both the rate and the
560 magnitude (Figure 7c). During the drop in sea level, pressure at all depths decreased by the
561 same amount (ca. 20 MPa, equivalent to ca. 2900 Psi). Similarly, rapid salt deposition (ca.
562 0.03 m/kyr) between 5.60-5.55 Ma caused a rapid increase in pore pressure. However, this
563 pressure increase varies from ca. 10 MPa (equivalent to 1450 Psi) in the shallow sediments
25
564 to ca. 20 MPa (2900 Psi) in the deeper basin. This is interpreted to be related to different
565 permeabilities and thus different flow rates during the dewatering: whereas pore water can
566 more easily escape in the shallow Miocene sediments, this is not the case for the well-
567 compacted deeper sediments, where slow water flow is modelled to cause additional
568 overpressure (ca. 10 MPa; 1450 Psi). The deep signatures of the progressive refill and the
569 Zanclean flooding are again instantaneous 5.55-5.33 Ma).
570
571 The temperature evolution of the shallow sediments during the MSC (Figure 7b) follow the
572 SWIT, evolving from temperatures of about 10℃ to 20℃ after the sea level drop and >40℃
573 after the deposition of the salt. In contrast to pressure, the temperature response to this
574 event with increasing depth is modelled to be of transient nature (Figure 7d). Heat
575 transmission is attenuated and delayed progressively with depth, both during heating
576 accompanying the water removal and in the cooling during refill. Also the magnitude of heat
577 peak gets reduced with depth to a point that the effect is insignificant at top basement.
578
579 6.2 Petroleum Systems Modelling
580 Building up on the PT framework of the basin model described above, petroleum systems
581 modelling allows to analyse the evolution of the biogenic and thermogenic petroleum
582 systems through geologic time. Therefore, source rocks need to be assigned to the basin
583 model. A review of the petroleum systems elements; source rocks, reservoir rocks and seal
584 rocks presented in this section.
585 (i) Source rocks 586 Source rocks have been assigned to the petroleum systems model at five stratigraphic
587 levels: pre-rift (Late Triassic-to-Early Jurassic), syn-rift (Mid-to-Late Jurassic) and post-rift
26
588 (Late Cretaceous, Oligocene-to-Mid Miocene and Late Miocene; Figure 2). Because there
589 is no publically available information any possible source rock in the basin, the source rock
590 intervals listed above are selected based on the available information from nearby active
591 source rock analogues around the Neo-Tethyan Margin (Ala & Moss, 1979; Carrigan et al.,
592 1995; Alsharhan & Abd El-Gawad, 2008; Moretti et al., 2010; Bou Daher et al., 2014). This
593 is considered to be a reasonable assumption as most of the organic rich intervals are
594 believed to be laterally continuous on a basin scale. A summary of these source rocks in
595 terms of the depositional environment, kerogen type, total organic carbon (TOC) and
596 hydrogen index (HI) is given in Table 1. As can be seen from the Table 1, more than one
597 analogue is provided for each stratigraphic interval. This is to show that the source rock
598 within that particular stratigraphic interval is present in different places around the margin of
599 the basin. The kinetics used for the modelling of source rock kerogen to hydrocarbon
600 transformation are shown in Table 1. These are selected based on published kinetics
601 available in the PetroMod library that best match the Kerogen type, TOC and HI ranges of
602 source rock analogues.
603 A schematic palaeo-facies map that represents a conceptual model for the facies
604 distribution in the eastern Mediterranean Basin during the Cretaceous and the Oligocene-
605 Miocene is shown in Figure 8. During the Cretaceous, the basin was dominated by
606 carbonate platforms on the margin, and the deposition of deep-water organic-rich
607 mudstones is postulated in the basin centre during the Late Cretaceous. During the
608 Oligocene-to-Miocene, however, a deep-water turbidite system depositing high-quality
609 reservoir sands and biogenic source rocks became established over much of the basin. The
610 Oligocene source rocks are interpreted as disseminated, largely land plant organic matter
611 derived form the Nile Delta plain when Northern Africa was experiencing a more humid
612 climate during this time and transported into the deepwater Eastern Mediterranean basin
27
613 via slope turbidite channels (Vilinski, 2013). Fossilised remnants of widespread Oligocene
614 forests have been described for the onshore Nile Delta area (Dolson et al, 2001). The
615 Oligocene source has been shown by Vilinski (2013) to be the major contributor to both the
616 thermogenic and biogenic charge systems in the Nile Delta and it is likely that similar
617 sediments have been responsible for the biogenic charge in the offshore Levant.
618
619 The authors acknowledge that these parameters carry a degree of uncertainty. However,
620 the focus of this study is neither a detailed hydrocarbon mass balance, nor a quantitative
621 prediction of charge volumes. Therefore, these parameters have not been varied for
622 sensitivity analysis. It should be noted that the kinetic assigned to the Oligocene-Miocene
623 biogenic source rocks is based on a simple temperature-dependent reaction.
624
625 (ii) Regional carrier beds and reservoirs 626 Recent Lower Miocene gas discoveries in the Levant Basin (i.e. Tamar, Leviathan and
627 Aphrodite) have revealed that the Oligocene-Miocene section is composed of reservoir-
628 quality deep-water sandstones (Skiple et al., 2012; Durham, 2013). These sandstones
629 occur in laterally continuous sheets, expressed on seismic data as packages of continuous
630 and parallel reflections, extending regionally from as far south as the Nile Delta to as far
631 north as northern Lebanon (Roberts & Peace, 2007; Steinberg et al., 2011). The high
632 lateral continuity of these sandstones means they may act as effective carrier beds that
633 assist hydrocarbon migration into traps. Reservoirs have been assigned at three
634 stratigraphic intervals; Mid Jurassic, Late Cretaceous and Oligocene-Miocene (Figure 2). In
635 the Jurassic and the Cretaceous, carbonates of speculative reservoir qality were dominant
636 either in the form of carbonate buildups (e.g. Gialo filed in central Sirt Basin; Gumati &
28
637 Schamel, 1988) in the basinal highs or as carbonate platforms on the margin (Gardosh et
638 al., 2011).
639 640 641 (iii) Regional Seals 642 The Messinian Salt is considered to be the regional seal for the entire eastern
643 Mediterranean region, forming a barrier between the dominantly biogenic, post-Messinian
644 Pliocene petroleum systems and the dominantly thermogenic, pre-Messinian system in the
645 Nile Delta. Although the Messinian Salt is a very effective regional seal, it has been
646 deposited very recently and can therefore only act as a seal for hydrocarbons that have
647 migrated in the last 5 Ma.
648 For the pre-Messinian section, intra-formational shales may act as very effective seals as
649 they have been encountered in many plays around the Mediterranean. For example, shales
650 of the Kheir Formation in Libya and Souar-Cherahil Formation in Tunisia form a very good
651 seal for Palaeocene and Lower Eocene carbonate reservoirs (Macgregor & Moody, 1998).
652 In our model, we have assigned seals within the Late Cretaceous and Oligocene-Miocene
653 stratigraphic intervals. Dolson et al. (2001) state that the widespread transgression during
654 the development of the Late Cretaceous passive margin led to regional deposition of shales
655 that acted both as source rocks and seals. In the Oligocene-Miocene, the thin sheet-like
656 source rocks, inter-bedded within the turbidite sands, are assumed to act also as high
657 capillary entry pressures seal rock.
29
658 Table 1: A summary of the five source rocks used in the petroleum systems model based on analogues around the Neo-Tethyan Margin
Initial TOC Initial HI Source rock Analogue Depositional Environment Kerogen Type (weight %) (mgHC/g TOC)
Kupferschiefer Formation, North Sea Stratified lake with restricted communication with open Type I 5-15% >600 water systems. (Glennie, 1998) (Glennie, 1998) (Glennie, 1998) (5) No kinetics has been assigned as this source rock is
Upper Upper thermally immature
Miocene
Information about Kerogen type, TOC and HI are available Based on Villinski (2013) findings from drilling (i.e. Biogenic 1-2% 150-300 from drilling results of the Oligocene-Miocene successions Satis-1 and Satis -3) of the Oligocene successions
- (Villinski, 2013) (Villinski, 2013) (Villinski, 2013) offshore Nile (Villinski, 2013) offshore Nile Delta
Niger or Mississippi systems could also be feasible analogue. Sandy turbidite basin floor fans delivered by proto-Nile
(4) river.
rift rift Kinetic is based on biogenic reaction scheme following a
– Miocene Gaussian distribution (mean Temperature: 50 °C; standard Oligocene deviation 10 °C) Both reservoir successions and organic matters were
Post carried by these turbidite systems (Villinski, 2013).
(3.1) Chekka Formation (northern Lebanon) (Bou Daher et Broad, isolated bathymetric lows forming restricted Type II 2% 413-610 al., 2014) euxinic basins flanked by carbonate platforms. (Bou Daher et al., 2014) (Bou Daher et al., 2014) (Bou Daher et al., 2014)
(3.2) Sirte Shale (Libya)(Hallett, 2002) Deposition occurred in an outer-shelf setting.
(3) (3.3) Bahloul Formation (Tunisia) (Klett, 2001)
Upper (3.4) Brown Limestone (Gulf of Suez) (Alsharhan, 2003)
Cretaceous
Kinetics is based on di Primio and Horsfield (2006)
Laminated organic-rich lime mudstone deposited in a Type II 3% 600-800 relatively short lived intra-shelf basin. (2.1) Hanifa and Tuwaiq Formation (Saudi Arabia) (Carrigan (Carrigan et al., 1995) (Carrigan et al., 1995) (Carrigan et al., 1995) et al., 1995) The basin was partially separated from neo-Tethys
Ocean by flanking paleo-highs composed of grainstone shoal/ barrier island facies.
rift
-
(2) Composed mainly of shales, sandstones with coal seams Type I/II 0.11-3.5% 36-766
Syn and minor limestones deposited in a deltaic (2.2) Khatabta Formation (South Alamein Western Desert) environment. (Alsharhan & Abd El- (Alsharhan & Abd El- (Alsharhan & Abd El-
Upper Jurassic Upper (Alsharhan & Abd El-Gawad, 2008) Gawad, 2008) Gawad, 2008) Gawad, 2008)
Kinetics used for the Upper Jurassic is based on Ungerer (1990)
Kurra Chine Formation (eastern Syria) (Jassim & Goff, 2006) Restricted intra-shelf basins with deposition of Type II and III 3% 500
condensed sections during the early transgressive phase. (Jassim & Goff, 2006) (Jassim & Goff, 2006) (Jassim & Goff, 2006) Kinetics used in the modeling is based on (Ungerer, 1990)
rift
-
(1)
Pre
Upper Triassic Triassic Upper
30
659 6.2.1 Source rock maturity
660 Due to the very high uncertainty of the kinetics (describing the kerogen to
661 hydrocarbon transformation) chosen for the modelling of the thermogenic
662 source rocks, we prefer describing general thermal maturity windows, based
663 on source rock kinetic-independent vitrinite reflectance calculation (Sweeney
664 and Burnham, 1990). The present-day thermal maturity as modelled is shown
665 in Figure 9a. Any potential Upper Triassic source rocks are assessed to be in
666 the “gas window”, as are some of the Lower Jurassic source beds. Upper
667 Jurassic source rocks are modelled to be currently in the “oil window” and, if
668 present, might be the most effective thermogenic source rocks at present-day.
669 The model suggests that potential Oligocene to Miocene source rocks are
670 immature for thermogenic oil and gas generation. The modelled evolution of
671 the thermal maturity of the source rocks intervals (Upper Triassic/Lower
672 Jurassic, Upper Jurassic, Upper Cretaceous, Lower Miocene and
673 UpperMiocene (Messinian)) is shown in Figure 9b. The rift-related heat peak
674 (black line) affects mainly the Upper Triassic (pink line) and marginally the
675 Jurassic (blue line) interval. General burial-related temperature increase is
676 modelled in post-rift times, accentuated during the Upper Miocene and, in
677 particular, during the Pliocene-Pleistocene.
678 As discussed above, the combined effect of elevated surface temperatures
679 and rapid burial by salt deposition during the MSC triggered a subsurface
680 temperature increase, which became less significant with depth. The
681 petroleum systems modelling, indicates that the thermogenic source rocks are
682 only marginally affected by the MSC (Figure 9c) since it is limited to a 31
683 relatively minor increase in thermal maturity in the still immature Oligocene-
684 Miocene. The impact of the subsurface temperature increase is however
685 much more significant on the biogenic system. Since the simple temperature-
686 dependent kinetic applied to the biogenic source rocks is very sensitive to
687 changes in the range 30 to 70°C, which corresponds the thermal window the
688 shallow Miocene sediments are in during the MSC. Dependent on the pre-
689 Messinian state of maturity, kerogen to gas transformation ratios can increase
690 by up to 80% during the MSC. Deeper Oligocene source rocks are not
691 modelled to be affected since they have been buried deep enough and
692 therefore have gone through the narrow biogenic gas generation window prior
693 to the MSC.
32
694 6.2.2 Shallow gas accumulations
695 The high sensitivity of biogenic source rocks to the rapid heating of the shallow sub-
696 surface during the Messinian sea-level drop lead to a massive gas generation rate
697 (Figure 9d). The modelling results further suggest that, after some of the newly
698 generated gas was adsorbed by the source rocks and saturated their pore space as
699 free gas, the majority of it was expelled rapidly from the source beds. In addition, the
700 sea-level drop destabilized possibly pre-existing gas accumulations, since the pressure
701 drop lead to a gas density decrease (gas volume increase), a rapid desorption of gas
702 from the source rocks and exsolution of gas previously dissolved within the pore water.
703 All those sudden changes are modelled to have had as a common consequence the
704 gas flooding of the sub-Messinian strata during the MSC. At the same time, the storage
705 capacity for that gas went dramatically down, with the load of the precipitating salt
706 leading to a rapid compaction of the sub-Messinian sediments.
707 Those effects have not been further quantified, since too many important unknowns
708 exist in particular around the characterization of the biogenic source rock (spatial
709 distribution, kinetic, richness, adsorption capacity). However, it can be stated that the
710 combined effect of newly generated biogenic gas, the destabilization of previously
711 accumulated gas and the modification of the physical environment lead to a massive
712 gas charge in a setting, which cannot easily contain the additional gas volumes.
713 The consequence of the gas flood is assumed to have been regional fracturing of
714 stratigraphically concentrated around the biogenic source rocks and gas-bearing layers,
715 triggering accelerated gas expulsion and migration. Whereas the gas has been
716 modelled to have migrated predominantly vertically prior to and during the sea-level
33
717 drop, with an accelerated surface leakage, the load of the Messinian salt fundamentally
718 changed that pattern. Since in the model the Messinian salt has been assumed
719 impermeable, the underlying sediments became quickly overpressured. From that
720 moment on, the gas was modelled to migrate laterally and escape to the sea floor along
721 the depositional edges of the salt. Similarly to the temperature effect, the overall
722 influence of the MSC on the shallow gas system is modelled to diminish with depth.
723 Sediments of the Upper Miocene are more exposed to the MSC, which is due to several
724 factors. Firstly, shallow biogenic source rocks had more gas generation potential prior to
725 the MSC as opposed to already deeper buried biogenic source rocks, which were
726 already partially to totally transformed. In addition, the temperature changes due to the
727 sea-level drop, which triggered the gas generation according to the simple kinetic used
728 for modelling, are of higher magnitude in those shallow sediments (Figure 7d).
729 Furthermore, porosity reduction due to the rapid compaction by the salt (Figure 7b), and
730 related fluid migration are accentuated in the shallow sediments, since they were
731 relatively freshly deposited, as opposed to partially compacted buried rocks. Finally,
732 since the gas escape occurred predominantly vertically, upper sediments were not only
733 exposed to the destabilization effects of “local” biogenic gas, but also received all the
734 fluids the deeper sediments could not hold.
735 Figure 10 presents the modelled present day hydrocarbon saturation and the migration
736 vectors of the last timestep. The general basin-wide fluid escape system can be
737 observed, with regional migration trends from the centre towards the eastern and
738 western edges of the Messinian salt, where fluids leak to the sea floor. Biogenic gas
739 accumulations have been modelled in the structural highs in the Miocene and Oligocene
34
740 levels, with saturations which progressively increase with depth. It can therefore be
741 interpreted that, as a result of the above-mentioned processes, the “preservation
742 window” of biogenic gas accumulation starts at a depth of approximately 1km below the
743 base of the Messinian salt. This “line” divides the Oligocene to Miocene into a domain
744 where the MSC had rather destructive effects on the potential for biogenic gas and a
745 domain where these effects where attenuated. All current discoveries are located below
746 that line when projected on the modelled cross section (Figure 10b).
747
748
749 6.2.3 Impact of the MSC on existing oil accumulations
750 Possible Mesozoic source rocks, yet to be proven, are modelled to be in the oil window
751 or below (Figure 9a) mostly before the MSC (Figure 9b). Thermogenic generation of
752 hydrocarbon is modelled, and the migration results indicate deep oil accumulations in
753 the Cretaceous focussed on the structural highs of the basin (Figure 10a). At the crest
754 of those structures and at the basin edges, the oil is modelled to leak into the Cenozoic.
755 That suggests the possibility of a mixture of thermogenic hydrocarbon, predominantly
756 oil, and biogenic gas in the Oligocene to Miocene sediments. In the following section,
757 hypothetical accumulation cases and their possible reaction to the MSC are discussed.
758 The authors insist on the fact that the model has been set up in a way to create those
759 “critical mixtures” to test and showcase their general behaviour during the MSC, and to
760 propose a methodology of risking the effect of the MSC on possible accumulations. In
761 no case, the model accumulations discussed below should be confused with existing
762 fields, discoveries or prospects.
35
763
764
765 (i) Phase Change
766 The phase of a sub-surface accumulation, i.e. which hydrocarbon components (oil, gas)
767 are in liquid or vapour state, is dependent on the in situ pressure and temperature (PT;
768 Dandekar, 2013). Dependent on the initial composition of an oil accumulation, PT
769 changes during production often causes phase changes. When the reservoir pressure is
770 depleted, gas previously contained in the oil (gas component in liquid phase, or
771 “associated gas”) can exsolve to form a gas cap. The same phenomena, at different
772 time scales, occur during the geological history. Generally, changes in pressure and
773 temperature are very progressive at geological timescales. However, the sea-level drop
774 associated to the MSC occurred can be considered as a geological “moment” due to its
775 very short duration (5.96 to 5.6 Ma).
776
777 Figure 11a shows the same PT extraction for one model cell (Upper Miocene at the
778 vicinity of the projected Tamar field) as in the time extraction Figure 7b, but in a PT
779 diagram. The order of the extraction points reflect the geological time aspect, each point
780 represents a 10 kyr modelling step. The following trends can be seen: pressure drop
781 and heating (1) during the sea-level fall, pressure increase and further heating by the
782 burial of the salt (2), followed by a progressive pressure increase and cooling phase
783 during the deposition of the Lago Mare Formation (3) and a very rapid pressure
784 increase as a response to the flooding event (4), and finally the post-MSC heating at a
36
785 near-constant pressure due to the post-MSC burial (5). On the same PT diagram, any
786 phase diagram of a possible analogue oil, in that particular case from the Al Shaheen
787 field offshore Qatar (Lindeloff et al., 2008, Al Ghafri et al., 2014), can plotted. The Qatar
788 crude oil has been selected as an analogue because it represents one of the Neo-
789 Tethyan margin basins that shares a very similar tectonic evolution history to the
790 eastern Mediterranean. More importantly, this crude is produced from the Cretaceous
791 carbonate reservoirs, which are speculated to be active, but as yet untested, in the
792 eastern Mediterranean. The bubble point curve of that phase diagram separates the
793 liquid domain from the mixed liquid and vapour domain, On this combined plot it can be
794 analysed when the PT path of the model cell crosses the bubble point line, and
795 therefore when and by how much the corresponding oil mixture is expected to change
796 phase, i.e., starts to build a gas cap.
797
798 PT extractions of other model cells, either at different locations along the model section
799 or at different depth, can be plotted on that diagram. Those locations could correspond
800 to existing fields, discoveries or prospects. None of those accumulations need to be
801 specifically simulated, since only the results of the pressure and temperature modelling
802 are extracted from the model. The PT plot on Figure 11b illustrates two important points.
803 Firstly, the first-order control on phase change during the MSC is not temperature
804 variation, but pressure changes. The pressure drop depends directly on the amount of
805 sea level drawdown assumed in the model. The two-step pressure increase is due to
806 the loading by the salt followed by the refill and flooding. Secondly, and even more
807 importantly, is the observation that not all existing pre-Messinian hydrocarbon
37
808 accumulations are sensitive to the modelled PT changes. Deep accumulations are
809 much more likely to be ‘protected’ from gas exsolution than shallow ones. However, the
810 main variable is the hydrocarbon composition and the corresponding phase diagram.
811 Neither a typical black oil containing hardly any associated gas (Dandekar, 2013,
812 pp.135) nor a pure gas accumulation already in the vapour phase would react to the
813 MSC with a phase change. Only specific hydrocarbon mixtures are concerned, and the
814 closer the reservoir PT conditions were to the bubble point curve prior to the MSC, the
815 more likely it was to react by gas exsolution (“bubble point oil”;Dandekar, 2013).
816 Obviously, many different possible analogue oils can be plotted in the diagram, together
817 with an uncertainty envelope.
818
819 (ii) Impact of the Phase Change
820 The combined PT plot (Figure 11b) can provide a good understanding whether a
821 possible accumulation in given model location, or trap, might have been subject to
822 phase change or not. However, a phase change can have drastically different
823 consequences on that trap, dependent on controlling parameters of the trap fill. To
824 investigate how phase changes could have affected pre-Messinian hydrocarbon
825 accumulations, three trap scenarios have been modelled, where each represents a trap
826 fill-controlling parameter: (A) amount of charge, or “charge limited”, (B) seal strength, or
827 “seal limited”, and (C) trap size, or “trap limited” (Figure 12). Those traps have been
828 filled by a “near bubble point” hydrocarbon mixture of mainly thermogenic oil with some
829 biogenic gas has been created to fill the trap, referred to as “reactive accumulations”.
38
830 The oil has essentially been sufficiently enriched with gas that those accumulations can
831 undergo gas exsolution during the MSC and its consequences can be modelled. The
832 migration method used in the simulation is Invasion Percolation (Wilkinson, 1984,
833 Hantschel and Kauerauf, 2009).
834
835 Scenario A: Charge-limited 836 837 A charge-limited accumulation represents an under-filled trap because of the little
838 amount of hydrocarbon charge it received with respect to the available pore volume,
839 which is assumed to be much larger than the charge amount in that scenario. The seal
840 is assumed to be very strong. Scenario A showcases the evolution of a reactive
841 accumulation within a charge-limited trap which experiences a phase separation and a
842 related increase in volume and column height during drawdown. At the peak of the
843 MSC, two phases could conceivably coexist because the fluid expansion was
844 accommodated by the under-filled and large trap. After the peak of drawdown, the
845 accumulation volume is modelled to decrease and the two phases to merge again into a
846 single liquid phase. Since the trap is large enough and the seal is effective, no
847 hydrocarbon losses are modelled to occur, and the post-Messinian composition is
848 comparable to the composition prior to the MSC.
849 850 Scenario B: Seal-limited 851 852 A seal-limited accumulation represents an under-filled trap because the buoyancy
853 pressure of the hydrocarbon column is in equilibrium with the capillary entry pressure of
854 the seal, and no further increase of the column height is supported. Scenario B
39
855 describes how a reactive accumulation within a seal-limited trap experiences a phase
856 separation and a related increase in volume and buoyancy pressure, leading to leakage
857 through the top seal. Theoretically, with preferential leakage of vapour gas, there is a
858 corresponding relative enrichment in the liquid oil component. However, because the
859 rates of buoyancy increase imposed by the rapidly falling sea level are abnormally high,
860 the maximum rate of capillary leakage is likely to be reached relatively quickly. If the
861 seal cannot release the vapour gas as fast as the buoyancy increases, it fractures and
862 there could be catastrophic seal failure.
863 864 Scenario C: Trap size-limited 865 866 A trap size-limited accumulation represents a trap which is filled to spill, with the
867 hydrocarbon charge amount exceeding the available pore space and a very strong seal.
868 Scenario C shows a reactive accumulation within a size-limited trap that is filled-to-spill
869 prior to the MSC. The accumulation experienced a phase separation and a related
870 increase in volume and column height during drawdown. The vapour cap (mostly gas
871 components) expels the liquid “leg” (mostly oil components) downwards via the
872 structural spill point and relative enrichment of vapour gas components occurs. Once
873 the lake level rises again, the increase in pressure compressed the gas cap, resulting in
874 an under-filled trap with higher gas content, possibly with a remaining gas cap if not
875 enough oil is present any more, than prior to the MSC.
876
877 It should be noted that these scenarios represent end-member cases, whereas in
878 nature, the controlling parameters on trap fill are likely to evolve. However, the three
40
879 modelling scenarios demonstrate that those parameters need to be individually
880 investigated to increase the understanding of the present day composition and column
881 height of fields and discoveries, and in the assessment of leads and prospects.
882
883 7. Discussion
884 The presented modelling results suggest that the sea-level fall associated to the
885 beginning of the MSC triggered a large drop in sub-surface pore pressure, immediately
886 and deeply penetrating the basin, accompanied by a surface temperature increase,
887 which attenuated with depth. These changes in PT conditions are modelled to
888 dramatically impact both the shallow biogenic and deeper thermogenic petroleum
889 systems, resulting in sudden fluid-related fracturing events which are stratigraphically
890 concentrated (around biogenic source rocks and gas bearing layers) and structurally
891 controlled (centred around the crest of paleo-accumulations). In particular, the effect on
892 traps with seal-limited “bubble point oil” accumulations are modelled to have triggered
893 catastrophic fluid escape events. Evidence for such fluid events is present in our data,
894 and has been documented in the field from several circum-Mediterranean locations
895 (Iadanza et al., 2013)
896
897 Pockmarks are geomorphic expression of focused subsurface fluid flow and common
898 features on the ocean floor (King & MacLean, 1970; Schroot et al., 2005; Judd &
899 Hovland, 2007). Ancient (i.e. buried) (e.g. Bizarro, 1998; Gemmer et al., 2002;
900 Andresen et al., 2008) pockmarks are also observed in many basins, and they are
901 commonly indicative of the escape of gas-rich fluids to the palaeo-seafloor. Ancient
41
902 pockmarks are thus critical elements in establishing the fluid flow history of a
903 sedimentary basin (e.g. Dimitrov, 2002; Loncke et al., 2004; Dupré et al., 2010). Buried
904 depressions occur in our dataset at the base Messinian surface, offshore Western
905 Desert, Egypt (section BB’, Figure 1) in present water depths of ca. 3000 m. In cross
906 section (Figure 13), the depressions are characterised by symmetrical ellipsoid, ca. 2-3
907 km wide and about ca. 500 m deep, which truncate underlying reflections. Although
908 sub-salt imaging can be challenging and is often associated with acoustic distortions
909 effects that can distort the primary geometry of underlying seismic reflections, such
910 acoustic disturbance has been documented above the South Arne field in the North Sea
911 where they are interpreted to be related to fluid escape features (Andresen et al., 2008).
912 Based on their similar geometry to the depressions observed in North Sea and
913 elsewhere (Pilcher & Argent, 2007; Andresen & Huuse, 2011; Ostanin et al., 2012), we
914 interpret that these depressions are pockmarks that are caused by focussed fluid flow
915 from deeper, pre-Messinian structures.
916
917 Our interpretation suggests that these pockmarks most likely formed in response to
918 focused fluid flows supported by the observation that other base Messinian pockmarks
919 are observed in 3D seismic data from the nearby Levant Basin (Lazar et al., 2012;
920 Bertoni et al., 2013). More than 35 pockmarks, ranging in diameter from 100 to 2000 m,
921 have been mapped at the base of the pre-Messinian Afiq Canyon by Bertoni et al.
922 (2013). Bertoni et al. (2013) interpreted these pockmarks as to have formed in response
923 to gas venting occurring during the Messinian drawdown (cf. Fraser et al. (2011)).
42
924 The reaction of hydrocarbon to changes in overburden load has been described in the
925 Barents Sea, offshore northern Norway (Henriksen et al., 2011), where regional uplift
926 occurred due to the isostatic rebound following Plio-Pleistocene deglaciations. Fluid and
927 gas escape to the seafloor is manifested by two giant pockmarks ca. 1-2 km wide in
928 addition to > ca. 300 pockmarks are mapped beneath the seabed on the Upper
929 Regional Unconformity. Six of these pockmarks are sufficiently large (ca. 1-2 km in
930 diameter) to be classified as ‘giant’. Nyland et al. (1992) and Skagen (1993) argue that
931 the pressure release during overburden unloading had a direct impact on existing
932 hydrocarbon accumulations in Hammerfest Basin. Similar to the size-limited trap
933 scenario presented in this study, the combined effect of gas exsolution from the oil
934 column and resulting gas cap expansion, is thought to have displaced the oil via the
935 structural spill point. This would have resulted in spilling of significant amounts of oil
936 from existing traps (Nyland et al., 1992; Henriksen et al., 2011). Residual oil columns
937 ca. 100 m below the present-day oil-water contact has been reported from the Snøhvit
938 gas field in the Hammerfest Basin (Henriksen et al., 2011) and might be related to the
939 gas cap expansion and oil spill mechanism.
940
941 Successions of vertically stacked pockmarks have also been documented in the Lower
942 Congo Basin, offshore West Africa (Andresen & Huuse, 2011). The timing of formation
943 of these pockmarks appears to coincide with periods of relative sea-level fall, leading
944 Andresen and Huuse (2011) to suggest that water load removal and subsurface
945 depressurisation was the key trigger for fluid escape and pockmark formation (Andresen
946 & Huuse, 2011).
43
947
948 Outcrop evidence for MSC-related palaeo-fluid escape comes from a Late Messinian
949 limestone-bearing unit (Brecciated Limestones Unit) exposed in Maiella, central Italy.
950 Iadanza et al. (2013) suggest upward migration of hydrocarbon-rich fluid through the
951 Messinian succession accompanied by brecciation was triggered by the sudden
952 depressurization associated with Messinian drawdown. The migration of hydrocarbon-
953 charged fluids is proven by distinct tar impregnation in the carbonate microfacies,
954 suggesting that the area experienced two main phases of fluid flow (Iadanza et al.,
955 2013). The early phase was characterised by light hydrocarbon fluid migration
956 ascending at high flow rates; this induced widespread brecciation of the overlying
957 sedimentary column. During the Late phase, oil and heavier hydrocarbons migrated
958 upwards, presumably assisted by the previously formed fluid flow pathways (Iadanza et
959 al., 2013).
960
961 The lack of general understanding of biogenic gas generation and its relatively weak
962 implementation in petroleum systems modelling software limits the predictions. The
963 “temperature only”-dependent source rock kinetic presents an extreme simplification.
964 The biogenic gas kinetic published by Middelburg et al. (1991), which mainly depends
965 on sedimentation rate, has been applied to the model. However, due to the lack of high-
966 resolution dating of the sub-Messinian sediments, which are necessary to define a
967 sedimentation rate at the resolution of the modelled process, the modelling results have
968 been judged not trustable. In addition, even if high resolution dating of the Miocene
969 sediments were available, since those are in great burial depth at present day, the
44
970 decompaction necessary to restore depositional sedimentation rate adds another big
971 uncertainty, which might be well above the resolution of the results. It should be noted
972 that the kinetic by Middelbourg (1995) does not have any temperature dependency.
973 However, even though the impact of the MSC on biogenic gas generation is
974 questionable due to a simplified kinetic, the other mechanisms modelled and described
975 above should be sufficient enough to heavily destabilise the biogenic gas accumulations
976 during the MSC.
977 Another big uncertainty, heavily disputed in literature, are the amplitude of the sea level
978 drop, the relative chronology and absolute timing of the MSC. The way the MSC has
979 been implemented in the basin model (Figure 5) presents only one possible simplified
980 scenario, with uncertainties are at many levels. Where no fundamental impact on the
981 modelling results has been assumed, e.g. a single phase sea-level drop instead of a
982 cyclic sea-level drop, those simplifications were taken into account without further
983 sensitivity analysis. One major uncertainty to be further investigated concerns the
984 relative timing between the sea-level drop and the salt precipitation, since the sea-level
985 drop triggers fluid escape, whereas the sealing salt is more likely to retain some of the
986 fluids, or at least complicate their way to the basin floor. In reality, the Messinian “salt” is
987 not as clean and sealing as assumed in the modelling, and recent observations of intra-
988 Messinian fluid escape structures (Eruteya et al., 2015) suggest a more complex
989 migration pattern, where fluid chimneys initiated during the MSC were reused until
990 recent times. However, the overall impact of the MSC on both the shallow biogenic
991 system and deeper oil accumulations remains undisputable and should be considered
992 in the individual understanding and risking of leads and prospects in the eastern
45
993 Mediterranean Sea, and possibly in other basins where similar events are reported to
994 have occurred.
995
996 8. Conclusions and implications for oil and gas exploration
997 In this study, 2D basin and petroleum systems modelling has been conducted to
998 demonstrate the impact of the geologically instantaneous Messinian Salinity Crisis
999 (MSC), with a water drawdown, subsequent salt deposition and re-flooding of the basin,
1000 on the proven biogenic and speculative thermogenic petroleum systems of the eastern
1001 Mediterranean Sea. The study demonstrates that significant changes in subsurface
1002 pressure and temperature are likely to have occurred. The sea level fall of 2070 m is
1003 assumed to have taken place in an extraordinarily short time period (i.e 360 kyr) and
1004 caused the in situ pore pressure of underlying sediments to drop by ca. 20 MPa
1005 (equivalent to ca. 2900 Psi) The pressure drop is modelled to be instantaneous and to
1006 affect even deeply buried layers. In contrast, the model suggests that the effect of the
1007 MSC on temperature was more transient, i.e. slightly delayed in time and attenuated
1008 with increasing burial depth. The deposition of ca. 1500 m of Messinian salt is assumed
1009 to have followed the sea-level drop, partly reloading and therefore pressurising,
1010 compacting and heating the underlying sediments very rapidly (45 kyr). The basin has
1011 been refilled with water in two steps, firstly a progressive fill while depositing the Lago
1012 Mare Formation, and finally the abrupt Zanclean flooding event, which marks the end of
1013 the MSC.
1014
46
1015 These changes in pressure and temperature are modelled to have severely affected the
1016 shallow sub-Messinian biogenic petroleum system. Rapid compaction and heating, an
1017 accelerated gas generation and expulsion, gas density changes and a decrease of
1018 adsorption capacity and water solubility are assumed to have led to fluid brecciation
1019 stratigraphically concentrated around biogenic source rocks and gas-bearing layers and
1020 triggered massive fluid escape events. It appears that the Upper Miocene is far more
1021 affected than the deeper Miocene and Oligocene, indicating a “preservation window” of
1022 biogenic gas accumulations starting at ca. 1km below the Messinian salt. All recent gas
1023 discoveries (Tamar, Leviathan, Aphrodite) are located within this window. Furthermore,
1024 the deeply penetrating pressure drop is modelled to have caused shallower, pre-
1025 Messinian hydrocarbon accumulations with a specific “bubble point oil” composition to
1026 undergo a phase change, i.e. to build a gas cap by exsolution from the oil. However,
1027 even though affected similarly by the pressure drop, deeper accumulations with similar
1028 composition are more likely to have been preserved from such phase changes, since
1029 accumulations in highly pressured depths are assumed to be always in the liquid PT
1030 domain. The impact of the phase change on the pre-Messinian hydrocarbon
1031 accumulations was found to be dependent on the trap-fill controlling parameters: seal-
1032 limited, size-limited and charge-limited. In the former case, catastrophic seal failure has
1033 been modelled as the most likely consequence of the rapidly increasing gas column
1034 triggered by the MSC. Size limited-traps, in comparison, were predicted to experience
1035 tertiary hydrocarbon migration, with the growing gas cap pushing the liquid oil down to
1036 spill out of the trap. In the latter case of charge-limited traps, the accumulations are
1037 likely to be of similar size and composition after the MSC.
47
1038
1039 This research focused on the Levant Basin, although it has implications for many of the
1040 Mediterranean basins affected by the MSC (e.g. Sirt Basin, offshore Libya). The authors
1041 strongly recommend that any future exploration activity should integrate the MSC in
1042 basin and petroleum systems models and prospect risk analysis. A comprehensive
1043 methodology has been suggested with the use of pressure and temperature extractions
1044 at prospect locations, in conjunction with phase diagrams of possible oil analogues.
1045 Shallow traps estimated to be charged prior to the MSC and capped by weak seals are
1046 predicted to be very high-risk prospects, especially if there are associated with palaeo-
1047 pockmarks in the overburden. Mapping these palaeo-pockmarks may therefore be a
1048 very useful tool to delineate the high and medium-risk prospects. Medium risk prospects
1049 are expected in similar traps within the vicinity of these palaeo-pockmarks but not
1050 directly beneath them. In contrast, deep accumulations are classified as low risk
1051 prospects as they are less likely to have been affected by the MSC drawdown.
1052
1053 Acknowledgments
1054 Petroleum Development Oman (PDO) is thanked for providing the funding for the first
1055 author PhD project at Imperial College London. British Petroleum Egypt is thanked for
1056 providing the data and allowing this study to be published. Schlumberger is also
1057 thanked for providing Petrel and PetroMod software. In particular, we would like to thank
1058 Bjorn Wygrala, Thomas Hantschel, Armin Kauerauf, Thomas Fuchs, Michael Fuecker
1059 and Adrian Kleine (Schlumberger Technology Centre at Aachen) for useful discussion.
1060 Saif Al-Ghafri from the department of chemical engineering at Imperial College is also
48
1061 acknowledged for providing the phase envelope for the Qatari crude oil we used in this
1062 study and for the useful discussion on the phase behaviour of reservoir fluids.
1063
1064
1065
1066
1067 1068 1069 Figure Captions
1070
1071 Figure 1: Location map of the eastern Mediterranean Basin showing the study area, the
1072 database available for this study and the distribution of the hydrocarbon fields (mainly
1073 gas) in the Nile Delta region. Blue lines show the seismic profiles we have used. Profile
1074 A-A’: 235 km long seismic profile that has been used to construct the petroleum
1075 systems model. The recent gas discoveries (Tamar, Leviathan and Aphrodite) in the
1076 vicinity of this seismic profile are indicated by the red squares numbered 1, 2 and 3
1077 respectively. Profile B-B’: 35 km long seismic profile showing evidence for pockmarks at
1078 base Messinian. Blue circles show the location of the Deep Sea Drilling Program
1079 (DSDP) holes drilled over the Eratosthenes Seamount and to the west of Cyprus.
1080
1081
1082 Figure 2: Tectono-stratigraphic chart summarising the pre-rift, syn-rift and post-rift
1083 geologic history of the present-day Levant Basin. The key seismic horizons (H1-H9)
1084 bounding the major seismic units (SU1-SU8) as described in this paper are shown in
49
1085 the chart. The stratigraphic intervals of the petroleum systems elements (source,
1086 carrier/reservoir and seal) assigned to the petroleum systems model are also shown.
1087
1088 Figure 3: (a) uninterpreted, (b) partially interpreted and (c) interpreted seismic reflection
1089 profile and (d) the model used in this study. The approximate location of well control
1090 from the Eratosthenes Seamount and the Levant Margin are shown in (a). Horizons with
1091 dashed lines and yellow question marks in (b) indicate areas where the data quality
1092 deteriorates and hence the interpretation is less certain.
1093
1094 Figure 4: A series of structural reconstructions describing the evolution of the Levant
1095 Basin through time with particular emphasis on the MSC period (reconstructions from
1096 (d) to (g)). (a) In the Late Jurassic, the basin is dominated by shallow marine carbonates
1097 with carbonate build-ups on the structural highs. (b) In the Late Cretaceous,
1098 convergence inverted some of the earlier rift structures and caused the development of
1099 anticlines. (c) In the Late Cretaceous to Eocene, organic rich source rock and chalk
1100 were deposited. (d) At the start of the MSC (5.96 Ma), the basin was about 3 km deep.
1101 (e) At the time of the maximum drawdown (5.60 Ma), the water depth was only about 1
1102 km and Messinian shale has already been deposited. The Eratosthenes Seamount was
1103 exposed subaerially during this time. (f) At end of salt deposition (5.55 Ma), the water
1104 depth was maintained at about 1 km and the Eratosthenes Seamount is in subaqueous
1105 condition because of the subsidence associated with salt deposition. (g) At the end of
1106 the MSC (5.33 Ma), the basin continued subsiding due to the water loading associated
50
1107 with the Zanclean flood. (h) At present-day, the Messinian salt is overlain by Plio-
1108 Pleistocene sediments that are thicker on the margin and thinner in the basin centre.
1109
1110
1111
1112 Figure 5: Schematic diagram showing how the MSC was incorporated in the modelling.
1113 Sketch sections from (1) to (4) summarise the sequential order of the MSC
1114 development. (1) Is the interpretation of the pre-Messinian setting in the basin with sea
1115 level at normal Atlantic levels. (2) The MSC is modeled to have started at 5.96 Ma
1116 where sea level started dropping below Atlantic level. The maximum drawdown of c.
1117 2070 m was reached at 5.60 Ma and we assumed that the water depth is c.1000 at the
1118 time of maximum drawdown. During this time (5.96-5.60 Ma), as the circulation in the
1119 basin became restricted, the Messinian shale was modelled to have been depositing as
1120 indicated by the thin black layer. (3) The Messinian salt deposition (c. 1500 m) was
1121 modeled to have occurred between 5.60-5.55 Ma. During the salt deposition, we
1122 maintained the water depth at c. 1000 m by steadily increasing the Messinian lake level
1123 as indicated by the red arrows. After Messinian salt deposition, the lake level continued
1124 to rise, and the deposition of the Lago-Mare facies was modeled to take place between
1125 5.6-5.33 Ma. (4) The end of MSC which was modelled by the Zanclean flooding to be at
1126 5.33 Ma. The Zanclean flooding was incorporated in our model by filling the basin
1127 instantaneously as indicated by the very steep slope of Messinian sea level curve.
1128 Subsequent Plio-Pleistocene deposition occurred from 5.33 Ma to present-day.
51
1129 Pressure and SWIT conditions associated with these steps are indicated on the
1130 diagram.
1131
1132 Figure 6: (a) The present-day overpressure and temperature as predicted by our basin
1133 model. Overpressure field as shown by the overlay (scale is in bottom left of the figure)
1134 predicts that the Messinian salt and Eocene are overpressured. Temperature variation
1135 across the profile is shown by the isolines (in yellow). To compare the effect of the
1136 Messinian salt on temperature across the profile, geothermal gradients have been
1137 extracted at the basin centre (b) and the basin margin (c). In the basin centre (b) where
1138 the salt is about 1200 m thick, the temperature is predicted to be c. 85°C at an
1139 approximate depth of c. 4500 m. At the basin margin (c) where the salt is thinner, the
1140 temperature is predicted to be c. 125°C for the same burial depth.
1141
1142 Figure 7: Pressure (c) and temperature (d) time extraction plots between 6.5 - 4.5 Ma
1143 at four different depths: Basement (7000m); red, Top Mesozoic (3650m); green,
1144 Oligocene/Miocene (Tamar: 1800m); brown and Top Miocene (close to surface);
1145 orange. On diagram (b), temperature (red line) shows a transient response to water
1146 unloading and evaporite loading whereas pressure (blue line) shows an instantaneous
1147 response.
1148
1149 Figure 8: Schematic palaeo-facies maps for the post rift megasequence in the eastern
1150 Mediterranean. (a) Carbonate dominated passive margin during the Cretaceous. (b)
1151 Clastic dominated margin (Oligocnene-Miocene). Source rocks are predicted in the
52
1152 Cretaceous deep-water deposits and Oligocene organic rich turbidite sands. The
1153 Oligocene and later Miocene Nile derived turbidites also act as major lateral carrier
1154 beds in the basin.
1155
1156 Figure 9: Present-day thermal maturity as modelled in this study. (a) An overlay of
1157 vitrinite reflectance (key in the bottom left) shows the maturation stages within various
1158 stratigraphic intervals. (b) The thermal maturity evolution. The black solid line shows the
1159 heat flow variation (input to the modeling) associated with the Middle Jurassic -Early
1160 Cretaceous rifting based on McKenzie stretching model. The modelled thermal
1161 evolution (dotted line: temperature; solid line: vitrinite reflectance is shown at the
1162 stratigraphic level of potential source rocks (i.e. Upper Triassic: pink, Upper Jurassic:
1163 blue, Upper Cretaceous: green, Miocene-Oligocene: orange, Upper Miocene
1164 (Messinain): purple. The location of the extractions along the cross section are shown in
1165 Figure 6. (c) A zoomed in view of plot (b) showing the Late Miocene (6.0-5.30 Ma)
1166 interval. (d) Transformation ratios for biogenic source beds. Kinetics modelled after
1167 Middleburg et al. 1991.
1168
1169 Figure 10: (a) The modelled thermogenic generation of hydrocarbons in the Levant
1170 Basin. The migration results indicate deep oil accumulations in the Cretaceous focused
1171 on the structural highs in the basin. At the crest of these structures the oil is modeled to
1172 leak vertically into the Cenozoic. (b) Modelled biogenic accumulations in the Levant
1173 Basin sitting above the 1km sub base Miocene salt preservation window.
1174
53
1175 Figure 11: (a) shows the modelled pressure-temperature extraction for one model cell
1176 (Upper Miocene at the vicinity of the projected Tamar field) as in the time extraction
1177 Figure 7b, but in a PT diagram. The order of the extraction points reflect the geological
1178 time aspect, each point represents a 10 kyr modelling step. The following trends can be
1179 seen: pressure drop and heating (1) during the sea-level fall, pressure increase and
1180 further heating by the burial of the salt (2), followed by a progressive pressure increase
1181 and cooling phase during the deposition of the Lago Mare Formation (3) and a very
1182 rapid pressure increase as a response to the flooding event (4), and finally the post-
1183 MSC heating at a near-constant pressure due to the post-MSC burial (5). On the same
1184 PT diagram, any phase diagram of a possible analogue oil, in that particular case from
1185 the Al Shaheen field offshore Qatar (Lindeloff et al., 2008, Al Ghafri et al., 2014), can
1186 plotted. The bubble point curve of that phase diagram separates the liquid domain from
1187 the mixed liquid and vapour domain, On this combined plot it can be analysed when the
1188 PT path of the model cell crosses the bubble point line, and therefore when and by how
1189 much the corresponding oil mixture is expected to change phase, i.e., starts to build a
1190 gas cap.
1191
1192 Figure 12: Models showing the impact of phase change for different trap geometries.
1193 Three trap scenarios have been modelled, where each represents a trap fill-controlling
1194 parameter: (A) amount of charge, or “charge limited”, (B) seal strength, or “seal limited”,
1195 and (C) trap size, or “trap limited”. Those traps have been filled by a “near bubble point”
1196 hydrocarbon mixture of mainly thermogenic oil with some biogenic gas has been
1197 created to fill the trap, referred to as “reactive accumulations”. The oil has essentially
54
1198 been sufficiently enriched with gas that those accumulations can undergo gas
1199 exsolution during the MSC and its consequences can be modelled. The migration
1200 method used in the simulation is Invasion Percolation (Wilkinson, 1984, Hantschel and
1201 Kauerauf, 2009).
1202
1203 Figure 13: Pockmark and fluid escape evidence from offshore Western Desert (refer to
1204 Figure 1 for location). The pockmark appears as a sub circular depression, 2-3 km wide
1205 and about 500 m deep, surrounded by truncational unconformities on either side. The
1206 chaotic distorted seismic character beneath and surrounding the pockmarks could be
1207 attributed to focussed fluid flow from the deeper pre-Messinian structures during the
1208 water drawdown which disrupted the overburden sediments.
1209
1210
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1510 Italy [PhD]. University of Cologne.
1511
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Figure 1 Click here to download Figure Fig_1.pdf Figure 2 Click here to download Figure Fig_2.pdf Figure 3 Click here to download Figure Fig_3.pdf Figure 4a Click here to download Figure Fig_4a.pdf Figure 4b Click here to download Figure Fig_4b.pdf Figure 4c Click here to download Figure Fig_4c.pdf Figure 5 ClickGeologic here Time (Ma)to download Figure Fig_5_new.pdf
6.0 5.9 5.8 5.7 5.6 5.5 5.4 5.3 5.2 1 Pre-Messinian 0
-20 Mpa 1 2900 psi Sea water 2 Sea level fall 13°C 2 13°C 55°F 55°F 40°C 104°F Messinian 3 salt 3 Salt deposition 4 Pre- Messinian 4 Re-flooding 5 1 – Pre-Messinian 2 – Sea level fall 3 –deposition Salt 4 – Re-flooding (km) (km) Depth Depth Geologic Time (Ma) Figure 6 Click here to download Figure Fig_6_new.pdf
94°C Figure 7 Click here to download Figure Fig_7_new.pdf Geologic Time (Ma) (a) 6.0 5.9 5.8 5.7 5.6 5.5 5.4 5.3 5.2
0
-20 Mpa 1 2900 psi Sea water
Depth [km] 13°C 2 8°C 55°F 46°F 40°C 104°F Messinian 3 salt
4 Pre- Messinian 5 4 – Re-flooding 4 – Re-flooding 1 – Pre-Messinian 3 –deposition Salt 2 – Sea level fall (b) 60
Porosity 60 40 40 Pressure
20 Porosity [%] Pressure [MPa] 20 Temperature [°C] Temperature 0 0
(c) 200
] Top Basement 150 (7000m) MPa Top Mesozoic 100 (3650m) Oligocene/Miocene (Tamar; 1800m) Pressure [ 50 Top Miocene (close to surface) 0
(d) 250 200 Top Basement (7000m) 150 Top Mesozoic (3650m) 100 Oligocene/Miocene (Tamar; 1800m) Top Miocene
Temperature [°C] 50 (close to surface) 0 Figure 8 Click here to download Figure Fig_8.pdf Figure 9a,b,c Click here to download Figure Fig_9a_c.pdf Figure 9d Click here to download Figure Fig_9d_new.pdf
(d) Figure 10 Click here to download Figure Fig_10_new.pdf
(a)
Messinian Salt
Depth [km]
0 Hydrocarbon Saturation [%] 100
(b) 3
4
5 Depth [km]
Aphrodite Leviathan Tamar Figure 11 Click here to download Figure Fig_11_new.pdf
(a) 100 5 4 3 2
] 1 MPa Pressure [ 5 - Burial until present day Liquid Phase envelope of oil accumulation 4 - Re-flooding Example from Al Shaheen Field 1 – Sea-level (offshore Qatar) drop 3 – Progressive Lindeloff et al., 2008 drowning Liquid & vapor Al Ghafri et al., 2014 2 - Salt deposition 0 0 Temperature [°C] 400
(b) 100 Top Jurassic (4500m) high
Top Cretaceous (3600m)
] MPa Oligocene (2000m) Probability of of Probability affected by MSC accumulation being Pressure [ Top Miocene low ??? (close to surface) Liquid Phase envelope of oil accumulation Example from Al Shaheen Field ??? (offshore Qatar)
Lindeloff et al., 2008 Liquid & vapor Al Ghafri et al., 2014
0 0 Temperature [°C] 400 Figure 12 Click here to download Figure Fig_12_new.pdf
Con nuous oil charge from Mesozoic
Con nuous oil charge from Mesozoic
Con nuous oil charge from Mesozoic
Con nuous oil charge from Mesozoic Figure 13 Click here to download Figure Fig_13.pdf