- Email: [email protected]
The Leviathan Miocene Diapir – A novel insight into the tectonic evolution of the Southern Levant Basin, Eastern Mediterranean
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Research paper
The Leviathan Miocene Diapir – A novel insight into the tectonic evolution of the Southern Levant Basin, Eastern Mediterranean Yuval Ben-Gai 7 Metsada St., Hod Hasharon, 4529418, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords: Eastern Mediterranean Levant Basin Seismic interpretation Mud diapirs Petroleum systems
The Leviathan gas field is the largest accumulation so far discovered in the Southern Levant Basin within the turbiditic Tamar Sand Complex. The discovery of this and other fields in the basin was made possible following acquisition of a vast amount of data since 2000. A consensus has been reached that the basin is dominated by Early Mesozoic extensional blocks related to opening of the Neotethys while Miocene folds formed as a result of its closing. Through the study of geophysical data over the Leviathan gas field and its vicinity, it is suggested that most of what we observe today has been formed since the Early Miocene and that the alleged old blocks are actually intrusive bodies. Their structural timing is accurately constrained by well and seismic data but not their composition, as none has yet been penetrated at any borehole. Mud diapirism is currently assumed, as volcanic or magmatic origin is discarded by magnetic data and salt is yet a remote possibility. The tectonic reconstruction suggests that the Levant Basin developed during the Mesozoic and Early Cenozoic as a set of regional highs and lows that were buried since the Early Tertiary until the Early Miocene by thick conforming sequences including potential reservoir rocks such as the gas-bearing turbiditic sands. It was then subject to an extensional tectonic regime associated with diapirs in the southern part of the basin that formed folded structures while rotational blocks developed to the north, offshore Lebanon. The implications of this tectonic setting on petroleum systems are of prime importance; widespread intrusions may have breached old traps, if they existed. On the other hand they formed potential traps such as the proven gas-filled structures and potentially others at depth. Presence of reservoir rocks and the timing of expulsion and migration from deep sources are the main geological risks.
1. Introduction The Levant Basin has recently gained a lot of attention owing to discovery of world-class biogenic gas accumulations in massive Lower Miocene turbiditic sands in the Southern Levant Basin (SLB), first in the Tamar Field and later in a few more. The largest field so far discovered is Leviathan. The term SLB is arbitrarily used in this contribution to describe the part of the Levant Basin bordered by the Eratosthenes Seamount to the northwest, the Nile Cone to the southwest, the IsraelLebanon maritime border to the north and the continental slope offshore Israel and Sinai to the southeast (Fig. 1). The Levant region evolved through several tectonic phases (Garfunkel, 1998; Gardosh et al., 2008, 2010 and references therein). The first episode involved extension during the Permian to Jurassic that was accompanied by normal faults on the Levant margin, and it has been suggested that these also occurred in the basin. Gardosh et al. (2010) presented a regional seismic section across the SLB, indicating two main elevated blocks: the Leviathan High and the Jonah Ridge. Roberts and Peace (2007) mainly studied the basin offshore Lebanon,
but presented a seismic section across the southern basin suggesting Leviathan High as a “Pop-up structure” which they defined as TriassicJurassic rifted terrain. Al-Balushi et al. (2016) showed in another section that basement highs form a NE-trending horst and graben system bounded by normal faults. These sections are further discussed below. Marlow et al. (2011) identified the Leviathan and Jonah Ridge as aggrading carbonate platforms topped by reefs and covered by Early Oligocene strata. Hodgson (2012) assumed that Leviathan and Tanin can be interpreted as basement drape structures. The Jonah High was studied by Sagy et al. (2015); they suggested that it originated during the Jurassic rifting stage forming a prominent seamount and that it has never been tectonically reactivated. In contrast to these works, a number of presentations suggested that the elevated blocks mentioned above represent diapirs of Lower to Middle Miocene age (Ben-Gai, 2012, 2017; Ben-Gai and Druckman, 2013). This model is herein used as a working hypothesis. The focus of this study is on the Leviathan structure. Attempt was made to drill deep below the Miocene gas layer into the Mesozoic in the Leviathan-1 Deep well that reached the Eocene at a depth of about 6500 m (Israel
E-mail address: [email protected]. https://doi.org/10.1016/j.marpetgeo.2017.11.002 Received 1 July 2017; Received in revised form 14 October 2017; Accepted 1 November 2017 0264-8172/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Ben-Gai, Y., Marine and Petroleum Geology (2017), http://dx.doi.org/10.1016/j.marpetgeo.2017.11.002
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 1. Map of the study area, denoted by the rectangle in the inset. Location of principal wells in the deep basin is shown as well as 2-D seismic surveys used in this study. The rectangle over Leviathan marks the location of the structural map shown in Fig. 2. Bathymetry is from the Ministry of Energy database, inset is courtesy of Google Earth. Coordinates are in UTM WGS 1984. All depth values in the figures and text are subsea. ES = Eratosthenes Seamount. Lev-1 = Leviathan-1, A-1 = Cyprus A-1 well in the Aphrodite gas field. The main structural elements discussed in the text are schematically shown, named by the wells (except for Jonah High and Ridge).
deposits over a top crystalline basement shown at depth of 10–11 km. Study of the Lebanese offshore suggested accumulation of deep-water carbonate-dominated sediments since the Middle Jurassic (Hawie et al., 2013). Depth migrated sections at the southeastern flank of the Eratosthenes Seamount indicate depth to the basement greater than 12.5 km (Fig. 10 in Klimke and Ehrhardt, 2014). The period since Middle Jurassic till the Late Cretaceous is marked by a transition to a passive margin stage as a result of crustal cooling and thermal subsidence (Garfunkel and Derin, 1984; Gardosh et al., 2008 and references therein). Based on borehole data from the continental shelf, Gardosh et al. (2008) suggested that deep marine environment was established throughout the basin since Oxfordian to Kimmeridgian times with deposition of carbonate mud. They further suggested that thick accumulations of this passive margin stage are located adjacent to the horsts formed during the rifting stage; the tops of these elevated blocks may have reached shallow water level and allowed the formation of carbonate buildups. The convergence of Africa and Eurasia since Late Cretaceous is clearly marked on land and in the Levant continental shelf as a set of asymmetric thrusted folds associated with reverse faults at their cores. These are well mapped on seismic data and many have been penetrated by drill holes as far west as the outer shelf. This phase has been termed the Syrian Arc by Krenkel (1924). See Şengor and Stock (2014) for reconstruction of Krenkel's map and for a regional overview. The termination of the Syrian Arc folding phase on the shelf is clearly marked by the seismic marker of the Base Tertiary that represents a regional erosional surface (Gardosh et al., 2008; Bar et al., 2013; Ben-Gai and Druckman, 2013). No folds of the Syrian Arc age have yet been reported in the deep basin. Steinberg et al. (2011) showed that the SLB
Ministry of National Infrastructures, Energy and Water Resources database). These exploration efforts make understanding of the Levant Basin tectonic evolution as a whole and Leviathan in particular of great interest to both the scientific community and the oil industry. 2. Geological setting The Levant Basin and Margin evolved since the Late Paleozoic or Early Mesozoic, when Northern Gondwana rifted away from a consolidated set of microcontinents (Şengor and Yılmaz, 1981; Robertson and Mountrakis, 2006; Frizon de Lamotte et al., 2011 and references therein). The crustal composition of the deep basin is subject to long and on-going debates. Refraction surveys indicate thinning of the crust in the basin compared to the margin, but the lack of magnetic anomalies implies that it may not be oceanic. Some reconstructions suggest that the actual Neotethys rifting occurred further west in the present Herodotus Basin forming an oceanic crust there and thinned continental crust in the Levant Basin (Netzeband et al., 2006; Cowie and Kusznir, 2012; Inati el al., 2016). The extension during the Permian to Jurassic is characterized by a transition from carbonate platform under the present margin to deep marine clastics in the basin (Ziegler, 2001). Gardosh and Druckman (2006) suggested that the top of the crystalline basement dips gradually from 7 km at the present Levant shoreline down to a depth of 14–16 km in the basin. This is translated to as much as 4–8 km thick pre-Middle Jurassic section in sub-basins in the central part of the SLB (Gardosh et al., 2010). Al-Balushi et al. (2016) presented a section across the basin suggesting that the pre-Middle Jurassic is up to 2 km thick and is composed of continental to shallow marine siliciclastic and carbonate 2
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
a Orientation of 2D lines along apparent structural dip and oblique crossing of faults may result with erroneous interpretation. b Upwelling liquefied material from deep strata may not penetrate or even push up overlying strata when observed on sections located away from their culmination. c Liquefied mass may portray large base at depth and very narrow culmination that penetrates younger strata. Sparse 2D seismic grid may miss the culmination, and may result with erroneous age constrain. d Seismic image of the internal part of the liquefied mass may be chaotic; overlying pushed-up strata may retain the original bedding, though expected to be deformed. e 3D seismic surveys produce far better images over 2D lines where complex structural features are present. 2D lines may fail to properly migrate such events and are prone to side swipes, consequently resulting in erroneous models.
experienced fast subsidence since the Early Tertiary manifested in accumulation of about 5 km thick Tertiary and Quaternary section of clastics and evaporites. Even greater thicknesses are evident immediately to the north in offshore Lebanon, as indicated by seismic sections (Roberts and Peace, 2007; Hodgson, 2012; Hawie et al., 2013). The SLB experienced two important events since Late Oligocene, coinciding with the breakup of Africa and Arabia along the Dead Sea Transform: the first was accumulation of turbidite sands known as the Tamar Sands during the Late Oligocene and Early Miocene. The second was a short and intense phase that resulted in the gas-bearing structures, forming elongated highs mostly oriented in a NE-SW direction. These are well constrained in age based on seismic and borehole data, though little has yet been published by the oil companies operating in the basin. The onset of this phase seems to be right after the end of the turbidite sand deposition in the Late Early Miocene and they continued to develop into the Middle Miocene. Owing to their resemblance to the older folds of the Syrian Arc as seen in the margin, they were named Syrian Arc II (Gardosh et al., 2008), following Walley (1998), though the latter assigned this term to structures of Late Eocene to Oligocene in onshore Lebanon. These structures are the topic of this study and are further discussed below. In general, the base of the Messinian salt is not affected by these folds in the SLB. The thick evaporitic layer of the Messinian is subject to neotectonic activity as a result of processes such as gravity-driven spreading and gliding (e.g. Gradmann et al., 2005; Cartwright et al., 2012; Allen et al., 2016). Though these are in general considered to be thin-skinned tectonics over detachment surfaces of base and intra-salt layers, some studies suggest that plate tectonics as well as faulting along the continental slope may have affected the Messinian and the overlying Plio-Pleistocene section (e.g. Gradmann et al., 2005). Gravity-spreading is assumed to be driven by post-Messinian basin subsidence and margin tilting (Cartwright et al., 2012).
The Leviathan Gas Field is located on the Leviathan High that occupies a large area in the central part of the SLB; the High probably stretches further to the northeast into the Tanin Field and to the southwest, into a yet undrilled region (Fig. 1). The Leviathan High is about 50 km long and 25 km wide over an area of about 800 sq.km., while the gas field occupies its top and spreads over about half of it. Its internal core was described by Noble Energy as a reflection-free unit and was titled “Structural High”, though not given an age or nature explanation. As discussed above, the Leviathan High is shown in the current literature as a prominent tectonic feature, related to Early Mesozoic extension during the Neotethyan rifting. Fig. 2 shows the location of seismic sections used for this study over a depth contour map of the top of the Leviathan's diapir as interpreted on these sections. Fig. 3 shows part of line EMED-54 in its depth migrated version; the same line, in time domain, was used by Roberts and Peace (2007) to demonstrate rifted terrain (see Fig. 8a below). Fig. 4 shows part of line EMED-50 (depth domain), used by Gardosh et al. (2010) in time domain (Fig. 8c below); it is located further to the south of the culmination of the diapir as marked by the map in Fig. 2. Fig. 5 shows part of line EMED-13 that runs sub-parallel to the strike of the diapir. Fig. 6 shows line 135-ISY of the TGS 2008 survey; it is probably the most vivid display of the Leviathan's culmination and is likely to remain so until 3D images are shown by the lease's operator. Fig. 7 shows line EMED-18 across the basin; this is the depth domain version of the line shown by Al-Balushi et al. (2016) in time domain (Fig. 8b below). Nine seismic markers are interpreted on these sections: Sea Bottom (SB), Top Messinian Salt (TST), Base of Salt (BST), Top Lower Miocene (TLM), Near Top Oligocene (TO), Base Tertiary (near Middle Upper Eocene) (BT), Base Senonian (BS), Middle Jurassic (MJ) and the top of the Leviathan's diapir (TLD). The markers down to the Base Tertiary are constrained by well data. Definition of deep markers is important in order to constrain the source of the intrusive bodies, but as yet these are speculative. The Base Senonian as marked is widely accepted in the literature; some of the works mentioned above (Roberts and Peace, 2007; Gardosh et al., 2008, 2010; Al-Balushi et al., 2016) attempted to define deeper markers, mainly Middle Jurassic and top of the Basement. The Middle Jurassic marker (MJ) is shown in the figures below as a dashed line to denote its unconstrained depth. Line EMED-54 (Fig. 3) runs close to the culmination of the diapir. It affects the section up to the Upper Oligocene and slightly above. No effect is observed on the Base Salt. The diapir's outline is defined by termination of high amplitude reflectors at and below Base Tertiary towards its internal, reflection-free core. Steep events are observed at the edge of the diapir at the Base Tertiary and Base Senonian, in particular to the east. These assumed to represent deformed beds of these corresponding strata and as such may constrain the age of the deformation. Line EMED-50 (Fig. 4) is located to the south of the diapir; its effect
3. The seismic database Being under a production lease currently operated by Noble Energy, the data over the Leviathan Field available for research are restricted to authorized materials, on which this study is based. The prime source is the Spectrum EMED-2000 spec survey, acquired with a 7200 m long cable and 12 s recording time. The lines spread over the entire basin and allow correlation to deep boreholes on the margin and to the recent wells drilled in the deep basin. Yet, their orientation is oblique to the predominant strike of the main structures as seen in Fig. 1. This survey is currently available in both time and depth migrated versions processed by Spectrum. The depth migrated version achieved a far better image than the time migration, as shown below. A TGS survey shot in 2001 with the same cable length and 9 s record length, also covers large parts of the basin, but not its western section. This makes it less appealing for the Leviathan area. The TGS survey also provided magnetic and gravity data that became public domain. Few additional 2D and 3D surveys were acquired over the Leviathan area by oil companies. They are mostly proprietary, but few were released for research down to the Base Messinian (e.g. Feng and Reshef, 2016); as such they cannot support studies of deeper strata. One that became available is a 2D survey shot by TGS in 2008. Though it is currently in time domain only, quality wise it appears to match the depth sections of the Spectrum survey. Moreover, the survey was shot in a direction offset about 23° to the Spectrum lines, and as such it crosses the general trend of the Miocene structures along the true dip and strike. 4. Seismic interpretation Seismic interpretation stands at the heart of this study. As such, a set of few simple rules relevant to the structural features under investigation is suggested: 3
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 2. Depth contour map of the top of the Leviathan's diapir and location of lines shown in the figures below. Note the location of the Leviathan's development wells around the diapir. Fig. 7 as noted here marks the location of line EMED-18 on this map, yet its full length is shown in Fig. 1. Countour interval is 500 m.
hence are their tectonic conclusions. When the corrected ages are introduced, their interpretation fits Fig. 3b, which is part of the depth version of this line. Likewise, marking seismic events below Base Senonian is not supported by the time domain version of line EMED-18 in Fig. 8b, in particular when it is compared to the same line in depth domain (Fig. 7a). Line EMED-50 (Fig. 4) clearly demonstrates the Lower Miocene tectonic event in Leviathan on the depth version, even though the diapir itself is not clearly observed on this line.
is observed in a similar way, i.e. deformation of beds up to the Base Tertiary, but it clearly shows folding of much later age, into the Miocene. EMED-13 (Fig. 5) is a strike line, crossing the southwestern edge of the culmination as mapped in Fig. 2. Line 135-ISY (Fig. 6) is a key element in understanding the Leviathan's feature. As seen in Fig. 2, it is oriented along the true structural dip and probably right over the culmination in a better position than any of the other lines. As such it may be less affected by sideswipes. Note that the strike is defined by the contours adjacent to the culmination and by the extension of the outlined structure towards the Tanin Field as shown in Fig. 1. This line clearly shows how the entire section up to the TLM and slightly above is affected. As mentioned above, this survey is available in time migrated domain and the section shown here was stretched from time to depth domain via interval velocity maps for the sake of mapping the diapir and for consistency with the rest of the sections. Fig. 7 shows line EMED-18 across the basin. As of the Leviathan's diapir, it is fairly well defined by termination of reflectors below Base Tertiary as in Fig. 3 and is likewise chaotic. This appearance in these sections is apparently a genuine image; even a seemingly good depth imaging failed to recover any coherent markers within. It may though need verification by a 3D survey, given the limitations of 2D surveys. The term “pop-up structure” assigned by Roberts and Peace (2007) to the Leviathan High based on line EMED-54 (Fig. 8a) might be appropriate. Published prior to the drilling in the deep basin, their defined ages below Base Messinian were erroneous (Steinberg et al., 2011) and
5. Discussion This work aims at the geophysical interpretation of data over and around the Leviathan High based on the working assumption that its image on seismic sections is mainly a result of upwelling of liquefied material. The discussion below addresses issues such as the nature of this material, the setting of the Leviathan high in the regional tectonic frame, suggesting an alternative tectonic evolution for the SLB over previous models and discussing potential implications on the petroleum system. 5.1. Mud diapirs in the Southern Levant Basin The content of the Leviathan internal core as defined above is yet unknown, and will probably remain so until reached by drilling. In this respect, this work adopts the approach suggested by Mascle et al. (2014) of identifying fluid-releasing features based on geophysical 4
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 3. Line EMED-54, depth migrated (part). (a) Non-interpreted section (b) Interpreted section (c) Section flattened on the Base Tertiary marker. Note the phantom horizons through the diapir's core. Markers are: SB=Sea Bottom, TST = Top Salt, BST=Base Salt, TLM = Top Lower Miocene, TO = Near Top Oligocene, BT=Base Tertiary, BS=Base Senonian, MJ = Middle Jurassic, TLD = Top Leviathan's Diapir. See location in Fig. 2. Vertical exaggeration on Fig. 3 through 6 and 9 is about 1:3.
mud, volcanics and salt. Volcanic or magmatic origin can be excluded based on the absence of significant magnetic anomaly over Leviathan. This is demonstrated by the residual magnetic field along the line in Fig. 7 and a map and cross section shown by Rybakov et al. (2011). The broad Bouguer gravity anomaly over Leviathan may be attributed to crust/mantle relations (e.g. Netzeband et al., 2006) and not to the
evidence and not only those observed in situ or sampled. It can be speculated that these features are composed of a mix of allochthonous material intruded into the strata and highly deformed the autochthonous beds. The deformation is probably reduced at shallower parts of the affected section. The ratio between the two may bear on volumetric considerations. As of the composition, a short list includes
Fig. 3. (continued)
5
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 3. (continued)
suggesting that deep marine clastics prevailed in the Levant Basin since Late Permian. The excess sediment loading since Early Tertiary (Steinberg et al., 2011) might have triggered deep unconsolidated strata to liquefy and flow due to gravitational instability. Depth-wise, the source originated at great depth; Robertson and Kopf (1998) suggested a Miocene clast-rich mud breccia at depth of 5–7 km at the East Mediterranean Ridge. The same depth may correspond to infra-Jurassic beds in the SLB. Note that seismic markers as deep as 12 km are observed in the vicinity of Leviathan (Fig. 7), subject to the reliability of the velocity model used for depth imaging. As discussed above, the
sedimentary section; as such it cannot contribute to the discussion on the diapir's composition. Presence of mobilized Upper Triassic salt was proposed in offshore Lebanon as an extension of the Kurra Chine Formation in Syria (Beydoun and Habib, 1995; Marlow et al., 2011; Soledad Velasco et al., 2015), but has no further support. Jonah High and Ridge are associated with a magnetic anomaly, suggested by Folkman and Ben-Gai (2004) and Rybakov et al. (2011) to be caused by deep intrusive volcanic body. Suggesting mud as a source for the Leviathan's diapir is therefore based on an elimination process. It may though be supported by the compilation made by Ziegler (2001),
Fig. 4. Line EMED-50, depth migrated (part). Interpreted section. The line is located on the southern flank of Leviathan's diapir, yet the Lower Miocene tectonic onset is clearly observed. It may also represent the location of a separate, deeper culmination. See text for discussion. Depth to Leviathan-4 well is from the Ministry of Energy database. See location in Fig. 2.
6
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 5. Line EMED-13, depth migrated (part). Interpreted section. This is a strike line crossing the southwestern flank of the culmination. A fault pattern probably trending SE-NW is evident. Depth to Leviathan-1 well is from the Ministry of Energy database. See location in Fig. 2.
offshore Lebanon experienced greater subsidence rates during the Tertiary also between the margin and the Eratosthenes. This differential tectonic behavior between the Levant sub-basins requires further study. Focusing on mud as a likely source, there is a vast literature on mud diapirs and volcanoes in the Mediterranean basins. Mascle et al. (2014) compiled many of these, relating to recently active or active features. They concluded that these are associated with accretionary wedges. Robertson and Kopf (1998) discussed the Eastern Mediterranean Ridge mud volcanoes. Gamberi and Rovere (2010) dealt with mud volcanoes in the Tyrrhenian Sea while Soto et al. (2012) described the Miocene diapirs in the Alboran Sea and suggested a gravity-driven mechanism as described above. None of the diapirs described in this study are currently active or
depth to the acoustic basement in the SLB may reach 14–16 km in places. The geographic distribution of the diapirs in the SLB may provide a clue to some of the questions. It appears that they are confined to a belt in the southern basin, as sections across the basin to the north (Roberts and Peace, 2007; Hodgson, 2012; Skiple et al., 2012; Hawie et al., 2013) do not show such prominent events. It should be noted that Robertson (1998) suggested that the Eratosthenes Seamount was tectonically uplifted by about 1 km during or before the early Miocene. He attributed the uplift to regional compression resulting from northward subduction beneath Cyprus. In terms of dating, this perfectly matches the events described here. The amplitude of the Miocene structure in Leviathan exceeds 700 m. On the other hand, as mentioned above,
Fig. 6. Line 135-ISY, depth converted time migration (see text for details). (a) Non-interpreted section (b) Interpreted section (c) Section flattened on the Base Tertiary marker. See location in Fig. 2. Note direction of this line compared to the other lines, that makes it favorable for imaging the diapir's culmination.
7
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 6. (continued)
reconstructed maps and sections of the SLB marked in particular the Leviathan High and Jonah Ridge as the principal extension blocks while the other structures are considered of later age. The seismic sections shown here demonstrate the alternative reconstruction by flattening on key markers. The lack of coherent reflectors inside these structures leaves the deep markers (i.e. below Lower Miocene) practically uninterpretable and as such force using phantom markers. Reconstruction of the lines in Figs. 3c and 6c by flattening on the Base Tertiary marker suggests that until this time the Leviathan structure may have formed a high and depocenters around. From the Early Tertiary until the Late Oligocene not much happened besides accelerated subsidence, as evident by the conformity of these beds. The interval between the Top Oligocene and the Top Lower Miocene clearly marks the onset of the tectonic events that are seen today. It seems to have been developed symmetrically around the Leviathan's diapir. Fig. 7a shows line EMED-18 accompanied by gravity and magnetic
have been active recently. In this respect they differ from most of those mentioned above. Their life span is assumed to be rather short, starting at the Late Early Miocene and continue into the Middle Miocene. Robertson and Shipboard Scientific Party (1996) defined the age of the now dormant Milano mud volcano as 1.75 Ma while the Napoli volcano is active for the past 1.25 m.y. Such age range may fit the case in the SLB. Detailed biostratigraphic data that is available from the deep basin boreholes has not yet been published but it may be speculated that the main phase of activity lasted for about 3–4 m.y.
5.2. Tectonic reconstruction As discussed above, the current literature suggests that the SLB is assumed to be dominated by a set of Early Mesozoic horsts and grabens formed during the Neotethyan rifting while the Miocene folds are attributed to the collision involved with its closing. Previous
Fig. 6. (continued)
8
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 7. Line EMED-18, depth migrated. Gravity and magnetic measurements are projected from TGS line 2069-IS that in most parts coincides with EMED-18. The Bouguer gravity (solid line) and residual magnetic (dashed line) measurements are the actual observations along the line and not a grid cut-out. (a) Non-interpreted section (b) Interpreted section (c) Section flattened on the Base Tertiary marker with the suggested phantom markers through. ES = Eratosthenes, Aph = Aphrodite, Lev = Leviathan, Tmr = Tamar, JR = Jonah Ridge. Note the speculative marking of an intrusion at the easternmost side of Eratosthenes. Depth to Leviathan-5 well is taken from the Ministry of Energy database. See location in Figs. 1 and 2. Vertical exaggeration is about 1:8.
distorted due to oblique crossing of the structure through a major SENW trending normal fault of Middle Miocene age. The same applies for Tamar; both these structures are quazi-symmetric and lack any obvious reverse fault at depth (see Ben-Gai and Druckman, 2013). The speculative marking of the Middle Jurassic in Fig. 7b and c is a key in the suggested model. Not only that no extensional blocks are observed, as previously suggested (e.g. Gardosh et al., 2010; Sagy et al., 2015), but a thick depocenter evolved east of Leviathan under the
measurements projected from a neighboring TGS line (2069-IS). Its interpretation is shown in Fig. 7b. The section starts from the flank of the Eratosthenes in the west and runs through the structures of Aphrodite, Leviathan, Tamar and Jonah Ridge. It provides a synoptic view of the basin that emphasizes the dominance of the Miocene structures. The section demonstrates the conformity of events up to the Top Oligocene, while the section above up to Top Lower Miocene marks the onset of the folding. The Aphrodite structure at depth is severely
Fig. 7. (continued)
9
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 7. (continued)
Aphrodite. Another diapir under the Dalit gas field is not dealt with in this study; see its location in Figs. 1 and 8c. The same stands for the speculative intrusion at the eastern flank of the Eratosthenes Seamount seen at the western end of the section in Fig. 7b and c. The Tamar and Aphrodite structures show much reduced chaotic image at their cores. Two possible models may be suggested: one is that they are associated with deep-seated swells that did not develop into full-scale diapirs; the other is that they resulted from shortening caused by adjacent massive
present location of the Jonah Ridge during the Jurassic to Middle Cretaceous and under Tamar throughout the Senonian and Eocene. This is seen in Fig. 7c as asymmetric basin, yet its eastern edge is not well constrained by this line. This asymmetry of was also observed by Gardosh et al. (2008) in what they defined as grabens located between the elevated faulted blocks of Leviathan and Jonah. Line EMED-18 (Fig. 7) illustrates the difference between the main bodies of Leviathan and Jonah Ridge and the other structures, namely Tamar and
Fig. 8. Compilation of the Southern Levant Basin models in previous works, (a-c from north to south, see location in Fig. 1). All are based on the time domain versions of the Spectrum EMED lines. (a) Roberts and Peace (2007), line EMED-54, showing the pop-up structure of Leviathan. Ages below Base Salt were adopted from the above figures and do not coincide with the original interpretation as indicated at the left hand side in brackets (note for example that TLM corresponds to Base Paleogene in the original interpretation). A marker corresponding to the Top Oligocene has not been picked on this section. See Fig. 3 for the part of this section in depth domain over the Leviathan High. (b) Al-Balushi et al. (2016), based on time migrated version of line EMED-18. Its depth domain version is shown in Fig. 7. Horizon names adjusted to fit the above figures (appear at the left hand side) but grossly agree with the original interpretation. Note the Jurassic and basement phantom markers within the elevated blocks. (c) Gardosh et al. (2010), line EMED-50. Note the location of the Dalit structure at the east. See Fig. 4 for part of this section in depth domain over the Leviathan High. Structures are denoted as in Fig. 7 above. Interpretation is as appears on the original section.
10
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 8. (continued)
upwelling affected this level and these sections may provide further constrain for the timing of this phase. This study attempts to suggest an alternative view of the tectonic evolution of the SLB. It is based primarily on seismic images, yet the intense Miocene activity as described above greatly disturbed the deep part. In contrast, seismic data from offshore Lebanon, where the deep section is not affected, may provide important constraints. In this respect, studies such as Hodgson (2012), Hawie et al. (2013) and Skiple et al. (2012) may provide a comprehensive view of the Levant Basin evolution as they are based on the most modern seismic database available. The main issue addressed here is a seemingly intensive episode that occurred during the Late Early and Early Middle Miocene in the SLB. Few key points regarding the tectonic evolution of the basin in conjunction with this event are addressed. During the Permian and Early Mesozoic the basin experienced extension that was accompanied by subsidence and deposition of deep marine sediments and maybe also evaporites (see discussion and references above). This phase might have been associated with normal faults in the SLB, as also observed on the margin, though seismic lines offshore Lebanon do not show any faulted blocks at that level (e.g. Hawie et al., 2013, Figs. 6 and 7 there). Local basins developed during
diapirs (Tamar, for example is located between Leviathan to the west and Dalit and the Jonah Ridge to the east). It may also be a combination of both (see Ben-Gai and Druckman, 2013). The volume of the intrusive material is poorly constrained by the sparse 2D seismic grid used here. This should be further studied based on the 3D data covering the field and its vicinity. As such, it cannot be determined whether the intrusions are the cause of the observed folding or are the consequence of a yet to be determined tectonic process. The short duration of this phase may though suggest that the former is more likely. It is worth noting that the intrusion is fairly restricted spatially as indicated by the elongated culmination as seen in Fig. 2, coupled with its potential continuation into the Tanin Field to the NE (Fig. 1). It may also be that there is more than a single culmination and that deeper diapirs are present, for example at the location of the Leviathan-4 well (Fig. 4). Fig. 9 indicates, by the presence of a deep-seated feature marked by the TLD, that it might be connected with the one under the Leviathan-4 well, consistent with the southwest extension of the Leviathan High as seen in Fig. 1. In such case the surface mapped in Fig. 2 will obtain a more complicated shape, maybe showing en echelon pattern of separate structures. In both these locations it appears that the diapir did not penetrate the Lower Miocene as in Fig. 6, yet the
Fig. 8. (continued)
11
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 9. Line 65-ISY, depth converted time migration. Interpreted section. This line suggests that the Leviathan Diapir extends to the southwest, maybe as a separate feature connected to the location of Leviathan-4. See text for further discussion. See location in Fig. 1.
5.3. Implications on petroleum systems
the Middle Mesozoic, such as the one described in Fig. 7, creating an accommodation space that may have trapped both source and reservoir rocks. During the Late Cretaceous, while the margin experienced Alpine-type tectonics and the formation of the Syrian Arc, the basin developed broad highs and lows manifested in thickness variations of the interval between Base Senonian and Late Eocene. No clear sign of compressional tectonics is evident in the basin during this period. It is worth noting that this change in structural style between the margin and the basin is concurrent with the change in crustal thickness, and maybe crustal composition. During the Early Tertiary the basin experienced accelerating subsidence but otherwise a quiescent period, that was interrupted at the Late Early Miocene by the intrusive phase. The predominant trend of these features is NE-SW, though further detailed mapping is required. It might be speculated that this trend follows weakness zones formed during the Early Mesozoic extensional stage. The intrusive phase was followed by closely spaced normal faults trending SE-NW, clearly shown on structures such as Tamar in Noble Energy's presentations (see Ben-Gai and Druckman, 2013). They are also evident on the strike line across Leviathan (Fig. 5). Offshore Lebanon extensional faults of the same age and orientation formed a multitude of potential traps in rotated blocks (Hodgson, 2012). The difference in structural style between the southern and northern basins of the Levant is herein attributed to the absence of intrusive activity offshore Lebanon, despite the thicker Tertiary section there. In both sub-basins, the tectonic activity ceased prior to the Late Miocene and is not observed at the Base Messinian salt. Tectonics of the massive Messinian salt is assumed to be thin-skinned as discussed above and it is probably not affected by the Miocene structures. Hodgson (2012) stated that the extensional regime in the deep basin is synchronous with the compression of the Syrian Arc system. Actually, he might have referred to the Syrian Arc II discussed above, as the original system is much older. To avoid confusion, Ben-Gai and Druckman (2013) suggested naming the Miocene system as the "Tamar Folding Phase".
The effect of the diapirs and the resulting structures described above on petroleum systems in the SLB is the accumulation of the biogenic gas so far discovered. This happened in places where the traps remained intact during and after the gas migration. As yet, no hydrocarbons have been found in the basin at deeper levels, though a few wells along the continental shelf of Israel and Northern Sinai encountered oil in the Lower Cretaceous and Jurassic (Gardosh and Tannenbaum, 2014). The implications of the suggested tectonic model on the petroleum systems are fairly obvious: if indeed the basin experienced such a wide-spread episode of diapirism during the Oligo-Miocene, then the integrity of traps older than Early Miocene is in jeopardy due to potential breaching. On the other hand, this phase formed a multitude of potential traps on and around these structures. The immediate candidates for deep targets are the interpreted closed structures of Tamar, Aphrodite and others alike. The flanks of diapirs may also form traps, such as those around Leviathan for example (Fig. 2). Presence of reservoir rocks and timing of expulsion and migration of thermogenic hydrocarbons comprise the main geological risk. As noted by Gardosh and Tannenbaum (2014), the relevant information regarding the SLB is limited to public announcements by the operators, and each component of the petroleum system there is currently speculative. 6. Conclusions In light of the working hypothesis, this study suggests an alternative tectonic evolution for the Southern Levant Basin based on seismic imaging. The principal conclusion is that most of what we observe today in the vicinity of the Leviathan Gas Field, and also in other parts of the basin, has been formed since the Early Miocene by a wide spread phase of diapirism. This has obvious implications for the proven biogenic and yet unexplored deep thermogenic petroleum systems. This interpretation poses a few questions that are yet to be answered with further seismic and well data. These questions are related first of all to the enigmatic composition of the diapirs and secondly to their geographic distribution and the associated tectonic processes. The 12
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
difference between the tectonic setting of the Levant margin and the basin and among the Levant sub-basins deserves further elaboration. The rock mechanics involved with these massive intrusions is another issue related to the question whether they formed the Miocene folding or were the consequence of a regional tectonic phase associated with these folds.
Gardosh, M., Tannenbaum, E., 2014. The petroleum systems of Israel. In: In: Marlow, L., Kendall, C., Yose, L. (Eds.), Petroleum Systems of the Tethyan Region, vol. 106. AAPG Memoir, pp. 179–216. Garfunkel, Z., 1998. Constrains on the origin and history of the eastern Mediterranean basin. Tectonophysics 298, 5–35. Garfunkel, Z., Derin, B., 1984. Permian–Early Mesozoic tectonism and continental margin formation in Israel and its implications to the history of the eastern Mediterranean. In: In: Dixon, J.E., Robertson, A.H.F. (Eds.), The Geological Evolution of the Eastern Mediterranean, vol. 17. Geological Society, London, Special Publications, pp. 18–201. Gradmann, S., Hübscher, C., Ben-Avraham, Z., Gajewski, D., Netzeband, G., 2005. Salt tectonics off northern Israel. Mar. Petroleum Geol. 22, 597–611. Hawie, N., Gorini, C., Deschamps, R., Nader, F.H., Mondaret, L., Granjeon, D., Baudin, F., 2013. Tectono-stratigraphic evolution of the northern Levant Basin (offshore Lebanon). Mar. Petroleum Geol. 48, 392–410. http://dx.doi.org/10.1016/j. marpetgeo.2013.08.004. Hodgson, N., 2012. The Miocene hydrocarbon play in Southern Lebanon. First Break 30, 93–98. Inati, L., Zeyen, H., Nader, F.H., Roure, F., 2016. Lithospheric architecture of the Levant Basin (eastern mediterranean region): a 2D modeling approach. Tectonophysics 693 (A), 143–156. http://dx.doi.org/10.1016/j.tecto.2016.10.030. Klimke, J., Ehrhardt, A., 2014. Impact and implications of the Afro-Eurasian collision south of Cyprus from reflection seismic data. Tectonophysics 626, 105–119. Krenkel, E., 1924. Der Syrische Bogen: Centralblatt fur Mineralogie. pp. 274–313 Geologie und Paleontologie. Jahrgang. Marlow, L., Kornpihl, K., Kendall, C.G.St.C., 2011. 2-D basin modeling study of petroleum systems in the levantine basin, eastern mediterranean. GeoArabia 16 (2), 17–42. Mascle, J., Mary, F., Praeg, D., Brosolo, L., Camera, L., Ceramicola, S., Dupre, S., 2014. Distribution and geological control of mud volcanoes and other fluid/free gas seepage features in the Mediterranean Sea and nearby Gulf of Cadiz. Geo-Marine Lett. 34 (2–3), 89–110. Netzeband, G.L., Gohl, K., Hubscher, C.P., Ben-Avraham, Z., Dehghani, G.A., Gajewski, D., Liersch, P., 2006. The Levantine Basin – crustal structure and origin. Tectonophysics 418 (3–4), 167–188. Roberts, G., Peace, D., 2007. Hydrocarbon plays and prospectivity of the Levantine Basin, offshore Lebanon and Syria from modern seismic data. GeoArabia 12, 99–124. Robertson, A.H.F., 1998. Mesozoic-Tertiary tectonic evolution of the Easternmost Mediterranean area: integration of marine and land evidence. In: In: Robertson, A.H.F., Emeis, K.-C., Richter, C., Camerlenghi, A. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 160. pp. 723–782. Robertson, A.H.F., Shipboard Scientific Party, 1996. Mud volcanism on the Mediterranean ridge. In: In: Emeis, K.-C., Robertson, A.H.F., Richter, C. (Eds.), Proceedings of the Ocean Drilling Program, Initial Reports, vol. 160. pp. 521–526. Robertson, A.H.F., Kopf, A., 1998. Tectonic setting and processes of mud volcanism on the Mediterranean Ridge accretionary complex: evidence from Leg 160. In: In: Robertson, A.H.F., Emeis, K.-C., Richter, C., Camerlenghi, A. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 160. pp. 665–680. Robertson, A.H.F., Mountrakis, D., 2006. Tectonic development of the Eastern Mediterranean region: an introduction. In: In: Robertson, A.H.F., Mountrakis, D. (Eds.), Tectonic Development of the Eastern Mediterranean Region, vol. 260. Geological Society, London, Special Publications, pp. 1–9. Rybakov, M., Goldshmidt, V., Hall, J.K., Ben-Avraham, Z., Lazar, M., 2011. New insights into the sources of magnetic anomalies in the Levant. Russ. Geol. Geophys. 52, 377–397. Sagy, Y., Gvirtzman, Z., Reshef, M., Makovsky, Y., 2015. The enigma of the Jonah high in the Middle of the Levant Basin and its significance to the history of rifting. Tectonophysics. http://dx.doi.org/10.1016/j.tecto.2015.09.037. Şengor, A.M.C., Stock, J., 2014. The ayyubid orogen: an ophiolite obduction-driven orogen in the late cretaceous of the neo-tethyan south margin. Geosci. Can. 41, 225–254. Şengor, A.M.C., Yılmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75, 181–241. http://dx.doi.org/10.1016/0040-1951(81)90275-4. Skiple, C., Anderson, E., Furstenau, J., 2012. Seismic interpretation and attribute analysis of the Herodotus and the Levantine Basin, offshore Cyprus and Lebanon. Pet. Geosci. 18, 433–442. Soledad Velasco, M., Snyder, H., Kidwai, M., 2015. Triassic Hydrocarbon Potential in the Eastern Mediterranean Region, Lebanon. pp. 3336–3341. http://dx.doi.org/10.1190/ segam2015-5906258.1. SEG Technical Program Expanded Abstracts 2015. Soto, J.I., Fernandez-Ibanez, F., Talukder, A.R., 2012. Recent shale tectonics and basin evolution of the NW Alboran sea. Lead. Edge 768–775 July 2012. Steinberg, J., Gvirtzman, Z., Folkman, Y., Garfunkel, Z., 2011. Origin and nature of the rapid late Tertiary filling of the Levant Basin. Geology 39 (4), 355–358. http://dx.doi. org/10.1130/G31615.1. April 2011. Walley, C.D., 1998. 1998. Some outstanding issues in the geology of Lebanon and their importance in the tectonic evolution of the Levantine region. Tectonophysics 298, 37–62. Ziegler, M.A., 2001. Late permian to holocene paleofacies evolution of the arabian plate and its hydrocarbon occurrences. GeoArabia 6 (3), 445–504.
Acknowledgements The partners of the Pelagic Joint Venture are acknowledged for providing access to the depth migrated sections. The Israel Ministry of National Infrastructures, Energy and Water Resources and M. Gardosh kindly allowed the use of the TGS 2008 survey. D. Craik greatly contributed in reviewing this manuscript. Y. Druckman and Y. Folakman are acknowledged for constructive discussions. The author wishes to thank two anonymous reviewers for highly constructive comments. References Al-Balushi, A.N., Neumaier, M., Fraser, A.J., Jackson, C.A.-L., 2016. The impact of the Messinian salinity crisis on the petroleum system of the Eastern Mediterranean: a critical assessment using 2D petroleum system modelling. Pet. Geosci. 22 (4), 357–379. http://dx.doi.org/10.1144/petgeo2016-054. Allen, H., Jackson, C.A.-L., Fraser, A.J., 2016. Gravity-driven deformation of a youthful saline giant: the interplay between gliding and spreading in the Messinian Basins of the Eastern Mediterranean. Pet. Geosci. 22 (4), 340–356. http://dx.doi.org/10.1144/ petgeo2016-034. Bar, O., Gvirtzman, Z., Feinstein, S., Zilberman, E., 2013. Accelerated subsidence and sedimentation in the Levant Basin during the Late Tertiary and concurrent uplift of the Arabian platform: tectonic versus counteracting sedimentary loading effects. Tectonics 32, 334–350. http://dx.doi.org/10.1002/tect.20026. Ben-Gai, Y., 2012. Mud diapirs in the deep Levant Basin - a proposed mechanism for the evolution of the gas-bearing tertiary structures. Isr. Geol. Soc. Ann. 31 Meeting, Ashkelon, abstract. Ben-Gai, Y., Druckman, Y., 2013. Mega-scale swells and diapirs in the deep Levant Basin, eastern mediterranean, and their association with recent world-class gas discoveries. In: AAPG ERC Conference, Barcelona, Search and Discovery Article #10518. Ben-Gai, Y., 2017. Seismic imaging of the Jonah high, Levant Basin – a new look at an old enigma. In: AAPG ERC Conference, Larnaca, Cyprus. Available at: https:// europeevents.aapg.org/ehome/202660/presentationspdf/. Beydoun, Z.R., Habib, J.G., 1995. Lebanon revisited: new insights into Triassic hydrocarbon prospects. J. Petroleum Geol. 18 (1), 75–90. Cartwright, J., Jackson, M., Dooley, T., Higgins, S., 2012. Strain Partitioning in Gravitydriven Shortening of a Thick, Multilayered Evaporite Sequence, vol. 363. Geological Society, London, Special Publications, pp. 449–470. http://dx.doi.org/10.1144/ SP363.21. Cowie, L., Kusznir, N., 2012. Mapping crustal thickness and oceanic lithosphere distribution in the Eastern Mediterranean using gravity inversion. Pet. Geosci. 18 (4), 373–380. Feng, Y.E., Reshef, M., 2016. The Eastern Mediterranean Messinian salt-depth imaging and velocity analysis considerations. Pet. Geosci. 22 (4), 333–339. http://dx.doi.org/ 10.1144/petgeo2015-088. Folkman, Y., Ben-Gai, Y., 2004. The ‘Jonah’ buried seamount: intrusive structure in the southeastern Levant Basin offshore Israel. Isr. Geol. Soc. Ann. 29 Meeting, Hagoshrim, abstract. Frizon de Lamotte, D., Raulin, C., Mouchot, N., Wrobel-Daveau, J.-C., Blanpied, C., Ringenbach, J.-C., 2011. The southernmost margin of the Tethys realm during the Mesozoic and Cenozoic: initial geometry and timing of the inversion processes. Tectonics 30http://dx.doi.org/10.1029/2010TC002691. TC3002. Gamberi, F., Rovere, M., 2010. Mud diapirs, mud volcanoes and fluid flow in the rear of the Calabrian Arc Orogenic Wedge (southeastern Tyrrhenian sea). Basin Res. 22, 452–464. http://dx.doi.org/10.1111/j.1365-2117.2010.00473.x. Gardosh, M., Druckman, Y., 2006. Seismic stratigraphy, structure and tectonic evolution of the Levantine Basin, offshore Israel. In: In: Robertson, A.H.F., Mountrakis, D. (Eds.), Tectonic Development of the Eastern Mediterranean Region, vol. 260. Geological Society, London, Special Publications, pp. 201–227. Gardosh, M., Druckman, Y., Buchbinder, B., Rybakov, M., 2008. The Levant Basin Offshore Israel: Stratigraphy, Structure, Tectonic Evolution and Implications for Hydrocarbon Exploration. pp. 121 Geological Survey of Israel Report GSI/4/2008. Gardosh, M., Garfunkel, Z., Druckman, Y., Buchbinder, B., 2010. Tethyan rifting in the levant region and its role in early mesozoic crustal evolution. In: In: Homberg, C., Bachmann, M. (Eds.), Evolution of the Levant Margin and Western Arabia Platform since the Mesozoic, vol. 341. Geological Society, London, Special Publications, pp. 9–36.
13
Contents lists available at ScienceDirect
Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Research paper
The Leviathan Miocene Diapir – A novel insight into the tectonic evolution of the Southern Levant Basin, Eastern Mediterranean Yuval Ben-Gai 7 Metsada St., Hod Hasharon, 4529418, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords: Eastern Mediterranean Levant Basin Seismic interpretation Mud diapirs Petroleum systems
The Leviathan gas field is the largest accumulation so far discovered in the Southern Levant Basin within the turbiditic Tamar Sand Complex. The discovery of this and other fields in the basin was made possible following acquisition of a vast amount of data since 2000. A consensus has been reached that the basin is dominated by Early Mesozoic extensional blocks related to opening of the Neotethys while Miocene folds formed as a result of its closing. Through the study of geophysical data over the Leviathan gas field and its vicinity, it is suggested that most of what we observe today has been formed since the Early Miocene and that the alleged old blocks are actually intrusive bodies. Their structural timing is accurately constrained by well and seismic data but not their composition, as none has yet been penetrated at any borehole. Mud diapirism is currently assumed, as volcanic or magmatic origin is discarded by magnetic data and salt is yet a remote possibility. The tectonic reconstruction suggests that the Levant Basin developed during the Mesozoic and Early Cenozoic as a set of regional highs and lows that were buried since the Early Tertiary until the Early Miocene by thick conforming sequences including potential reservoir rocks such as the gas-bearing turbiditic sands. It was then subject to an extensional tectonic regime associated with diapirs in the southern part of the basin that formed folded structures while rotational blocks developed to the north, offshore Lebanon. The implications of this tectonic setting on petroleum systems are of prime importance; widespread intrusions may have breached old traps, if they existed. On the other hand they formed potential traps such as the proven gas-filled structures and potentially others at depth. Presence of reservoir rocks and the timing of expulsion and migration from deep sources are the main geological risks.
1. Introduction The Levant Basin has recently gained a lot of attention owing to discovery of world-class biogenic gas accumulations in massive Lower Miocene turbiditic sands in the Southern Levant Basin (SLB), first in the Tamar Field and later in a few more. The largest field so far discovered is Leviathan. The term SLB is arbitrarily used in this contribution to describe the part of the Levant Basin bordered by the Eratosthenes Seamount to the northwest, the Nile Cone to the southwest, the IsraelLebanon maritime border to the north and the continental slope offshore Israel and Sinai to the southeast (Fig. 1). The Levant region evolved through several tectonic phases (Garfunkel, 1998; Gardosh et al., 2008, 2010 and references therein). The first episode involved extension during the Permian to Jurassic that was accompanied by normal faults on the Levant margin, and it has been suggested that these also occurred in the basin. Gardosh et al. (2010) presented a regional seismic section across the SLB, indicating two main elevated blocks: the Leviathan High and the Jonah Ridge. Roberts and Peace (2007) mainly studied the basin offshore Lebanon,
but presented a seismic section across the southern basin suggesting Leviathan High as a “Pop-up structure” which they defined as TriassicJurassic rifted terrain. Al-Balushi et al. (2016) showed in another section that basement highs form a NE-trending horst and graben system bounded by normal faults. These sections are further discussed below. Marlow et al. (2011) identified the Leviathan and Jonah Ridge as aggrading carbonate platforms topped by reefs and covered by Early Oligocene strata. Hodgson (2012) assumed that Leviathan and Tanin can be interpreted as basement drape structures. The Jonah High was studied by Sagy et al. (2015); they suggested that it originated during the Jurassic rifting stage forming a prominent seamount and that it has never been tectonically reactivated. In contrast to these works, a number of presentations suggested that the elevated blocks mentioned above represent diapirs of Lower to Middle Miocene age (Ben-Gai, 2012, 2017; Ben-Gai and Druckman, 2013). This model is herein used as a working hypothesis. The focus of this study is on the Leviathan structure. Attempt was made to drill deep below the Miocene gas layer into the Mesozoic in the Leviathan-1 Deep well that reached the Eocene at a depth of about 6500 m (Israel
E-mail address: [email protected]. https://doi.org/10.1016/j.marpetgeo.2017.11.002 Received 1 July 2017; Received in revised form 14 October 2017; Accepted 1 November 2017 0264-8172/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Ben-Gai, Y., Marine and Petroleum Geology (2017), http://dx.doi.org/10.1016/j.marpetgeo.2017.11.002
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 1. Map of the study area, denoted by the rectangle in the inset. Location of principal wells in the deep basin is shown as well as 2-D seismic surveys used in this study. The rectangle over Leviathan marks the location of the structural map shown in Fig. 2. Bathymetry is from the Ministry of Energy database, inset is courtesy of Google Earth. Coordinates are in UTM WGS 1984. All depth values in the figures and text are subsea. ES = Eratosthenes Seamount. Lev-1 = Leviathan-1, A-1 = Cyprus A-1 well in the Aphrodite gas field. The main structural elements discussed in the text are schematically shown, named by the wells (except for Jonah High and Ridge).
deposits over a top crystalline basement shown at depth of 10–11 km. Study of the Lebanese offshore suggested accumulation of deep-water carbonate-dominated sediments since the Middle Jurassic (Hawie et al., 2013). Depth migrated sections at the southeastern flank of the Eratosthenes Seamount indicate depth to the basement greater than 12.5 km (Fig. 10 in Klimke and Ehrhardt, 2014). The period since Middle Jurassic till the Late Cretaceous is marked by a transition to a passive margin stage as a result of crustal cooling and thermal subsidence (Garfunkel and Derin, 1984; Gardosh et al., 2008 and references therein). Based on borehole data from the continental shelf, Gardosh et al. (2008) suggested that deep marine environment was established throughout the basin since Oxfordian to Kimmeridgian times with deposition of carbonate mud. They further suggested that thick accumulations of this passive margin stage are located adjacent to the horsts formed during the rifting stage; the tops of these elevated blocks may have reached shallow water level and allowed the formation of carbonate buildups. The convergence of Africa and Eurasia since Late Cretaceous is clearly marked on land and in the Levant continental shelf as a set of asymmetric thrusted folds associated with reverse faults at their cores. These are well mapped on seismic data and many have been penetrated by drill holes as far west as the outer shelf. This phase has been termed the Syrian Arc by Krenkel (1924). See Şengor and Stock (2014) for reconstruction of Krenkel's map and for a regional overview. The termination of the Syrian Arc folding phase on the shelf is clearly marked by the seismic marker of the Base Tertiary that represents a regional erosional surface (Gardosh et al., 2008; Bar et al., 2013; Ben-Gai and Druckman, 2013). No folds of the Syrian Arc age have yet been reported in the deep basin. Steinberg et al. (2011) showed that the SLB
Ministry of National Infrastructures, Energy and Water Resources database). These exploration efforts make understanding of the Levant Basin tectonic evolution as a whole and Leviathan in particular of great interest to both the scientific community and the oil industry. 2. Geological setting The Levant Basin and Margin evolved since the Late Paleozoic or Early Mesozoic, when Northern Gondwana rifted away from a consolidated set of microcontinents (Şengor and Yılmaz, 1981; Robertson and Mountrakis, 2006; Frizon de Lamotte et al., 2011 and references therein). The crustal composition of the deep basin is subject to long and on-going debates. Refraction surveys indicate thinning of the crust in the basin compared to the margin, but the lack of magnetic anomalies implies that it may not be oceanic. Some reconstructions suggest that the actual Neotethys rifting occurred further west in the present Herodotus Basin forming an oceanic crust there and thinned continental crust in the Levant Basin (Netzeband et al., 2006; Cowie and Kusznir, 2012; Inati el al., 2016). The extension during the Permian to Jurassic is characterized by a transition from carbonate platform under the present margin to deep marine clastics in the basin (Ziegler, 2001). Gardosh and Druckman (2006) suggested that the top of the crystalline basement dips gradually from 7 km at the present Levant shoreline down to a depth of 14–16 km in the basin. This is translated to as much as 4–8 km thick pre-Middle Jurassic section in sub-basins in the central part of the SLB (Gardosh et al., 2010). Al-Balushi et al. (2016) presented a section across the basin suggesting that the pre-Middle Jurassic is up to 2 km thick and is composed of continental to shallow marine siliciclastic and carbonate 2
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
a Orientation of 2D lines along apparent structural dip and oblique crossing of faults may result with erroneous interpretation. b Upwelling liquefied material from deep strata may not penetrate or even push up overlying strata when observed on sections located away from their culmination. c Liquefied mass may portray large base at depth and very narrow culmination that penetrates younger strata. Sparse 2D seismic grid may miss the culmination, and may result with erroneous age constrain. d Seismic image of the internal part of the liquefied mass may be chaotic; overlying pushed-up strata may retain the original bedding, though expected to be deformed. e 3D seismic surveys produce far better images over 2D lines where complex structural features are present. 2D lines may fail to properly migrate such events and are prone to side swipes, consequently resulting in erroneous models.
experienced fast subsidence since the Early Tertiary manifested in accumulation of about 5 km thick Tertiary and Quaternary section of clastics and evaporites. Even greater thicknesses are evident immediately to the north in offshore Lebanon, as indicated by seismic sections (Roberts and Peace, 2007; Hodgson, 2012; Hawie et al., 2013). The SLB experienced two important events since Late Oligocene, coinciding with the breakup of Africa and Arabia along the Dead Sea Transform: the first was accumulation of turbidite sands known as the Tamar Sands during the Late Oligocene and Early Miocene. The second was a short and intense phase that resulted in the gas-bearing structures, forming elongated highs mostly oriented in a NE-SW direction. These are well constrained in age based on seismic and borehole data, though little has yet been published by the oil companies operating in the basin. The onset of this phase seems to be right after the end of the turbidite sand deposition in the Late Early Miocene and they continued to develop into the Middle Miocene. Owing to their resemblance to the older folds of the Syrian Arc as seen in the margin, they were named Syrian Arc II (Gardosh et al., 2008), following Walley (1998), though the latter assigned this term to structures of Late Eocene to Oligocene in onshore Lebanon. These structures are the topic of this study and are further discussed below. In general, the base of the Messinian salt is not affected by these folds in the SLB. The thick evaporitic layer of the Messinian is subject to neotectonic activity as a result of processes such as gravity-driven spreading and gliding (e.g. Gradmann et al., 2005; Cartwright et al., 2012; Allen et al., 2016). Though these are in general considered to be thin-skinned tectonics over detachment surfaces of base and intra-salt layers, some studies suggest that plate tectonics as well as faulting along the continental slope may have affected the Messinian and the overlying Plio-Pleistocene section (e.g. Gradmann et al., 2005). Gravity-spreading is assumed to be driven by post-Messinian basin subsidence and margin tilting (Cartwright et al., 2012).
The Leviathan Gas Field is located on the Leviathan High that occupies a large area in the central part of the SLB; the High probably stretches further to the northeast into the Tanin Field and to the southwest, into a yet undrilled region (Fig. 1). The Leviathan High is about 50 km long and 25 km wide over an area of about 800 sq.km., while the gas field occupies its top and spreads over about half of it. Its internal core was described by Noble Energy as a reflection-free unit and was titled “Structural High”, though not given an age or nature explanation. As discussed above, the Leviathan High is shown in the current literature as a prominent tectonic feature, related to Early Mesozoic extension during the Neotethyan rifting. Fig. 2 shows the location of seismic sections used for this study over a depth contour map of the top of the Leviathan's diapir as interpreted on these sections. Fig. 3 shows part of line EMED-54 in its depth migrated version; the same line, in time domain, was used by Roberts and Peace (2007) to demonstrate rifted terrain (see Fig. 8a below). Fig. 4 shows part of line EMED-50 (depth domain), used by Gardosh et al. (2010) in time domain (Fig. 8c below); it is located further to the south of the culmination of the diapir as marked by the map in Fig. 2. Fig. 5 shows part of line EMED-13 that runs sub-parallel to the strike of the diapir. Fig. 6 shows line 135-ISY of the TGS 2008 survey; it is probably the most vivid display of the Leviathan's culmination and is likely to remain so until 3D images are shown by the lease's operator. Fig. 7 shows line EMED-18 across the basin; this is the depth domain version of the line shown by Al-Balushi et al. (2016) in time domain (Fig. 8b below). Nine seismic markers are interpreted on these sections: Sea Bottom (SB), Top Messinian Salt (TST), Base of Salt (BST), Top Lower Miocene (TLM), Near Top Oligocene (TO), Base Tertiary (near Middle Upper Eocene) (BT), Base Senonian (BS), Middle Jurassic (MJ) and the top of the Leviathan's diapir (TLD). The markers down to the Base Tertiary are constrained by well data. Definition of deep markers is important in order to constrain the source of the intrusive bodies, but as yet these are speculative. The Base Senonian as marked is widely accepted in the literature; some of the works mentioned above (Roberts and Peace, 2007; Gardosh et al., 2008, 2010; Al-Balushi et al., 2016) attempted to define deeper markers, mainly Middle Jurassic and top of the Basement. The Middle Jurassic marker (MJ) is shown in the figures below as a dashed line to denote its unconstrained depth. Line EMED-54 (Fig. 3) runs close to the culmination of the diapir. It affects the section up to the Upper Oligocene and slightly above. No effect is observed on the Base Salt. The diapir's outline is defined by termination of high amplitude reflectors at and below Base Tertiary towards its internal, reflection-free core. Steep events are observed at the edge of the diapir at the Base Tertiary and Base Senonian, in particular to the east. These assumed to represent deformed beds of these corresponding strata and as such may constrain the age of the deformation. Line EMED-50 (Fig. 4) is located to the south of the diapir; its effect
3. The seismic database Being under a production lease currently operated by Noble Energy, the data over the Leviathan Field available for research are restricted to authorized materials, on which this study is based. The prime source is the Spectrum EMED-2000 spec survey, acquired with a 7200 m long cable and 12 s recording time. The lines spread over the entire basin and allow correlation to deep boreholes on the margin and to the recent wells drilled in the deep basin. Yet, their orientation is oblique to the predominant strike of the main structures as seen in Fig. 1. This survey is currently available in both time and depth migrated versions processed by Spectrum. The depth migrated version achieved a far better image than the time migration, as shown below. A TGS survey shot in 2001 with the same cable length and 9 s record length, also covers large parts of the basin, but not its western section. This makes it less appealing for the Leviathan area. The TGS survey also provided magnetic and gravity data that became public domain. Few additional 2D and 3D surveys were acquired over the Leviathan area by oil companies. They are mostly proprietary, but few were released for research down to the Base Messinian (e.g. Feng and Reshef, 2016); as such they cannot support studies of deeper strata. One that became available is a 2D survey shot by TGS in 2008. Though it is currently in time domain only, quality wise it appears to match the depth sections of the Spectrum survey. Moreover, the survey was shot in a direction offset about 23° to the Spectrum lines, and as such it crosses the general trend of the Miocene structures along the true dip and strike. 4. Seismic interpretation Seismic interpretation stands at the heart of this study. As such, a set of few simple rules relevant to the structural features under investigation is suggested: 3
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 2. Depth contour map of the top of the Leviathan's diapir and location of lines shown in the figures below. Note the location of the Leviathan's development wells around the diapir. Fig. 7 as noted here marks the location of line EMED-18 on this map, yet its full length is shown in Fig. 1. Countour interval is 500 m.
hence are their tectonic conclusions. When the corrected ages are introduced, their interpretation fits Fig. 3b, which is part of the depth version of this line. Likewise, marking seismic events below Base Senonian is not supported by the time domain version of line EMED-18 in Fig. 8b, in particular when it is compared to the same line in depth domain (Fig. 7a). Line EMED-50 (Fig. 4) clearly demonstrates the Lower Miocene tectonic event in Leviathan on the depth version, even though the diapir itself is not clearly observed on this line.
is observed in a similar way, i.e. deformation of beds up to the Base Tertiary, but it clearly shows folding of much later age, into the Miocene. EMED-13 (Fig. 5) is a strike line, crossing the southwestern edge of the culmination as mapped in Fig. 2. Line 135-ISY (Fig. 6) is a key element in understanding the Leviathan's feature. As seen in Fig. 2, it is oriented along the true structural dip and probably right over the culmination in a better position than any of the other lines. As such it may be less affected by sideswipes. Note that the strike is defined by the contours adjacent to the culmination and by the extension of the outlined structure towards the Tanin Field as shown in Fig. 1. This line clearly shows how the entire section up to the TLM and slightly above is affected. As mentioned above, this survey is available in time migrated domain and the section shown here was stretched from time to depth domain via interval velocity maps for the sake of mapping the diapir and for consistency with the rest of the sections. Fig. 7 shows line EMED-18 across the basin. As of the Leviathan's diapir, it is fairly well defined by termination of reflectors below Base Tertiary as in Fig. 3 and is likewise chaotic. This appearance in these sections is apparently a genuine image; even a seemingly good depth imaging failed to recover any coherent markers within. It may though need verification by a 3D survey, given the limitations of 2D surveys. The term “pop-up structure” assigned by Roberts and Peace (2007) to the Leviathan High based on line EMED-54 (Fig. 8a) might be appropriate. Published prior to the drilling in the deep basin, their defined ages below Base Messinian were erroneous (Steinberg et al., 2011) and
5. Discussion This work aims at the geophysical interpretation of data over and around the Leviathan High based on the working assumption that its image on seismic sections is mainly a result of upwelling of liquefied material. The discussion below addresses issues such as the nature of this material, the setting of the Leviathan high in the regional tectonic frame, suggesting an alternative tectonic evolution for the SLB over previous models and discussing potential implications on the petroleum system. 5.1. Mud diapirs in the Southern Levant Basin The content of the Leviathan internal core as defined above is yet unknown, and will probably remain so until reached by drilling. In this respect, this work adopts the approach suggested by Mascle et al. (2014) of identifying fluid-releasing features based on geophysical 4
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 3. Line EMED-54, depth migrated (part). (a) Non-interpreted section (b) Interpreted section (c) Section flattened on the Base Tertiary marker. Note the phantom horizons through the diapir's core. Markers are: SB=Sea Bottom, TST = Top Salt, BST=Base Salt, TLM = Top Lower Miocene, TO = Near Top Oligocene, BT=Base Tertiary, BS=Base Senonian, MJ = Middle Jurassic, TLD = Top Leviathan's Diapir. See location in Fig. 2. Vertical exaggeration on Fig. 3 through 6 and 9 is about 1:3.
mud, volcanics and salt. Volcanic or magmatic origin can be excluded based on the absence of significant magnetic anomaly over Leviathan. This is demonstrated by the residual magnetic field along the line in Fig. 7 and a map and cross section shown by Rybakov et al. (2011). The broad Bouguer gravity anomaly over Leviathan may be attributed to crust/mantle relations (e.g. Netzeband et al., 2006) and not to the
evidence and not only those observed in situ or sampled. It can be speculated that these features are composed of a mix of allochthonous material intruded into the strata and highly deformed the autochthonous beds. The deformation is probably reduced at shallower parts of the affected section. The ratio between the two may bear on volumetric considerations. As of the composition, a short list includes
Fig. 3. (continued)
5
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 3. (continued)
suggesting that deep marine clastics prevailed in the Levant Basin since Late Permian. The excess sediment loading since Early Tertiary (Steinberg et al., 2011) might have triggered deep unconsolidated strata to liquefy and flow due to gravitational instability. Depth-wise, the source originated at great depth; Robertson and Kopf (1998) suggested a Miocene clast-rich mud breccia at depth of 5–7 km at the East Mediterranean Ridge. The same depth may correspond to infra-Jurassic beds in the SLB. Note that seismic markers as deep as 12 km are observed in the vicinity of Leviathan (Fig. 7), subject to the reliability of the velocity model used for depth imaging. As discussed above, the
sedimentary section; as such it cannot contribute to the discussion on the diapir's composition. Presence of mobilized Upper Triassic salt was proposed in offshore Lebanon as an extension of the Kurra Chine Formation in Syria (Beydoun and Habib, 1995; Marlow et al., 2011; Soledad Velasco et al., 2015), but has no further support. Jonah High and Ridge are associated with a magnetic anomaly, suggested by Folkman and Ben-Gai (2004) and Rybakov et al. (2011) to be caused by deep intrusive volcanic body. Suggesting mud as a source for the Leviathan's diapir is therefore based on an elimination process. It may though be supported by the compilation made by Ziegler (2001),
Fig. 4. Line EMED-50, depth migrated (part). Interpreted section. The line is located on the southern flank of Leviathan's diapir, yet the Lower Miocene tectonic onset is clearly observed. It may also represent the location of a separate, deeper culmination. See text for discussion. Depth to Leviathan-4 well is from the Ministry of Energy database. See location in Fig. 2.
6
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 5. Line EMED-13, depth migrated (part). Interpreted section. This is a strike line crossing the southwestern flank of the culmination. A fault pattern probably trending SE-NW is evident. Depth to Leviathan-1 well is from the Ministry of Energy database. See location in Fig. 2.
offshore Lebanon experienced greater subsidence rates during the Tertiary also between the margin and the Eratosthenes. This differential tectonic behavior between the Levant sub-basins requires further study. Focusing on mud as a likely source, there is a vast literature on mud diapirs and volcanoes in the Mediterranean basins. Mascle et al. (2014) compiled many of these, relating to recently active or active features. They concluded that these are associated with accretionary wedges. Robertson and Kopf (1998) discussed the Eastern Mediterranean Ridge mud volcanoes. Gamberi and Rovere (2010) dealt with mud volcanoes in the Tyrrhenian Sea while Soto et al. (2012) described the Miocene diapirs in the Alboran Sea and suggested a gravity-driven mechanism as described above. None of the diapirs described in this study are currently active or
depth to the acoustic basement in the SLB may reach 14–16 km in places. The geographic distribution of the diapirs in the SLB may provide a clue to some of the questions. It appears that they are confined to a belt in the southern basin, as sections across the basin to the north (Roberts and Peace, 2007; Hodgson, 2012; Skiple et al., 2012; Hawie et al., 2013) do not show such prominent events. It should be noted that Robertson (1998) suggested that the Eratosthenes Seamount was tectonically uplifted by about 1 km during or before the early Miocene. He attributed the uplift to regional compression resulting from northward subduction beneath Cyprus. In terms of dating, this perfectly matches the events described here. The amplitude of the Miocene structure in Leviathan exceeds 700 m. On the other hand, as mentioned above,
Fig. 6. Line 135-ISY, depth converted time migration (see text for details). (a) Non-interpreted section (b) Interpreted section (c) Section flattened on the Base Tertiary marker. See location in Fig. 2. Note direction of this line compared to the other lines, that makes it favorable for imaging the diapir's culmination.
7
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 6. (continued)
reconstructed maps and sections of the SLB marked in particular the Leviathan High and Jonah Ridge as the principal extension blocks while the other structures are considered of later age. The seismic sections shown here demonstrate the alternative reconstruction by flattening on key markers. The lack of coherent reflectors inside these structures leaves the deep markers (i.e. below Lower Miocene) practically uninterpretable and as such force using phantom markers. Reconstruction of the lines in Figs. 3c and 6c by flattening on the Base Tertiary marker suggests that until this time the Leviathan structure may have formed a high and depocenters around. From the Early Tertiary until the Late Oligocene not much happened besides accelerated subsidence, as evident by the conformity of these beds. The interval between the Top Oligocene and the Top Lower Miocene clearly marks the onset of the tectonic events that are seen today. It seems to have been developed symmetrically around the Leviathan's diapir. Fig. 7a shows line EMED-18 accompanied by gravity and magnetic
have been active recently. In this respect they differ from most of those mentioned above. Their life span is assumed to be rather short, starting at the Late Early Miocene and continue into the Middle Miocene. Robertson and Shipboard Scientific Party (1996) defined the age of the now dormant Milano mud volcano as 1.75 Ma while the Napoli volcano is active for the past 1.25 m.y. Such age range may fit the case in the SLB. Detailed biostratigraphic data that is available from the deep basin boreholes has not yet been published but it may be speculated that the main phase of activity lasted for about 3–4 m.y.
5.2. Tectonic reconstruction As discussed above, the current literature suggests that the SLB is assumed to be dominated by a set of Early Mesozoic horsts and grabens formed during the Neotethyan rifting while the Miocene folds are attributed to the collision involved with its closing. Previous
Fig. 6. (continued)
8
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 7. Line EMED-18, depth migrated. Gravity and magnetic measurements are projected from TGS line 2069-IS that in most parts coincides with EMED-18. The Bouguer gravity (solid line) and residual magnetic (dashed line) measurements are the actual observations along the line and not a grid cut-out. (a) Non-interpreted section (b) Interpreted section (c) Section flattened on the Base Tertiary marker with the suggested phantom markers through. ES = Eratosthenes, Aph = Aphrodite, Lev = Leviathan, Tmr = Tamar, JR = Jonah Ridge. Note the speculative marking of an intrusion at the easternmost side of Eratosthenes. Depth to Leviathan-5 well is taken from the Ministry of Energy database. See location in Figs. 1 and 2. Vertical exaggeration is about 1:8.
distorted due to oblique crossing of the structure through a major SENW trending normal fault of Middle Miocene age. The same applies for Tamar; both these structures are quazi-symmetric and lack any obvious reverse fault at depth (see Ben-Gai and Druckman, 2013). The speculative marking of the Middle Jurassic in Fig. 7b and c is a key in the suggested model. Not only that no extensional blocks are observed, as previously suggested (e.g. Gardosh et al., 2010; Sagy et al., 2015), but a thick depocenter evolved east of Leviathan under the
measurements projected from a neighboring TGS line (2069-IS). Its interpretation is shown in Fig. 7b. The section starts from the flank of the Eratosthenes in the west and runs through the structures of Aphrodite, Leviathan, Tamar and Jonah Ridge. It provides a synoptic view of the basin that emphasizes the dominance of the Miocene structures. The section demonstrates the conformity of events up to the Top Oligocene, while the section above up to Top Lower Miocene marks the onset of the folding. The Aphrodite structure at depth is severely
Fig. 7. (continued)
9
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 7. (continued)
Aphrodite. Another diapir under the Dalit gas field is not dealt with in this study; see its location in Figs. 1 and 8c. The same stands for the speculative intrusion at the eastern flank of the Eratosthenes Seamount seen at the western end of the section in Fig. 7b and c. The Tamar and Aphrodite structures show much reduced chaotic image at their cores. Two possible models may be suggested: one is that they are associated with deep-seated swells that did not develop into full-scale diapirs; the other is that they resulted from shortening caused by adjacent massive
present location of the Jonah Ridge during the Jurassic to Middle Cretaceous and under Tamar throughout the Senonian and Eocene. This is seen in Fig. 7c as asymmetric basin, yet its eastern edge is not well constrained by this line. This asymmetry of was also observed by Gardosh et al. (2008) in what they defined as grabens located between the elevated faulted blocks of Leviathan and Jonah. Line EMED-18 (Fig. 7) illustrates the difference between the main bodies of Leviathan and Jonah Ridge and the other structures, namely Tamar and
Fig. 8. Compilation of the Southern Levant Basin models in previous works, (a-c from north to south, see location in Fig. 1). All are based on the time domain versions of the Spectrum EMED lines. (a) Roberts and Peace (2007), line EMED-54, showing the pop-up structure of Leviathan. Ages below Base Salt were adopted from the above figures and do not coincide with the original interpretation as indicated at the left hand side in brackets (note for example that TLM corresponds to Base Paleogene in the original interpretation). A marker corresponding to the Top Oligocene has not been picked on this section. See Fig. 3 for the part of this section in depth domain over the Leviathan High. (b) Al-Balushi et al. (2016), based on time migrated version of line EMED-18. Its depth domain version is shown in Fig. 7. Horizon names adjusted to fit the above figures (appear at the left hand side) but grossly agree with the original interpretation. Note the Jurassic and basement phantom markers within the elevated blocks. (c) Gardosh et al. (2010), line EMED-50. Note the location of the Dalit structure at the east. See Fig. 4 for part of this section in depth domain over the Leviathan High. Structures are denoted as in Fig. 7 above. Interpretation is as appears on the original section.
10
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 8. (continued)
upwelling affected this level and these sections may provide further constrain for the timing of this phase. This study attempts to suggest an alternative view of the tectonic evolution of the SLB. It is based primarily on seismic images, yet the intense Miocene activity as described above greatly disturbed the deep part. In contrast, seismic data from offshore Lebanon, where the deep section is not affected, may provide important constraints. In this respect, studies such as Hodgson (2012), Hawie et al. (2013) and Skiple et al. (2012) may provide a comprehensive view of the Levant Basin evolution as they are based on the most modern seismic database available. The main issue addressed here is a seemingly intensive episode that occurred during the Late Early and Early Middle Miocene in the SLB. Few key points regarding the tectonic evolution of the basin in conjunction with this event are addressed. During the Permian and Early Mesozoic the basin experienced extension that was accompanied by subsidence and deposition of deep marine sediments and maybe also evaporites (see discussion and references above). This phase might have been associated with normal faults in the SLB, as also observed on the margin, though seismic lines offshore Lebanon do not show any faulted blocks at that level (e.g. Hawie et al., 2013, Figs. 6 and 7 there). Local basins developed during
diapirs (Tamar, for example is located between Leviathan to the west and Dalit and the Jonah Ridge to the east). It may also be a combination of both (see Ben-Gai and Druckman, 2013). The volume of the intrusive material is poorly constrained by the sparse 2D seismic grid used here. This should be further studied based on the 3D data covering the field and its vicinity. As such, it cannot be determined whether the intrusions are the cause of the observed folding or are the consequence of a yet to be determined tectonic process. The short duration of this phase may though suggest that the former is more likely. It is worth noting that the intrusion is fairly restricted spatially as indicated by the elongated culmination as seen in Fig. 2, coupled with its potential continuation into the Tanin Field to the NE (Fig. 1). It may also be that there is more than a single culmination and that deeper diapirs are present, for example at the location of the Leviathan-4 well (Fig. 4). Fig. 9 indicates, by the presence of a deep-seated feature marked by the TLD, that it might be connected with the one under the Leviathan-4 well, consistent with the southwest extension of the Leviathan High as seen in Fig. 1. In such case the surface mapped in Fig. 2 will obtain a more complicated shape, maybe showing en echelon pattern of separate structures. In both these locations it appears that the diapir did not penetrate the Lower Miocene as in Fig. 6, yet the
Fig. 8. (continued)
11
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
Fig. 9. Line 65-ISY, depth converted time migration. Interpreted section. This line suggests that the Leviathan Diapir extends to the southwest, maybe as a separate feature connected to the location of Leviathan-4. See text for further discussion. See location in Fig. 1.
5.3. Implications on petroleum systems
the Middle Mesozoic, such as the one described in Fig. 7, creating an accommodation space that may have trapped both source and reservoir rocks. During the Late Cretaceous, while the margin experienced Alpine-type tectonics and the formation of the Syrian Arc, the basin developed broad highs and lows manifested in thickness variations of the interval between Base Senonian and Late Eocene. No clear sign of compressional tectonics is evident in the basin during this period. It is worth noting that this change in structural style between the margin and the basin is concurrent with the change in crustal thickness, and maybe crustal composition. During the Early Tertiary the basin experienced accelerating subsidence but otherwise a quiescent period, that was interrupted at the Late Early Miocene by the intrusive phase. The predominant trend of these features is NE-SW, though further detailed mapping is required. It might be speculated that this trend follows weakness zones formed during the Early Mesozoic extensional stage. The intrusive phase was followed by closely spaced normal faults trending SE-NW, clearly shown on structures such as Tamar in Noble Energy's presentations (see Ben-Gai and Druckman, 2013). They are also evident on the strike line across Leviathan (Fig. 5). Offshore Lebanon extensional faults of the same age and orientation formed a multitude of potential traps in rotated blocks (Hodgson, 2012). The difference in structural style between the southern and northern basins of the Levant is herein attributed to the absence of intrusive activity offshore Lebanon, despite the thicker Tertiary section there. In both sub-basins, the tectonic activity ceased prior to the Late Miocene and is not observed at the Base Messinian salt. Tectonics of the massive Messinian salt is assumed to be thin-skinned as discussed above and it is probably not affected by the Miocene structures. Hodgson (2012) stated that the extensional regime in the deep basin is synchronous with the compression of the Syrian Arc system. Actually, he might have referred to the Syrian Arc II discussed above, as the original system is much older. To avoid confusion, Ben-Gai and Druckman (2013) suggested naming the Miocene system as the "Tamar Folding Phase".
The effect of the diapirs and the resulting structures described above on petroleum systems in the SLB is the accumulation of the biogenic gas so far discovered. This happened in places where the traps remained intact during and after the gas migration. As yet, no hydrocarbons have been found in the basin at deeper levels, though a few wells along the continental shelf of Israel and Northern Sinai encountered oil in the Lower Cretaceous and Jurassic (Gardosh and Tannenbaum, 2014). The implications of the suggested tectonic model on the petroleum systems are fairly obvious: if indeed the basin experienced such a wide-spread episode of diapirism during the Oligo-Miocene, then the integrity of traps older than Early Miocene is in jeopardy due to potential breaching. On the other hand, this phase formed a multitude of potential traps on and around these structures. The immediate candidates for deep targets are the interpreted closed structures of Tamar, Aphrodite and others alike. The flanks of diapirs may also form traps, such as those around Leviathan for example (Fig. 2). Presence of reservoir rocks and timing of expulsion and migration of thermogenic hydrocarbons comprise the main geological risk. As noted by Gardosh and Tannenbaum (2014), the relevant information regarding the SLB is limited to public announcements by the operators, and each component of the petroleum system there is currently speculative. 6. Conclusions In light of the working hypothesis, this study suggests an alternative tectonic evolution for the Southern Levant Basin based on seismic imaging. The principal conclusion is that most of what we observe today in the vicinity of the Leviathan Gas Field, and also in other parts of the basin, has been formed since the Early Miocene by a wide spread phase of diapirism. This has obvious implications for the proven biogenic and yet unexplored deep thermogenic petroleum systems. This interpretation poses a few questions that are yet to be answered with further seismic and well data. These questions are related first of all to the enigmatic composition of the diapirs and secondly to their geographic distribution and the associated tectonic processes. The 12
Marine and Petroleum Geology xxx (xxxx) xxx–xxx
Y. Ben-Gai
difference between the tectonic setting of the Levant margin and the basin and among the Levant sub-basins deserves further elaboration. The rock mechanics involved with these massive intrusions is another issue related to the question whether they formed the Miocene folding or were the consequence of a regional tectonic phase associated with these folds.
Gardosh, M., Tannenbaum, E., 2014. The petroleum systems of Israel. In: In: Marlow, L., Kendall, C., Yose, L. (Eds.), Petroleum Systems of the Tethyan Region, vol. 106. AAPG Memoir, pp. 179–216. Garfunkel, Z., 1998. Constrains on the origin and history of the eastern Mediterranean basin. Tectonophysics 298, 5–35. Garfunkel, Z., Derin, B., 1984. Permian–Early Mesozoic tectonism and continental margin formation in Israel and its implications to the history of the eastern Mediterranean. In: In: Dixon, J.E., Robertson, A.H.F. (Eds.), The Geological Evolution of the Eastern Mediterranean, vol. 17. Geological Society, London, Special Publications, pp. 18–201. Gradmann, S., Hübscher, C., Ben-Avraham, Z., Gajewski, D., Netzeband, G., 2005. Salt tectonics off northern Israel. Mar. Petroleum Geol. 22, 597–611. Hawie, N., Gorini, C., Deschamps, R., Nader, F.H., Mondaret, L., Granjeon, D., Baudin, F., 2013. Tectono-stratigraphic evolution of the northern Levant Basin (offshore Lebanon). Mar. Petroleum Geol. 48, 392–410. http://dx.doi.org/10.1016/j. marpetgeo.2013.08.004. Hodgson, N., 2012. The Miocene hydrocarbon play in Southern Lebanon. First Break 30, 93–98. Inati, L., Zeyen, H., Nader, F.H., Roure, F., 2016. Lithospheric architecture of the Levant Basin (eastern mediterranean region): a 2D modeling approach. Tectonophysics 693 (A), 143–156. http://dx.doi.org/10.1016/j.tecto.2016.10.030. Klimke, J., Ehrhardt, A., 2014. Impact and implications of the Afro-Eurasian collision south of Cyprus from reflection seismic data. Tectonophysics 626, 105–119. Krenkel, E., 1924. Der Syrische Bogen: Centralblatt fur Mineralogie. pp. 274–313 Geologie und Paleontologie. Jahrgang. Marlow, L., Kornpihl, K., Kendall, C.G.St.C., 2011. 2-D basin modeling study of petroleum systems in the levantine basin, eastern mediterranean. GeoArabia 16 (2), 17–42. Mascle, J., Mary, F., Praeg, D., Brosolo, L., Camera, L., Ceramicola, S., Dupre, S., 2014. Distribution and geological control of mud volcanoes and other fluid/free gas seepage features in the Mediterranean Sea and nearby Gulf of Cadiz. Geo-Marine Lett. 34 (2–3), 89–110. Netzeband, G.L., Gohl, K., Hubscher, C.P., Ben-Avraham, Z., Dehghani, G.A., Gajewski, D., Liersch, P., 2006. The Levantine Basin – crustal structure and origin. Tectonophysics 418 (3–4), 167–188. Roberts, G., Peace, D., 2007. Hydrocarbon plays and prospectivity of the Levantine Basin, offshore Lebanon and Syria from modern seismic data. GeoArabia 12, 99–124. Robertson, A.H.F., 1998. Mesozoic-Tertiary tectonic evolution of the Easternmost Mediterranean area: integration of marine and land evidence. In: In: Robertson, A.H.F., Emeis, K.-C., Richter, C., Camerlenghi, A. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 160. pp. 723–782. Robertson, A.H.F., Shipboard Scientific Party, 1996. Mud volcanism on the Mediterranean ridge. In: In: Emeis, K.-C., Robertson, A.H.F., Richter, C. (Eds.), Proceedings of the Ocean Drilling Program, Initial Reports, vol. 160. pp. 521–526. Robertson, A.H.F., Kopf, A., 1998. Tectonic setting and processes of mud volcanism on the Mediterranean Ridge accretionary complex: evidence from Leg 160. In: In: Robertson, A.H.F., Emeis, K.-C., Richter, C., Camerlenghi, A. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 160. pp. 665–680. Robertson, A.H.F., Mountrakis, D., 2006. Tectonic development of the Eastern Mediterranean region: an introduction. In: In: Robertson, A.H.F., Mountrakis, D. (Eds.), Tectonic Development of the Eastern Mediterranean Region, vol. 260. Geological Society, London, Special Publications, pp. 1–9. Rybakov, M., Goldshmidt, V., Hall, J.K., Ben-Avraham, Z., Lazar, M., 2011. New insights into the sources of magnetic anomalies in the Levant. Russ. Geol. Geophys. 52, 377–397. Sagy, Y., Gvirtzman, Z., Reshef, M., Makovsky, Y., 2015. The enigma of the Jonah high in the Middle of the Levant Basin and its significance to the history of rifting. Tectonophysics. http://dx.doi.org/10.1016/j.tecto.2015.09.037. Şengor, A.M.C., Stock, J., 2014. The ayyubid orogen: an ophiolite obduction-driven orogen in the late cretaceous of the neo-tethyan south margin. Geosci. Can. 41, 225–254. Şengor, A.M.C., Yılmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75, 181–241. http://dx.doi.org/10.1016/0040-1951(81)90275-4. Skiple, C., Anderson, E., Furstenau, J., 2012. Seismic interpretation and attribute analysis of the Herodotus and the Levantine Basin, offshore Cyprus and Lebanon. Pet. Geosci. 18, 433–442. Soledad Velasco, M., Snyder, H., Kidwai, M., 2015. Triassic Hydrocarbon Potential in the Eastern Mediterranean Region, Lebanon. pp. 3336–3341. http://dx.doi.org/10.1190/ segam2015-5906258.1. SEG Technical Program Expanded Abstracts 2015. Soto, J.I., Fernandez-Ibanez, F., Talukder, A.R., 2012. Recent shale tectonics and basin evolution of the NW Alboran sea. Lead. Edge 768–775 July 2012. Steinberg, J., Gvirtzman, Z., Folkman, Y., Garfunkel, Z., 2011. Origin and nature of the rapid late Tertiary filling of the Levant Basin. Geology 39 (4), 355–358. http://dx.doi. org/10.1130/G31615.1. April 2011. Walley, C.D., 1998. 1998. Some outstanding issues in the geology of Lebanon and their importance in the tectonic evolution of the Levantine region. Tectonophysics 298, 37–62. Ziegler, M.A., 2001. Late permian to holocene paleofacies evolution of the arabian plate and its hydrocarbon occurrences. GeoArabia 6 (3), 445–504.
Acknowledgements The partners of the Pelagic Joint Venture are acknowledged for providing access to the depth migrated sections. The Israel Ministry of National Infrastructures, Energy and Water Resources and M. Gardosh kindly allowed the use of the TGS 2008 survey. D. Craik greatly contributed in reviewing this manuscript. Y. Druckman and Y. Folakman are acknowledged for constructive discussions. The author wishes to thank two anonymous reviewers for highly constructive comments. References Al-Balushi, A.N., Neumaier, M., Fraser, A.J., Jackson, C.A.-L., 2016. The impact of the Messinian salinity crisis on the petroleum system of the Eastern Mediterranean: a critical assessment using 2D petroleum system modelling. Pet. Geosci. 22 (4), 357–379. http://dx.doi.org/10.1144/petgeo2016-054. Allen, H., Jackson, C.A.-L., Fraser, A.J., 2016. Gravity-driven deformation of a youthful saline giant: the interplay between gliding and spreading in the Messinian Basins of the Eastern Mediterranean. Pet. Geosci. 22 (4), 340–356. http://dx.doi.org/10.1144/ petgeo2016-034. Bar, O., Gvirtzman, Z., Feinstein, S., Zilberman, E., 2013. Accelerated subsidence and sedimentation in the Levant Basin during the Late Tertiary and concurrent uplift of the Arabian platform: tectonic versus counteracting sedimentary loading effects. Tectonics 32, 334–350. http://dx.doi.org/10.1002/tect.20026. Ben-Gai, Y., 2012. Mud diapirs in the deep Levant Basin - a proposed mechanism for the evolution of the gas-bearing tertiary structures. Isr. Geol. Soc. Ann. 31 Meeting, Ashkelon, abstract. Ben-Gai, Y., Druckman, Y., 2013. Mega-scale swells and diapirs in the deep Levant Basin, eastern mediterranean, and their association with recent world-class gas discoveries. In: AAPG ERC Conference, Barcelona, Search and Discovery Article #10518. Ben-Gai, Y., 2017. Seismic imaging of the Jonah high, Levant Basin – a new look at an old enigma. In: AAPG ERC Conference, Larnaca, Cyprus. Available at: https:// europeevents.aapg.org/ehome/202660/presentationspdf/. Beydoun, Z.R., Habib, J.G., 1995. Lebanon revisited: new insights into Triassic hydrocarbon prospects. J. Petroleum Geol. 18 (1), 75–90. Cartwright, J., Jackson, M., Dooley, T., Higgins, S., 2012. Strain Partitioning in Gravitydriven Shortening of a Thick, Multilayered Evaporite Sequence, vol. 363. Geological Society, London, Special Publications, pp. 449–470. http://dx.doi.org/10.1144/ SP363.21. Cowie, L., Kusznir, N., 2012. Mapping crustal thickness and oceanic lithosphere distribution in the Eastern Mediterranean using gravity inversion. Pet. Geosci. 18 (4), 373–380. Feng, Y.E., Reshef, M., 2016. The Eastern Mediterranean Messinian salt-depth imaging and velocity analysis considerations. Pet. Geosci. 22 (4), 333–339. http://dx.doi.org/ 10.1144/petgeo2015-088. Folkman, Y., Ben-Gai, Y., 2004. The ‘Jonah’ buried seamount: intrusive structure in the southeastern Levant Basin offshore Israel. Isr. Geol. Soc. Ann. 29 Meeting, Hagoshrim, abstract. Frizon de Lamotte, D., Raulin, C., Mouchot, N., Wrobel-Daveau, J.-C., Blanpied, C., Ringenbach, J.-C., 2011. The southernmost margin of the Tethys realm during the Mesozoic and Cenozoic: initial geometry and timing of the inversion processes. Tectonics 30http://dx.doi.org/10.1029/2010TC002691. TC3002. Gamberi, F., Rovere, M., 2010. Mud diapirs, mud volcanoes and fluid flow in the rear of the Calabrian Arc Orogenic Wedge (southeastern Tyrrhenian sea). Basin Res. 22, 452–464. http://dx.doi.org/10.1111/j.1365-2117.2010.00473.x. Gardosh, M., Druckman, Y., 2006. Seismic stratigraphy, structure and tectonic evolution of the Levantine Basin, offshore Israel. In: In: Robertson, A.H.F., Mountrakis, D. (Eds.), Tectonic Development of the Eastern Mediterranean Region, vol. 260. Geological Society, London, Special Publications, pp. 201–227. Gardosh, M., Druckman, Y., Buchbinder, B., Rybakov, M., 2008. The Levant Basin Offshore Israel: Stratigraphy, Structure, Tectonic Evolution and Implications for Hydrocarbon Exploration. pp. 121 Geological Survey of Israel Report GSI/4/2008. Gardosh, M., Garfunkel, Z., Druckman, Y., Buchbinder, B., 2010. Tethyan rifting in the levant region and its role in early mesozoic crustal evolution. In: In: Homberg, C., Bachmann, M. (Eds.), Evolution of the Levant Margin and Western Arabia Platform since the Mesozoic, vol. 341. Geological Society, London, Special Publications, pp. 9–36.
13