The Phanerozoic history of the Western Desert was shaped by the opening of first Paleotethys and then Neotethys, which morphed into the modern Mediterranean Sea when the seaway between Arabia and Eurasia closed about 15 Ma. Extensional structures related to Paleotethys are present in the subsurface of the Western Desert but are presently not well known. Neotethyan rifting, however, left a complex legacy of multi-phase basins along the northern margin of Gondwana (Fig. 2). Further west in Algeria and Tunisia initial opening began in the Permian and by the Triassic had reached northern Egypt and the Levant and a seaway extended into Syria (Şengör, 1979; Stampfli et al., 2001; Garfunkel, 2004; Berra and Angiolini, 2014). Permian and Triassic continental strata are encountered in wells in the far Western Desert and in outcrops along the Gulf of Suez. Like the Paleozoic section, relatively little is known regarding the structural setting of these units.

The earliest well-defined rifting event in the Western Desert occurred during the Middle to Late Jurassic and established the general basin configuration that persisted through most of the Mesozoic (Keeley and Wallis, 1991; Guiraud, 1998). Most faults active in the Jurassic are oriented E–W to ENE–WSW with an ∼ N–S extension direction (Fig. 3). This structuration is generally attributed to the distal effects of the continued opening of Neotethys further to the north. However, potential fields and seismic datasets acquired over the past several decades suggest that the eastern Mediterranean basin segment opened with a WNW–ESE extension direction and that the Egyptian margin was a transform boundary (Longacre et al., 2007). Resolving the apparent disconnect between the Egyptian offshore and Western Desert onshore basin kinematics will be important to establishing a better understanding of the geodynamic evolution of NE Gondwana.

In the western Faghur and Shushan sub-basins, Jurassic rifting was marked by an early phase of volcanism, mostly in the form of local basaltic flows, tuffs, and volcaniclastics (Abbas et al., 2019). The volcanics are overlain and interfinger with siliciclastic rocks that are ascribed to the Khatatba Formation (Norton, 1967; Fig. 3). The Khatatba Formation is both an important reservoir objective and the most important source rock in the Western Desert (Keeley et al., 1990).

Western Desert “Jurassic” rifting was relatively short-lived and ended in the earliest Cretaceous, spanning a period of ∼ 10 Myr or less (∼ 150–14 Ma). The syn-rift stratigraphy varies dramatically in thickness and facies from sub-basin to sub-basin. In general, the section is much thinner in the west and south and thickens toward the north. At the end of the Jurassic to earliest Cretaceous, a widespread but brief marine incursion resulted in the deposition of Masajid Formation open marine limestone facies over most of the Western Desert, except on a few, high-standing platform areas (Fig. 3; Norton, 1967; Keeley et al., 1990).

Immediately following Masajid flooding, during which active extensional faulting is not recognizable in most sub-basins, a second phase of rifting initiated with strata assigned to the Alam el Bueib Member of the Burg el Arab Formation (Fig. 3; Norton, 1967). This is the most pronounced extensional phase in most Western Desert sub-basins and lasted about 14 Myr (∼ 139–125 Ma). Extension was also initially N–S directed, but midway through the rift event, extension rotated to NE–SW (Fig. 3).

In addition to the strong clockwise rotation of the extensional stress field, which is also recognized in many other basins of north and central Gondwana (Guiraud and Bellion, 1995; Guiraud, 1998; Guiraud and Bosworth, 1999; Guiraud et al., 2001, 2005), the Western Desert experienced a pulse of compression at about 138 Ma, which we refer to as the late Cimmerian event (Fig. 3). This shortening only affected a small number of faults, an example of which is discussed below.

The Alam el Bueib phase of rifting, like the Khatatba, ended with a second even more regionally extensive marine flooding event, which deposited the Alamein and Dahab members (Norton, 1967). NE–SW-oriented extension renewed in the mid-Aptian at about 120 Ma, and marine deposition was replaced by predominantly fluvial deposits of the Kharita Member and Bahariya Formation (Said, 1962; Norton, 1967). Kharita–Bahariya rifting was prolonged, lasting about 20 Myr (∼ 120–100 Ma), but generally occurred at slower extension rates that gradually dissipated in lower Bahariya times. In other parts of Gondwana, the Albian–Aptian was the most important phase of extension, as was the case in much of the central African rift system (Schull, 1988; McHargue et al., 1992; Bosworth, 1992).

Sea-level rise in the Cenomanian and Turonian resulted in flooding of all the Western Desert and establishment of an epeiric sea that would last into the early Cenozoic (Said, 1962; Kerdany and Cherif, 1990). These marine strata are assigned to the upper Bahariya and Abu Roash formations (Fig. 3; Norton, 1967) and were deposited during a relatively quiescent period in the Western Desert. In the Sirt basin to the west (Fig. 2), this was a time of significant extension and subsidence in its NW–SE-trending sub-basins (Wennekers et al., 1996; Abadi et al., 2008). The Western Desert calm was abruptly terminated at 84 Ma with the onset of the main pulse of regional basin inversion, the Santonian event (Moustafa and Khalil, 1995; Guiraud and Bosworth, 1997; Guiraud, 1998; Bevan and Moustafa, 2012). Santonian compression, shortening, and inversion were of true plate-scale significance, as was recognized long ago by Burke and Dewey (1974).

Santonian inversion can be interpreted to be a consequence of a change in relative movement between the Eurasian and African plates, with N–S divergence switching to N–S slightly oblique convergence (Savostin et al., 1986; Le Pichon et al., 1988; Dewey et al., 1989). Convergence continues to the present day and was manifest in North Africa by a series of compressional pulses, interspersed with periods of quiescence or extension that were spatially complex (Bevan and Moustafa, 2012). The most pronounced post-Santonian shortening occurred at the end-Cretaceous and within the late Eocene, corresponding to coeval compressional maxima in the Alpine belt of Eurasia (Fig. 3; Guiraud et al., 1987; Guiraud and Bosworth, 1997; Guiraud, 1998).

During and following Santonian inversion, shallow marine carbonate environments continued across the Western Desert with deposition of the Khoman Formation (Fig. 3; Norton, 1967). The Khoman, which is commonly a chalky facies, is completely missing from the crests of some major Santonian inversion structures. Apollonia Formation (a term borrowed from Libyan stratigraphy) limestone deposition commenced following the base Cenozoic unconformity and generally continued until the late Eocene deformation when the northern Western Desert epeiric seas began to retreat and siliciclastic deposition returned (Dabaa Fm.; Norton, 1967). Mixed carbonate and siliciclastic deposition continued through the Oligocene and Miocene (Moghra and Marmarica Fms.; Said, 1962), punctuated by a very brief period of basaltic volcanism at 24–22 Ma that was related to Red Sea rift initiation (Fig. 3; Meneisy, 1990; Bosworth et al., 2015a). Most of the Western Desert, excluding some coastal regions, experienced gradual uplift and erosion from the late Miocene to the present day.

Faghur is the westernmost sub-basin of the northern Egyptian Western Desert rift system (Fig. 2). Exploration started there in the late 1950s encouraged by success to the west in the basins of Libya. However, the first commercial discovery was not made until 2006. The only documented functioning source rock in the Faghur basin is the Khatatba Formation (Bosworth et al., 2015b; Fig. 3), which was deposited during the short-lived Late Jurassic first phase of rifting. The Alam el Bueib-6 unit at the base of the second more profound Early Cretaceous rifting event may also have local source potential at Faghur.

Extensional faults that affect the Khatatba and immediately overlying Masajid Formation strike predominantly in two orientations: E–W or ENE–WSW (Fig. 4). Like the other main sub-basins of the Western Desert, most of these faults dip to the south, which is significant as the coeval Neotethyan margin stepped down to the north. The south dip probably reflects reactivation of a pre-existing basement or Paleozoic (Hercynian?) structural fabric.

The only places where an early phase of shortening and inversion have been observed are on a few of the ENE–WSW-striking faults, as along the Tayim-/Phiops trend (Fig. 4). There the inversion affected a small segment of the fault system that dipped NW, just north of the large Kalabsha horst block. Early syn-rift growth on this fault was small but resolvable (Fig. 5). Inversion occurred during deposition of the basal units of the Alam el Bueib Member, so in very early Cretaceous times. Based on this timing and in accordance with the better documented tectonic phases of SE Europe we designate the inversion a “late Cimmerian” event (Nikishin et al., 2001; Stampfli et al., 2001). Minor folding and local erosion of this age have been observed elsewhere in North Africa, the Benue trough, the Levant margin, and the Arabian platform (summarized in Guiraud et al., 2005).

Alam el Bueib syn-rift phase 2 strata drape over and seal the inversion anticline (Fig. 5). Differential compaction across the structure affected most of the mid- and upper Alam el Bueib strata resulting in four-way dipping (domal) unfaulted closures higher in the section. In detail, the hinge of the Phiops fold is doubly plunging and not exactly parallel to the underlying contractional fault (Fig. 6). The hinge curves away from the ENE–WSW-striking fault becoming NE–SW trending. This suggests that the shortening direction was approximately NW–SE oriented. Along strike several other smaller inversion anticlines are recognized, and to the north the fold trend steps to the east across another major down-to-the south early extensional fault.

In the Faghur basin oil migration commenced in the Late Cretaceous (Bosworth et al., 2015b; Abdelbaset et al., 2019), long after the late Cimmerian inversion structure was formed. Reserves are trapped in both pre- and post-inversion siliciclastic reservoirs. The amount of shortening at Phiops is not large, although it did remove all the early extension on the fault and all units now display reverse offset. The Phiops inversion is restricted to a single fault trend and had no noticeable effect at the scale of the Faghur sub-basin. No reserves have so far been recovered from the overthrust footwall block.

The products of younger inversion are present in the Faghur basin but are very minor. Structures formed by Santonian shortening include small folds of the Abu Roash strata (Fig. 3) along the large basin-bounding faults. This was of no consequence to the hydrocarbon system of the sub-basin. Slightly more significant was renewed NE–SW-directed extension and accompanying sedimentation during the Campanian and Maastrichtian, which provided additional overburden and therefore helped to accelerate maturation of the deep Khatatba source rocks. Late Cretaceous NW–SE shortening and NE–SW extension were probably at least in part coeval at Faghur.

Late Cretaceous shortening in the Western Desert has been extensively documented, both in outcrop (Moustafa, 1988; Abdel Khalek et al., 1989; Moustafa et al., 2003) and the subsurface (Kerdany and Cherif, 1990; Moustafa et al., 1998; Yousef et al., 2010, 2019; Bevan and Moustafa, 2012). In the eastern sub-basins of the Western Desert, inversion is manifest at both the scale of individual faults and across complete sub-basin profiles. Shortening was intense in the Alamein, Abu Gharadig, and Matruh sub-basins (Fig. 2) but as discussed above largely absent from the westernmost regions. Near the border with Libya, almost all the Late Cretaceous shortening occurred further to the north in Cyrenaica, which acted as a promontory or indenter during the Eurasia–Gondwana collision (Bosworth et al., 2008).

A regional transect of the Alamein and East Abu Gharadig basins illustrates the scale and significance of Late Cretaceous and younger inversion in the eastern Western Desert (Fig. 7; see also Bevan and Moustafa, 2012, their Fig. 19.7). The stratigraphy of the eastern sub-basins is very similar to that of Faghur in its overall framework. In detail, several differences can be noted: (1) the pre-Jurassic stratigraphic section is much thinner or absent completely; (2) particularly in the north, depositional facies in the Jurassic and Cretaceous tend to display more marine affinities; (3) thickness variations in the Cenozoic section are much more dramatic, in part due to the effects of late inversion; and (4) a gentle, regional northward tilt of the late Miocene to Holocene section, particularly in the Alamein basin (not observed in Faghur).

The most pronounced shortening at the longitude of Fig. 7 occurred at the Mubarak inversion where crystalline basement now structurally overlies part of the early syn-rift stratigraphy. The inverted fault at this position was not the original basin-bounding fault but rather cuts through the axis of the early basin. The area is covered by good-quality 3D seismic reflection data, and along-strike the basin-bounding and inverted faults merge to produce a more “typical” inverted extensional fault. Other less prominent inversion structures, more akin to the scale of Early Cretaceous Cimmerian shortening described at Faghur, are present at Misaada and Gondul (Fig. 7).

The most prominent inversion-related unconformity across all these eastern basin structures initiated in the Santonian. Pronounced onlap of the Campanian–Maastrichtian Khoman chalk onto the Mubarak fold is evident in seismic lines (Bevan and Moustafa, 2012). A second, dramatic unconformity at Mubarak developed at the end of the Mesozoic, indicating renewed shortening and denudation that continued into the late Eocene.

The total Jurassic to present-day stratigraphic thickness of the Alamein, East Abu Gharadig, and Faghur basins are quite comparable, generally 5–6 km in the vicinity of the main basin axes. However, the geothermal gradients at Alamein and East Abu Gharadig are higher than at Faghur, and therefore oil generation commenced earlier, generally in the mid-Cretaceous. Migration was well underway by the time of the Santonian inversion and more so for the later pulses of compression. Breaching of some reservoirs that had already trapped hydrocarbons was inevitable. Fortunately for the inversion structures in Fig. 7, numerous reservoir horizons remained intact and Early Cretaceous syn-rift exploration targets were successful.

The exploratory wells drilled on the inversion structures of Figs. 5 and 7 were all successful. Along strike, other wells were not so lucky. Other parts of the Mubarak inversion were uplifted and eroded more deeply than at the EB-32A location. In some cases, wells encountered reservoirs with residual hydrocarbons suggesting that oil migration and trapping occurred and then was lost. The Phiops, Misaada, and Gondul trends are much smaller structures than Mubarak, and all display very complex local fault patterns. Offset locations and similar play types to these wells were not always successful either.

In addition to breaching structurally shallow reservoirs, the large Western Desert inversions such as Mubarak also interrupt hydrocarbon maturation processes, at least over the region undergoing significant uplift. Estimating how much stratigraphic section was removed, rather than non-deposited, is a complex and difficult problem to address as relevant data (e.g., thermochronometric) are often lacking. The potential effects of inversion-driven denudation on paleo-heat flow are another consideration, generally not well-constrained or even considered.

Basin-scale inversions like Mubarak also drastically impacted migration pathways (Bevan and Moustafa, 2012). Prior to Santonian inversion, almost all hydrocarbons being generated and expelled from the Jurassic Khatatba Formation in the East Abu Gharadig basin were flowing through carrier beds up-dip to the south, toward Misaada and Gondul (Fig. 7). During Santonian and younger inversion, the basin axis progressively migrated to the south, with more and more of the deeper stratigraphic section rotating and ultimately dipping to the south, refocusing migration to the north. Understanding these changes in migration paths, which can occur at both local and regional scales, is important to successful exploration strategies.

The Black Sea is a Cretaceous basin complex superposed on the northern margin of the Tethys/southern margin of Laurussia (Nikishin et al., 2001; Okay and Nikishin, 2015). The Mesozoic pre-rift tectonostratigraphy of the WBSB is quite complex as it has elements of Early to Middle Triassic rifting, Late Triassic–Early Cimmerian orogenesis, Jurassic back-arc extension, and the Late Jurassic–late Cimmerian regional compressional phase (Fig. 3; Nikishin et al., 2001). These alternating extensional and compressional cycles produced inverted structures, like those of the Triassic rifts on the Scythian Platform and in Dobrogea (Saintot et al., 2006), but these are typically poorly understood subsurface features.

The Black Sea basin complex is traditionally thought to be a marginal or back-arc basin with active rifting beginning in the mid-Cretaceous (Finetti et al., 1988; Nikishin et al., 2015a, b). In terms of geodynamic models of modern back-arc basin formation, this extension was driven by slab roll-back (Stephenson and Schellart, 2010). However, a debate in the literature is still ongoing regarding not only the geodynamic reason for the basin opening but also its timing and kinematics (Tari, 2015; Tari et al., 2015).

The Black Sea basin opened in a complex manner and Tari (2015) distinguished two major rifting periods within the Cretaceous. Initial rifting started as soon as the Barremian and became regionally widespread in a “wide-rift” mode by the Aptian–Albian (syn-rift stage 1; Fig. 3) with numerous rift sub-basins trending NW–SE or E–W (Robinson and Kerusov, 1997; Krezsek et al., 2017). There is surface and subsurface evidence for Albian volcanics in the area, including western Crimea (Nikishin et al., 2013), and the mostly andesitic volcanism appears to be limited to the E–W-trending Karkinit basin. The trend of rifting changed to NE–SW at the end of the Albian and a new rifting period occurred during the Cenomanian to Santonian (syn-rift stage 2; Fig. 3). During this time a “narrow-rift” style of much larger-scale regional volcanic back-arc extension was superimposed on the Early Cretaceous, mostly a non-volcanic extensional system. By the Santonian, the WBSB opened to its full extent and in our study area the top Santonian is considered by Khriachtchevskaia et al. (2010) as the ultimate break-up unconformity.

Since the first basin-wide distributed volcanics are Turonian in age, the WBSB evolved as a sensu stricto back-arc basin only during the Turonian–Santonian interval (Tari, 2015). The subsequent widespread Campanian volcanism in the Pontides, and its assumed equivalent in the Turkish offshore area (Nikishin et al., 2015a), was interpreted by Tari (2015) as being arc-related but post-dating the opening of the WBSB.

The uppermost Cretaceous and lower Paleogene (Paleocene to middle Eocene) stratigraphy of the Odessa Shelf is dominated by chalks (Figs. 3, 9), reflecting tectonic quiescence in a post-rift setting. The first compressional event disrupting the waning subsidence pattern happened at the end of the middle Eocene at about 38.6 Ma (Khriachtchevskaia et al., 2010), and the deposition of carbonates was replaced by shales (Figs. 3, 9). During the late Eocene at about 35.4 Ma, another basin-wide shortening episode produced the bulk of the inverted structures (Khriachtchevskaia et al., 2010). This “Pyrenean” event (Fig. 2) is considered as the most significant one in the broader Black Sea area, and it can be correlated with the last phase of overthrusting in the Balkans (Doglioni et al., 1996; Bergerat et al., 2010). The Crimean Mountains also experienced shortening-related uplift during this time based on apatite fission-track studies (Panek et al., 2009).

Regionally, the Oligocene to lower Miocene Maykop Formation (Vernyhorova and Ryabokon, 2020; Figs. 3, 9) postdates the two Eocene discrete inversion events as can be deduced from the onlap geometries seen on reflection seismic data (Fig. 10). The early and middle Miocene saw another two inversion events (circa 16.3 and 10.4 Ma) in our study area (Khriachtchevskaia et al., 2010). The pronounced diapir-looking structure (Gamburtsev) in the middle of the regional seismic line (Fig. 10) is an extreme example of the multiple contractional reactivation of an already existing inverted structure. The inversion process was quite selective spatially and temporarily across the Odessa Shelf, as not all the pre-existing Cretaceous master faults were reactivated in any given cross section (Fig. 10). However, the large border fault on the northern margin of the Karkinit basin did experience reactivation along strike to the east in the area of the Golytsina gas-condensate field (Fig. 11). The seismic expression of both the footwall and hanging wall is clear and even the position of the null point can be determined with confidence. The inversion clearly post-dated the Maykop Formation and therefore is post-early Miocene in age.

The Sudak folded belt offshore Crimea (Tari and Simmons, 2018) formed during the Miocene (Stovba et al., 2009, 2013, 2017; Sheremet et al., 2016a, b) in multiple stages (Fig. 3). The corresponding Miocene compressional episodes with slightly rotating but generally N–S-oriented compressional stress fields were documented by micro-tectonic studies in onshore Crimea (Murovskaya et al., 2014; Hippolyte et al., 2018). The challenge onshore, just like in the offshore, is that these stages or events cannot be precisely dated, i.e. with a resolution of less than 1–2 Myr, and separated thus far. This limitation is primarily due to the lack of Miocene sediments in the Crimean Mountains onshore and the lack of sufficiently dense sampling of the stratigraphy in offshore industry wells.

Khriachtchevskaia et al. (2010) argued that the period of discrete inversion ended by the late Miocene or at least was suspended. This is contrary to the models of Robinson et al. (1995) and Nikishin et al. (2003), who suggested an accelerated period of subsidence in the Black Sea basin complex since the late Miocene or Pliocene, respectively, as the result of an overall N–S-directed compressional stress regime down-bending the basin center. Whereas this subsidence is difficult to document given the resolution of the biostratigraphic dating, a closer look at the available seismic reflection data does provide definitive evidence for ongoing post-late Miocene compression in the Gulf of Odessa.

We chose a legacy 2D seismic line across the Shtormovoe inversion anticline to show how this particular feature displays signs of repeated and also neotectonic shortening (Fig. 12). This broad, 20 km wide structure is a composite one at depth; below about 1 s two-way travel time, it splits into two inversion anticlines, 4 and 12 km across. These correspond to earlier Eocene inversion events. With the continuous thickening of the sedimentary cover, the earlier Eocene inversion anticlines were incorporated into a broader, single Miocene to Pliocene anticline.

A key observation regards the geometry of a prograding shelf margin sequence over the apex of the structure which postdates the pronounced regional intra-Sarmatian (Khersonian) unconformity dated as circa 7.5 Ma in the Black Sea (Fig. 12; Popov et al., 2010). The clinoforms in this prograding unit are slightly back-rotated to the north and their top laps, which should be sub-horizontal, show ∼ 2–4∘ northward tilt (Fig. 12). There are also two onlapping reflectors on the northern flank above the prograding sequence. Given the dimensions of the structure and the timing, this back-rotation cannot be attributed to differential compaction. These observations underscore the reactivation of the inversion process during the Pliocene.

The present-day stress field in the area, based on earthquake focal mechanism studies, is a compressional-to-strike-slip one (Murovskaya et al., 2018) which is consistent with other regional observations of ongoing N–S-directed compression in the broader Black Sea area (Tsereteli et al., 2016).

There are several hydrocarbon fields in the Gulf of Odessa and the adjacent Crimean Peninsula (Fig. 13). The Odessa Shelf was explored for the last 5 decades, and eight gas and gas-condensate fields have been discovered, all drilled in jack-up water depth (less than 100 m). Exploration was historically focused on the inverted structural highs. The productive horizons are related to Upper Cretaceous (Maastrichtian), Paleocene, Eocene, and Oligocene–lower Miocene reservoirs found at depths of 480–3000 m (Khriachtchevskaia et al., 2009; Stovba et al., 2009). The two largest gas and condensate finds – Golitsyno and Shtormovoe, with recoverable gas or condensate reserves of 420 bcf (bcf) (3 mmboe) and 777 bcf (21 mmboe), respectively – have been developed.

Nedosekova et al. (2008) reviewed the drilled structures and concluded that all the prospects and leads associated with simple four-way closures have been tested. As a general observation, there seems to be a trap-fill issue as the inverted structures could hold much larger hydrocarbon volumes than the discovered resources. The map-view four-way closures of the Golitsyno and Shtormovoe anticlines are 680 and 440 km2, respectively (Sergey Stovba, personal communication, 2010) with multi-trillion cubic feet recoverable gas potential. However, these structures are clearly not filled to spill and the observed gas columns are in the range of tens of meters. The underfilled trap issue can be explained by charge limitations, trap timing versus charge and/or by trap failure or breaching. Given the multiple (up to four) inversional events shaping these anticlines individually, losses to several remigration periods between the inversions probably played a role. These risks also explain why some of the large inverted structures in the area turned out to be dry, like the prominent Gamburtsev anticline (Fig. 10).

There are two schools of thought as to finding more hydrocarbons in this seemingly mature petroleum province. One suggestion was made by Burchell (2008), who described a new gas play type associated with the deeper part of the Golitsyno anticline, beneath the producing lower Paleocene chalks (Fig. 11; Robinson and Kerusov, 1997). Four possible gas-charged Lower Cretaceous sand targets were considered within the rift basin fill of the Karkinit trough at 4500 to 5500 m depth (Fig. 11). These targets with structural dip in three directions that closes against a fault have a large map-view extent, on the order of about 300 km2. Hydrocarbon charge was deemed to be relatively low risk by Burchell (2008) given the gas-condensate finds in adjacent fields and assuming thick gas-mature Lower Cretaceous shales in between the sand units in the so far undrilled rift basin center. Upper Cretaceous calcareous black shales, as potential source rocks, have been documented on the western part of the Crimean Peninsula (Kitchka et al., 2016). The obvious exploration risks of this deep, inversion-related play include side seal against crystalline basement across the large inverted fault, the presence and quality of the Lower Cretaceous reservoir objectives and trap definition due to the lack of 3D seismic data. This deep play remains untested to date.

The other line of thought is represented by Nedosekova et al. (2008) emphasizing the underexplored nature of stratigraphic and combination traps in the region, such as sand body pinchouts along the flank of paleo-highs. To show the impact of the Eocene inversion events shaping the paleo-relief of the basin, we reproduce here the isopach map of the Oligocene to lower Miocene Maykop sequence (Fig. 13; Gozhik et al., 2010). Contrary to what basin-scale well correlations can indicate, incorporating data points from basin highs (Fig. 9) the Maykop isopachs shows dramatic variations between 0 and 1700 m across the Gulf of Odessa (Fig. 13). Given this range, we interpret a deepwater sedimentary environment for the Karkinit trough.

Whereas the Maykop sequence overall is dominated by shales (Fig. 9), as in the rest of the Black Sea (Tari and Simmons, 2018), there are reservoir quality deepwater sandstones in it, as in the Krymska field (Fig. 13) and in the undeveloped Subbotina oil discovery south of the Kerch Peninsula (Fig. 8), reported by Khriachtchevskaia et al. (2009) and Stovba et al. (2009), respectively. Therefore, we speculate that future regional-scale 3D seismic surveys could image potential longitudinal and transversal intra-Maykop turbiditic systems within the Karkinit trough offering various stratigraphic traps along the basin margin (Fig. 13).