Inversion tectonics: a brief petroleum industry perspective

The concept of structural inversion was introduced in the early 1980s. By definition, an inversion structure forms when a pre-existing extensional (or transtensional) fault controlling a hangingwall basin containing a syn-rift or passive fill sequence subsequently undergoes compression (or transpression) producing partial (or total) extrusion of the basin fill. 10 Inverted structures provide traps for petroleum exploration, typically four-way structural closures. As to the degree of inversion, based on large number of worldwide examples seen in various basins, the most preferred petroleum exploration targets are mild to moderate inversional structures, defined by the location of the null-points. In these instances, the closures have a relatively small vertical amplitude, but simple in a map-view sense and well imaged on seismic reflection data. Also, the closures typically cluster above the extensional depocentres which tend to contain source rocks providing petroleum 15 charge during and after the inversion. Cases for strong or total inversion are generally not that common and typically are not considered as ideal exploration prospects, mostly due to breaching and seismic imaging challenges associated with the trap(s) formed early on in the process of inversion. Also, migration may become tortuous due to the structural complexity or the source rock units may be uplifted above the hydrocarbon generation window effectively terminating the charge once the inversion occurred. 20 For any particular structure the evidence for inversion is typically provided by subsurface data sets such as reflection seismic and well data. However, in many cases the deeper segments of the structure are either poorly imaged by the seismic data and/or have not been penetrated by exploration wells. In these cases the interpretation of any given structure in terms of inversion has to rely on the regional understanding of the basin evolution with evidence for an early phase of substantial crustal extension by normal faulting. 25


Introduction
Whereas the concept of structural inversion has been around for a century (e.g. Lamplugh, 1919), the term has been specifically used for the first time by Glennie and Boegner (1981) to explain the evolution of the Sole Pit structure located in the UK sector of the southern North Sea. The first generalized description of structural inversion was offered by Bally (1984) https://doi.org/10.5194/se-2020-33 Preprint. Discussion started: 30 March 2020 c Author(s) 2020. CC BY 4.0 License.
We chose another example of the Sava Folds which provided an important oil and gas find in the region. One of the first 160 major oil fields discovered in Hungary, Lovászi, is also an inverted anticline delineated by potential field data and surface dip measurements in the western Pannonian Basin in 1940. As this particular exploration play was relatively simple, i.e. E-W trending anticlines with relatively shallow Pliocene to Miocene clastic reservoir targets, all the prospects of this play were drilled up as early as in the 1940's (Dank, 1985) and most of them are essentially depleted by now.
Based on abundant well control, the Pliocene to Miocene succession in the broader area was studied by Juhász (1994,1998). 165 Her sub-regional lithostratigraphic transect, crossing the Lovászi field (Fig. 6), clearly shows a prominent surface anticline with a vertical relief of about 800-1000 m associated with the Pliocene to Upper Miocene (Pannonian; Sarmatian to Badenian) strata compared to their regional levels in this part of the Pannonian Basin. The Lovászi anticline is depicted as a slightly asymmetric one, therefore, in our interpretation, suggesting an underlying master syn-rift fault on its southern flank (Fig. 6). However, given the lack of deep wells penetrating the entire syn-rift core of the inverted anticline, the geometry of 170 the inferred master fault and the location of a null-point along it cannot be established using well data only.
As there is modern 3D seismic data available covering the entire Lovászi field (Tóth and Tari, 2014) the structural history of this anticline can be studied in the context of its inverted nature (Fig. 7). The interpretation of the seismic data ( Fig. 7a) reveals the growth of anticline in the manner depicted in Bally's cartoon (Fig. 1). In particular, the thickening/thinning geometries within the Upper Pliocene (Pannonian) strata in the apex of the anticline show the switch from extension to 175 compression (Fig. 7a). Interestingly, flattening on multiple seismic horizons demonstrated the early growth of the anticline during the early Pannonian already focusing hydrocarbon migration into the structure (Tóth and Tari, 2014). The main period for the formation of the anticline, however, is clearly post-Pannonian as all the Pannonian reservoirs levels are gently folded ( Fig. 7c) into low-amplitude 4-way closures (Fig. 7b).
Historical production from the multiple Pannonian reservoirs of the Lovászi field ( Fig. 7c) was about 50 mmbbl oil and 230 180 bcf gas. Current exploration efforts are focusing on the deeper parts of these anticlines where reservoir quality prediction and imaging of viable traps are the main challenges (Tóth and Tari, 2014). As most of these anticlines are the products of Late Pannonian (Pliocene) to Quaternary inversion of Middle Miocene syn-rift half-grabens, the proper structural understanding of the core of the anticlines is critical for any future exploration efforts.

A case study from the East Med: Tamar gas field 185
Another region where inverted anticlines have been described is located in the Eastern Mediterranean (Fig. 8). These Syrian Arc structures, as named by Krenkel (1925), extend from the Sinai to the Palmyrides with a typical trend of ENE-WSW to NNE-SSW. These prominent features formed by the inversion of pre-existing Mesozoic extensional structures from the late Cretaceous to Oligocene times. Two main phases of folding have been documented so far (e.g. Walley, 1998). The first one can be dated as an intra-Santonian phase of deformation (early Syrian Arc phase) and the second one is dated dominantly as 190 a late Eocene series of events (late Syrian Arc phase).
From an exploration point of view, the Syrian Arc structures are very important. For example, the traps within several onshore Egyptian hydrocarbon fields are formed by Syrian Arc events (e.g. Dolson, 2003). Also, Middle to Late Cenozoic Syrian Arc style compressional features are present in the deepwater of the Eastern Mediterranean providing the traps for many deepwater discoveries during the last decade (e.g. Gardosh and Tannenbaum, 2014). 195 It is important to emphasize, that not all Syrian Arc anticlines are basement-involved structures and therefore, not all of them are inverted features in the strict sense of the word (e.g. Cooper et al., 1989). A regional Upper Triassic salt sequence provided an effective detachment surface for numerous anticlines in the Damascene segment of the Arc in Syria (Fig. 8).
In northern Egypt (Fig. 8), sedimentation during the Late Cretaceous was interrupted during the Santonian by the The summary below is largely based on the work of Bevan and Moustafa (2012) who used the examples of three onshore 255 Egyptian fields (e.g. Razzak, Mubarak and Kattaniya) to generalize some observations. We note that these cases specifically capture the learnings from inverted structures in a failed wide rift setting in an onshore basin where the post-rift basin fill is very thin, especially compared to the syn-rift sequence (Fig. 12).
Inversion structures which are relatively mild develop low-amplitude but robust 4-way closures in the hangingwall of the master fault responsible for the structure (Fig. 12a). The master fault does not necessarily have to manifest itself at the level 260 of the reservoirs. As described earlier in the case of the Lovászi and Tamar fields inverted structures could have large closures higher up in the unfaulted sequence (Figs. 7 and 11, respectively). As to charge, the position of source and seal rocks in the hangingwall side of the fault is quite critical. Whereas the source rocks located beneath the inverted anticline may actually migrate away from the structure, the source rocks on the flank of the footwall may generate hydrocarbons which then migrate to the tip of footwall and travel along the fault to the ultimate trap in the apex (Fig. 12a). 265 In the more advanced inverted structure (Fig. 12b) the same basic charge limitation occurs, i.e. the majority of mature hydrocarbons from within the source rocks within the deeper syn-rift sequence will migrate away from the hangingwall closure associated with the reactivation of the master fault. However, the smaller closures that could develop above antithetic faults on the subsidiary side of the half-graben (Fig. 1e) may receive charge (Fig. 12b). This asymmetric arrangement of traps associated with near null-point inversion is informally called the butterfly structure (see Fig. 1e). 270 In the most advanced cases of structural inversions (Fig. 12c), the reservoir units in the hangingwall become uptilted and potentially exposed on the paleo-surface, therefore becoming breached. As noted by many, the vertical uplift of source rocks, potentially generating hydrocarbons prior to the inversion, may switch off the kitchen as the source rocks may reach shallower depth where they are not generating any more (e.g. Turner and Williams, 2004;Cooper and Warren, 2010). In these more severe cases of inversion, the smaller, subsidiary structures on the flank should be targeted (Fig. 12c). These 275 smaller closures may remain unbreached and could receive charge from downdip source rocks as Bevan and Moustafa (2012) pointed out.
As to the regional scale impact of structural inversion we would like to highlight here the simple point made by Tari and Jabour (2011). The large gas discoveries of the last decade in the deepwater Levant Basin are all associated with inverted structures which strike parallel with the margin (Fig. 10). From a trapping point of view this translates to an optimum 280 situation as the closures of the four-way anticlines are not significantly affected by the regional basinward dip trending perpendicular to the anticlinal axes (Fig. 13). In contrast, on passive margin where the inversional anticlines have the same trend as the regional dip, the four-way closures on the updip end of the structures tend to be much smaller (Fig. 13). Late coastline, in water depth of 2,000-4,000 m. The anticlines have a general WNW-ESE trend, perpendicular to the overall strike of the Central Atlantic margin, but parallel with the regional dip of the margin. Therefore we believe that the regional-290 scale trend of the inverted structures versus the regional dip in a passive continental margin or in a foredeep setting is quite important (Tari and Jabour, 2011).

Discussion and outlook
Whereas inversion tectonics could produce spectacular traps, inversion tectonics is a process which has profound implications on other elements of the petroleum systems and, therefore, the prospectivity, both in a positive and a negative 295 sense (e.g. Macgregor, 1995;Turner and Williams, 2004;Cooper and Warren, 2010). The most negative impact is attributed to the fact that during inversion source-rock sections are brought much closer to the paleo-surface and therefore previous mature source-rocks switch-off and become non-generative. Also, the main reservoir and source-rock sections are many times being brought to the surface and therefore breached. There are many other negative, but valid impacts listed by Turner and Williams (2004) giving the impression that inverted features may be more challenging for exploration than "regular" 300 anticlines formed by simple contraction. Perhaps, their view might also somewhat biased by considering examples from exhumed European Atlantic margins (e.g. Dore et al., 2002). In these regionally inverted rifts basins there are plenty of evidence for underfilled fields and former petroleum accumulations which were breached and leaked away due to inversion tectonics (Turner and Williams, 2004).
Yet, in many other basins of the world, inverted structures provided repeatable and highly successful plays. In particular, the 305 examples we chose for this paper located in the Sava Folds region of the western Pannonian Basin and the Syrian Arc anticlines in the deepwater Eastern Mediterranean basin turned out to be very successful, especially in the Levant.
We believe that the key for the success in these basins is that source rocks are not constrained to the extensional basin fill but rather occupy a higher, but pre-inversion stratigraphic position. These source rocks tend to be unconfined to the underlying extensional basins and more regional in character. Indeed, as Bevan and Moustafa (2012)  2). We believe that this number should be significantly higher as many inverted structures may not be recognized as such.
Finally, we would like to emphasize the need to better quantify the degree of inversion for any given structure in order to find the optimum trapping situation for exploration efforts on a global scale. With other words, what degree of inversion 320 provided the largest number of HC fields worldwide?
However, this analysis requires a quantitative description of the inversion. There are two ways of doing this (Fig. 14). Williams et al. (1989) introduced the concept of inversion rate, i.e. the magnitude of contraction due to inversion versus the magnitude of extension. In seismic profiles, this is equivalent to the ratio between the thickness of syn-rift deposits above the null point parallel to the fault plane and the total thickness of syn-rift deposits parallel to the fault plane on the hanging-wall 325 (Fig. 14a). However, the inversion rate may be difficult to calculate in cases when the null-point cannot be located with confidence. An alternative method was proposed by Song (1997) to calculate the inversion rate (Fig. 14b), but this method also requires good handle on many elements of the stratal geometry along the master fault (e.g. Yang et al., 2011). In our opinion, the quantification of inversion degree is a challenge as the deeper section beneath an inverted anticline is typically not well imaged seismically and/or not drilled due to the greater depth. The inversion rate in both of the case studies 330 described in this paper, i.e. Lovászi and Tamar fields (Figs. 7 and 11, respectively) would be difficult to determine, at best.

Conclusions
Inverted structures provide traps for petroleum exploration, typically four-way structural closures. As to the degree of inversion, based on large number of worldwide examples seen in various basins, the most preferred petroleum exploration targets are mild to moderate inversional structures. In these instances, the closures have a relatively small vertical amplitude, 335 but simple in a map-view sense and well imaged on seismic reflection data. Also, the closures typically cluster above the extensional depocentres which tend to contain source rocks providing petroleum charge during and after the inversion. Cases for strong or total inversion are generally not that common and typically are not considered as ideal exploration prospects, mostly due to breaching and seismic imaging challenges associated with the trap(s) formed early on in the process of inversion. Also, migration may become tortuous due to the structural complexity or the source rock units may be uplifted 340 above the hydrocarbon generation window effectively terminating the charge once the inversion occurred.
For any particular structure the evidence for inversion is typically provided by subsurface data sets such as reflection seismic and well data. However, in many cases the deeper segments of the structure are either poorly imaged by the seismic data and/or have not been penetrated by exploration wells. In these cases the interpretation of any given structure in terms of inversion has to rely on the regional understanding of the basin evolution with evidence for an early phase of substantial 345 crustal extension by normal faulting. In some cases, where the regional geology has not been properly appreciated, the simple reactivation of pre-existing structures related to earlier episodes of shortening in the area was erroneously classified as inversion.
There might be a negative bias towards the prospectivity of inverted structures using examples from exhumed margins.
Another bias may stem from the typical assumption that the generating kitchen tends to be in the syn-rift sequence of the inverted structure. Highly successful exploration cases in basins which have not experienced severe uplift and exhumation, like the giant gas discoveries in the deepwater Levant Basin, highlighted the importance of source rocks not being constrained to the syn-rift basin fill but rather located in the post-rift regional sequence, but in a pre-inversion stratigraphic position.

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Interestingly, within these 2,000 cases we have found only about 60 matches for inversion as a "trap forming mechanism". This translates to only about 3%, a strikingly low proportion. We believe that inversion tectonics may be unrecognized in many fields globally and therefore it remains underreported. Courtesy of IHS Markit.