Gravity-induced deep-water carbonate deposits: Potential new plays in the Eocene of the Sirte Basin, Libya

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1 Gravity-induced deep-water carbonate deposits: Potential new plays in the Eocene of the Sirte Basin, Libya Uwe P. Baaske 1, Paul Tricker 1, Jimmy Van Itterbeeck 2, Helena Griffiths 2 and Jim Pickens 1 (1) Shell International Exploration and Production B.V., Kessler Park 1, 2288 GS Rijswijk, Netherlands; contact: uwe.baaske@shell.com (2) Shell Exploration and Production Libya GmbH, Abunawas 2, Gagaresh Road, Tripoli, Libya Keywords: Sirte Basin, Eocene, deep-water carbonate systems, calciclastic fan systems, mass transport complexes Introduction Eocene carbonates of the Sirte Basin in Libya house economically significant hydrocarbon accumulations (e.g. Zella, Gialo, and Sahl fields). Most of the fields and discoveries are located on the platform/shallow ramp part of the carbonate system in plays focusing on reservoir-seal pairs draped over deep structures. Generally these plays can be considered as becoming creamed (Figure 1) and new play concepts are needed to extend exploration outside the proven platform area. Significant potential to develop new play concepts exists in under-explored areas in the deep water depositional settings of the Eocene carbonate system. Figure 1: Creaming curve of the Eocene reservoirs of the Sirte Basin; blue: Lower Eocene; green: Middle Eocene; red: combined curve (graph based on IHS data). Tectonostratigraphic setting and stratigraphic framework of the Eocene The Sirte Basin in north Libya is a rift basin, with Early Cretaceous rift topography in-filled by Upper Cretaceous post-rift and Paleocene sag sequences. The Eocene is dominated by carbonate ramps that were part of the extensive carbonate system developed on the North African Tethyan margin. A Mid- to Late Eocene compression caused regional basin tilting and subsidence. Aside from that event, the Eocene was a period of only minor tectonic activity. Barr and Weegar (1972) divided the Eocene of the Sirte Basin into three formations: (1) the Gir (Al Jir) Formation, the (2) Gialo (Jalu) Formation and the Augila (Awjilah) Formation. Based on sequence stratigraphic analysis the Eocene of the Sirte Basin comprises six sequences of mean duration of 4 Myrs, of which five cover the interval of interest (Figure 2). The two sequences within the Gir Formation are independent of the lithostratigraphic scheme. Within the Gialo Formation there is a link between the three lithostratigraphic members and the three sequences (Al Gata Mbr. = ME1; Jakhira Mbr. = ME2; Gialo Mbr. = ME3).

2 Time [Ma] Series Stages Fms. Sea-Level Curve [m] Accomm. cycles Long Short term term Short term SL trend + - Seq EOCEN E Ea rly Middle La te Priabonian Bartonian Lutetian Ypresian Augila Fm. Gialo Fm. Gir Fm.?? FR elements in seismic LE ME3 ME2 ME1 EE2 EE1 55 Figure 2: Sequence stratigraphic framework of the Eocene of the Sirte Basin. Sea-level curves from Haq and Al-Qahtani (2005): light blue curve: 2 nd order sea-level trend, dark blue curve: 3 rd order sea-level trend. EE = Early Eocene; ME = Middle Eocene; LE = Late Eocene; red triangles: decreasing accommodation; blue triangles: increasing accommodation; red horizontal lines: sequence boundaries; blue horizontal lines: maximum flooding surfaces. Conceptual depositional model and typical plays The Eocene carbonate deposystem of the Sirte Basin can be classified either as a non-rimmed platform or as a distally steepened carbonate ramp. For the eastern flank of the Ajdabiya Trough, the focus area of this work, the ramp model is considered most appropriate. Wide depositional facies belts, mapped based on available regional well data, and the absence of margin stabilizing reef systems supported the carbonate ramp model. Most regional seismic lines also support the ramp model though a steeper margin is seen on some lines, due to influence of underlying structure. Figure 3: Conceptual depositional model for the Eocene formations of the eastern flank of the Ajdabiya Trough. A distally steepened ramp is seen as the most representative model for the complex carbonate system (modified after Al-Hawat et al., 2007).

3 Typical Eocene plays in the eastern part of the Sirte Basin comprise a nummulitic reservoir section draped over a deep-seated, potentially inverted structure and sealed by upper Eocene Augila Formation shales (e.g. Gialo and Sahl fields). Within the ramp model most of these fields sit within the inner to middle ramp facies belts suggesting that reservoir was deposited on the shallow ramp section and comprises coarser-grained, typically nummulitic limestones. Potential new plays Large parts of the Eocene section within the northern Sirte Basin are dominated by more distal carbonate settings and lie in an under-explored area with only speculative prospectivity. New regional 2D and 3D seismic data reveal the presence of widespread gravity-induced carbonate depositional elements like mass transport complexes (MTCs) and calciclastic fan systems (CFSs), especially along the eastern flank of the Ajdabiya Trough. Though good, comparable outcrop and subsurface analogues of such deepwater resedimented carbonate systems are not very common, an understanding of these systems is starting to develop, and play concepts can also be applied to other deep-water carbonate systems in other basins worldwide. The Lower Eocene calciclastic fan systems Large-scale calciclastic fan systems (CFSs) occur commonly in the basinal setting of the Lower Eocene Gir Formation of the Sirte Basin but are usually absent in the Gialo Formation. In the Ajdabiya Trough CFSs are predominantly characterized by extensive channel-levee elements and lobe systems. In that respect they are comparable to siliciclastic submarine fan systems, in terms of depositional processes and environments, and therefore can be seen as potential targets for hydrocarbon exploration. Unless draping older or inverted structural features, CFSs are mainly a stratigraphic play that can be sealed by fine-grained pelagic drapes. Charge could be provided from potential intra-formational, basinal source rocks, similar to the Eocene Bou Dabbous Formation, offshore Tunisia (e.g. Beavington-Penney et al., 2005). The reservoir comprises dominantly re-deposited fine-grained carbonate material with occasional intercalated coarser-grained material stemming from the shallow ramp. Reservoir quality will be poor (potentially high to moderate porosities but very low permeabilites) but might be improved by fracture systems and/or dolomitization (e.g. Kaufman et al., 1991). Analogues with Chalk fields might be useful. Figure 4: A map view of a sculpted seismic volume focusing on basinal Lower Eocene Gir Formation in the Ajdabiya Trough. A larger scale, amalgamated calciclastic fan system, with feeder and lobe elements, can be seen on the left. Sinuous channel-like features (channel-levee complexes, blue dashed lines) can be interpreted along the feeder systems and crosscutting the lobe system. The ramp and slope features on the right side of the picture are part of the Gialo Formation.

4 CFSs are difficult to identify in 2D seismic data but can be mapped in 3D data sets by using seismic attribute and/or seismic volume sculpting methods (Figure 4). These approaches usually highlight the main depositional elements of the CFSs, like channel-levee systems and lobe facies, that can be used to map the overall extent of the fan systems. The Middle Eocene Mass Transport Complexes Though mass transport complexes (MTCs) are common along the slopes of the Eocene carbonate systems, they are especially frequent in the Middle Eocene Gialo Formation. Observed MTCs in that area include debrites, slumps, slides and similar gravity-driven features along the slope and toe-of-slope. Turbidites and small-scale CFSs are common on the toe-of-slope to basin floor setting. If MTCs are detached from the ramp system, they often form valid stratigraphic traps on the steepened part of the ramp ( slope ) and toe-of-slope. Assuming that such systems can be charged by potential intraformational source rocks (e.g. Bou Dabbous equivalent, Beavington-Penney et al., 2005) and/or Sirte sources, e.g. via faults, reservoir quality is the main issue for the MTC play as seal can be provided by fine grained pelagic strata. Reservoir quality of carbonate MTCs is usually considered to be poor. This can be related to the grain-size mix, transport processes but also to the high diagenetic potential of such deposits. Still, if fracturing and potential late stage dolomitisation are considered, the Gialo MTCs might prove to be prospective (e.g. Poza Rica Field, C&C Reservoirs, 2003). Early charge and overpressure might also improve the reservoir quality by limiting burial diagenesis (e.g. Neilson et al., 1998). Figure 5: Oblique view on multiple mass transport complexes (MTCs) along part of the eastern flank of the Ajdabiya Trough. The seismic volume sculpting highlights the presence of large-scale MTCs on the upper part of the slope. The small-scale calciclastic fans on the toe-of-slope are turbidite systems and partly younger than the upper slope features. Play potential and sequence stratigraphic framework The dominance of CFSs in the Gir section and their absence within the Gialo Formation might be linked to the high-level sequence stratigraphic setting of the two formations (Figure 6). During the Gir Formation the bulk of the northern Sirte Basin is flooded and even wide parts of the shallow shelf areas are dominated by finer-grained carbonate deposition. Fine-grained, carbonate mud is typically highly water saturated and instable and can be transported over long distances in turbidity currents particularly in channel-levee complexes. Over time these depositional systems form large-scale calciclastic fan systems reaching far into the basin (Figure 6A).

5 Continued carbonate production on the shallow shelf and the overall regressive sea-level trend during the Lower and Middle Eocene (Figure 2) caused a continuous, partly forced progradation of the carbonate system towards the steepened part of the ramp (Figure 6B). Enhanced off-bank transport of coarsergrained shelf material (e.g. nummulitic fragments) in combination with the tectonically steepened slope along the Ajdabiya Trough, led to development of extensive MTCs along the Gialo slope/outer ramp. Turbidites and small-scale CFSs form on the toe-of-slope. Due to the presence of more coarse material during Gialo times, CFSs are less prominent and considerably smaller than during the deposition of the Gir Formation but might be better targets in terms of reservoir quality. Figure 6: Simplified cartoons showing the different depositional settings of the gravity driven Eocene plays. (A) The Gir calciclastic fan play: large areas of the Gir shelf are flooded and covered with finergrained carbonate sediments (e.g. pelagic sediments and fine-grained off-bank material). This allows the development of large calciclastic fan systems. (B) The Gialo mass transport play: continuing carbonate production and continued lowering of the sea-level leads to progradation of the carbonate system and brings coarser-grained material close to the shelf break. Off-bank transport causes the deposition of coarser-grained MTCs and associated, finer-grained turbidites and small-scale calciclastic fan systems. Summary and implications Resedimented deep-water carbonate deposits are a common feature in the Eocene of the Sirte Basin in Libya. Two types can be identified in seismic data: calciclastic fan deposits on the basin floor, forming a potential play in the Gir Formation, and mass transport complexes within the slope setting of the Gialo Formation, also a potential play. Both plays focus on resedimented carbonate material as reservoir and fine-grained pelagic strata as seal. Charge can be provided from intra-formational sources in the deeper basin or from classical Sirte sources. The trapping mechanism in both plays is mainly stratigraphic and therefore difficult to derisk. These deep-water carbonate plays in the Sirte Basin have yet to be drilled and therefore are not proven and resedimented carbonate reservoirs might be more difficult than more typical carbonate targets. However the stratigraphic nature of the systems, especially the CFSs, could also allow relatively large volumes. Play concepts related to resedimented carbonates in deep water settings are not new but have not been fully applied to the Eocene carbonates of the Sirte Basin before. The presented work shows that seismic volume sculpting is a valuable tool to help highlighting resedimented carbonate deep-water plays. Combined with seismic geomorphological analysis it allows to get the most out of the seismic and to identify under-explored potential in the deep-water carbonate setting. This approach could also be applied in other basins to develop and implement new play concepts.

6 Acknowledgements We are grateful to the National Oil Corporation of Libya for their input to the study and the permission to publish data and the results. Especially we thank Monem Albatikh and Eisa Alfurjani (both NOC of Libya) for their input. The authors would also like to thank the many people within Shell that were involved in the study of the Eocene carbonates: C. Nicolai, I. van der Molen, C. Boynton, the Exploration Team in Tripoli, J. Karlo, P. Burgess, T. van Hoek, H. Williams, G. Kleemayer, S. Chowdhury, S. Witte, M. de Roij, S. van Heck, B. Barkmeijer and M. Al-Mugheiry. References Barr and Weegar, 1972, Petroleum Exploration Society of Libya, 179 p. Beavington-Penney et al., 2005, Sedimentology, 52, C&C Reservoirs, 2003, Field Evaluation Report, 39 p. El-Hawat et al., 2007, 3 rd North African/Mediterranean Petroleum & Geoscience Conference, EAGE, A26. Haq and Al-Qahtani, 2005, GeoArabia, v. 10, No. 2, Kaufman et al., 1991, Sedimentology, 38, Neilson et al., 1998, Marine and Petroleum Geology, 15,