2D cross-section restoration in the onshore Nile Delta, Egypt. Nico Hübner (DEA Deutsche Erdoel AG)

Transcription

1 2D cross-section restoration in the onshore Nile Delta, Egypt Nico Hübner (DEA Deutsche Erdoel AG)

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3 Introduction A 2D cross-section restoration was carried out as part of a hydrocarbon play assessment of the Nile Delta. The restoration comprises fault block restoration and backstripping and restoration, performed with the structural modelling software MOVE. The section is located in the onshore central Nile Delta and covers almost the entire part in Southwest-Northeast orientation (Figure 1). The 78km long fully interpreted composite 2D/3D time-migrated seismic line is representative of the Pre- Messinian structural style (Figure 2). The section connects the Mesozoic to Oligocene platform in the South with the Cenozoic basin in the North. Platform and basin are separated by the Hinge line, a nearly East-West oriented complex fault zone, which is affected by booth, high angle normal faulting in the footwall and low angle listric faulting in the hangingwall. Subject of investigation is the Pre-Messinian Oligocene to Middle Miocene interval north of the Hinge line. Extensive 3D Seismic coverage reveals several structural styles and fault geometries accompanied with various depositional architecture and facies environments. However, poor seismic imaging, low well control and structural complexity induce uncertainty in interpretation of faults and horizons. Only a few deep wells have been drilled to test the Pre-Messinian in basin marginal position, e.g. Helal-1X and El Tayifa-Deep-1X which were both dry. The area North of the Master fault, however, remains frontier and offers great potential for finding hydrocarbon accumulations. The aim of this study is to validate or adjust an existing structural framework and horizon interpretation. Furthermore, the restoration is used to improve the knowledge of fault geometries, fault linkage and timing of structuring. Figure 1 A - schematic structure map with major tectonic units in the Pre-Messinian, B regional overview of the SE-Mediterranean with major faults, modified after Harrison et al Method The preparation of the 2D section is based on a regional seismic interpretation and well data from 6 offset wells (see Figure 1 and 2). A number of adjustments and assumptions were made for the preparation of the 2D section: 1) definition of 18 stratigraphic layers, 2) definition of Top Mesozoic and 3) position and shape of the Master fault as it forms the detachment from the fixed footwall. Assumptions are supported by observations from diverse offset 2D/3D seismic data (i.e. seismic character, multitrace attributes).

4 The section can structurally be divided into a slightly deformed fixed footwall, a heavily folded and faulted hangingwall (above the Master fault) and a post-extensional Plio-Pleistocene sequence. The term syn-extension is only used for the Qantara-Abu-Madi interval. Due to poor seismic imaging in the central part of the section, seismic interpretation is highly uncertain. The interpretation of the footwall north of the hinge zone is conceptual and in alignment with Dolson et al. (2005). In order to validate the seismic interpretation a fault block restoration was applied for two appropriate intervals to be present along the whole section, the Oligocene-Qantara and the Mesozoic footwall. This method highlights any geometric inconsistencies (gaps and overlaps) or thickness variations of beds across faults, but it does not take decompaction into account. The Oligocene-Qantara interval was validated by application of the flexural slip unfold technic. The result is satisfying as a more or less constant thickness without large gaps or overlaps was gained. However, some restored Oligocene fault blocks do not fit exactly because of shale mobilisation, secondary structural trends (out of section) or minor miss- interpretations. Restoration of the Mesozoic footwall (Figure 2-C) was executed using a combination of basic transform tools and flexural slip unfold. As a result the section has been restored with some mismatch at the Cretaceous collapse zone with an amount of extension of 3km. In contrast, the amount of extension within Oligocene-Qantara is approximately 11km. In preparation for the sequential restoration the section was depth-converted using a pseudo-check shot based on El Tayifa-1X. A single check shot is certainly not ideal for a long line with strong lateral contrasts in lithology and interval velocity. However, the result was acceptable as projected wells tied to the section and only minor effects of distortions appeared in the footwall. Backstripping and restoration was performed on the depth-converted cross-section. The workflow involves the sequential removal and decompaction of the overburden, the restoration of fault displacements and folds and subordinated the rebuilding of eroded material. Decompaction parameters (initial porosity, porosity-depth coefficient) are estimated from bulk lithology of the units derived from well logs. Restored porosities were calculated by the Sclater & Christie formula (Sclater & Christie, 1980). The restoration procedure refers to the application of Move on fold algorithms simple shear and faul-parallel flow. It should be noted that distortion and artificial folding could not be avoided to some extent. Figure 2 A - 2D balanced cross-section, onshore Nile Delta, B Qantara/Oligocene interval restored to pre-deformation stage via flexual slip unfold technique. Strong condensed Qantara (blue) in the central part refers most likely to erosion (see Figure 3), C Mesozoic/Basement interval restored to pre-deformation stage in combination of basic transform tools and flexural slip unfold.

5 Conclusions In backstripping and restoration all in all 23 steps have been done, starting from the removal of Pleistocene up to the restoration of the Mesozoic footwall. As a result the section could be completely restored revealing an amount of extension of about 10km in the Oligo-Miocene complex and 4km in the Mesozoic Basement. This is in line with the flexural slip unfold exercise. The procedure of backstripping and restoration gives not only insight to the timing of tectonic activity or sedimentological events, it also helps to infer paleo-geometries of faults, folds and beddings. A key uncertainty in this study is the shape and role of the Master-fault. After removal of the overburden up to Sidi Salim and partly restoration of Top Qantara the Master fault appears to have a low angle listric ramp-flat geometry (Figure 3). The hangingwall shows a roll-over anticline with a breached frontal limb. Further subdivision can be made by taking analogue models from Ellis & McClay (1988) and McClay (1995) into account. Thus the hangingwall can be further subdivided into crestal collapse block and translational block. A second trend in the North of the section has similiar geometry. This model explains the shallow position of the Oligocene in the central part of the section. Other models have different approaches, e.g. thin-skinned with deeper detachment (Kellner et al. 2009) and thick-skinned (Abdel Aal et al. 2000). The structuring defined in the 2D cross-section can be mapped in lateral extend. Thus the Cenozoic Pre-Messinian of the central onshore Nile Delta can be divided into major structural units: roll-over anticline, crestal collapse zone and translational block (Figure 2 and 3). The units have a general WNW-ESE strike and follow more or less the hinge line (Figure 1). Figure 3 Step 13 in the workflow of backdstripping and restoration, overburden up to Sidi Salim was removed and Qantara partly restored showing a paleo-structure of a ramp-flat listric detachment geometry. The black line is top pre-extension and gives the regional level. Stratigraphy see Figure 2. With regard to tectonic phasing the following major observations were derived from sequential restoration: At the Hinge zone, roll-over anticlines started to develop initially in Qantara times in a ramp/flat setting (Figure 3). Anticlines were overprinted by ongoing extension in Sidi Salim 1 to 5. Locally, the ramp anticlines were breached by faults which accommodated considerable throw. Further subordinate collapse occurred in Qawasim times with apparently more subsidence at the northern roll-over anticline. In Abu Madi times the whole area north of the Master fault was affected by subsidence of up to 400m. After that tectonic activity ceased and regional subsidence took place. The collapse of the central part started probably in Qantara and continued until Abu Madi times. Bed rotation of late Sidi Salim and thickening of Qawasim age beds suggests strong fault activity at this time, in particular at the southward dipping faults. The collapse of the Upper Mesozoic at the Hinge line is probably attributed to an earlier event in Eocene to Early Oligocene time. This interpretation is in line with a study from the Matruh trough, NW Egypt (Tari et al. 2012). Based on the above mentioned observation a tectonic model is suggested: (1) Jurassic Extension, horst and graben development, subsidence along the active hinge zone; (2) Cretaceous no fault activity, ongoing deposition;

6 (3) Eocene-Early Oligocene - subsidence and development of local troughs north of the Hinge line, collapse at the Hinge zone above a Lower Cretaceous detachment; Oligocene relief-equation; (4) Early Miocene (Qantara) Extension and formation of a graben system north of the Hinge line in connection with roll-over anticline and crestal collapse structures; (5) Middle Miocene (Sidi Salim) breaching and ongoing crestal collapse of the southern roll-over anticline; (6) Late Miocene (Lower Qawasim) sharp decrease in extension, minor activity along the northern roll-over anticline; (7) Late Miocene (Upper Qawasim) minor extension and ongoing subsidence along some faults in the crestal collapse zone; (8) Latest Miocene (Abu Madi) subsidence of the whole area north of the Master fault; (9) Pliocene to recent cease of tectonic activity and regional subsidence. Time-depth conversion and decompaction parameter are significant technical risk factors in sequential restoration, especially when rock composition is speculative. Improvements will be gained by depth conversion based on velocity models and scenarios with different decompaction parameters and formula. Finally, the knowledge of prevailing structural styles is crucial for the development of play concepts. Acknowledgements The author thanks colleagues and contractors from DEA Cairo Egypt Branch for cooperation and DEA Deutsche Erdoel AG for permission to publish the data. References Abdel Aal, A., El Barkooky, A., Gerrits, M., Meyer, H., Schwander, M. Zaki, H Tectonic evolution of the Eastern Mediterranian Basin and its significance for hydrocarbon prospectivity in the ultradeepwater of the Nile Delta. The Leading Edge, pp Dolson, J.C., Boucher, P.J., Siok, J., Heppard, P.D Key Challanges to realizeing full potential in an emerging giant gas province: Nile Delta/ Mediterranian offshore, deep water, Egypt. In: Doré, A.G. & Vining, B.A. (eds) Petroleum Geology: North-West Europe and Global Perspectives Proceedings of the 6th Petroleum Geology Conference, pp Ellis, P.G. & McClay, K.R Listric extensional fault systems-results of analogue model experiments. Basin research Vol. 1, pp Kellner, A., Khawaga, H.E., Brink, G., Brink-Larsen, S., Hesham, M., Abu El Saad, H., Atef, A., Young, H., Finlayson, B Depositional History of the West Nile Delta Upper Oligocene to Upper Pliocene. AAPG Search and Discovery Article #30092 (2009). McClay, K.R D and 3D analogue modelling of extensional fault structures templates for seismic interpretation. Petroleum Geoscience Vol. 1, pp Sclater, J.G.& Christie, P.A.F Continental stretching: an explanation of the post-mid- Cretaceous subsidence of the Central North Sea basin. Journal of Geophysical Research, Vol.85, pp Tari, G., Hussein, H.; Novotny, B., Hannke, K. and Kohazy, R Play types of the deep-water Matruth and Herodotus basins, NW Egypt. Petroleum Geoscience, Vol. 18, pp Harrison, R., Newell, W., Batıhanll, H., Panayides I, McGeehin, J., Mahan, S., Özhür, A., Tsiolakis, E., Necdet, M Tectonic framework and late Cenozoic tectonic history of the northern part of Cyprus: implications for earthquake hazards and regional tectonics. Journal of Asian Earth Science Vol. 23, pp