Lower-Middle Miocene Play Fairway Opening Multi TCF Gas Discovery: Raven Field, Nile Delta
Peter Butterworth and James Saxton
Up until the early part of the 21st Century, the majority of the gas discoveries in the offshore Nile Delta were amplitude anomaly driven, Plio-Pleistocene reservoirs. ‘Shallow gas’ slope system and shelf edge delta play fairways were delineated in the West and East Nile Delta respectively. The discovery of the Lower Miocene Raven Field in 2004 was the first discovery of a multi-TCF, pre-Messinian gas-bearing slope reservoir play fairway in the Nile Delta cone.
Structurally, the Western Nile Delta is underlain by Jurassic rift blocks associated with the opening of the Mediterranean. The structural setting of the basin is characterised by steep, fault-bounded margins that exert a fundamental control on deposition of slope canyon and channel systems of the Miocene play fairway. The most prominent structural feature is a large SW to NE plunging anticlinorium that extends offshore for 160 km. The Raven discovery consists of slope channel complexes draped over a 20 km by 10 km anticline along this structure. Pre-drill the stratigraphy was unknown, with the closest well 130km to the east. Given the absence of well control, charge risk, the effectiveness of reservoirs at >4000m BSL and the lack of a 4 way closure were issues of considerable concern pre-drill.
Raven Field is a Burdigalian – Langhian aged reservoir comprising stacked channel complexes draping a 3 way closure. The source rock is an Oligocene aged terrestrial kerogen, and the field is sealed by the Qantara Middle Miocene marine flooding surface. High resolution 3D multi azimuth seismic data has allowed for the definition of internal channel architecture, as well as recognition of a fluid response in the Lower Miocene section. The reservoir sedimentology of the field comprises a series of stacked channel complexes infilling a palaeo-low generated by large scale upper slope collapse. Stacking patterns are controlled by this basin slope palaeotopography, and is analogous to the present day Rosetta channel fill (which overlies the field). Internally individual channel complexes display a systematic fill sequence. Following incision, early sediment by-pass is followed by aggradation of individual channel elements stacking vertically to form fining upward sequences backfilled by the deposits of high density turbidity currents. Abrupt switchoff of sediment supply occurs systematically within all channel complexes into a debrite – silty facies prior to ultimate abandonment as low relief constructional leveed channel fill.
Reservoir quality and deliverability are governed by facies, diagenetic cementation (quartz, kaolinite) and depositional stacking patterns. The key net:gross issue is in the delineation of the thick aggradational channel fill which comprises the bulk of the reserves. Thin bedded internal levees are typically totally cemented and non-reservoir. Pressure data from the gas and aquifer legs of the channel complexes suggests that channel complexes will deplete as separate reservoir entities.
The success at Raven has derisked a significant volume of the Nile Delta’s yet to find volumes. The opportunity for export or supply to a growing domestic market means that Egypt is well placed to benefit from a new play fairway in a mature basin.
AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013