Sedimentology and depositional setting of the Asl and Hawara Members, October Field, Egypt
The authors gratefully acknowledge the management of BP and GUPCO for permission to publish this manuscript.
PART 1: INTRODUCTION
The study was initiated by BP/GUPCO in order to:
• assess the sedimentology and petrology of the cored Asl and Hawara Members in October Field, Egypt (Fig. 1A),
• determine the depositional framework and diagenetic overprint and how they relate to the tectonic setting,
• place the October Field into a depositional and reservoir framework to facilitate prediction of deposits with optimal reservoir properties.
Figure 1A: Location and Structural Content
• Detailed 1:50 scale logging of 936ft of core from 7 wells of the October Field (Fig. 1A).
• Examination of 539 thin-sections to assist with lithological classification and provide an initial assessment of diagenetic modification and reservoir quality.
• Two principal lithofacies were observed: carbonate-rich packstones and grainstones and medium-grained quartz-rich sandstones; the sandstones occur repeatedly as coarse-grained and fine-grained elements that reflect the evolution of a turbidity current/fan system.
• Gravity flows are the dominant depositional processes.
• Uplifted fault blocks provided sediment sources and established sediment bypass zones along cross faults and relay-ramps.
• Carbonates formed on uplifted blocks while siliciclastic material was derived from those fault blocks as well as from exposed older source areas and erosion of the sidewall stratigraphy along transform faults.
• Reservoir quality is controlled by a combination of primary depositional fabric and subsequent diagenetic modification.
• Thick, porous sandstones retain their primary interparticle porosity whereas thinner sandstone lithologies are more extensively cemented by calcite and contain relatively isolated macropores.
• Uplifted fault blocks provide poor reservoir targets due to the lack of siliciclastic material, abundance of cemented carbonate material and sediment starvation. Palaeotopographic lows, however, will provide the best reservoir targets due to the presence of thicker, relatively uncemented sandstones.
Figure 1B: Stratigraphy and Geological Summary
Initial isostatic uplift and rotation of rift shoulders rearranged drainage systems that delivered sediment to the basin. Consequently fluvial flow was directed away from the rift causing sediment starvation in deeper parts.
As drainage systems re-equilibrated, deltas formed on the rift margins and transported coarse clastic sediments into the rift. This phase of tectonic evolution of the Suez Rift is documented by the Mheiherrat Member (designated T20, Fig. 1B). Following initial isostatic uplift, thinning of the crust caused further isostatic uplift of the basin and increased fault block rotation.
New sedimentary patterns were established within, and on the margins of the rift, and deposition is characterised by the Asl and Hawara Members (designated within T30, Fig. 1B).
Within these members, carbonates accumulated on uplifted parts of fault blocks and sands accumulated in low areas in front of the faults.
At Wadi Baba, (Fig. 1A) the change in sedimentation is marked by shallower water deposition and an increase in sand content.
Latest Oligocene to Late Miocene
A NW-SE rift formed during the Latest Oligocene and extension continued through the Late Miocene when active extension ceased.
GEOLOGICAL CONTEXT AND SUMMARY OF THE TERTIARY IN THE OCTOBER FIELD REGION
The Asl and Hawara Members have traditionally been placed as part of the Lower Miocene Rudeis Formation (Fig. 1B). Structural complexity (Fig. 1A), and resulting stratigraphic complexity, arose from the Gulf of Suez as a rift system.
PART 2: SEDIMENTOLOGY - FACIES & DEPOSITIONAL ELEMENTS
Detailed description of the cored intervals of the Asl and Hawara Members reveals:
• A succession of mixed carbonate-siliciclastic allochthonous lithologies (Fig. 2A).
• A four-fold hierarchical division of lithological heterogeneities (Fig. 2B).
Figure 2A: Lithofacies Scheme
Figure 2B: Sedimentological Hierarchy
Lithofacies classification is up-scaled into two types of depositional element which are defined by (intervals of similar) texture and grain-size; type-A (coarse-grained and laminated) and type-B elements (fine-grained and bioturbated). Both elements may be either clastic (Fig. 2C) or carbonate dominated. (Fig. 2D).
Observations and conclusions:
• Repeated stacking of elements suggests that depositional energy levels were fluctuating on a regular basis.
• The alternation between carbonate and clastic lithologies suggests multiple sediment pathways.
• This alternation of energy regimes is, in the palaeogeographical context of the Gulf of Suez, best explained by a submarine fan system.
Figure 2C: Clastic Dominated Mid-Fan Deposition
Figure 2D: Carbonate Dominated Mid-Fan Deposition