Meeting the challenge of Mesozoic exploration
Authors: Laake Andreas , Quigley John , Strobbia Claudio , Velasco Larry , Vermeer Peter L. , Van Baaren Peter Michiel , Cogan Mike , Shabrawi Ayman 
Published: March 2015

The Western Desert of Egypt challenges the exploration for oil and gas both from the reservoir as well as from the surface. The reservoir rocks, Jurassic sandstones and Cretaceous sandstones and carbonates, are often intensely faulted and fractured as a result of the long tectonic history of the area. Often the entire overburden is heavily faulted. The reservoir structures comprise inversion anticlines and fault blocks. Mapping these reservoirs requires high quality 3D seismic data of broad bandwidth with frequencies above 40 Hz to provide sufficient vertical resolution and lateral discrimination of reservoir features. A rich contents of low frequencies, lower than 10 Hz benefits depth penetration and inversion for reservoir properties, such as porosity in clastic reservoirs. A high signal-to-noise ratio is needed for fault mapping.

This paper addresses the challenges of the Western Desert through the introduction of point-receiver based seismic technology. Conventional receiver arrays, which attenuate surface waves and high frequency signal, are replaced by pointreceivers. Coherent noise attenuation can be carried out on point-receiver traces, whilst additional signal enhancemnt is taken care of in subsequent steps. The result is often a substantial increase in high frequency content of the desired signal. Traditional geophones are replaced with accelerometers with a flat spectral response down to 3 Hz. Combining these receivers with heavy (80 klbs) vibrator emitting a maximum displacement sweep, the low frequency contents can be enhanced by more than one octave.

Reducing multiple contamination (particularly interbed multiples) is one of the greatest challenges in land seismic processing. Demultiple algorithms and innovative workflows are presented, specifically designed to handle 3D land geometries and attack multiples in a 3D sense. In practice, these methods, relating to both surface and interbed multiples may be combined and cascaded to obtain the optimum solution. We will illustrate the benefits of point-receiver technology through examples from Egypt and the Middle East.

Definition of challenges

The challenges for exploration of the Mesozoic reservoirs in the Western Desert of Egypt can be grouped into; (1) challenges imposed by surface and near-surface reservoirs, (2) solving overburden effects, and (3) resolving the reservoirs themselves.

A large portion of the Western Desert is covered by the Qattara Depression, Egypt's biggest sabkha plain located at up to 100 m below sea level (see figure 2). Sabkha in the depression and rough terrain along its western rim have a significant impact on logistics of seismic operations. Tertiary limestones from the Marmarica formation, which are often karsted as a result of chemical erosion, act as wave guides for surface waves. If this layer is fractured or broken up by erosion, significant surface scattering occurs. The interface of the Marmarica to the underlying Moghra sandstones and mudstones shows a negative impedance contrast. This interface produces the first strong refraction and is often several hundred meters deep, which makes first break picking and static corrections difficult. In contrast, sandy gravel plains show the best data quality. Figure 1 shows a selection of the surface types found in the Western Desert of Egypt.