Integrated prospect evaluation and characterisation using pre-stack seismic data, Mansoura area, onshore Nile Delta, Egypt
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The Mansoura area locates in the north of the Nile Delta Hinge Zone. It geologically comprises a thick sequence of deltaic sediments from Recent to Oligocene age overlying older Mesozoic sequences which are probably too deep for hydrocarbon exploration. The middle and late Miocene in the onshore Nile Delta is dominated by siliclastic sediments with excellent reservoir quality, which allows the researchers to introduce different techniques for reservoir properties prediction for the sensitivity of this type of sediments to the reservoir characterization studies. West Dikirnis (WD) and West Khilala (WKH) fields were selected for the reservoir characterization studies done in this thesis because they are typical examples for the onshore Nile delta geological setting. In 2003, a successful aggressive drilling program started in the Mansoura area in the onshore Nile Delta with a high success ratio depending on the direct hydrocarbon indicators from the post-stack seismic data. However, after proving hydrocarbon presence and many fields discovered, some challenges appeared. Lately, the most important ones are the lithology and fluid discrimination due to the inconvenient shale behavior, reliability of more than half the available pre-stack seismic data, and the delineation of different reservoir properties like clay content, water saturation, and porosity. One of the big challenges facing different operators in the on-shore Nile Delta is the discrimination between residual gas saturations and mobile commercial gas. Analysis of pre-stack seismic data for different reservoir properties prediction is commonly used for reservoir geophysics. As a result of the problems facing the acquisition and processing in the Mansoura area, the output pre-stack seismic data needs some improvements especially for the ultra-far data; consequently, the gathers pre-conditioning becomes essential before proceeding in any reservoir delineation process. After a crucial review of the gathers, it was found that there is a potential to improve the signal to noise ratio and to prove the reliability of the ultra-far data, after several iterations and testing several processing parameters, a conditioning workflow was developed and applied to the prestack seismic data. The clean ultra-far data was an output that resulted from the gathers conditioning developed a workflow that was applied to the whole common depth point (CDP) gathers for Mansoura area in the onshore Nile Delta. This workflow can be applied simply in the whole onshore Nile Delta, not just the Mansoura area. The application of spectral decomposition and frequency attributes on pre-stack seismic data has opened the door to think about the relationship between frequency attenuation and reservoir properties. We will see later in the thesis how spectral decomposition can be used to discriminate between low gas saturations and mobile, commercial gas in the area of study. The popped-out amplitudes are vital to be analyzed. Understanding the phase and polarity of the seismic data is critical; the seismic survey design should take into consideration the special geophysical techniques. Building up a rock physics model in the onshore Nile Delta can play an important role in linking the elastic properties to the reservoir properties and applying the results in the unexplored areas within the concession, which help in di-risking the delineated prospects. The rock physics model built in the study area helps significantly in the discrimination of sand and shale, wet and gas sands. The amplitude versus offset (AVO) simultaneous inversion applied to the ultra-far seismic data helped in qualitative interpretation for the reservoir in both WD and WKH fields by combining the results of the Zp, Zs, Vp/Vs and density above, within and below the reservoir. The middle and late Messinian are typically sand-rich sections with excellent reservoir quality encountered in the drilled wells within both fields. Different reservoir properties have been predicted using advanced inversion techniques. The well data showing strong correlation at the well locations increases the confidence in using this prediction in prospects drilling. Developing workflows to increase the reliability of the far and ultra-far seismic data (one third of the recorded seismic data was useless before) is one of the innovations of this study. The cross-plot of the acoustic impedance versus shear impedance represents one of the best tools in the onshore Nile Delta for sand and shale discrimination. Using Elastic impedance logs, we can discriminate the water bearing sand, hydrocarbon sand and shale, especially at the ultra-far angle (in this case, 45 degrees). The application of spectral decomposition, frequency attributes and amplitude attributes on the ultra-far stacked data shows excellent results in terms of the tuning frequency response. The strong relationship between gas saturation and frequency attenuation was proven and will be shown in this study. The work done in this study will broaden the role of spectral decomposition and frequency attribute analysis beyond its use as a hydrocarbon indicator by further emphasizing its role in reservoir properties delineation.