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INTRODUCTION
Summary A recent reprocessing of a large Ocean Bottom Cable (OBC) seismic data set of a Middle East Offshore Carbonate Oilfield in U.A.E. resulted in significant seismic imaging, signal/noise ratio, and detectability improvements of faults and horizon geometries which included new fault system sets never recognized before. In addition, seismic amplitude fidelity was improved significantly and it has been confirmed by better well-ties and subsequent acoustic impedance inversion, which enabled seismic quantitative analysis possible. This paper will focus on the description of a successful application of porosity prediction based on the data and a workflow that consists of four major steps: rock property analysis, acoustic impedance inversion, porosity prediction from multi-attribute analysis, and validation based on well data. Hundreds of regular wire-line and/or cross dipole sonic logs were acquired and dozens of ultrasonic measurements from core samples were performed across the field. Data was conditioned and analyzed to understand the porosity versus impedance and other rock physics trends. A relatively narrow porosity versus impedance trend was observed in the data set, which laid the foundation for our subsequent analysis. Acoustic impedance inversions were performed in a commercial software package by 1) well-to-seismic tie and wavelet estimation, 2) earth model building based on interpretation and well data, 3) band-limited impedance inversion and total impedance derivation. The inversion results and seismic stacks were loaded into a seismic attribute analysis software package where seismic multiattribute analysis was performed and porosity versus impedance and other attribute relationship was established. The porosity volume was then generated across the entire field based on the established relationships. The resulted porosity volumes based on the inversion results and other attributes were validated with well data where high consistency was observed. These volumes are layered properties rather than reflectivity tied to interfaces; therefore they can be very easily used for integration between geological and engineering data. Also higher frequency was observed within the volumes. Examples of application of the impedance and porosity volumes for thinner reservoir mapping, well planning, and input for geological models will be shown.
The field presented in this paper is a giant carbonate oilfield located in the offshore area of the Abu Dhabi, United Arab Emirates. This region contains predominantly carbonate sediments that were folded during the Neogene tectonic phase (El- Awawdeh et al., 2008). The field is a very gentle dipping (2-3 degrees) east-west elongated anticline structure with extensional fault systems developed across the region (Ikawa et al., 2008). The reservoir zones are porous shallower water shelf ramp grain-dominated carbonate deposits, interbedded with low porosity deeper water clay/mud rich dense limestone intervals, interpreted to be Lower Cretaceous (Azer et al., 1993; Boichard et al, 1994). The Lower Cretaceous high porosity reservoir intervals are characterized by low acoustic impedance whereas the dense intervals are characterized by high acoustic impedance. Many low impedance Tertiary and Mid-Cretaceous Karsts and Upper Cretaceous high impedance channel deposits (Figure 1) were observed in the overburden formations (El-Awawdeh et al., 2008).
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