3D Seismic Characterization of UER Karst, Offshore Qatar

Abstract

Abstract Produced water from the Al Shaheen Field is currently re-injected into the Umm Er Radhuma (UER) Formation. To model the impact of future increase of water disposal into this unit a detailed and multidisciplinary characterization of the karst system was done. In this work the spatial distribution, connectivity and geometry of the karst network in the Umm Er Radhuma Formation present in the Al Shaheen Field has been mapped in detail using horizon extractions and time slices on full-stack, spectral whitened, discontinuity, curvature and spectral decomposition seismic volumes fully integrated with drilling and regional geological data. The presence of faults and fractures associated with the interpreted karst features was also investigated and provided alternative scenarios for the distribution of subtle (sub-seismic) areas of karstification. This 3D seismic characterization of the Umm Er Radhuma karst network helped to generate a static model. The model serves as basis for the hydrodynamic modeling that will generate different scenarios for the expected increase of injection of produced water into this karstified unit. This integrated workflow has proven to be successful for the characterization of geobodies related to karstification of carbonate rocks; providing key insights for porosity and permeability distribution in similar karst reservoirs in the region. Introduction In the Al Shaheen field, Block 5 and extension area offshore Qatar (Fig. 1), the detection of borehole enlargements and occurrence of partial or total losses (drilling mud and cuttings) while drilling suggests that the Umm Er Radhuma Formation (UER) has a complex pore system that may include multiscale vugs, intercrystalline porosity and probably fractures. Regional studies (e.g. Sadiq and Nasir, 2002; Trice, 2005; El Awawdeh et al., 2008) relate this complex pore system to meteoric diagenesis that resulted in dissolution of the carbonate rocks (karstification). The main objective of this work is to characterize this complex pore system of the UER in the Al Shaheen Field in order to assess the potential pore volume available for water disposal and the spatial distribution of the pores. Rocks of the Umm er Radhuma, north of Al Batin, Saudi Arabia, are accurately represented by the reference sequence measured and described in Wadi al Batin (Powers et al., 1985). In this section the Umm er Radhuma consists of a repetitive series of light-colored foraminiferal fine grained and calcarenitic limestone, dolomitic limestone, and dolomite. The calcarenitic and locally siliceous beds are more common in the upper part of the formation. Partially to completely dolomitized, aphanitic limestones are more common in the lower portion of the formation. Karst features have been recognized, particularly northwest of Al Batin (Powers et al., 1985). Regionally, there seems to be a progressive replacement of limestone by dolomite from south to north. The maximum known thickness of the Umm er Radhuma on outcrop, measured at the reference sequence was 243 meters. The Umm Er Radhuma Formation falls into three main subdivisions on fauna, the upper unit is early Eocene in age and the other two lower subdivisions are Palaeocene in age (Fig. 2). In the neighboring fields around Block 5, mud logs from offshore wells show a light grey to light cream dolomite with sucrosic texture and very good intergranular / intercrystalline porosity. The dolomite becomes more argillaceous towards the base of the unit. Offshore of the UAE, the Umm Er Radhuma Formation thickens and consists of black shale, thin, argillaceous anhydrites and thick dolomitic limestone suggesting deposition in somewhat deeper water (Pabdeh Formation).