Print this pageAAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Resolving Thin Beds: A New Approach in the Cantabrian Sea
(1) Geotrace Technologies Ltd.,, Woking, United Kingdom.
(2) Repsol Madrid, Madrid, Spain.
Thin sandstone bodies, ranging between 6.8 m and 11.8 m thickness were identified as reservoirs in a turbiditic environment of Upper Cretaceous age, located in the Cantabrian Sea, Northern Spain. The sandstone has extremely high porosity (up to 30%) but the net thickness of the sandstone includes shale stringers. The sandstone was poorly resolved by recently acquired, high-resolution seismic reflection data with a typical seismic bandwidth extending from ~0.5 to 60 Hz, peaking at ~22 Hz. A previous acoustic impedance inversion showed that the sandstone had low acoustic impedance but the unit was difficult to map due to surrounding shales. Geotrace utilised its Bandwidth Extension (BE)® algorithm to first improve the resolution of the thin sandstone, before applying RockRes® to better define the geometric extent of the sandstone. BE® extends the bandwidth of conventional seismic reflection data on a trace-by-trace basis, at both the high and low frequency ends of the spectrum. It operates in the Continuous Wavelet Transform domain, using a convolution-like process to extend the spectrum by a specified number of octaves from harmonics present within the seismic trace. Broader frequency content of the data serves to resolve thin beds as well as normalising the seismic wavelet for inversion. RockRes® is a high-fidelity seismic inversion, utilising a three-term linearized approximation of AVO attributes to derive rock properties. BE® applied to a stacked dataset increased the viable high frequencies to ~96 Hz and the peak frequency to ~58 Hz. The mapability of the sandstone unit was markedly increased, revealing the presence of two separate sand bodies. The stratigraphically overlapping sands have slightly different density and acoustic impedance properties. BE® also aided definition of small faults in the reservoir interval. Having successfully parameterised BE® on stacked data Geotrace applied BE® to six angle stacks, which were input into RockRes® in order to better characterise the reservoir. Acoustic and density logs enabled calibration of both the acoustic impedance and density inversion terms. The inversion revealed the low acoustic-impedance sands in exquisite detail and the depositional model for the sandstone has been refined. Where the previous inversion suggested that the sandstone was one contiguous body, possibly offset by small faults and fed by a channel-like structure, RockRes® reveals two separate lobes that are compartmentalised.