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Detecting Morrow A Sandstone by Multi-Attribute Analysis of Multicomponent Seismic

Abstract

The Upper Morrow sandstone reservoir in the western Anadarko Basin is a major oil-producing reservoir. These sandstones have plagued operators and investigators alike, because of their irregular distribution. P-wave studies have been mostly done for characterizing the Morrow sandstones. It is difficult to detect these thin, discontinuous reservoir sandstones using P-wave datasets because of insufficient acoustic impedance contrast between the Morrow sandstone and surrounding shales. But the contrast in rigidity between the Morrow sandstone and surrounding shale causes a strong seismic expression on the shear wave data. This paper shows how S-wave data can help improve the detection of reservoirs with low acoustic impedance contrast. The Reservoir Characterization Project (RCP) at Colorado School of Mines, in conjunction with Whiting Petroleum Corporation, acquired 9C multicomponent seismic survey over a 6.25 square mile area within the Hovey Morrow Unit (HMU) of Postle Field, Oklahoma. The field is undergoing CO2 flood and field rejuvenation is difficult if we are unable to see the reservoir. The primary producing units in the Morrow at Postle Field are Upper Morrow A, A1 and A2 sands. The main reservoir in our study area is the Morrow A sandstone which are at a depth of around 6100 ft and have an average thickness of 30 ft (0–90 ft). The P, SV and SH stacks obtained after processing are interpreted. The Sum of Positive Samples (SPS) amplitude map at Morrow A sandstone peak and the RMS amplitude map at Morrow shale trough are analyzed for all the stacks. The attribute maps are correlated with the Morrow A sandstone gross thickness obtained from well logs. Single attribute analysis shows that SV- and SH-wave attributes give better prediction of Morrow A sandstone thickness compared to P-wave attributes. We find that multi-attribute combination of P, SV and SH data provides better correlation with sandstone thickness as compared to single attribute. Morrow A sandstone thickness map is constructed using a collocated cokriging procedure based on a linear combination of the three attributes obtained from P, SV and SH data. The seismic-guided isopach map is a good way of estimating the sandstone thickness in the study area.