INTEGRATION OF ROCK PHYSICS AND SEISMIC INVERSION FOR LITHOFACIES DFERENTIATION AND RESERVOIR FLUID PREDICTION OF “AA” FIELD, NIGER-DELTA

Abstract

Integrated approach of rock physics and seismic inversion analysis were ultilised in enhancing lithology and fluid differentiation in “AA” field, offshore Niger Delta. The data set used for the study includes prestack seismic and a suite of well logs from two wells. Rock physics concepts were utilised to understand the sand and fluid trend from background trends. Seismic inversion was ultilised to unravel the elastic responses of the sand saturated with the fluid and that of non-hydrocarbon bearing. Computation of reservoir properties from well log data using well-established equations was also executed. Two horizons were identified and mapped from seismic sections using seismic attributes such as amplitude and reflection geometry. The petrophysical parameters of the two reservoirs (RES-1 and RES-2) were also estimated to evaluate the quality of reservoirs. RES-1 has average volume of shale of 15%, effective porosity 29% and permeability of 755 mD while RES-2 has average volume of shale of 23%, effective porosity of 24% and permeability 589 mD. Rock physics diagnostics was carried out for quantitative analysis of lithologies and fluids. The crossplots analyzed classify the study area into hydrocarbon sands, brine and shale based on large separation between the gas sand and background trend. Simultaneous inversion revealed that the elastic parameters at the zone of interest are low compare to the surrounding rocks indicating low compressional wave velocity, low shear wave velocity, low density and low impedances in the two reservoirs. These responses showed that the reservoirs are gas saturated. Integration of rock physics and seismic inversion has added another dimension in the differentiation of lithologies and different fluids within the hydrocarbon bearing sands and also helped in characterizing the study field.