Abstract
This study presents the result of a Model-based seismic inversion technique which was used to invert an acoustic impedance structure within a reservoir interval by intergrating well logs and 3D post stack seismic data obtained from XY field offshore Niger Delta. The purpose was to delineate lateral and vertical alternations in subsurface rock properties which is caused by difference in lithofacies within the reservoir interval. This would help to define hydrocarbon fairways better and constrain the range of hydrocarbon zones for field development. The inversion workflow used in this study includes forward modelling of reflection coefficients from a low frequency impedance model driven from well logs and convolution of the reflection coeffiecients with a source wavelet derived from the seismic data. Acoustic impedance cross section obtained from the inversioin algorithim showed impedance values increasing from 4112 to 7539 (m/sec*g/cm 3 ) from top to bottom of the reservoir with gas filled sand facies observed at the top of the reservoir within time window 1900-2100msec. Below time window 2100msec, there is variation in impedance values observed within the anticlinal structures seen at this interval which suggests porous sand facies containing little shale intercalations. This is characteristic of sandstone reservoirs within the Agbada formation in the Niger Delta. These sands were most likely deposited through distributaries channel deposits, distributaries mouth bars, barrier bars, alluvial fans and crevasse which characterize the reservoir rocks (sandstones) in the Niger Delta. At time window 2100-2200msec, anticlinal structures containing porous sand facies with little shale intercalations was observed again. At time window 2200msec, water bearing sand facies (clean sandstone) was observed and at the bottom of the reservoir within time window 2300- 2500msec, the impedance was dominantly high which suggests the presence of shale facies at the bottom of the reservoir. Gas-oil contact (GOC) was observed between time window 2100-2200msec of the acoustic impedance section. These variations in acoustic impedance amplitude is due to lateral changes in lithofacies within the reservoir. The results obtained gave enhanced structural disposition of the reservoir and are important for accurate stratigraphic imaging interpretation to lower the risk in drilling of exploratory and development wells. Keywords : Acoustic impedance, Seismic inversion, Model-based, Lithofacies
Highlights
IntroductionThe inference of subsurface properties from measured data (seismic data) is identified with the solution of “inverse problem”
The inference of subsurface properties from measured data is identified with the solution of “inverse problem”
The fundermental essence of seismic inversion is to retrieve a quantitative rock property from seismic reflection data that is characteristic to a reservoir (Yi Zhang et al, 2015; Pendrel, 2006)
Summary
The inference of subsurface properties from measured data (seismic data) is identified with the solution of “inverse problem”. The observation consists of the physical signature of a subsurface structure i:e the structure’s reflected (or scattered) as a wave field due to a seismic source signal. The fundermental essence of seismic inversion is to retrieve a quantitative rock property from seismic reflection data that is characteristic to a reservoir (Yi Zhang et al, 2015; Pendrel, 2006). Reflections are due to contrast in acoustic impedance in the subsurface caused by difference in physical properties of rocks which can be density and compressional wave velocity and can be explained in terms of lithology, porosity and porefill (Karbalaali et al, 2013; Hansen et al, 2008, Ukaigwe, 2000).
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