Quantification of Reservoir Thickness Reduction Using Seismic Attributes and Inversion
Authors: Contreras David , Sellar Christopher , Abdalla Alnaqbi Dhabia , Verma Ravi 
Published: November 2020

The objective of this paper is to define the relationship between diagenetic processes observed in core, reservoir thickness measured in wells and the thickness distribution at the field scale using seismic attributes. The delineation of a robust geological reservoir quality concept honouring all scales of observation is of extreme importance when defining an FDP based on a horizontal drilling programme.

Pressure dissolution features measured in core provide evidence that the intensity and abundance of chemical compaction varies across Reservoir 2 in Oilfield A. Reservoir thickness was measured in wells across the field. A negative correlation was found between reservoir thickness and the abundance of chemical compaction features field wide. This observation about reduction in reservoir thickness was subsequently validated using 3D seismic interpretation and attributes and ultimately implemented in structural reservoir modeling. Mapping of Reservoir 2 thickness from wells (point data) identified a trend of decreasing thickness across the field from the crest in the north-west (thickest) to the flank in the south-east (thinnest). The seismic interpretation of top and base reservoir reflections in both the time and depth domain allow the definition of the thickness trend to be calibrated with well data and extrapolated between them with increased confidence. Furthermore, a thickness trend is observed in seismic attributes (e.g. Amplitude and Sweetness) and in P-impedance from seismic inversion. By performing multivariate analysis on the seismic attributes it is possible to establish a strong relationship between geological process, thickness reduction and seismic response at the field scale.

The key finding from reconciling these different types and scales of data was the observation that reservoir thickness is positively correlated to reservoir quality. The presented methodology is a practical solution to estimate reservoir thickness variation and therefore the range of expected reservoir properties using multilinear regression of a combined set of seismic attributes. Quantification of the compaction effect on reservoir thickness reduction and reservoir quality shows a relationship that can be predicted with a sufficient level of confidence. Multiple realizations of expected thickness values are used to define various structural framework realizations within a distribution of equiprobable reservoir property scenarios.

The cross validation of direct measurements from core and logs, facilitated the development of a strong geological concept in Oilfield A. The integration of these observations with indirect measurements of density and velocity (impedance) from 3D seismic data enabled the concept to be reconciled field wide. The product has the ability to predict reservoir property distribution field wide using a simple tool (reservoir thickness from 3D seismic) grounded in a detailed understanding of the diagenetic overprint. The ability to generate a validated reservoir property distribution concept allows confident predictability and the anticipation of complex reservoir heterogeneities away from well control using 3D seismic. This tool will be used for efficient well placement, based on robust geomodeling, high confidence well planning and ultimately a more robust Field Development Plan.