Influence Of Solid Pore Fills On Reservoir Characterization Studies Calibrated On The Triassic Volpriehausen Formation, Southern North Sea Basin

ABSTRACT

In the Buntsandstein gas fields of the Southern North Sea, the Volpriehausen reservoir is the

main producing unit. This reservoir suffers often from the in-filling of the reservoir pore spaces

by solid materials (salt plugs). Determining the reservoir quality of these fields based on seismic

data alone has proved to be a major exploration challenge; the presence of salt-plugs in reservoir

pore spaces seems to produce seismic responses that are similar at first sight to that of gas-filled

reservoirs. Some wells drilled in the area have failed to yield the expected volumes of

hydrocarbons because of this problem. Thus, in this study, seismic-based methodical approach

was developed for reservoir characterization and modelling that could determine the difference

(s) between solid-filled versus gas-filled reservoir intervals. To that effect, six methodical steps

were adapted involving: Data (3D seismic, well and core) integration; Well data analyses

involving well Log correlation, petrophysical evaluation and cross plot analyses; Seismic data

analysis and interpretation; Development of rock-physical framework using petro-elastic

modelling algorithm; Forward seismic modelling using 2D forward modelling simulator

algorithm; and Amplitude Variation with Offset (AVO) analysis with AVO modelling algorithm.

Two seismic surveys were merged in order to establish single global filters and scalars, and

interpreted for subsurface structures and reservoir quality. Forward seismic modelling simulator

algorithm was used for seismic simulation and verification calibrated in post-stack and pre-stack

seismic domains. Amplitude Variation with Offset (AVO) modelling algorithm was used for

AVO-based pore-fill characterization. Results of petrophysical evaluation of well data showed

evidence of salt-plugging in the Volpriehausen reservoir, at intervals with anomalous log

signatures between 3912 m and 3918 m. The petro-elastic modelling algorithm developed

enabled fluid- and solid-substitutions based on numerical simulation. The substitution between

gas, water and salt correctly predicted the elastic moduli and therefore the AVO behaviour of

salt-plugged intervals. Also, by applying the new formula in performing fluid- and solid-substitutions

on the seismic data, subtle differences in seismic response between the different pore-fills were

determined. AVO modelling shows AVO response with clear distinctions between the different porefills,

mostly expressed by the gradient behaviour. The significance of these findings is that the

characteristic differences in elastic parameters and AVO attributes based on the methodical

approach adapted in this study have proven that this method can be used as a better hydrocarbon

reservoir characterization and prediction tool, as well as in correcting interpretation errors within

the Buntsandstein gas fields, thereby preventing drilling into dry wells.