By Darren Kondrat, Sr. Technical Advisor at geoLOGIC systems ltd.

Reservoir geophysics as a discipline is a continuously evolving technology, developing new techniques that enable high quality, multi-dimensional mapping of hydrocarbon reservoirs to substantially improve productivity and maximize returns.

The goal of reservoir geophysics is straightforward: to integrate well data with seismic data to provide detailed information and visualizations of reservoir properties — notably area, thickness, porosity, lithology and saturation. This data is vitally important to assess the value of an asset and optimize field development. A fully three-dimensional perspective of key reservoir parameters can reduce the number of wells required to develop a prospect and materially improve operational decisions and overall production from a play.

What sets reservoir geophysics apart?

Reservoir geophysics is quite different to traditional geophysics. For the last half century, the main purpose of seismic exploration has been to prospect for hydrocarbon accumulation. This was accomplished by interpreting amplitude data that represents the boundary of the subsurface formations. Picking the horizons and faults provided the geophysicist with the ability to identify potential hydrocarbon traps. Changes in seismic amplitude were also used to infer changes in stratigraphy, porosity, or saturation. This information provided an excellent tool for mapping large areas that had little or no well control.

Reservoir geophysics seeks to create a better physical representation of the reservoir by integrating the detailed subsurface information from well data with the seismic data. It requires good well control and excellent seismic coverage of the reservoir. The interpretation and merging of these data sets delivers detailed three-dimensional volumes of reservoir properties, including porosity, hydrocarbon saturation and lithology. These 3D volumes provide asset teams with the ability to map the reservoir in much more detail away from the known reservoir at the wells. For engineers, this information is hugely valuable when calculating reserves and optimizing field development.

Calculation techniques

Techniques used in reservoir geophysics have evolved over the last 20 years. Seismic data is now converted from recorded amplitude data to more meaningful information. This “seismic inversion” is a key technique that creates elastic parameters, compressional velocity (Vp), shear velocity (Vs) and density, from the seismic data. These parameters are used to calculate additional parameters such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, rigidity and incompressibility. These parameters are used to identify lithologies and reservoir properties using petrophysical analysis and rock physics.

New crossplotting insights

Crossplotting, when used in conjunction with petrophysics and rock physics, is an excellent method to bridge from elastic seismic parameters to lithology. (Note: Seismic data pictured below provided courtesy of Pulse Seismic Inc.)

Figure 1 shows the well data crossplotted with zones used to identify various lithologies. Created using geoROCK’s wells analysis tools.

Figure 1 is an example of a crossplotted well. The points in the crossplot are coloured by a gamma ray curve. The plot clearly shows that higher gamma values (representing shale) cluster in the middle. Lower gamma values represent sands, carbonates and coals. Coals are the least rigid rocks and data from them cluster on the base of the crossplot. Porous sands are compressible and cluster on the left side. Carbonates are the rigid and incompressible rocks with low gamma values. The facies track shows the information from the crossplot as a lithology type display. The wiggle trace synthetic is displayed beside it as a comparison to the additional information gained in a reservoir geophysics workflow.

Figure 2 shows the seismic data crossplotted with zones used to identify the various lithologies from the well log data. Created using geoROCK’s seismic analysis tools.

Figure 3 is a time slice of the 3D volume within the zone of interest. The analysis can provide a powerful map view of the reservoir. The highest value zones of the reservoir can be identified with precision. This can allow operators to optimize development. Created using geoROCK’s seismic analysis tools.

The same data can be crossplotted using seismic data derived from an Amplitude-Variation-With-Offset (AVO) inversion. Figure 2 shows the seismic Lambda-Mu-Rho (LMR) data crossplotted with zones used to identify various lithologies from well log data. Lithologies are underlaying the stacked data on the section display. Figure 3 is a time slice demonstrating a porous sand reservoir.

Darren Kondrat is a subject matter expert on geophysics and the geoROCK product lead with geoLOGIC systems ltd. He is based in Calgary and can be reached at dkondrat@geologic.com. More information about geoROCK can be found here.