Rock physics represents the link between geology and geophysics. Increasingly over the last decade, rock physics stands out as a key technology in petroleum geophysics, as it has become an integral part of quantitative interpretation of seismic and electromagnetic data. Ultimately, the application of rock physics tools can reduce exploration risk and improve reservoir forecasting in the petroleum industry.
Mind the gap
Traditionally, rock physics has focused on the understanding of how elastic properties and seismic signatures change as a function of hydrocarbon saturation, porosity, and pressure. With great breakthroughs in laboratory experiments and theoretical modelling, rock physics has extended its turf and today plays an important role in the basin scale characterization of the subsurface, being an integral part of well log, seismic, and electromagnetic data analysis.
The role of rock physics as a bridge between geology and geophysics poses new challenges and opportunities. The introduction of rock physics ‘templates’ (Ødegaard and Avseth 2004) as a tool for interpretation and communication has proven beneficial to the oil industry. The nifty thing with the templates is that rock physics models can be constrained by local knowledge from experienced geologists. Furthermore, the templates force the geological interpretation of well log and seismic data to be made within rigorous physical bounds. We can also use the rock physics templates to extrapolate away from a few observed wells in the earth and say something about expected rock properties and seismic signatures for various lithology and pore-fluid scenarios.
The sound of geology
Recent research studies have highlighted the importance and benefit of linking rock physics to geologic processes, including depositional and diagenetic trends (e.g. Dræge et al. 2006, Avseth et al. 2010). These studies have proven that lithology substitution can be as important as fluid substitution during seismic reservoir prediction. It is important during exploration and appraisal to extrapolate away from existing wells, taking into account how the depositional environment changes as well as burial depth trends. In this way rock physics can better constrain the geophysical inversion and classification problem in underexplored marginal fields, surrounding satellite areas, or in new frontiers.
It turns out that the rock texture we observe in cores and thin sections at the microscale strongly affects the seismic reflection amplitudes that we observe at the scale of tens of metres. We can apply rock physics templates to interpret sonic well-log measurements or seismic inversion data. In a way, we are using our effective rock physics models to downscale our geophysical observations to geological information about subsurface rock and fluid properties.
The memory of rocks
It is important to honour not only the present-day geology when we use rock physics templates for geological interpretation of well and seismic data. We should also know the burial history of the rocks. The rocks have ‘memory’ of the stress and temperature history since deposition, from mechanical and chemical compaction to episodes of perhaps uplift and burial. Therefore we occasionally observe well-cemented and high velocity rocks not corresponding with present-day temperatures and depths.
In other words, it is important to take into account the memory of rocks, as temperature and stress history make a significant imprint on reservoir and seal rocks. This is particularly important in areas with complex tectonics and uplift. With a better integration of basin analysis and geophysical interpretation, via rock physics models, we can more reliably interpret lithology and fluid parameters during hydrocarbon exploration and production.
Let’s rock it!
In a world in which energy insecurity is at the forefront of global challenges, building bridges across disciplines is a requirement for new discoveries and improved oil recovery. The field of rock physics has evolved to become one of these bridges, bringing geology and geophysics closer together. The time when large gaps separated our earth science disciplines is definitely over. So let’s rock the future together!
Avseth, P, T Mukerji, G Mavko, and J Dvorkin (2010). Rock-physics diagnostics of depositional texture, diagenetic alterations, and reservoir heterogeneity in high-porosity siliciclastic sediments and rocks — A review of selected models and suggested work flows. Geophysics 75, 7531–47, DOI 10.1190/1.3483770.
Dræge, A, T A Johansen, I Brevik, and C Thorsen Dræge (2006). A strategy for modeling the diagenetic evolution of seismic properties in sandstones. Petroleum Geoscience 12 (4), 309–323, DOI 10.1144/1354-079305-691.
Ødegaard, E, and P Avseth (2004). Well log and seismic data analysis using rock physics templates. First Break 22, 37–43, DOI 10.3997/1365-2397.2004017.