Ultimately all exploration starts with the rocks

The statement, ultimately all exploration is related back to rock properties and the only way to connect rock properties to seismic amplitude or response is to use rock physics (Ronny Hoffman, Shell International), has always been true, but as the industry moves to more complex reservoirs, making it harder to find oil and gas, this principle becomes increasingly relevant. Professionals in each discipline (geophysics, petrophysics, and petroleum engineering) are interested rock properties, which are dependent on the characteristics of the reservoir.

However, over the past 10-15 years, due to a lack of technology and understanding, rock physics modelling has remained in the background, with only specialist teams in the major companies taking advantage of the fact that these are properties of the same rocks. Traditional seismic interpreters will tend to treat the rock properties in models either as independent quantities (only the position in the model is common) or as being related by simple transforms. A consistent set of rock properties within a model is likely to be more accurate than an unconstrained set, and the constraints of each subsurface discipline can, and should, inform the interpretation of data in other disciplines. Rock physics models provide the internal connections in a new generation of shared earth model, so different model properties at the same position in the model are consistent. They're properties of the same piece of rock, not just representative values for that type of rock.

From wiggles to properties. "The only way to connect rock properties to seismic amplitude or response is to use rock physics."

Rock Physics and Quantitative Interpretation (QI)

Rock physics models provide the link between the reservoir rock properties and the Vp, Vs, Rho, acoustic and elastic properties picked up by the seismic sound waves. The reservoir rock properties include the description and makeup of the rock, i.e. the lithology (type, mineral mix), porosity (%, type), pore fluid (type, mix), fracturing and the rock’s present environmental conditions such as the pore pressure, confining pressure and temperature. Reservoir rock properties vary tremendously, from cold, shallow marine sands to hot, high pressured, deeply buried, fractured sandstones; from quartz grain suspensions to solid, cemented rocks; and from clastic sandstones to carbonate reefs to fractured volcanics. RokDoc provides a selection of rock physics models with matched fluid and lithology substitution methods. This enables seismic interpreters to forward-model the seismic data that would have been recorded in different geological scenarios. A quantitative interpretation is made by making comparisons with the real seismic data.

A RokDoc user in Perth commented: “Using rock physics we can forward model any scenario we want. If you want to know what would happen if your reservoir was filled with chocolate instead of oil you can! (Provided you can work out the Vp, Vs and Rho of chocolate.) Using rock physics we directly link the seismic and the rocks. Forward modelling allows us to find out what the effect of changing any characteristic of geology or petrophysics has on the seismic. If we have a water-wet well we can find out what the seismic would have looked like if it was gas bearing, or oil bearing (or even full of chocolate). It allows us to de-risk potential reservoirs that have been identified on seismic. We can forward model to see what the reservoir 'should' look like if filled with hydrocarbons. If our model of seismic and the actual seismic match, there is a very good chance that our model is correct”.

A rock physics model is a set of equations and statistics capturing the relationships between physical properties for a particular rock type or formation. Rock physics modelling makes use of the interdependencies between these properties and creates an opportunity to synthesize a more accurate and less uncertain picture, thus optimising the output of a shared model. Developments in technology are making it easier for rock physics modelling to be more accessible. Nowadays petrophysical knowledge can be captured and made available to every geoscientist through tools and workflows.

RokDoc is being used more and more by non-specialist interpreters and in smaller companies where specialist departments don't exist. 

Alan Mur, Geoscientist at Ikon Science Americas, explains a rock physics equation to a colleague

Rock physics models can also be used to guide inversion of Vp, Vs, Rho properties from seismic to reservoir rock properties. This requires correctly processed and conditioned pre-stack seismic (AVO partial stacks) as well as optimised rock physics models.

RokDoc provides seismic data conditioning along with unique inversion capabilities to ensure the best seismic rock property volumes. These can then be mined for likely hydrocarbon-filled zones by searching for hydrocarbon-filled, rock property crossplot character, seen in the wells. The capability of using AVO analysis to look at the more subtle changes in seismic is one of many advantages of understanding rock properties and using rock physics. A low porosity gas-saturated sand and a high-porosity water-saturated sand often have very similar impedances and seismic amplitudes. However, by analysing all angles of seismic response (AVO) it is possible to discriminate between the two.

QI can also be applied with care in areas where there is limited or no well control. If the geology is approximated (i.e. sand-shale sequences), simple rules of thumb can be taken from analogue environments, and QI techniques such as burial trends and depth trending can be used. Depth-trending basin study comparisons allow the interpreter to build a library of analogue information which can be used in highly unexplored areas.

Petrophysics and Ikon Science

Ikon Science has a global team of petrophysicists who are able to deliver high quality petrophysical interpretations across a broad range of geological environments.  When combined with our rock physics and seismic expertise, the petrophysical evaluations can be calibrated in such a way as to provide a set of data that are consistent with seismic observations, so called seismic petrophysics.

As well as providing turnkey projects, Ikon Science also provides for purchase a global database of interpreted well data, available as regional studies or as individual wells or groups of wells.  The wells in our studies have been analysed in a consistent and repeatable manner, providing consistency across large geographical regions.  This makes their use in analogue studies ideal.

Routine petrophysical analyses can be combined with detailed pressure, rock physics and geomechanical workflows to deliver an integrated study which can provide you with a detailed post mortem of your well(s) and provide invaluable information for future seismic interpretation (AVO analysis), well planning, pore pressure prediction and borehole stability studies.