Statistical Rock-Property Estimates from Inverted Impedances and Rock-Physics Modeling

Abstract

AbstractThis work provides an example of a technique to estimate reservoir properties from inverted seismic impedances. High-quality seismic data has made inversion of that data for elastic properties reliable. Petrophysical and elastic properties from well-log data, along with a rock-physics model, explain the relationships among the elastic properties and the reservoir properties of interest. However, ambiguity and non-uniqueness are present in those relationships. The inversion of seismic data for P-impedance (IP) and S-impedance (IS) requires pre-stack seismic data, and a particular algorithm to combine low-frequency information. The inversion provides IP and IS for every time sample at each CDP. A calibrated rock-physics model translates seismic-scale impedances to reservoir properties. The calibration of the model typically is done using well-log curve information around the interval of interest. This paper demonstrates the seismic inversion routine and the subsequent mapping of the inverted impedances to rock properties using data from the Marco Polo field. The rock-physics model chosen was the soft-sand model because of the geological trends identified from well data and the interpretation of the depositional environment. In addition, the well-log data indicated the presence of five facies, including a gas-sand, oil-sand, two brine-sands, and shale facies. A Bayesian classification technique mapped the seismic impedances to the most likely facies. A statistical technique was necessary to account for the non-unique relationships among the elastic and reservoir properties. The results are realizations of the most likely facies. Probabilistic estimates of porosity and saturation for the hydrocarbon-bearing facies came from joint conditional distributions of IP and the ratio of P- to S-velocity (VP/VS). Maps of the probabilities contain the associated uncertainty in each results. Limitations to this technique are three-fold. First is that the relationship is non-unique between impedances and the rock properties, whereby one value of impedance relates to different combinations of rock properties. Second, the resolution of the seismic data is the resolution of the rock properties. Third, the rock-property estimates have errors in them due to errors in the rock physics model, errors in the inverted data, and errors in the match between the data and the model.