Wednesday, December 20, 2017

Synthetic AVOs from Logs

In much the same way that a blindfolded expert can identify a wine and its vintage, or an X-ray diffraction lab technician can identify mineral components in a rock sample, the key to using AVO for fluid identification is comparison of real data with a standard - in this case a synthetic seismogram. This is an artificial seismic trace manufactured by assuming that some pulse travels through an earth model - rock layers of given thickness, density and velocity- and returns to be recorded. The earth model that produced the synthetic can be modified, sometimes repeatedly, until the synthetic matches the measured data, indicating the earth model is a reasonable approximation of the earth.

The densities and velocities of fluid saturated rocks necessary for the creation of synthetic traces preferably come from logs or cores. Missing data can be estimated using theoretical or empirical equations. The synthetic traces show the expected AVO effect for each fluid type.

Take, for example, the AVO effect of gas in sandstone predicted from logs in a gas field operated by Texas-based Royal Oil&Gas. Here, acoustic velocities were measured with the DSI Dipole Shear Sonic Imager tool. The seismic event of interest is the circled blue reflection corresponding to the interface between an overlying shale and the gas sand. The trace recorded at zero offset - 0 degree from vertical, directly above the reflecting point - begins with a small negative amplitude. The amplitude becomes more negative as offset increases. The AVO response to oil is the same . But when hydrocarbons are replaced with water, the AVO response changes. Now polarity becomes positive (amplitude deflection to the right), and amplitude decreases with offset. 

 The AVO effect at any interface can be quantified with the Zoeppritz formulas, and plotted as a curve. At the top of the gas sand in Royal Oil & Gas well, and for most gas sands, Zoeppritz calculations predict an increasingly negative amplitude with offset. In this case, the predicted negative amplitude increases 100%. Also shown are the Zoeppritz-predicted AVO effects for the gas-water contact deeper in the sand, and for a nonfluid, lithologic contact higher in the section.

These curves of amplitude versus angle of incidence can be used to make quantitative comparisons between synthetic predictions and amplitudes from real data, once the data have been processed for true amplitudes. This is made easier by plotting amplitude versus angle of incidence squared, which converts Zoeppritz curves to straight lines. AVO behavior can then be succintly described by the line's gradient, G, and normal incidence intercept , P.

For typical reservoir rocks, the reflection at an interface between a water-bearing layer and a hydrocarbon-bearing layer is such that a negative polarity reflection becomes more negative - intercept and gradient both negative - or a positive polarity reflection becomes more positive - intercept and gradient both positive.

The simplest indicator of hydrocarbons is therefore the product of gradient and intercept. A positive product most likely indicate oil or gas. A product trace for the Royal Oil & Gas example clearly reveals gas. G traces, P traces or product traces can be plotted next to each other to produce sections, similiar to stacked sections, for AVO interpretations.


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