Friday, December 15, 2017

Hydrocarbon Detection With AVO

Imagine a geophysical technique with the volumetric coverage of surface seismic that could delineate zones of gas, oil and water. In many ways, that summarizes the potential of interpreting seismic reflection amplitude variation with offset, or AVO.

In the late 1920s, the seismic reflection technique  became a key tool for the oil industry, revealing shapes of subsurface structures and indicating drilling targets. This has developed into a multibillion dollar business that is still primarily concerned with structural interpretation. But advances in data acquisition, processing and interpretation now make it possible to use seismic traces to reveal more than just reflector shape and position. Changes in the character of seismic pulses returning from a reflector can be interpreted to ascertain the depositional history of a basin, the rock type in a layer, and even the nature of the pore fluid. This last refinement, pore fluid identification, is the ultimate goal of AVO analysis.

Early practical evidence that fluids could be seen by seismic waves came from "bright spots" -streaks of unexpectedly high amplitude on seismic sections - often found to  signify gas.

Bright spots were recognized in the early 1970s as potential hydrocarbon indicators, but drillers soon learned that hydrocarbons are not the only generators of bright spots. High amplitudes from tight or hard rocks look the same as high amplitudes from hydrocarbons, once seismic traces have been processed conventionally.  Only AVO analysis, which requires special handling of the data, can distinguish lithology changes from fluid changes.

An analogy for the physics of AVO is the skipping of a stone across a pond. Everyone knows that if a stone is dropped or thrown into water directly above, it sinks instantly. But skimmed nearly horizontally, it bounces off the surface of the water. The amplitude of the bounce, which was zero at vertical incidence, increases with the angle of incidence.

Now replace the water with rubber and repeat the process. This time the vertical bounce is high, and the high-angle bounce is low. The amplitude of the bounce decreases with angle of incidence, a dramatically different behaviour from the water case.

Analogous concept applied to seismics form the basis for inferring formation properties - density and compressional and shear velocities - from seismic reflection amplitude variation with angle of incidence. And because formation density and velocity depend on the fluid saturating the formation, reflection amplitude variation also permits identifaction of pore fluid. 

Conventional treatment of seismic data, however, masks this fluid information. The problem lies with the way seismic traces are manipulated in order to enhance reflection visiblity. In a seismic survey, as changes are made in the horizontal distance between source and receiver, called offset, the angle at which a seismic wave reflects at an interface also changes. Seismic traces -recordings of transmitted and reflected sound - are sorted into pairs of source-receiver combinations that have different  offset but share a common reflection point midway between each source-receiver pair. This  collection of traces is referred to as a common midpoint (CMP) gather. In conventional seismic processing, in which the goal is to create a seismic secion for structural or stratigraphic interpretation, traces in a gather are stacked - summed to produce a single average trace.

Stacking enhances signal at the expense of noise, making reflections visible , and compress data volume. But it destroys information about amplitude variation with offset. Consider two reflections in the section : one has amplitude increasing with offset, such as in the case of the stone bouncing off the ater, and the other has amplitude decreasing with offset, similiar to the stone bouncing off rubber. Once the reflection traces are stacked, they may have identical amplitudes - they may even be bright spots -while their AVO signatures are completely different. AVO analysis can usually distinguish fluid contrasts from lithology contrast, but it requires carefully processed gathers that have not been stacked.

A litte theory

The general expressions for the reflection of compressional and shear waves at a boundary as a function of the densities and velocities of the layers in contact at the boundary are credited to Karl Zoeppritz.  Zoeppritz found that amplitude increase, decrease, or remain constant with changing angle of incidence, depending on the contrast in density, compressional velocity, Vp, and shear velocity, Vs, across the boundary.

Conventional seismic surveys deal exclusively with the reflection of compressional waves. When a compressional seismic wave arrives vertically at a horizontal interface, the amplitude of the reflected wave is proportional to the amplitude of the incoming wave, according to the normal incidence reflection coefficient. When the seismic wave arrives obliquely, the situation is more complicated. The compressional reflection coefficient is now a tortuous function of the angle of incidence, the densities, and Vp and Vs of the two layers in contact. 

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