Hydrocarbon indicators

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Problem

Figure 10.17a displays both peaks and troughs in black so that we can not tell easily the polarity. It shows two lines across a high-amplitude bright spot, presumably indicating a hydrocarbon reservoir. The top of the reservoir is indicated by a single event, but the flat spot on part (i) seems to have three side lobes on either side of the central strongest part, which we pick as a flat spot. What is the maximum reservoir thicknesses shown on each line? Assume 1800 m/s velocity. Why do the reflections from the reservoir top and bottom not converge at the pinchout edge of the reservoirs? Why does the flat spot disappear over the center of the reservoir in (ii)?

Background

Under normal circumstances rock interstices are filled with salt water, but in a reservoir hydrocarbons replace the water in the upper portions because they have lower density. Hydrocarbon-filled rocks almost always have lower velocities and lower densities, than water-filled ones. If the overlying rocks have a higher acoustic impedance than the brine-filled reservoir rocks, the impedance decrease at the top of the reservoir becomes larger when hydrocarbons replace brine, increasing the magnitude of the negative reflection coefficient, thus creating a bright spot. If the overlying rocks have a lower impedance than the brine-filled rocks, the lowering of impedance when hydrocarbons replace brine may lower the reflection coefficient, creating a dim spot, and sometimes may also reverse the polarity.

Figure 10.17a.  Two lines over a reservoir in the North Sea (courtesy of Elf Aquitaine Norge a/s).

Where a fluid contact is present in a reservoir, as at a gas-oil, gas-water, or oil-water interface, there is a positive impedance contrast that may produce a nearly horizontal positive reflection (flat spot) that is distinctive where the bedding reflections are not horizontal. Flat spots, which are sometimes tilted because of overlying velocity changes, are among the best hydrocarbon indicators, but they are often not seen where the resolution is inadequate or where the change in hydrocarbon saturation is gradual. Sometimes other features such as porosity or cementation changes associated with a former horizontal reflector produce a flat spot that is not associated with a current fluid contact.

The change in velocity associated with hydrocarbon-filled rock sometimes produces velocity anomalies beneath a reservoir. The large amplitudes associated with bright spots may result in gain changes in processing that produce decreased amplitudes (amplitude shadows) in the underlying and overlying sections. A lowering of frequency immediately below a reservoir is sometimes seen (low-frequency shadow) and gas leaking from a reservoir into overlying sediments may cause a deterioration of reflections and even a velocity anomaly (gas-chimney effects). One of the most important evidences is to observe the structural conditions that could produce a trap.

All of the hydrocarbon indicators can be caused by situations other than hydrocarbon accumulations, so one cannot rely on any one indicator. However, the case for a hydrocarbon accumulation is considerably strengthened where several indicators are present.

Solution

The strong reflections indicating the reservoir have limited horizontal extent and appear to be located at the top of an anticline, i.e., they are hydrocarbon indicators. The nearly horizontal reflection that cuts across other events, which we interpret as a fluid-contact flat spot, produces a simple positive reflection; we often examine it to learn about the embedded wavelet. In Figure 10.17a(i) it appears to be nearly symmetrical, suggesting that this section is almost zero-phase. We expect the single half-cycle at the top of the reservoir to be a negative reflection because the event is a bright spot. As the negative top-reservoir reflection approaches the positive flat-spot reflection, they interfere and tend to form an antisymmetric 90 wavelet; the top reflection is pushed upward and the flat spot reflection downward because the high frequencies required to show a pinch out are not present.

In Figure 10.17a(ii), the flat spot disappears over the highest part of the reservoir because here the reservoir is completely filled with hydrocarbons so that no hydrocarbon-water contact exists.

The largest time between the centers of the reflections indicating the reservoir top and the flat-spot reservoir base is about 30 ms on (i) and perhaps 40 ms on (ii), giving thicknesses of 27 and 36 m.

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