What do seismic 'bright fractures' indicate?

Fabrizio Rigo
International exploration consultant
Houston

Seismic reflection technology has improved dramatically the last few years. Besides offering geologists a good image of geological cross sections of sedimentary basins otherwise impossible to achieve, seismic sections sometimes indicate peculiar features.

The best known features are anomalies in the amplitude of reflections, so-called "bright spots," used to indicate gas accumulations, especially in stratigraphic traps such as pinchouts and truncations. Another known seismic expression of contact between fluid is called a "flat spot" and runs subhorizontal in the middle of inclined reflections of the rocks in the trap.

The author of this brief article has noticed a third seismic expression he believes might indicate the presence of hydrocarbons and would like to name this feature "bright fractures."

The scope of writing and publishing this article is to propose an explanation of this feature that the great majority of geophysicists calls noise or disturbances. It is only by statistically verifying that "bright fractures" occur frequently in areas where hydrocarbon migration is present that the idea would prove acceptable. This statistical check-up is impossible for one individual or company and has to be proposed to the large community of geologists and geophysicists.

The author of this article first noticed these phenomena many years ago in seismic lines over a faulted anticline, which after being drilled flowed more than 10 MMcfd of gas and 1,500 b/d of oil. Fig. 1 [54189] represents a seismic line across this anomaly, showing an evident down to the north-northeast (right side of the line) major fault accompanied by several other minor faults. In Fig. 2 [31974] a magnification of a part of Fig. 1 [54189], are marked in yellow numerous reflections crossing each other in a regular grid showing acute angles in perpendicular direction and obtuse angles in horizontal direction. This pattern is typical of fractures in faulted anticlines.

Fig. 3 [52239] represents an anomaly very similar to that of Fig. 1 [54189], drilled in the same basin at a distance of approximately 20 km. The well penetrated the same formation that tested hydrocarbons in the Fig. 1 [54189] anomaly but resulted in a dry hole.

In Fig. 3 [52239] there is not the slightest indication of the fracture pattern shown in Fig. 2. [31974] The two anomalies are quite similar in size and amount of tectonization, and certainly a dense grid of fractures must be present also in the anomaly of Fig. 3 [52239], but it has not been evidenced by the seismic. Why the Fig. 1 [54189] anomaly shows these "bright fractures" can be explained by the fact that fractures must be filled by a thin film of gases.

This is not necessarily indicative of a bad seal in the trap associated with this phenomenon but rather of a general migration of hydrocarbons in the area. By the same token that migration has filled the reservoir, the lightest portion of hydrocarbons, especially gas, has infiltrated along fractures up to the surface. Surface geochemical anomalies demonstrate that this happens very frequently. For the same reason we often find oil and gas shows above a producing reservoir while drilling.

Are "bright fractures" in seismic indicative of a nearby accumulation? Not necessarily, but certainly they must be indicative of hydrocarbon migration in the area. Bright fractures have been useful to the author of this article in selecting anomalies "not to be drilled." In some cases their presence has encouraged the drilling of successful wells.

A suspicion is, for example, that not all known oil and gas fields show this phenomenon of "bright fractures" for various reasons. One reason could be that they do not have open fractures, as may happen in sand and shale sequences. Having worked mostly in carbonate reservoirs, the author of this article did not have a chance to statistically work out this case.

Geophysicists should also verify if this "bright fractures" phenomenon is related to certain geophysical acquisition or processing parameters. Certainly there is a lot of work to be done to better comprehend the occurrence of "bright fractures."

As a future test of the role of "bright fractures" the author of this article has proposed to a company to consider drilling a well in a nearly unexplored basin in North Africa.

Fig. 4 [50300] shows a fractured anticline in this area that was drilled and tested dry without any oil or gas shows. No evidence of bright fractures is present along the seismic line crossing this anomaly. Fig. 5 [63326] anomaly, located 14 km from this dry anomaly, has been recommended for drilling due to the presence of bright fractures above it.

This faulted anticline has a main objective in Middle Triassic sandstone, located at approximately 700 msec immediately above the purple horizon, which represents the Hercynian unconformity. Secondary objectives are Upper Permian clastics and carbonates below the unconformity.

If only good oil and gas shows, not necessarily production, will be found in the well, another good proof of the possible usefulness of bright fractures will have been acquired. n

The Author