The challenge of dim spots

Alistair R. Brown, Associate Editor

The easy oil and gas has been found; the more diffi-cult oil and gas is to be found. We have all heard thisadage and have extended our exploration efforts insome way — to deeper targets, in deeper water, tohostile environments, to unconventional reservoirs.

My favorite “difficult oil and gas” is to be found byseismic dim spots. For a dim spot, the amplitude dimin-ishes where the hydrocarbon is located. It does not at-tract any attention, it is difficult to study withconfidence, and it is a type of play which is very difficultto sell to management.

Let us consider the situation in which hydrocarbondim spots occur. When water in the pores of a rock isreplaced by hydrocarbon, the acoustic impedance uni-versally goes down. If the water-filled reservoir rock isalready lower acoustic impedance than the embeddingrock, then this change of fluid causes an increase incontrast top and base and thus an increase in amplitude(a bright spot). If the reservoir rock is already higherimpedance than the embedding rock, then this changeof fluid causes a decrease in contrast top and base andthus a decrease in amplitude (a dim spot). There areplenty of reservoir rocks of higher impedance than theirembedding rocks, and therefore there should be plentyof dim spots.

Compaction trends of sands and shales show sandslower impedance than shales in younger rocks andhigher impedance than shales in older rocks. The cross-over occurs at an age ranging from Pliocene to Permianbut is commonly somewhere in the Lower Tertiary. Be-low this crossover, porous rocks are prone to dim spots.Also carbonate reservoirs often are embedded in shaleand are thus higher impedance than their embeddingmedium. So it is clear that there are many circumstan-ces in sedimentary basins where dim spots may occur.One of the great opportunities of the future lies in hy-drocarbon dim spots!

There are many methods of geophysical reconnais-sance for bright spots, one of which is RMS amplitudeover a window. This works effectively because the am-plitude of the objective is dominant within the window.But there is no equivalent reconnaissance technique fordim spots; there is no indicator to easily attract our at-tention. We must develop a greater understanding ofpossible hydrocarbon effects in the basins in whichwe work and thus sharpen our thinking about whereto look. They will be difficult to find, but I am convincedthey exist. Dim spots will occur deeper than bright

spots and also deeper than hydrocarbon polarity rever-sals. At these depths, the signal-to-noise ratio may bepoorer and the resolution will certainly be less. Themagnitude of the effect on impedance by changing flu-ids also decreases with depth. Improved data andsharper thinking should help overcome these hurdles.

Let’s study one proven dim spot from the North Seashown in Figure 1. This is a Lower Tertiary sand reser-voir, higher impedance than the embedding shale whenfilled with water. The data are zero phase European po-larity so that the reflection at the top of the water-filledsand is red (on the right). Notice that the track on thetop of the sand follows a reflection which reduces inamplitude greatly as the position of the fluid contactis passed. This is because the impedance of the reser-voir has been greatly reduced by replacing water in thepores by gas, reducing contrast with the overlying shaleand thus yielding a dim spot. Because the difference inimpedance between gas and water here is great, there is

Figure 1. Dim spot from a known gas reservoir in the NorthSea. Data are European polarity zero phase. (Courtesy ShellU.K. Limited and ExxonMobil International Limited)

This piece appears in INTERPRETATION, Vol. 1, No. 1 (August 2013); p. 6A–7A, 2 FIGS.http://dx.doi.org/10.1190/INT2013-0052-PS.1. © 2013 Society of Exploration Geophysicists and American Association of Petroleum Geologists. All rights reserved.

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a very prominent high amplitude red fluid contact re-flection or flat spot. The apparent continuity of the datawould cause an unwary interpreter to follow the red re-flection from the top of the water-filled sand onto theflat spot and possibly miss the hydrocarbon effect com-pletely. Clearly extra vigilance is required!

Figure 2 illustrates dim spots on the Northwest Shelfof Australia. Notice there are reductions in amplitudealong the unconformity surface, and this is where gassands subcrop. The water-filled sands and shales below

the unconformity have higher imped-ance than the shale above. When thesubcropping sand contains gas, theimpedance is reduced to a value verysimilar to that of the shale above theunconformity, and a dim spot results.

Clearly, dim spots are difficultalthough many should exist. We all needmore experience in recognizing them,and management needs a greater appre-ciation of this type of play. Few havebeen published. You will find two moreexamples in Brown (2012) and three

more in Brown (2011). The future of explorationis dim!

ReferencesBrown, A. R., 2011, Interpretation of three-dimensional

seismic data, seventh edition: AAPG Memoir 42 andSEG Investigations in Geophysics no. 9, p. 646.

Brown, A. R., 2012, Dim spots: Opportunity for futurehydrocarbon discoveries?: The Leading Edge, 31,682–683, 10.1190/tle31060682.1.

Figure 2. Dim spots from Goodwyn gas field, Northwest Shelf, Australia. [Cour-tesy Woodside Offshore Petroleum Pty., BP Development Australia Ltd., BHPPetroleum Pty. Ltd., Shell Development (Australia) Pty. Ltd., California AsiaticOil Company, Japan Australia LNG Pty. Ltd., and Woodside Petroleum Ltd.]Modified from Brown (2011).

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