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    Case Study

    Fundamentals of Petroleum Geoscience

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    Fundamentals of Petroleum Geoscience


     Fundamentals of Petroleum Geoscience

    Background

    The purpose of this exercise is to get an understanding of the fundamental processes behind making a contour map, and to appreciate how such maps are used to interpret the subsurface. This exercise also tests out your understanding of one of the key components of the petroleum system, namely the Trap. We will achieve this by a simple example of a hand-drawn contour map of depth to the top of a reservoir sandstone.

    You should also gain from this experience a greater appreciation of the importance of having correct and reliable datasets – as in this exercise, the interpretations made from a map can be the difference between economic success or complete failure of a petroleum exploration project, and errors can cost hundreds of millions of dollars in dry wells or risk losing billions in future revenue if traps were missed.

    Your task – make a depth map of the top of an Upper Cretaceous-age sandstone reservoir to determine if there is a suitable petroleum trap present to justify buying into a field.

    Your company is considering buying into an oil field that another company (UNOC) claims to have discovered in a deep Jurassic-age sandstone. This is a common situation: small companies may discover a field through their own technical skills but then not have enough money to be able to bring it onto production.

    In this example, your colleagues have a suspicion a second possible target (trap) has been missed by UNOC, one that is nearer the surface (i.e. cheaper to drill for) and in younger Upper Cretaceous-age sandstone. Such a target could be missed during drilling because, even though a well has passed through it, when that well was drilled the drilling mud was too dense (we talk about “mud weight” in this context), thus over-compensating for the natural fluid pressure in the rock and forcing drilling fluid into the pore spaces, all of which would have prevented oil flowing into the borehole and being seen. If it is true there is another and shallower trap present, it would make this prospect much more economically attractive.

    Data:

    UNOC have released to you the depths to the top of each major geological unit in 15 offshore boreholes in the region of this prospect. You therefore have available to you the depth to the top of the Upper Cretaceous sandstone (in metres below sea-level) that might form a trap, together with the geographic coordinates of the boreholes (e.g. latitude and longitude, although the exact details are not essential for this exercise). Map 1 at the end of this document shows the relative location of each borehole and alongside each is written the depth (in m) to the top of the Upper Cretaceous sandstone.

    Remember that depth values are the distance below the surface reference datum (sea level in this case), measured for example as distance down a borehole or from seismic surveys by converting (reflection) time to depth. These values are zero at the surface and get larger as we go down deeper into the Earth. Smaller numbers are nearer to the surface than larger ones. The top of a trap must be nearer the surface than its sides because oil is less dense than water and the rocks it passes through, and thus tries to rise up to the surface.

    We normally choose a datum to make sure that depth is always a positive number. But bear in mind that if the datum is sea-level and a borehole is drilled on land, we will have negative depths for those points in the borehole that are above sea-level (a potentially confusing situation).

     

    Actions required:

    Carry out the actions listed below and answer the question asked by writing on the map and showing the location with a star. Also, clearly write your name on the map where indicated at the top right corner – no name, no mark.

    Make a scan or careful photograph of the contour map you have drawn (with the question answered on it), and upload this to My Aberdeen through the relevant assessment link on My Aberdeen.

    If you need a reminder of the method, look at the Practical Demonstration 4 video (Unit 7) between times 20:40 and 32:41.

    1)      You are required to make a contour map of the depth to the top of the Upper Cretaceous sandstone. Using this map, identify if there is – or is not – an anticline trap present. An outline of how to proceed follows:

    2)      Inspect the data: find and discretely mark the largest and smallest depth values (there may be more than one well with the same or similar depth).

    3)      You should construct contour lines at sensibly-spaced 100m depth intervals between the minimum and maximum values you have located. It is not possible to draw contours (reliably) for depth values that fall outside this range, so do not try!