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Introduction to Petroleum Exploration:
The Brendon Bombshell Background You are a fresh-faced, ultra keen geology graduate who has joined Rainbow Oil, a small hydrocarbon exploration company. It’s your first week at work, and you’ve been assigned to an asset team looking at the Brendon region in the central North Sea. Rainbow recently acquired this acreage from a major oil company that has moved out of the North Sea. The Rainbow top brass hope to find some small hydrocarbon accumulations that slipped under the radar of the major but could be economical if developed by a smaller, more specialized outfit. On Friday morning you arrive at your usual early hour to find an empty office. As you scratch your head, trying to remember a meeting you might have forgotten, the exploration manager bursts in looking flustered. “Ah, good, you’ve arrived at last. Your team leader and all the rest of your asset team have come down with swine flu. The team leader was supposed to report to me first thing Monday morning to make the case for drilling Brendon. You’ll have to make the pitch for him. Email me your report before you leave this evening so I can get the background over the weekend”. Before you have time to explain that you’ve been in the job less than a week, the manager rushes out, shouting over her shoulder. “We meet at half eight Monday morning. You’ll have 5 minutes to make your pitch and you’ll be up against the leaders of our other teams working North Sea assets”.
You only have 8 hours before emailing your report to the manager, so you have no time to feel sorry for yourself. But what should go in the report? Something stirs in your memory: source, reservoir, top-seal; structure, migration, timing. Yes, all you need do is systematically establish that each element of the petroleum system is in place at Brendon. Simple! Information You rummage across your office mates’ desk-tops and find the following materials on which you can base your report. 2 seismic lines extracted from a 3D seismic volume shot over the Brendon prospect by the major oil company. One is an in-line (that is a line in the direction that the acquisition vessel steamed) and the other is a cross-line (at right angles to the in-line). Stratigraphical and geophysical logs from 2 dry wells drilled in the area by the major oil company. These wells lie on your seismic cross-line and are labelled X and Y. 4 hand specimens from a core taken in well X. Resources of the Earth Petroleum Exercise Tasks You decide to interpret the seismic images first so as to get an idea of the structure. You need to correlate the geological horizons found in the wells with the seismic images. Fortunately, a geophysical check shot survey was carried out in well X, and used to measure the seismic two-way travel time (TWTT) to each horizon.
1. Mark all the stratigraphical boundaries encountered in well X on the seismic cross-line using the measured TWTTs. Unfortunately, no check shot survey was performed in well Y, so no TWTT information is available. You will have to calculate the TWTTs yourself from the depth information before you can correlate the horizons with the seismic section.
2. Calculate the TWTT to each horizon using the following procedure to fill in the table next to well log Y. a. Calculate the thickness of each layer. b. Calculate the interval travel time in each layer using distance = speed x time. For each layer, use the speed measured by the sonic well log.
c. Sum the interval travel times to find the total time for a seismic wave to travel from the sea surface to each stratigraphic boundary.
d. Double the one-way travel times you calculated in (c) to get the TWTTs to each horizon (that is, the time it takes for a seismic wave to travel from the sea surface down to each horizon and back up to the sea surface again).
e. Mark all the stratigraphical boundaries encountered in well Y on the seismic cross-line. Now you have correlated the well information with the seismic data, you are in a position to map the regional structure.
3. a. Carefully interpret the cross-line seismic section by tracing all the horizons encountered in the wells across the entire section using coloured pencils. Start with the youngest horizons, which are easiest to interpret.
b. The oldest horizons are difficult to interpret where they cross a major fault. Interpret the fault and trace the horizons up to it. Bear in mind that steeply dipping features such as faults (or indeed dykes or up-turned bedding) cannot be imaged directly using seismic reflection surveying (why is this?). Instead, you must infer the position of the fault where you can see disruption to gently dipping reflections. This difficulty is part of the exploration geologist’s job. Accept the uncertainty and do your best.
c. Correlate the cross-line section with the in-line section. An easy method is to fold the in-line section along its intersection with the Resources of the Earth Petroleum Exercise cross-line. You can then place the folded in-line on the cross-line and mark off each interpreted horizon.
d. Trace the horizons across the in-line. e. Briefly describe the structure. Why did this structure form? Remembering the critical importance of timing in the analysis of petroleum systems, you finish the structural work by interpreting the time period during which the structure formed.
4. a. Annotate the age of each layer on the representative stratigraphic column to the left of the cross-line.
b. Based on the geometry of the layers, when did the structure begin to form? When was its formation complete? Is there any uncertainty in your interpretation? Having mapped the structure in a trifling hour or so, you turn to work on the stratigraphy. You quickly notice that the lithological information in the well logs in incomplete. A zealous work-mate must have taken parts of the lithology logs home to study in bed. Luckily you can reconstruct the lithologies from the box of core samples.
5. a. Identify the lithologies represented by samples A to D and complete the lithology columns on the well logs.
b. On the representative stratigraphic column to the left of the crossline, colour the potential reservoir layers yellow, potential top-seal layers red and potential source rocks green. If a layer might play more than one role (for example, mud-rocks might act as a top-seal and a source) then use vertical stripes of alternating colours. The only remaining task before you begin to assemble your report is to work out the timing of source maturation. You find a report on organic geochemistry that identifies the main potential source rock in the area: Source Rock P, Upper Jurassic organic-rich mudrocks. You need to construct burial history plots for source P in the hanging wall and foot wall.
6. a. Source Rock P started life at the surface, so enter 0 km in the row corresponding to the base of the source rock unit in source burial column of well X. Working upwards, calculate the depth of burial at each younger time by adding successive layer thicknesses. At the present day (0 Ma) the depth should be the present day burial depth of your chosen source beneath the seabed.
b. Repeat part
(a) for Source Rock P in Well Y.
c. Plot a graph of source rock burial against time. Put time on the horizontal axis from 180 Ma (left) to 0 Ma (right). Put depth on the vertical axis from 0 km (top) to 4 km (bottom). Plot both burial curves on the same graph.
d. You find a report on the thermal history of well Y which states that the top of the oil window is at 2.5 km and the bottom of the window Resources of the Earth Petroleum Exercise is at 3.5 km. Mark these two contours on your burial plot. Hence label the time period when source P is in the oil window.
e. Draw 2 vertical lines to show the main period of trap formation you deduced in part 4(b). Did the sources become mature before or after the trap formed?
7. Where would you drill? Mark a vertical line on the cross-line in your preferred well location(s). In each case, clearly mark the reservoir target(s). Also mark the source kitchen (that is, the part of the source rock that is mature), and the migration pathway between source kitchen and reservoir.