Finding an Oil Deposit Activity You’ve just graduated with your B.S. in petroleum geology

| March 14, 2016

Question
Finding an Oil Deposit Activity

You’ve just graduated with your B.S. in petroleum geology, and been hired by a prospecting oil company. They have great faith in you as a geologist, so they’ve dropped you off in the field to do a survey of an area. It’s an area of sedimentary rock layers which might contain oil deposits. Your first task is make observations and measurements of the rock outcrops that you can see at the surface, across a transect line that has been mapped out for you.

Your transect line runs east-west across the field area and is 1 km (1000 m) long. Your observation of the geology along this transect line is that the area has conglomerate rock at the surface, and that the conglomerate layer is dipping (not horizontal) along the line. From the western end of the transect to about 600 m, the conglomerate layer is dipping down to the west; around 600 m the conglomerate layer is horizontal; and from about 600 m to the eastern end of the transect, the conglomerate layer is dipping down to the east.

Given these observations, the oil company decides to drill five boreholes along the transect. They give you the well log observations at the five boreholes.

Use the grid on the last page to make a cross-section of the transect. Draw a very thin rectangle at the locations of the five boreholes, listed below, and indicate (make a mark) in the rectangle the depth and orientation of all the rock layers observed in the boreholes. Your marks should be at the appropriate depth, and should be angled in the direction (west or east) of the dip of the rock layer. You should assume that all of the rock layers follow the conglomerate layer in their dip, i.e. where the conglomerate dips west, the shale under it also dips west.

BOREHOLE 1: 100 meters from the western end of the transect

From surface to 230 meters deep

conglomerate

230 – 275 meters

shale

275 – 400 meters

sandstone

BOREHOLE 2: 300 meters from the western end of the transect

From surface to 130 meters deep

conglomerate

130 – 170 meters

shale

170 – 290 meters

sandstone

290 – 450 meters

shale

BOREHOLE 3: 500 meters from the western end of the transect

From surface to 50 meters deep

conglomerate

50 – 90 meters

shale

90 – 210 meters

sandstone

210 – 350 meters

shale

350 – 440 meters

sandstone

BOREHOLE 4: 700 meters from the western end of the transect

From surface to 45 meters deep

conglomerate

45 – 80 meters

shale

80 – 190 meters

sandstone

190 – 340 meters

shale

340 – 420 meters

sandstone

BOREHOLE 5: 900 meters from the western end of the transect

From surface to 110 meters deep

conglomerate

110 – 145 meters

shale

145 – 270 meters

sandstone

270 – 415 meters

shale

415 – 500 meters

sandstone

Connect the borehole marks on your transect to draw in all the layers – a conglomerate layer at the surface, followed by a shale layer, followed by a sandstone layer, followed by a second shale layer, followed by a second sandstone layer. Using the layers as you’ve drawn them, answer the following questions about the study area.

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Which of the rock layers reported in the well logs is likely to be a source rock?

* Which of the rock layers reported in the well logs is likely to be a reservoir rock?

* Which of the rock layers reported in the well logs is likely to contain oil?

* What type of oil trap exists?

* At what location (how many meters from the western edge) across the top of the transect should a well derrick be constructed to drill for oil?

* At what depth would well operators expect to hit oil?

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