You have been hired by an oil company, and your first assignment is to take over a

Question:

You have been hired by an oil company, and your first assignment is to take over a project from another employee. Figure G-48 contains a geologic map and topographic profile of an area that your company has leased. The oil-bearing unit in the region is an Eocene sandstone that is not exposed in the map area. It is overlain by unit To (Tertiary, Oligocene shale).

As indicated on the map and profile, an exploratory well has been drilled. Some drill bits produce a solid core of rock for the geologist to examine, but that type of drilling is very slow and expensive. Usually, oil companies use a drill bit that chews up the rock into little chips. It is more difficult to recognize and interpret the rocks in the well, but it is much cheaper and faster. Such a drill bit was used for this well. Your predecessor was the on-site geologist who logged the well. Unfortunately, she was unable to identify the rock units within the well, nor was she able to determine their attitudes. She did record the depth at which the drill bit encountered different lithologies. These are indicated by the three short horizontal lines on the vertical line that represents the well. These indicate the depth of each lithologic contact within the well, but not the attitude of each contact. You will have to determine which rock units occur above and below these contacts in the subsurface. The heavy line lower in the well indicates the position of a major fault.

Complete the cross section. Start by projecting the dips and contacts onto the topographic profile. Then determine the orientations of the axial surfaces of the folds (remember, axial surfaces bisect adjacent fold limbs or panels). Be sure to complete the geology in the footwall of the thrust, as well as in the hanging wall. Follow the rules for fold construction.

After completing the cross section, do the following:

1. Determine what type of fold is present.

2. Calculate the amount of shortening accommodated by the fold.

3. On both the map and cross section, indicate where you recommend drilling a production oil well. In one or two succinct sentences, explain why this is the best place to drill for oil. 19 Tp AL 30 19 Scale 1mm = 100 m 30 30 Tmu Well Well 30 Tml 30 To Tml Tmu g 80 10 Tp 19 10 Tmu HA' A' Fig.