Making a geological map and creating a subsurface model

Imagine that you work for an oil company which will be evaluating a proposal for the possible development of an oil reservoir in the Osborn Quadrangle. At the end of this exercise, you will submit a report, which is your recommendation on the economic viability of this prospect. Your report will consist of a completed geologic map, two cross sections (which will be your model of the subsurface geology) calculations, and your recommendation (the calculations and recommendation will be recorded as your answers to the lab questions. There is no need to submit anything else).  

Part 1: Making a geological map (23pts)

On the Osborn Quadrangle map, there are outlined areas which represent the rock outcrops (rock that is exposed at the surface for you investigate); the rest of the area is covered by soil and vegetation. A previous group of geologists has already mapped the position and size of these outcrops, labeled them A-J on the map, and provided rock descriptions (table 1). 

A. The geologists have deemed that rocks with the same lithology are so similar that they belong to the same unit (layer of rock). On the map under key, list the lithologies and draw a square beside each. Assign each lithology a colour by colouring in the square. Then, colour in the outcrops based on your key. (3pts)

The rocks in the outcrops are all layered sedimentary rocks. The group of geologists also measured the orientations of the layers in the outcrops for you and recorded them in table 1. (Strike and dip are always recorded as 000/00. So, a dip of 32° and a strike of 50° is recorded as 050/32.) 

B. Transfer the strike and dip data onto the map. Using a protractor, carefully plot the strike and dip symbols for the outcrops A through J. Remember that the top of the map is north (as it is labelled) and your strike and dip symbols need to be oriented properly. If a symbol does not fit within the outcrop outline, it can be placed beside the outcrop. (10 pts)

C. Now you have the basis for constructing a geologic map. 

The goal here is to draw boundaries between different types of rocks. Because the rocks are only exposed in the outcrops (and covered by soil and trees everywhere else), you have no idea where the actual boundaries between different rock types are. So, you have to use the data you have at the outcrops to make an interpretation. You should consult the stratigraphic column at the end of this handout when you are interpreting where to place the contacts, but keep in mind that the unit thickness will be affected by the deformation; thus the distance between contact lines does not equal unit thickness. Color the belts associated with each rock type with the appropriate colors. 

Try to keep the interpretation as simple as possible.  There is often no single completely correct answer in the interpretation of geological data. Some interpretations, however, are closer to the truth than others, which is why some oil companies are more successful than others. You should end up with 3 belts of 3 different lithologies. Contacts and colouring should be neat and easy to understand. Make sure you add the lithology contact line to the map’s key. (10 pts)

Table 1: Lithology and structure of outcrops A to J

Outcrop

Lithology

Lithology description

Strike and dip 

A: Sandstone

Coarse-grained, tan-coloured quartz arenite with planar beds 1-2 cm thick

225/30

B: Lime mudstone

Massive, pale grey lime mudstone (reacts with HCl)

180/40

C: Sandstone

Medium to coarse-grained, tan-coloured quartz arenite with planar beds 1-2 cm thick

135/30

D: Lime mudstone

Massive, white-grey lime mudstone (reacts with HCl)

090/50

E: Sandstone

Medium-grained, tan-coloured quartz arenite with cross-laminations

045/30

F: Lime mudstone

Massive, pale grey lime mudstone with rare fossils (reacts with HCl)

000/40

G: Lime mudstone

Massive, pale grey lime mudstone (reacts with HCl)

270/50

H: Shale

Thinly laminated, black-coloured mudstone with minor pyrite.

horizontal

I: Sandstone

Coarse-grained, yellow to tan coloured quartz arenite with planar beds 1-2 cm thick

315/40

J: Sandstone

Coarse-grained, tan-coloured quartz arenite with planar beds 1-3 cm thick 

270/40

Part 2: Creating a subsurface model (20pts)

We will now use the geologic map and other data from the subsurface to develop a model of the geology that is below the surface. Our model will consist of the two cross sections, X-X’ and Y-Y’ as indicated on the geologic map.  

Before we create the cross sections, we need to examine the stratigraphic column of the Osborn Quadrangle provided at the end of this lab. A stratigraphic column is a list of all the possible geologic formations (mappable units of rock) within an area. In this case, the stratigraphic column shows the layers of rock that exist at and below the ground for a region extending 100s of km around our oil exploration site. The units are in order from the oldest at the bottom to the youngest at the top, thus obeying the principle of superposition. 

A. Construction of the cross sections

Remember that a cross section is a vertical slice through the Earth showing the subsurface geology, much in the same way that you are able to see the layers of onion when you slice one in half. The cross sections are constructed by transferring the known surface data to the subsurface, and then combining this with the additional subsurface data.(16pts)

1. Fold along the bold line of the X-X’ cross section and place it along the line of the X-X’ cross section on the geologic map and plot the location of the lithologic contacts along the top edge of the paper. Extrapolate these contacts to the subsurface by extending them in their dip direction (e.g. Is the contact dipping to the East? West?) at their dip magnitude (e.g. 45°). Use a protractor to extend the contacts below the surface.  

2. Remembering the principles of original horizontality and superposition, add the additional subsurface information that is given in the stratigraphic column. Remember that most of the rocks in the subsurface below the Osborn Quad are not seen at the surface. Use the thicknesses of the units in the stratigraphic column to determine the thicknesses on your cross section. Remember that the units will keep a consistent thickness in the subsurface, even when the units are folded. Neatly colour and label your cross section. Make sure to create a legend with all of the units. 

3. After you have completed cross section X-X’, draw the other cross section Y-Y’.