Question

An exploratory well is planned to be drilled in a basin that extends up to a depth of 5000 m. The surface temperature is 30°C. The geothermal gradient of the basin is 0.025°C/m. Select the possible range(s) of depth at which the potential oil-bearing zones can be encountered.

• A. 800 m to 950 m
• B. 1500 m to 1650 m
• C. 3100 m to 3150 m
• D. 4550 m to 4600 m

Hint:

The geothermal gradient is crucial in determining the temperature at different depths, which helps in estimating the potential depth for oil-bearing zones. The typical temperature range for oil-bearing zones is between 60°C and 120°C.

Answer 1

The Correct Option is B, C

Step 1: Understanding the geothermal gradient.
The geothermal gradient represents the rate at which the temperature increases with depth. The given geothermal gradient is 0.025°C/m, which means that for every meter drilled, the temperature increases by 0.025°C.
Step 2: Calculating the temperature at various depths.
Using the formula for temperature at depth: \[ T = T_{\text{surface}} + (\text{Geothermal Gradient} \times \text{Depth}) \] where \(T_{\text{surface}}\) is the surface temperature (30°C), the geothermal gradient is 0.025°C/m, and Depth is the depth at which the oil-bearing zones may be encountered.
- For depth 1500 m: \[ T = 30°C + (0.025°C/m \times 1500m) = 30°C + 37.5°C = 67.5°C \] - For depth 3100 m: \[ T = 30°C + (0.025°C/m \times 3100m) = 30°C + 77.5°C = 107.5°C \]
Step 3: Temperature range for oil-bearing zones.
Oil-bearing zones typically form at temperatures between 60°C and 120°C, making the depths between 1500 m to 1650 m and 3100 m to 3150 m potential candidates.
Step 4: Conclusion.
The correct ranges are 1500 m to 1650 m (Option B) and 3100 m to 3150 m (Option C).