Depending on the underground properties like geology and fluid properties while there is no exact level of pressure for the reservoirs
Both type of reservoirs can have a high initial reservoir pressure, but the role of fractures would be to release that pressure more effectively. On the other hand, even fractured reservoirs can have lower effective permeabilities than unfractured reservoirs. E.g. Ekofisk is extensively fractured, but only has ca 100 mD effective permeabiliy. It depends on the overall reservoir properties.
Please clarify the question. What do you mean by pressure difference: difference between initial and abandonment pressure, difference between a type of reservoir and another, etc. What do you mean by unconventional reservoir: naturally fractured / unfractured, the trendy shale oil / shale gas, something else? Difference between initial and abandonment pressure can be as large as ... initial pressure in natural depletion strategies (e.g. moderately deep heavy oil with activation: Emeraude). Difference of typical recovery methods (and associated differences in pressures handling policies) could be spelled out would the reservoir types you refer to be much clearer. Depth is strong factor that would determine differences in development strategies (as it is a first order determinant for initial pressure).
In general, even if they are equally fractured their performance will never be the same. In conventional reservoir the permeability in milli darcy order and it charge the fractured network in the reservior faster than the unconventional reservoirs such as shale where the permeability in shale in nano darcy order and it charge the fracture network at slower rate. therefore, the pressure differences is a function of production and you could compare on this basis.
If you are talking about the same reservoir which contains conventional and unconventional units (intercalation), means the same reservoir that has the same depositional environment, same structural and geodynamic framework, you can expect a rising of pressure reservoir nearby the unconventional unit due to the over-pressure and stress confining their.
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