Frequently Asked Questions
Fractured basement reservoirs can be comprised of igneous or metamorphic rocks. Fractures form naturally through a combination of thermal contraction as a result of cooling of the igneous melt and later tectonic activity that enhances pre-existing weaknesses within the rock. Tectonic processes can occur over millions of years, reactivating fractures successively.
Fractured basement reservoirs are naturally fractured and do not require additional fraccing. Oil is stored within the fracture network rather than in pore space within the rock, as would be seen in most other reservoirs.
The basement of the Rona Ridge is comprised of granitic rocks which have been dated at between 2.7 and 2.8 billion years old.
In the Rona Ridge area, the basement exists as a structural high which is located next to a deep sedimentary basin containing the prolific Kimmeridge Clay source rock. The Kimmeridge Clay onlaps the basement at depth allowing the hydrocarbons to migrate directly from the claystone into the basement fracture network. Due to hydrocarbons being lighter than water, over geological time they migrate upwards to the crest of the basement structure where they are held in place by a thick package of Upper Cretaceous clays that surround the basement ridge and acts as a seal.
There are many examples of fractured basement reservoirs globally. Rock type varies with each field, although granitic examples are common. Porosities in basement reservoirs are typically less than 10% and can be low as 1%. Oil column heights can be significant.
What differentiates Hurricane’s basement assets from other basement fields globally is that they were drilled with basement as a primary target, whereas most other basement fields are found by accident when the well operator was looking for a different reservoir overlying the basement. As a result, Hurricane’s wells have been designed to target the fault network identified from 3D seismic interpretation to maximise the chance of encountering open fractures. The use of horizontal wells as producers ensures the productivity from the reservoir is also maximised.
Hydrocarbons have been produced to surface from Lancaster, Halifax and Whirlwind. Hydrocarbon indicators (shows, geochemistry, mudlogging and log data) are seen on Lincoln which are analogous to those seen on Lancaster where it has been demonstrated that mobile hydrocarbons are present through production logging. Open fractures have been identified in all Hurricane basement wells, which form part of a well-connected fracture network and provide the porosity and permeability for the reservoir.
Hurricane have undertaken well tests on all of the Lancaster wells drilled to date. The two wells designed for production (205/21a-6 and 205/21a-7Z) have demonstrated high productivity from the basement reservoir. The wells are designed to be flowed using electrical submersible pumps (ESPs) to aid control of bottomhole pressure and for flow assurance. However, the wells are able to flow at high rates without assistance from artificial lift.
Understanding the fracture network is crucial to understanding the basement reservoir. Hurricane use standard industry techniques to evaluate the reservoir such as image log data for fracture quantification and 3D seismic for delineation of the basement structure and for fault interpretation. Porosity is assessed using either neutron & density logs or using NMR, consistent with assessment of other reservoirs. The reservoir performance is modelled using standard industry software in order to generate production profiles.
There are examples of basement fields globally that have produced significant volumes of oil and over extended timeframes. Bach Ho in Vietnam has been producing oil since 1986, with production plateauing at 268,000 bopd from 2001 – 2003. Aguila Nafoora in Libya, Szeghalom in Hurgary and Zdanice-Krystalinikum in the Czech Republic have all be producing for over 20 years.
For further information, refer to our Technical library.