Naturally Fractured Basement Reservoir: An Example from Ruby Field

The occurrence of naturally fractured basement reservoirs has been known within the hydrocarbon industry for many years. Generally regarded as non-productive, they have failed to draw the attention of the explorationists. Often passed over as of ‘no economic potential’, their investigation by exploratory drilling has been left to chance. Interesting enough there are still many companies that stop drilling the operations the minute basement is found. Yet, they are commonly distributed in various petroliferous regions throughout the world. Some examples on the fractured basement reservoirs from various countries have been discussed in detail and showed there are commercial excellent basement reservoirs. Basement reservoirs are typically complex with multiple lithologies, possible two or more fracture systems and multiple oil-gas and oil-water contacts and hence are challenging reservoirs for the geologists, explorationists and reservoir engineers. An attempt has been made in this article to understand hydrocarbon production fractured basement formations, particularly in Ruby field (Vietnam), which is a granite fractured reservoir, along with review of naturally fractured basement reservoirs in different countries (Table 1), assembled primarily from published literatures. This project has clearly described the nature of the fractured basement reservoir including how the trap is formed, the source rock, the properm properties, fracture intensity and aperture, by taking an example of fracture basement reservoir from Ruby Field. Moreover, there are oil and gas fields “left behind” in areas where basement was not entered by the drill bit but where mature oil or gas source rocks are close to basement and where basement is fractured or weathered and occurs within structural closure. Therefore, there needs to be a paradigm shift in the mentality regarding basement. These reservoirs need to be studied closely. Coring is typically difficult due to the fractured nature of the reservoirs, and provides more challenges to geoscientists and reservoir engineers.