The typical log response in the igneous and metamorphic rocks according to Kansas Geological Survey is as under,
The neutron log
Open pores typically have very low volumes in igneous and metamorphic rocks.
- low neutron porosity values in acid igneous rocks
- fairly low neutron porosity values in basic igneous rocks, except for sub aerially weathered basaltic lavas
- Low values in silica-rich metamorphics but increased values in micaceous rocks and very high values in chlorite schists.
The density log
The bulk density is a valuable diagnostic of igneous and metamorphic rock type.
- acid igneous rocks have a lower bulk density
- basic igneous rocks have a much higher bulk density
- Siliceous metamorphic rocks generally have a lower bulk density than micaceous metamorphic rocks.
In the Archean metamorphic rock, the density is high, and the neutron count, a direct indicator of hydrogen atoms, also increases. However, in the Kongdian Formation, both logs almost overlay. The elevated neutron value suggests additional hydrogen in the logged interval that is provided by either hydrocarbons or sufficient clay content. Additionally, there is notable porosity (5-6%) in the cores, with permeability below 2 milli darcy, as depicted in the figure above. Let’s assess if this aligns with the core details and petrology.
Sidewall cores, thin sections, and SEM analysis reveal well-developed karstification and weathering dissolution zones within the metamorphic rocks. Furthermore, Hou et al. (2019), asserts that there’s evidence suggesting the possibility of cryptoexplosion of supercritical fluid within the Archean buried-hill metamorphic granite. This event could have led to the formation of cryptoexplosive breccia or cryptoexplosive tuff, promoting thermal fluid filtrated alteration. Consequently, high-permeability reservoirs may have formed, containing abundant pores and fractures.
The Bohai 26-6 oilfield is situated primarily within a buried hill (geological feature), its reservoir part comprises of fractured and weathered basement rocks. Additionally, an overlying layer of coarse-grained clastics hints at the potential for additional oil reserves in this stratum. This scenario mirrors similar observations made by Henk Kombrink, GEO ExPro, such as the Lancaster field in the West of Shetlands, UK waters. In such cases, production extends beyond fractured basement rocks to include overlying coarse-grained clastic rocks. It’s intriguing to explore whether this phenomenon repeats itself in China, especially given the thicker upper section.
This find not only holds significant economic potential for China but also represents a crucial step forward in the diversification of global energy sources. As the world continues to navigate the complexities of the energy landscape, the Bohai 26-6 discovery serves as a reminder of the potential for innovation and the ongoing quest for sustainable energy solutions.
Key Points:
- CNOOC discovered a large oilfield in the Bohai Sea, claimed to be the world’s largest in metamorphic rock.
- The oil is found in a buried hill, where the reservoir is primarily fractured and weathered basement rock.
- The presence of overlying coarse-grained clastics raises the possibility of additional oil reserves in this layer.
References
- Characteristic and controlling factors of deep buried-hill reservoir in the BZ 19-6 structural belt, Bohai sea area:
Mingcai Hou a b, Haiyang Cao a b, Huiyong Li c, Anqing Chen a b, Ajuan Wei c, Yang Chen a b, Yuechuan Wang c, Xuewei Zhou a b, Tao Ye c
