Complex Reservoir Typing by Dual Imaging Log, Thin Sections and Advanced Mud Gas Analysis for a Weathered Granite Basement in Transformation Zone, Pearl River Mouth Basin, Offshore South China
Authors: Guan Lijun , Xiong Ting , Liu Bo , Shao Meng , Zeng Xianlei , Zhou Jiheng 
Published: February 2024

Typing basement reservoir is quite challenging due to rotated stress field, different tectonic evolution, and weathering. The target LF-A area is a granite basement reservoir located in the transformation zone of buried hill with high stress concentration and strongly weathered crust. Previously identified only fractured or pore-controlled reservoir type can't appropriately provide guidance for further exploration adjustment or field development. In addition, the complexity of fracture network brings ambiguity and uncertainty for the reservoir typing. There are also vugs caused secondary porosity developing near or along fractures which increased the difficulty for reservoir typing.

Normally in most basement formation, high saline mud is used to protect formation and ensure drilling safety, thus deployment of advanced logging such as Nuclear Magnetic Logging and spectroscopy logs is limited, which decreases the accuracy of basement formation evaluation. In this case, reservoir porosity can only be obtained by conventional logs is of high uncertainty in basement formation. Advanced mud gas analysis was integrated to provide a lithology independent porosity after calibration with conventional log-derived porosity.

Dual image logs and advanced mud gas analysis were firstly proposed and integrated for the fracture network characterization. Ultrasonic and electricity resistivity image logs are utilized for fracture identification and evaluation. Ultrasonic image logs clearly identify open and closed fractures by amplitude image and transit time image, while electricity resistivity image logs can calculate fracture parameters including fracture porosity, fracture aperture and secondary(vuggy) porosity. Fracture indicator from advanced mud gas analysis was compared to the fracture development from dual image logs and is used to analyze fractures in non-image acquisition section. Finally, the basement formation is classified into 5 reservoir types: porous reservoir, fractured reservoir, and porous-vuggy reservoir, porous-fractured reservoir, and fractured-vuggy reservoir. Thin sections were used to validate the reservoir typing in a micro perspective, and multi-well correlation indicate that the best reservoir quality holds the vertical combination of porous, porous-fractured and fractured-vuggy reservoir from top to bottom.

The newly proposed reservoir typing method deepened the understanding on the complex reservoir type in terms of matrix porosity, fracture patterns and secondary (vuggy) porosity. It can be easily expanded to multi-well correlation for regional analysis on reservoir distribution. Vertical combination of various reservoir type was also firstly utilized to analyze reservoir quality wholly, but not solely. This approach will refine geological modelling in mapping the good reservoir distribution and enhance the ultimate reserve replacement for future exploration strategy.