Relationship between geophysical log characteristics and lithofacies of the Lan Krabu Formation, Sirikit Oil Field, Phitsanulok basin, Thailand

Abstract

Sirikit oil field is situated about 400 km north of Bangkok. This field is in the Phitsanulok basin, which appears to have been constructed by major Oligocene extensional structures overlying Mesozoic basement. Oil has been stored in sandstone reservoir of the Miocene LanKrabu Formation. Recently, subsurface exploration in this area is still active, but core sampling is declining due to high cost of operation. Consequently, core lithofacies have not been investigated during this period. The main objective of this study is to assess the relationship between geophysical logs and core lithofacies in the LanKrabu Formation. Therefore, this leads to the establishment of criteria to interpret lithofacies using geophysical logs data. Thirteen wells with core samples are employed as database for calibration. A generalized method is then carried out; it consists of four significant steps: recognizing gamma ray log shape; investigating geophysical log values; selecting key bed; and constructing vertical relationship of various lithofacies. Hence, most lithofacies are identified. This application is verified using two sets of data (e.g. well with known lithofacies and well with unknown lithofacies). Testing result reveals reasonable achievement, although a few lithofacies could not be detected. The undetectable lithofacies commonly occur as thin layers with approximately less than 30 cm thick. Therefore, the application is conclusively restricted by thickness of some lithofacies. In addition, organic material is a crucial assemblage effecting geophysical log record especially the gamma ray log, particular in thin-beds sandstone. Higher-resolution log or high-performance computing system are recommended to identify and describe small-scale details such as thin-beds.