Interpreting Electro-facies And Fractures For An Oligocene Sandstone Reservoir Using Borehole Images Acquired In Oil-base Mud With Full Core Calibration: A Case Study From Songkhla Basin, Thailand

Abstract

Abstract The Oligocene reservoirs in the Songkhla Basin of Thailand are predominantly fluvial-deltaic deposits. There are several geological challenges describing these reservoirs. Firstly, it is sometimes difficult to determine lithology from conventional open-hole logs. Secondly, defining facies can be challenging due to the lack of sedimentary structure information from any of the conventional open-hole logs. Thirdly, natural fractures and sub-seismic faults exist in the formation which adds more complexity to the reservoirs. Geoscientists have traditionally incurred high operating costs to acquire conventional cores needed to resolve these challenges. This paper presents an alternative technique for sedimentary characterization of the Oligocene sandstone reservoirs in the Songkhla Basin utilizing electrical image logs acquired in synthetic-based drilling fluids, along with Elemental Capture Spectroscopy (ECS) and Nuclear Magnetic Resonance (NMR) data. Lithology has been defined a lot more accurately using ECS data than by using conventional open-hole logs alone. Electro-facies have been successfully determined by integrating sedimentary structure and texture information interpreted from image and NMR logs with lithology information from ECS. In addition, the natural fractures have been interpreted from image logs. These interpretations of lithology, electro-facies and fractures have been calibrated with core data acquired in the same well. The output from this pilot study was a cost-effective solution for electro-facies and fracture analysis. It also provided input to constrain the sedimentological settings and reservoir modeling for the field.