Enhancing Reservoir Insight with Real Time LWD Azimuthal Density Imaging, Wireline Advanced Log Acquisition & Core/Cuttings Analysis: a Case Study of Complex Lithology in Deepwater Sabah, Malaysia
Authors: Nordin T. B. , Ong L. W. , Saadon D. , Yaakob A. B. 
Published: February 2025

This paper presents a case study of evaluating deepwater exploration well with complex lithology and no suitable nearby reference/analog wells by integrating Core/Cuttings analysis (utilizing XRD/XRF), Wireline Formation Tester, Advanced Well Logging (3D Nuclear Magnetic Resonance & Resistivity Image Logs) and real-time LWD azimuthal density imaging.

Deepwater exploration well X-1 was drilled with target carbonate reservoir as objective & the studied field, located offshore deepwater Sabah, Malaysia. In this well, the data acquisition program was planned according to expected carbonate prospect with potential complex lithology. Advance wireline logging tools Nuclear magnetic resonance (3D NMR), water-based mud borehole image, formation tester with advanced DFA (downhole fluid analyzer) as well as drilling cuttings real-time advance analysis (XRD/XRF) was part of the data acquisition program. The well encountered complex lithology instead of conventional carbonate.

The synergic use of advance logging tools data with the aid of real-time LWD azimuthal density imaging & cuttings analysis XRD/XRF allowed for identification of lithology with formation bearing fluids, reservoir quality, invasion, and nearly saturated formation fluids with complex mineralogy. Applying conventional evaluation approach without thorough integration of advanced wireline log data with processing workflow would not achieve the formation evaluation objectives of the complex lithology. The integrated workflow delivered robust interpretation helps reduce uncertainties in answering complex reservoir characterization.

Key input data for petrophysical evaluation of the well revolves around the 3D Nuclear magnetic resonance, Image logs and real-time well cutting with advance analysis (XRD/XRF). A new wireline Nuclear magnetic resonance (NMR) logging has been routinely used to measure T1, T2 and Diffusion in addition to mineralogy independent porosity, irreducible water saturation, and permeability indication of reservoir. The T1 and T2 distributions inverted from NMR logging data are often composed of several fluid type components. Each fluid type component can be represented by a uniqueT1 and T2 peaks in the T1 and T2 distribution. This paper first discusses the application of NMR fluid characterization using T1-T2-Diffusion (3D model) & T1-T2 and D-T2 (2D models) from latest generation multi frequencies NMR tool. The (T2-T1) method increases the contrast between gas and oil, distinguishing gas from the wide viscosity range of oil. The (D-T2) method utilized diffusion analysis to identify oil and water.

The water-based resistivity image log is acquired with borehole coverage of approximately 60% in an 8.5-inch hole, with vertical resolution down to 5 mm. The image log raw data was processed with speed correction, eccentricating correction, and normalization. The post processed high resolution resistivity images were used for sedimentary interpretation together with LWD azimuthal density images.This paper also describes a method for assessing complex reservoir lithology that uses onsite XRD/XRF analysis on drill cuttings, samples, and sidewall core that are obtained during well drilling. As compared to submitting to an outside lab, on-site XRD/XRF analysis yields faster findings and costs less.

Complex lithology/volcanic can host significant hydrocarbon resources but are poorly understood in terms of their reservoir properties: especially their porosity and permeability characteristics. Despite low permeability, volcanic rocks are good hydrocarbon prospects and understanding the distribution of these key internal zones and their characteristic porosity and permeability will enhance hydrocarbon exploration within these unconventional reservoirs.

In summary, the integration of available multi-disciplinary data (latest generation of advanced wireline logging and LWD data) and the application of new in-house integrated workflow and solid geosciences knowledge added value to our further understanding of this surrounding area, especially in deepwater Sabah, Malaysia.