A Case Study from Offshore Australia: Reducing Reservoir Structural Uncertainties with the Help of Ultra-Deep Azimuthal Resistivity Data Inversion
Authors: Kushnir D. , Sviridov M. , Mosin A. , Sarmiento J. , Thompson R. , Sage J. 
Published: November 2024

The Thylacine raw gas and condensate field is in the offshore Otway Basin, SE Australia. This field has a complex geology with rapidly changing formation dips, further complicated with the presence of inter-fingerings and lateral discontinuities in the reservoir. Such geology leads to high reservoir structural uncertainties, complicating field development strategies. This paper presents how the combination of an ultra-deep azimuthal resistivity (UDAR) tool coupled with a high-tier vendor-independent inversion software resolved the reservoir uncertainties in the Thylacine West 2 well.

Initially the reservoir's architecture was defined by seismic sections with significant depth uncertainty. A modern LWD UDAR tool has been applied to explore the reservoir at a large scale and delineate its internal structure. Prior to spud, the tool parameters (spacings and frequencies) were specially adjusted to ensure optimal detection and resolution capabilities for the tool measurements under the field's resistivity contrasts. Next, a special multi-parametric inversion was applied to recover formation parameters within measurements' sensitivity region. Working with numerous measurements simultaneously, the inversion was able to extract enough information to reduce uncertainty in reservoir properties and keep ambiguity low enough to enable confident real-time decisions for desired well placement.

The section of interest of the Thylacine field is categorized into two target formations: Unit-1 and the underlying Unit-2. The best quality sand bodies of Unit-1 are usually 2-15 m thick and are separated vertically by mudstone and siltstone layers with thicknesses of 10-20 m. Unit-2 has an overall thickness of 20-60 m and represents a sequence of thick to faintly laminated sandstones separated by thin siltstone and claystone layers. The well was landed into Unit-1 intersecting ~400 m of net pay, prior to dropping into Unit-2, targeting an additional 1400 m of pay. Dealing with such massive layers, the UDAR tool demonstrated sufficient depth of investigation to detect presence of several targets simultaneously. The inversion of the data was able to map the boundaries of these targets, evaluate their properties and track structural dip changes. For instance, the inversion clearly resolved all sand bodies indicated in real time in both units. Moreover, it could identify a transition zone that is used to delineate between the lower quality facies in the upper part of Unit-2 and higher relative quality facies in the lower part. The inversion algorithm was able to automatically adjust the model's complexity, allowing for continuous monitoring of lateral and vertical changes in the geology within proximity to the wellbore.

Presented advanced multi-parametric inversion software allows simulation of various geological scenarios and evaluation of the capabilities of different UDAR tool configurations. The software may help the operators to evaluate and standardize various geosteering technologies as well as facilitate the use of the UDAR services through an improved understanding of their potential. This study illustrates that, being applied to the UDAR data, the inversion can help field operators to resolve high structural uncertainties and improve their understanding of full-scale reservoir structures as well as better delineate pay zones. The inversion demonstrates high performance confirming that it can be used in real time to facilitate making of geosteering decisions and optimize field development.