An Integrated Multi-Well Rock Physics Diagnostic and Depth Trend Approach for Better Facies Understanding and Prediction: A Case Study from Sabah Basin
Authors: Tay Guan Chuan , Mathur Akash 
Published: February 2022

An understanding of seismic reflections can be developed from depth trend analysis. Often, an average best fit line is generated across well data using quick facies cutoff and that basically summarize the depth trend analysis over an area. However, this approach may lead to misinterpretation of reservoir response especially in a complex geological setting. In this study, an integrated approach of rock physics diagnostic and traditional depth trend analysis is presented to identify different rock type clusters which in turn translates into different depth trends per cluster. This knowledge will then be applied to understand elastic response and facies prediction away from the well locations, as conceptualized in Figure 1.

Figure 1 Schematic diagram to extend the knowledge from pre-existing wells (Well A, B, C, D & E) to understand elastic response at Well F location.

The main differences between the proposed workflow and conventional workflow are demonstrated in Figure 2 which strongly emphasizes rock physics crossplot diagnostics as an important step that segregates the well data into different rock physics clusters before any depth trend analysis. Using the proposed approach, interpreters will gain a better subsurface understanding of a formation for the same environment of deposition, reservoir response at different depth (from depth trend analysis) and appropriate attributes extraction for reservoir.

Figure 2 Proposed workflow in comparison with conventional workflow

In principle, subsurface rocks that were deposited within the same chronostratigraphic episode should have similar sediment source and compacted at similar rate within the same basin. Therefore, a normal compaction trend would be expected from diagnostic crossplot analysis unless impacted by other post depositional processes such as tectonic event, overpressures, cementations and diagenesis that could modify the rock physics relationships.

In this paper, the proposed workflow focuses on facies evaluation within the same depositional environment using rock physics integrated depth trend approach. Segregation of wells into different clusters based on rock physics diagnostic crossplot was considered as the first step for this regional reservoir characterization work. This helps to correlate elastic properties with the petrophysical properties. A conceptual model of clustering exercises based on rock physics diagnostics is displayed in Figure 3. In the final step, depth compaction trend is developed and analyzed for each cluster.

Figure 3 Different clusters identified from rock physics diagnostic exercise. (Conceptual Model)

Potentially more than 1 cluster might be identified from rock physics diagnostic exercises and depth trend analysis will be debeloped per cluster. The depth trend could be extended into different fluid scenarios and potential AVO response at different depth could be developed, as shown in Figure 4.

Figure 4 Depth trend analysis, seismic reflectivity & AVO response at different depth. (A conceptual Model)