Structural Modelling of Complex Fault Relationships: A Volume Based Modelling Approach

Pal, Arindam (Cairn India Limited) | Nazhri, Ahmad (Cairn India Limited) | Phinney, Eric (Cairn India Limited) | Sunder, V. (Cairn India Limited) | Goodlad, Stephen (Cairn India Limited) | Dwivedi, Nikhilesh (Cairn India Limited)



Complicated structural relationships within gravity collapse, growth fault and thrust regimes have always been challenging while modelling with conventional Pillar Based Structural Modelling methods. The complex fault-fault and fault-horizon relationships are more accurately modelled with Volume Based Modelling (VBM) technique that results in robust structural grids. In terms of orthogonality, the quality of structural grid cells is superior compared to the corner point grids. Nearly all complex fault relationships can be modelled using this technique.

In the extensional Barmer Basin, north-western India, the Barmer Hill Formation of Paleocene-Eocene age is affected by syn-sedimentary faulting events. Gravity collapse structures developed synthetically along a down-to-the-west major boundary fault are expressed as a series of cuspate, low angle listric faults. Seismically mapped, depth converted conjugate faults and horizons are sequentially modelled as geological events using a pillarless structural framework algorithm within the volume of interest. The gravity collapse structure bound by main boundary fault and several λ and Y-shaped listric faults are accurately modelled using the fault framework modelling process. The conformable Barmer Hill horizon inputs are well tied, converted to point datasets, filtered with suitable fault cut-backs and modelled using horizon modelling process. The algorithm uses an interpolation technique that creates triangulated meshes for modelled surfaces ensuring high degree of data consistency, retaining complex truncation relationships in the model. Residual calculation of the structural framework shows excellent coherence with input dataset.

Structural framework modelling is followed by structural gridding performed at an optimum resolution to capture the reservoir heterogeneities. Depending on the end usage and fault complexity required in the model, multiple realizations of geological and simulation grids are created. The layering scheme is optimized with an integrated approach, honoring reservoir heterogeneity and simulation constraints. The structural grid is quality checked with parameters like negative volumes, distorted - isolated cells and cell width. By virtue of grid orthogonality, significant improvement is observed in simulation run time.

Volume Based Modelling is a workflow operated, automated, repeatable and geologically consistent technique compared to conventional pillar based structural modelling methods. The structural grid generated from a volume based model provides an excellent platform for dynamic simulation. With examples from Barmer Hill structural model, this paper showcases volume based modelling as a robust alternative technique for modelling complex fault relationships.