Theoretical evaluation of utilizing rock physics inversion for hybrid models of gas-hydrate distribution in deep water sediments of Oman sea

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Most of the hydrate assessment methods are developed based on relating the elastic properties to the hydrate saturation via establishment of robust and calibrated rock physics models and among them the most practical one is Effective Medium Theory (EMT) with four models of gas hydrate distributions. Generally, considering only one of these models in hydrate characterization could be an unrealistic assumption and therefore, rock physics inversion may cause erroneous assessments (over- or under-estimation).In this paper, hybrid models of hydrate distributions have been introduced for different combinations of two possible classical models by applying some modifications in the EMT workflow. Then, based on the modified EMT for hybrid models, a statistical rock physics inversion approach has been developed for estimation of gas hydrate saturations along with porosity, simultaneously. The result shows that theoretically it is possible to combine two models of distribution in both rock physics modeling and inversion processes. This developed approach was applied on a 2-D marine pre-stack time migrated (PSTM) seismic line passing through an anticlinal-ridge type structure in deep Oman sea sediment. For comparison, this approach was applied for single hydrate distribution as well. The results recommend to use the hybrid models of cementing and non-cementing distributions in the characterization to simultaneously consider all changes on the elastic properties of the host sediment imposed by hydrate presence. The results showed even in an un-explored area (without well information), this method can provide reliable robust result for porosity (independent of hydrate distribution assumption) and qualitative output for hydrate sweet-spots detection. Finally and after drilling the first exploration well, by having well log data in hydrate zone, along with possibility of validations and parameters calibrations for hybrid models, more reliable quantification could be achieved with this method; as the in-situ data increases the certainty of the determined elastic properties in model-based inversion process and also accuracy of defined mineralogical compositions.