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Quantitative seismic characterization for low saturation gas reservoirs: a case study in Magallanes formation, Austral basin, Argentina
Bufaliza, Nicolas
Bufaliza, Nicolas
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2023
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Abstract
Distinguishing low gas saturation reservoirs from producible ones can be difficult as they exhibit similar responses in seismic data [e.g., post-stack seismic attributes or amplitude versus offset analysis (AVO) fail to quantify gas and lead in unproductive wells]. The “Estancia Chiripa” is an oil field in Magallanes/Austral basin, south of Argentina, characterized by partial gas reservoirs with a strong seismic response in the lower Magallanes formation. The reservoirs consist of shallow marine deposits identified by low permeability (lower than 1mD) and porosities between 13-17%. Previously, AVO and seismic attributes were critical for defining drilling positions, but these analyses failed and led to dry wells, and further
evaluation is required. This study aims to characterize the seismic response to identify producible gas saturation zones and determine optimal drilling positions for the lower Magallanes formation. To address this challenge, the combination of comprehensive forward models and reservoir characterization techniques were employed, culminating in a decision tree workflow that enables efficient decision-making.
The first stage in the analysis examined the AVO response from single-component seismic data and compared it to synthetic models where elastic and anelastic phenomena were studied due to potential high attenuation in gas reservoirs. In addition, wedge models were built to account for the proximity of the reservoir thickness to tuning effect. The lack of physical phenomena able to explain the exaggerated AVO response led to the conclusion that processing artifacts had affected the provided seismic data. To solve this issue, seismic data was scaled comparing main seismic events with the forward models. The scaling equation changed the gradient in the seismic AVO response, adjusting the pre-stack provided seismic data.
In the second stage, fluid substitution theory and rock physics templates (RPT) were incorporated to characterize the reservoir and find the optimal approach to assess gas saturation for two main intervals in Magallanes formation. From the fluid substitution and RPT, a water saturation attribute dependent on Vp/Vs and P-impedance was obtained. Consequently, simultaneous seismic inversion was conducted for the upper seismically resolvable interval (depth of investigation of 1800m). In contrast, a qualitative pre-stack seismic attribute was used to characterize the lower reservoir (2200m). The seismic analysis was combined with seismic interpretation techniques such as geometric attribute and isochron maps to improve the understanding of the gas accumulation in the reservoirs. Finally, it was possible to delineate the upper
reservoir gas zone, concluding that the entire area of interest had a low chance of being productive due to the low gas content. For the lower interval, optimal positions were suggested based on the qualitative analysis of the attribute of multiplying the intercept and gradient. In conclusion, the Magallanes formation in the “Estancia Chiripá” oil field showed low probability of being producible and successful gas production would rely on drilling technologies capable of producing from partial gas reservoirs and low permeability formations.
The analysis presented in this thesis was summarized in a decision tree workflow that would be valuable for other oil fields facing similar partial gas challenges. The workflow helped to ensure representative seismic data of the formation, assess reservoirs qualitatively or quantitatively, build a water saturation attribute, and obtain representative reservoir maps for identifying prospective areas and associated risks. Although the current area of study remains unproductive, the approach presented here is significant given that the same formation is commercially being produced in other fields within the basin, and new seismic data would be acquired in neighboring positions. Adopting the decision tree workflow outlined in this thesis would facilitate successful drilling operations or wise decisions regarding the continuation of drilling the exploration area or not.
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