Rock Physics Template–Based Fluid Detection in Tight Sandstone Reservoirs

ABSTRACT Seismic fluid discrimination plays a critical role in sweet spot detection, reservoir characterization, reserve evaluation and well placement. Tight sandstone reservoirs are typically characterized by low porosity, poor pore connectivity, complex pore types, non‐uniform gas–water distribution and strong heterogeneity, which often lead to inaccurate fluid discrimination. In this study, we develop a double‐porosity equivalent medium model for tight sandstone reservoirs using the Keys–Xu model combined with Gassmann's equation. We systematically investigate the effects of pore structure, porosity and water saturation on elastic responses. On the basis of this model, a rock physics template (RPT) is constructed using the P‐wave modulus and the P‐ to S‐wave modulus ratio. Polynomial fitting is then applied to derive mathematical expressions for both water‐ and gas‐saturated trendlines. On the basis of these trendlines, an RPT‐based fluid indicator is defined to quantify deviations from the gas‐saturated sandstone trendline. We further apply the proposed fluid indicator to a tight gas sandstone reservoir in the central Sichuan Basin, Southwest China. The strong agreement between the extracted fluid indicator and well log‐based water saturation interpretation demonstrates that this method significantly improves the accuracy of fluid content quantification compared with traditional semi‐quantitative RPT‐based approaches. Application to seismic data further shows that our method yields a reasonable estimation of gas distribution in tight sandstone reservoirs, confirming its reliability and practical applicability for fluid characterization. This approach offers promising potential for quantifying fluid content in deep‐buried tight reservoirs.