SEISMIC CHARACTERISTICS OF COMPLEX HALO-ANHYDRITES AND THEIR EFFECTS ON THE UNDERLYING CARBONATITES: A CASE STUDY OF THE RIGHT BANK OF THE AMU DARYA RIVER

Research on the impact of complex halo-anhydrites on seismic imaging of underlying carbonates has been a focus of seismic exploration. Here, this research analyzed for the first time the seismic reflection characteristics of sandwich-structured halo-anhydrites on the right bank of the Amu Darya, having three anhydrite layers and two halite layers. The reflections from the upper halite displayed a set of weak amplitude reflection and gentle seismic events. Reflections from middle anhydrite, lower halite, and lower anhydrite showed intense reflected energy and distorted events with poor continuity. Three theoretical models, an "eyeball" salt dome model, thick salt model, and two-layer thin salt model, and two actual complex halo-anhydrite models were designed. Seismic forward modeling and illumination analysis were performed on the theoretical and empirical models using a staggered-grid high-order finite-difference method on an acoustic equation in an inhomogeneous medium. The results indicated several important points. First, interference waves generated by the halo-anhydrites, such as the strong energy multiples, wide-angle guided waves, scattered waves, and bow-ties, reduced the SNR (signal-to-noise ratio) and continuity of reflections from underlying strata. Second, bow-ties formed by complex halo-anhydrites causing false bright spots on the reflection amplitude from the underlying strata. Third, the number of halo-anhydrite layers had a more remarkable shielding effect on energy from underlying strata than thickness. Finally, the complex structure of halo-anhydrites led to heterogeneity in the downward propagating seismic energy and formed illumination shadows. In the study area, deformed halo-anhydrites demonstrated seismic characteristics like "eye-ball shaped halite" and "anhydrite caps". The effects of thickness and level of deformation of these halo-anhydrites on the formation of gas reservoirs in the underlying carbonatite, the height of the gas column, and the structure of the top of the gas reservoir, are noteworthy. Lateral sealing of the strongly deformed halo-anhydrites play a key role in the underlying patch reef gas reservoir. Underlying anhydrites near the gas-water interface are beneficial for improving the porosity of the porous carbonate reservoir.