Vertical Positioning of Sweet-Spot Intervals in Shunbei Fractured-Vuggy Reservoirs Constrained by Seismic Forward Modeling

Ordovician fault-controlled carbonate fractured-vuggy reservoirs in the Shunbei area are jointly controlled by fault activity, karstification and structural modification. Because fracture-vug systems are commonly vertically superposed and strongly heterogeneous, the relationship between string-of-beads seismic reflections and the relative positions of principal sweet-spot intervals is ambiguous. This study aims to clarify the indicative relationship between peak-trough phase patterns within reflection wave groups and favorable fractured-vuggy intervals. Constrained by typical well-seismic data and fracture-vug development patterns, a vertically superposed double-fracture-vug parametric model and five actual well-tie seismic-geological models were constructed. P-wave finite-difference forward modeling and prestack depth migration imaging were used to compare the effects of vertical spacing, infill velocity, fracture-vug thickness and combination style on peak-trough phase, waveform interference and energy distribution. The modeling results show that, when adjacent fracture-vug bodies are located within the resolution scale of the seismic wavelet, their reflection wave groups tend to undergo coherent enhancement, destructive interference, wave-group broadening and energy redistribution. These processes can generate typical phase combinations, including two troughs sandwiching one peak, two peaks sandwiching one trough, two peaks and two troughs, three troughs sandwiching two peaks, and three peaks sandwiching two troughs. Different phase combinations correspond to different vertical stacking styles of principal fracture-vug bodies, secondary caves and fracture systems. Integrated interpretation of drilling breaks, mud losses, overflows and gas shows further constrains the relative vertical position of the principal sweet-spot interval within the reflection wave group. String-of-beads reflections should not be interpreted only by their external morphology or by a single strong-amplitude phase. Relative vertical positioning of sweet-spot intervals should be conducted under the joint constraints of forward modeling, well-seismic calibration and regional geological understanding. The proposed phase-pattern interpretation provides seismic-geological evidence for fine sweet-spot positioning, target-interval prediction in undrilled areas and sidetrack trajectory optimization in Shunbei fault-controlled fractured-vuggy reservoirs.