Identification of Thin Gas-Bearing Beds in an Ultra-Deep Carbonate Reservoir through High-Resolution Seismic Inversion

Zhang, Tingting; Sun, Yuefeng; Zhang, Hanrong; Guo, Tonglou; Cai, Xiyuan

The Puguang gas field, with a depth >5km, is one of the deepest carbonate gas reservoirs discovered in Sichuan Basin, China. One of the most important features of this field is that there exists high porosity (10-28%) and permeability (up to 3300 mD), but very thin-bedded reservoir units of about 4-10 m consisting of sucrosic dolomite. The detailed spatial distribution and geological origin of such thin, high-porosity, high-permeability beds at such great depths is still not well understood with current seismic technologies. This study reports preliminary results of the fine-structure of the thin bedded reservoirs obtained using high-resolution seismic impedance inversion.

Resolution-enhanced seismic data reveals both vertical and lateral high-frequency lithology changes better than conventional seismic. The observation of a transgressive process at the beginning of the Lower Triassic Formation can clearly interpret a relative sea level rise. Overall, synthetic seismogram with a band-pass wavelet (5-10-110-120 Hz) has a good correlation with high-resolution seismic data in accordance with the high-frequency stratigraphy. Within a thick gas reservoir zone of about 300 m, more than three thin, high-porosity, high-permeability gas-bearing beds, characterized by low density and low P-wave velocity, are identified on both synthetic and high-resolution seismic data but not on conventional seismic data. One typical feature of high-porosity, high-permeability gas beds of sucrosic dolomite is their low p-impedance value. On the other hand, oolitic dolomite beds have high p-impedance, low permeability (<1mD), with similar porosity. Analysis of the high-resolution seismic inversion result demonstrates that porous gas-bearing beds are well connected horizontally but vertically barricaded by non-porous beds, controlled by depositional environment, diagenesis, and paleohydrology. Further investigation could implement the high-resolution seismic inversion result to interpret the flow direction of the paleo-current and map carbonate reservoir heterogeneity for production monitoring.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013