Abstract:

For the purposes of precisely delineating the characteristics of fractured-vuggy carbonate reservoirs marked by diverse genetic types and complicated internal structure, the paleo-karst is subdivided into three types in terms of their major control factors and origins, namely the epi-karst, fault-controlled karst and underground river system.Formed by weathering and leaching, the epi-karst has a wide range of distribution with a reservoir architecture composed of small seam-cavern bodies, predominantly small karst caves as well as dissolved pores and fractures.Fault-controlled karst dominated by complex faults and fractures, large fault controlled karst caves, dissolved pores and dissolved fractures, are formed by dissolution enlargement mainly along the fault zone.The large pipe system of the underground paleo-rivers mainly dominated by the fluctuation of water table, came into being by long-term current corrosion, and its reservoir architecture mainly makes up of karst pipes, chambers, and dissolved fractures and vugs along the pipe.In the light of distinct characteristics of karst in origin, amplitude spectrum gradient seismic attributes combined with sequential indication simulation were applied to characterize the epi-karst distribution.Features of fault-controlled karst were depicted based on impedance inversion and gradient structure tensor attributes.The model of ancient underground river was constructed by means of frequency-division attributes and object-based simulation method.Consequently, logging, geologic and dynamic data were integrated to delineate the connectivity, filling and physical properties of the paleo-karst; in addition, a 3D model was constructed by a fusion of the above-mentioned karst types.The results show that this method can effectively characterize the complex internal structure and strong heterogeneity of such reservoirs.Unit S67 comes in quite complicated reservoir architectures and various connectivity styles.The epi-karst reservoir bodies are quite small in scale and locally connected almost in laminar distribution, with a thickness ranging from 0 to 50 m, better in physical property and connectivity.The fault-controlled karst reservoirs are large in scale in various distribution patterns along fault zones, with great discrepancy in physical property and good connectivity following the fault direction.The pipe system mainly developed in paleo-subterranean river, is marked by two layers of river course, with heavy filling, poor physical property and connectivity.Based on the fusion model, the unit reserve constitution was refined and ascertained.The model was applied to reservoir numerical simulation and development plan adjustment with remarkable effect.

Key words: strong heterogeneity, reservoir architecture, epi-karst, fault-controlled karst, underground paleo-river system, fractured-vuggy reservoir