Electrical and acoustic image logs in 18 wells were integrated for prediction of buried-hill productive carbonate reservoirs in a field in China. The logs showed the reservoirs are associated with paleotopographic features in a very complex and heterogeneous fracture-vug-cave karst system that provides their fluid-flow pathways and storage capacity. The fluid-flow pathways were provided mainly from high-angle permeable fractures and vugs with good connectivity that were caused by a long period of weathering and leaching. The storage components of the reservoirs were composed predominantly of the secondary development of a complex mixture of solution-enhanced fractures, vugs, and pores and occasionally of karst caves. The reservoirs had also undergone karstic erosion and fracturing during the Ordovician period.
In addition to detecting and classifying buried-hill carbonate reservoir types, the combination of log data with data from 16 well production tests allowed us to establish reliable fracture and vug property ranges for a series of reservoir types and to calibrate the productivity of the various reservoir types by production test and core analysis, or local production experience from similar buried-hill carbonate reservoir fields. The main contributors to oil reserves and production are from the three types of buried-hill carbonate reservoirs, a fracture reservoir, a fracture-vug reservoir, and a vug reservoir.
We used a multistep process to predict the buried-hill productive carbonate reservoirs in the case study. First we detected the structural faults and fractures from seismic data to better understand the fracture-vug-cave karst system in the field scale. We then recognized and classified the fractures and vugs in the 18 wells from borehole electrical and acoustic image logs to indicate the types of reservoirs. Third, we quantitatively defined the fluid-flow pathways and storage capacity of the reservoirs through the fracture and vug properties to analyze the relationship between the reservoir types and production. Finally, from the multiwell correlation of the fracture and vug properties with reservoir production, we predicted the levels of the productivity of the reservoirs. The case study in this paper improves the conventional approaches to the definition of the fluid-flow pathways of the reservoirs by adding hydraulic fracture aperture, fracture porosity, and connected porosity to the fracture and vug properties, which have a close relationship with the reservoir production in the field.