Investigation On Construction And Filling Model Of Carbonate Paleokarst Fracture-caves In The Tahe Oilfield

The carbonate paleokarst fracture-caves developed in the paleo-weathered crust which suffered dissolution from atmospheric fresh water, and have complex spatial distribution and interconnectivity. The Ordovician fracture-caves in the Tahe Oilfield, which deeply buried more than 5300 m, have very complex anisotropy in vertical and horizontal direction, so characteristic the distribution of fractures and caves, identifying their construction models and filling rules are urgent scientific and practical problems. Using karst geological theory and applied geophysical methods, the construction and filling models of fracture-caves were studied in detail.Identifying the locations of fracture-caves is the first step of this study. Using conventional well logging data which demarcated by drilling, core samples, image logging and well testing data, the equations were built to identify the caves, fractures and fracture-cave combinations, and the caves with height less than 0.5m can be recognized effectively. DEN-Vsh、RLLD-Vsh cross charts were built to recognize the filling types. Synthetic the interpretations of signal well to acoustic impedance dataset, and the vertical resolution in somewhere is about 3 m.The epikarst-vadose-runoff zones and their construction types were identified. The epikarst zone can be divided into surface river, sinkhole, fossil soil and fracture-cave combinations; the water flowed laterally along the hillside. The vadose zone developed high-level and low-level hanging cave, fault-dissolved well; the water flowed vertically to the water table. The run-off zone developed buried river, including chamber caves, main channels, branch channels and tip channels; the water horizontal flowed along the water table. The main and branch channels developed along bedding. The tip channels connected with branch and main channels, or they maybe decreased as fracture-cave combinations and fractures.The filling model of fracture-caves was built. The filling of surface river has obvious rhythmicity, and has a typical fluvial face filling character. The filling of sinkhole is disordered combining with breccias. The fault-dissolved wells contained endogenous fillings and breccias. The high-level hanging caves had obvious low filling degree. The low-level hanging caves had high filling degree with filling characters similar to buried rivers. The filling characters in run-off zone were very complex. The chamber caves had many multiphase fillings and typical double structure, combining with breccias. The main channels filled with typical buried river fillings. The branch channels and tip channels filled with sedimentary and had low filling degree with chemical fillings. The fracture-cave combinations developed in the three zones, and filled with endogenous and chemical fillings, had good connectivity.Based on construction and filing models of the Tahe Oilfield, combined with reevaluation of reservoir property, the steps for fracture-cavity unit were proposed. The methods were applied in 7th and 4th well area. All the results clarify the basically geological characters of carbonate fracture-caves.