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Visualization and Quantification of Deeply Buried Paleokarst Reservoirs in Tahe Oilfield, Tarim Basin, China


Mapping and quantify 3-D construction of the paleokarst reservoirs is a challenge in deeply buried (>5500m) heterogeneous carbonate system, as their irregular geometry and complex filling materials. Well-logging constrained acoustic impedance seismic dataset and seismic attribute analysis combined with 3-D visualization technology provide a significant amount of visible information about paleokarst reservoirs’ features in Tahe oilfield, Tarim basin. This paper describes an integrated approach to visualize and quantify the paleokarst reservoirs. First, caves’ recognize equation is developed using conventional well-logging data, which is demarcated from core and image logs with a cave resolution of approximately 0.5 m. Second, time-depth conversions for 97 wells are identified one by one, and the recognition results in the signal wells are tied to seismic dataset. Third, after determining the cutoff values of the host rocks and the caves in acoustic impedance, the impedance inversion volume can recognize the spatial construction of the paleokarst reservoirs effectively. Forth, the 3-D mapping and visualization of the paleokarst reservoirs are achieved by tracing the distribution of caves. Fifth, based on the 3-D ‘geobody’ and karst genetic theory, comparing with spatial geometry of the Mammoth Cave, the Tahe paleokarst reservoirs are divided into epikarst, vadose and runoff zones. Additionally, the genetic types of them are identified, i.e. chamber caves, main channel, branch channel etc. Using 3-D visualized geobody, the length, width, area, volume of different genetic types are calculated, and the chamber caves and main channels are pointed out as prior targets for hydrocarbon exploration. Using 3-D visualization technology, the spatial construction of paleokarst reservoirs is delineated; combined with the karst hydrodynamics theory, vertical zones and genetic types of the reservoir are divided; the entrances, exits, collapses and relative high points of the cave systems are identified; the quantification of each genetic type is calculated from line, area to volume. All achievements above provide detailed information of the reservoir for structural model, geological model, hydrocarbon exploration and can be applied to other similar paleokarst oilfields.