Characterization of underground river reservoirs in Tahe Oilfield, Tarim Basin, China.

This study provides a method for characterizing underground river reservoirs while integrating outcrop, core, drilling, logging, and seismic data, and mainly including outcrop analogues, single-well identification, the prediction of inter-well locations, and the establishment of three-dimensional geological modeling. Outcrop analogues provide a better geological understanding of the spatial distribution and the internal filling characteristics of underground rivers. There exist unfilled and fully filled caverns, both of which indicate that a single well has encountered underground river reservoirs. The core fillings of underground river caverns are sand, mud, breccia, and calcite, which have logging response characteristics of enlarged diameter logs, high neutron values, low natural gamma values, and high-resistivity logging curves. Inter-well prediction of underground river reservoirs is mainly based on seismic impedance for depicting the spatial distribution of the river. Shallow and deep underground rivers have been identified in unit T of the Tahe Oilfield, and the major of the underground river caverns are partially filled. The initial three-dimensional reservoir model of the underground river reservoir is created based on the truncation of the seismic impedance value and has the characteristics of a discontinuous channel. On considering the statistical parameters of the real reservoir, the trend line, source point, and vertical development probability as constraints, the model is corrected via object-based simulation. For the filling of caverns in single well, the porosity are calculated according to the formula obtained by the core calibration logging curve. For the unfilled caverns in a single well, the porosity is comprehensively assigned according to cumulative production. The sequential Gauss simulation method is then used to establish a property model of underground river reservoirs. [ABSTRACT FROM AUTHOR]