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Bureau Seminar, January 8, 2010

Deeply-buried (18,000-20,000’) paleokarst-related drainage system and collapsed paleocaves, Ordovician, Tarim Basin, Western China


Link to streaming video: available 01.08.2010 at 8:25am

Hongliu Zeng, Bob Loucks, and Xavier Janson
Bureau of Economic Geology

China provides unique opportunities for energy-related research. In the past two years, a team of BEG and BGP (CNPC of China) scientists conducted an integrated study of a deeply-buried paleokarst reservoir in western China. Our presentation will present some unique and incredible geology from this study shown on images of 3D seismic, core, and outcrop.

Typical paleokarst reservoirs in US are generally in dolomite at depth because karst related pore networks in limestone are occulted at relatively shallow depths. In China, however, some highly productive reservoir that may be related to karst can be as deep as 20,000 ft! A seismic traveltime map illustrates extremely detailed erosional topography on the Upper Ordovician unconformity, revealing numerous paleosink holes and a mature paleodrainage system (hills, valleys, canyons, and associated fluvial channels; see figure below). Segments of meandering fluvial channels tend to follow topographic lows (valleys) initiated by clusters of sink holes and collapsed caves. Features associated with collapsed caves, such as circular and linear faults, local high-subsidence areas, and distorted beds, can be observed in outcrop and in seismic data. Low-impedance cave sediments, described in the cores as deep as 100 m beneath the unconformity, are commonly shown as amplitude bright spots. A collective showing of bright spots, circular or linear faults, and suprastratal deformation (both above and between bright spots) is a reliable indicator of collapsed cave systems.


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