Yu, Jingbo; Li, Zhong
Carbonate reservoirs related to early karstification are well-developed in Ordovician System in the central Tarim basin, Northwest China. Stratigraphic framework and potential paleo-karsts of these carbonates, however, were not clearly revealed in past studies due to deep-burying, few drilling data, and low resolution seismic profiles of weak amplitude, low signal-to-noise ratio and poor continuous reflections.
In this research, firstly, structure-oriented filtering which smooth seismic data parallel to reflections was employed to highly improve the 3-D seismic data quality. Then, stratigraphic framework of the deep buried carbonates was established through seismic interpretation. Secondly, the distribution of paleo-karsts within this stratigraphic framework was determined from integrated 3-D seismic records. Two methods were presented to identify the distinct reflection characteristics of paleo-karsts. For large preserved karst caves characterized by bead-like seismic anomaly, a typical waveform was extracted from the records and used to calculate similarities between the waveform and the other trace records through a running window. Then an index PCI (Preserved Cave Index) was introduced for recording the maximum similarity value of the trace. If PCI is greater than a certain threshold value (70%), it can be identified as a preserved karst cave. For collapsed caves or a group of small karst caves characterized by clutter reflection, the index PCI is invalid. So another index CCI (Collapsed Cave Index) was introduced for quantifying the variability of the reflection orientations. CCI is the sum of vector products of each unit vector parallel to reflections with the unit vector of average reflection orientation within a window. The large value of CCI denotes collapsed caves or a group of small karst caves. According to drilling data, more than 90% of the wells confirmed the calculated distribution of the paleo-karsts.
Based on this study, deep-buried carbonates in central Tarim basin were divided into three segments from northwest to southeast. The northwest segment is characterized by relatively weak structural deformation and undeveloped paleo-karsts. The middle segment with proper structural highs indicates a suitable setting for developing supergene karsts preserved during later deep-burial stage. Stronger structural deformation with more karst caves collapsed occurred in the southeast segment, which resulted in few good reservoirs preserved.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013