Characterizing seismic bright spots in deeply buried, Ordovician Paleokarst strata, Central Tabei uplift, Tarim Basin, Western China

Abstract

Anomalous seismic-amplitude bright spots are a common feature in deeply buried (5500 to 6500 m) Ordovician limestone strata in the Central Tabei Uplift area of the Tarim Basin in northwest China. Those anomalies have proven to be useful indicators of reservoir quality. The bright spots as seen on seismic data are tied to high-gamma ray, low-velocity zones in wireline logs, and correspond to clastic cave sediment-fills in the host limestone in core. Synthetic seismic models confirm this relationship between seismic bright spots and cave-sediment fills. A seismic traveltime map of the top Ordovician unconformity illustrates erosional topography and seismic geomorphologic patterns associated with the unconformity with numerous sinuous fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. A mature surface drainage system interacted with a near-surface karst system and allowed terrigenous sediments to enter an underground cave system. Karst-related bright spots probably correspond to paleocaves that initiated along an early regional fracture network and later were enhanced and altered by additional discharges in the surface drainage system. A few examples of bright spots are interpreted to be related to postkarstification faults that might have had a hydrothermal origin. Distribution of bright spots provides a useful reference in mapping regional collapsed-paleokarst systems. Bright spots typically are associated with circular and linear faults, and V-shaped depression patterns are related to a collapsed paleocave complex. Seismic-scale mapping and visualization of the paleokarst system can be achieved by interpreting circular and linear faults using continuity attributes and by analyzing the relationship among faults, local depressions, and bright spots.