Ligui Xu

Three-dimensional seismic geomorphology and analysis of the Ordovician paleokarst drainage system in the central Tabei Uplift, northern Tarim Basin, western China

High-quality three-dimensional seismic data acquired in the central Tabei Uplift, Tarim Basin, western China, provide a rare opportunity to characterize in exceptional detail the three-dimensional geomorphology of a deeply buried (5500–6500 m [18,045–21,325 ft]) Ordovician unconformity and the related paleokarst drainage system. An integrated approach was applied that emphasized integration of seismic data with available conventional core, wireline logs, and age-equivalent outcrops. The exceptional quality of the seismic data allowed a seismic detection limit of karstified features of less than 75 75 m (246 246 ft) horizontally and 6 m (20 ft) vertically. Interpreted geomorphologic and depositional elements include fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. The modern tower karst-drainage system in Guilin, China, is very similar to the mapped Ordovician karst-drainage system and is used as a modern analog. The interaction between the surface karst-drainage system and the shallow-subsurface cave-passage system is evidenced by the observation that surface canyons appear to initiate in areas associated with intense sinkhole development. Also, surface river valleys tend to correspond to dip-oriented surface depressions partly related to near-surface cave collapse. During burial into the deeper subsurface, the combination of intrastratal collapse (karstified strata) and suprastratal collapse (postkarst-deposited strata) created large damage zones hundreds of meters thick and kilometers wide. Coalesced-collapsed paleocave systems can be interpreted from the unique circular pattern of faults (observed in map view) that are associated with seismic bright spots.

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

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 correspo...

An Ultra-Deep Paleokarst System In the Ordovician, North-Central Tarim Basin, China: Outcrop Analog And Synthetic Seismic Models

Summary Exposed Ordovician strata in the western Tarim Basin in Northeastern China show a karst system that consists of karst towers and paleocave breccias. These outcrops are used to build 3D geocellular model and 3D synthetic seismogram. The low impedance paleocave breccias create distinctive high amplitude seismic anomalies below the unconformity reflection. A larger model is built with the top Ordovician surface mapped in the subsurface and with randomly distributed ellipses to simulate the paleocaves. The resulting seismograms show realistic reflection character and geometries. Impedance model with 150 m to 250 m wide by at least 20m thick paleocaves with a 175 % impedance contrast between the paleocave and host rock results in 3D synthetic seismogram that mimic the amplitude anomalies observed in the subsurface data.

Karst cavity mapping using CSAMT: A case history of tunnel investigation in China

GEM Beijing 2011: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Beijing, China. October 10-13, 2011. Karst cavity mapping using CSAMT: A case history of tunnel investigation in China Lanfang He, Xiumian Hu, Nanjing University; Ligui Xu, BGP; Xuben Wang, Chengdu University of Technology; Jian Li, Xuyou Lei, Donghua Wei, China Railway Eryuan Engineering Group

Challenges and solutions for land seismic exploration in China

There are more than 500 sedimentary basins in China, covering about 6.7 million km2 and containing an estimated 94 billion tons of oil and 38.14 trillion m3 of natural gas. The undiscovered volumes of inland oil and gas are about 49.42 billion tons and 27.74 trillion m3, respectively. These figures represent 68.02% and 77.99% of the total undiscovered oil and gas resources, respectively.