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An Ultra-Deep Paleokarst System In the Ordovician, North-Central Tarim Basin, China: High-Resolution 3D Seismic Interpretation
Zeng, Hongliu (The University of Texas at Austin) | Loucks, Bob (The University of Texas at Austin) | Janson, Xavier (The University of Texas at Austin) | Wang, Quizhong (BGP Inc., CNPC) | Xia, Yiping (BGP Inc., CNPC) | Xu, Ligui (BGP Inc., CNPC)
Summary Integration of 3D interpretation, core description, and outcrop analogs reveals numerous paleokarst features in the Lower and Middle Ordovician limestone in north-central Tarim Basin, China. This study describes seismic and core evidence of karst development, including karst tower- and sinkholelike features on the unconformity, chaotic collapse features, bright spots derived from cave sediments, and core photos of cave sandstones in collapsed limestone clasts. The paleokarst system was developed on the top and slope of a regional uplift beneath an Ordovician unconformity. An interpreted seismic traveltime map illustrates erosional topography and seismic geomorphologic patterns on top of the unconformity for sinkholes, rivers, canyons, and hills, revealing a mature surface drainage system related to the paleokarst system. Seismic-scale mapping and visualization of the paleokarst system can be achieved by interpreting circular and linear faults using continuity attributes and analyzing the relationship between faults and bright spots. Introduction General features of paleokarst systems in the subsurface, along with their outcrops and modern analogs, have been studied by many authors (e.g., Kerans, 1988; Lucia, 1995; Loucks 1999; Loucks et al., 2004; McDonnell et al., 2007). Seismic-interpretation-oriented studies have concentrated on a formationโs deformation (sag structures, faults, etc.) caused by karst collapse and sinkhole geomorphology (Hardage et al, 1996; McDonnell et al., 2007; Sullivan et al., 2007). However, paleokarst formations are typically difficult to interpret because of their generally low frequency and the inadequate quality of seismic data in complex carbonate sequences. A 3D seismic data set in the north-central Tarim Basin, western China (Figure 1), offers a rare opportunity to interpret seismic-scale collapsed paleocave systems with confidence. The seismic data (Figure 2b), acquired and processed by BGP of CNPC, are superior in quality, considering a frequency content of 10 to 80 Hz and a predominant frequency of 30 Hz at 6,700 m (4s, 22,000 ft) (typically 10โ15 Hz in other areas). Detailed (seismicscale) paleokarst features, including unconformities, sinkholes, and collapsed paleocaves, are observable in a 2000-ft limestone section in the Lower and Middle Ordovician, equivalent to the Ellenburger Group in the U.S. In contrast, the Ellenburger paleokarst system in Texas (e.g., Boonsville 3D, Figure 2a) is typically poorly imaged, with weak, unstable unconformity reflections, even at the clasticโcarbonate contact. Internal architecture is mostly invisible, even in the shallow formation (1.2s, 6,500 ft) with higher frequency content (40 Hz). Evidence of collapsed paleocave systems Typical near-surface karst terrain is well summarized in Figure 3a in a block diagram depicting the four basic elements of a karst system. On an exposed unconformity related to karst development, the surface drainage system can be seen, including surface rivers and streams eroding into carbonate host rocks and sinkholes, as well as karst towers that have resulted from erosion and dissolution. In the subsurface (mostly in the vadose zone), cave development has produced cave chambers (rooms) with breakdown breccia and stream sediments in caves and underground streams (vadose canyon and phreatic tubes). Preserved cave sediments are distorted and mixed with breccia during collapse.
- North America > United States > Texas (1.00)
- Asia > China > Xinjiang Uyghur Autonomous Region (0.82)
- Geology > Geological Subdiscipline > Stratigraphy > Lithostratigraphy (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.77)
- Geology > Sedimentary Geology > Depositional Environment > Continental Environment > Spelean Environment (0.49)
- Geophysics > Seismic Surveying > Surface Seismic Acquisition (1.00)
- Geophysics > Seismic Surveying > Seismic Interpretation (1.00)
- North America > United States > Texas > Fort Worth Basin (0.99)
- Asia > China > Xinjiang Uyghur Autonomous Region > Tarim Basin (0.99)
An Ultra-Deep Paleokarst System In the Ordovician, North-Central Tarim Basin, China: Outcrop Analog And Synthetic Seismic Models
Janson, Xavier (Bureau of Economic Geology) | Zeng, Hongliu (Bureau of Economic Geology) | Loucks, Bob (Bureau of Economic Geology) | John, A. (School of Geosciences, The University of Texas at Austin) | Jackson, Yiping G. (School of Geosciences, The University of Texas at Austin) | Wang, Quizhong (BGP Inc., CNPC) | Wang, Chunyang (BGP Inc., CNPC) | Li, Shunxin (BGP Inc., CNPC) | Yang, Tao (BGP Inc., CNPC) | Xia, Yiping (BGP Inc., CNPC) | Xu, Ligui (BGP Inc., CNPC)
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. Introduction Paloekarst reservoirs are not common worldwide but can contain significant hydrocarbon reserve (Kerans, 1988; Fritz et al., 1993; Tinker et al, 1995; Dembicki et al. 1996; Guachet and Corre, 1996; Guangpo et al., 2005; and Li et al, 2008). The karst system associated with the long-term Ordovician unconformity hosts hydrocarbons in both Texas and China. In the Tarim Basin in Western China, the Ordovician karst play is actively producing hydrocarbons and is the target of an ongoing exploration effort by CNPC. In seismic data the karst system can be difficult to indentify because of the generally low-frequency nature and inadequate quality of seismic data of complex carbonate sequences. This study is using an outcrop analog in the Tarim Basin to build 3D stratigraphic and velocity models that are in turn used to build a 3D synthetic seismic model of complex karst features associated with the Lower Ordovician unconformity. The results of these modeling experiments are then compared to the actual subsurface data to improve the seismic characterization of the deep Ordovician karst target in the Tarim Basin. Outcrop expression of Ordovician unconformity and associated paleocave systems Because of the complex tectonic structure of the Tarim basin, the equivalent Ordovician strata that are actively explored in the subsurface are exposed on the basin edge along the Kalpintag Mountains. The outcrops are located approximately 600 kilometers west of the actual subsurface dataset in the Xikeer area (Figure 1). There, the Ordovician strata are exposed within a large anticlinal structure (figure 2A) in the thrusted nappes of the Kalpintag Mountains. The Lower Ordovician limestones are unconformably overlain by Silurian shallow marine siliciclastic deposits. The unconformity surface is highly irregular and shows up to 45 m of relief along these buried karst towers (Figure 2B). Approximately 60 meters below the unconformity, large breccia bodies can be mapped on the outcrop (Figure 2A). These breccias contain blocks of Ordovician limestone embedded in red laminated silt interpreted as cave sediment of Silurian age (Figure 2 C and D). The breccia size ranges from 150 to 500 m long by 70 to 250 m wide. Outcrop-based 3D synthetic seismic models Using ground-based lidar, the unconformity and breccia morphology is mapped in detail within a digital outcrop model.
- Asia > China > Xinjiang Uyghur Autonomous Region (1.00)
- North America > United States > Texas (0.95)
- Geology > Geological Subdiscipline > Stratigraphy > Lithostratigraphy (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.57)
- Geophysics > Seismic Surveying > Seismic Processing (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling (1.00)
- North America > United States > Texas > Permian Basin > Delaware Basin > Yates Field > Whitehorse Group > Word Group > San Andreas Formation (0.99)
- North America > United States > Texas > Permian Basin > Delaware Basin > Yates Field > Whitehorse Group > Grayburg Formation > San Andreas Formation (0.99)
- North America > United States > Texas > Permian Basin > Central Basin > Word Group > San Andres Formation (0.99)
- (5 more...)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Geologic modeling (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)