Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Digital Outcrop Analogues Model of Ordovician Paleokarst in North-Central Tarim Basin, China
(1) Bureau of Economic Geology, Jackson School of Geosciences, UT Austin, Austin, TX.
(2) BGP, CNPC, Korla, China.
These three linked abstracts present results of an integrated reservoir characterization project in the Ordovician carbonate succession in the north-central Tarim Basin in Western China. To better identify and characterize this complex Ordovician karst reservoir, our project combined an outcrop-analog 3D model and synthetic seismic (this abstract), core interpretation (Loucks et al.), and 3D seismic interpretation (Zeng et al).
The spectacular Lower Ordovician carbonate succession crops out in the Kalpin Uplift on the north rim of the Tarim Basin. There, the Lower Ordovician is unconformably overlain by shallow-marine Silurian siliciclastic deposits. The Ordovician-Silurian unconformity is spectacularly exposed along the anticline flanks and crest, where reddish Silurian mudstone and sandstone is overlain by light-gray steep-sided Ordovician paleokarst towers. More than 50 m high, these towers have a nearly vertical edge. Evidence of a paleocliff with erosional features such as vertical gullies and whirlpools can be seen at the unconformity’s surface. In addition, large polimict breccia bodies, with Ordovician carbonate clasts and a laminated reddish-muddy to sandy matrix possibly Silurian in age, can be found approximately 100 m stratigraphically below the unconformity’s surface. These breccia bodies are clearly elongated along fold and fault directions. The age of these tectonic features is still unclear, but they could be younger than the Ordovician unconformity.
A ground-based lidar survey captures both the unconformity and the breccia bodies. On the basis of field mapping and the lidar data, we built a 3D geocellular model. We reproduced the karst towers using Gaussian simulation conditioned by outcrop mapping. We extrapolated the breccia bodies using a kriging algorithm. The geocellular model was populated with subsurface petrophysical data from a nearby field in which Ordovician karst produces hydrocarbon that we used to build a 3D synthetic seismogram having various cave-fill properties. We used the synthetic seismic models to investigate the nature of cave fill and saturation fluid in the subsurface by reproducing amplitude anomalies observed in real 3D seismic data. These synthetic results are easily applicable to other karst reservoirs, such as the Grosmont and Ireton Formations in Canada or the Ellenburger Formation in Texas.