AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Facies Distribution in Collapsed Carbonate Karsts: A Mechanical Approach
(1) Bureau of Economic Geology, University of Texas at Austin, Austin, TX.
(2) Center for Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, TX.
Paleokarst are economically viable hydrocarbon reservoir and aquifer. For instance, the Kirkuk Field (Northern Iraq) had produced up to 9.1 BBO until 1980 and the Idd Shargi North Dome Field in Qatar is producing 3500 BOPD on sub-vertical crestal wells (Trice, 2005). In China, the estimated reserve of the Jingbian field is evaluated at 25 BBO (Li et al, 2008), whereas in North America, the Devonian carbonate reservoirs in Alberta show very large potential with heavy oil production due to the dissolution of the evaporitic Hondo Formation (Huebscher, 1996), in the Grosmont carbonate section. Characterization of such geobodies, performed for reservoir simulation, must carry the typical sediment architecture and facies properties, most often observed within collapsed paleocaves. Although collapsed chambers may be compared with modern analogs (Loucks, 2001), only a few paleocave outcrops are available with very few of them providing insight about the 3-dimensional architecture. We propose to use numerical simulations of cave collapse to study the architecture and reservoir properties of paleokarsts carbonates to improve the accuracy of the static reservoir model in such reservoir setting.
The object of this study is to perform a mechanical analysis of collapsed sediment patterns within carbonate paleocaves, using a continuum/discrete forward modeling approach (Zienkiewicz and Taylor, 2000; Klerck, 2000; Vyazmensky et al, 2007). These patterns are associated, through the distribution of the fracture network, to observed facies within collapsed paleokarst: chaotic and clast breccias, disturbed and undisturbed strata facies (sediment-fill facies are related to flow of sediments within the caves). The strength of this numerical approach is that it allows us to examine and quantify porosity redistribution from an open void to the complex juxtaposition of different types of breccia and the enhanced fracture network associated with the collapse of the initial cave. First, results have been produced on a synthetic model showing typical carbonates stacking patterns. The generated fracture network shows concordant features with that observed in natural collapsed paleocaves. Then, this analysis is conducted on an outcropping collapsed paleocave situated in the Mallorca Island, Western Mediterranean. Bed markers are digitized and restored to a presumed pre-collapsed state. Following the collapse simulation, comparisons with the observed facies distributions are presented.