Multiple-point simulation of channelized and unconfined fluvial deposits: an exploratory model for Cretaceous oil-bearing sandstones of central Patagonia

UMAZANO, A. M., C. LARRIESTRA AND E. BELLOSI.

Tucuman

Conferencia; 9 Int Conf Fluvial Sedimentology - Acta Geologica Lilloana; 2009

Acta Geologica Lilloana, 21 Supl. 3: 64-65

<!-- /* Font Definitions */ @font-face {font-family:"Cambria Math"; panose-1:2 4 5 3 5 4 6 3 2 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-1610611985 1107304683 0 0 159 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin:0in; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:ES; mso-fareast-language:ES;} .MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; font-size:10.0pt; mso-ansi-font-size:10.0pt; mso-bidi-font-size:10.0pt;} @page Section1 {size:8.5in 11.0in; margin:1.0in 1.0in 1.0in 1.0in; mso-header-margin:.5in; mso-footer-margin:.5in; mso-paper-source:0;} div.Section1 {page:Section1;} --> The results of a multiple-point simulation of the lower member of the Bajo Barreal Formation (San Jorge Basin) are shown using outcrop data from the San Bernardo Range. Five localities were studied defining a 68 km length N-S transect. This member is equivalent to the main hydrocarbon producing unit of the basin. Multiple-point stochastic sequence simulation is a statistical procedure that allows to simulate categorical variables, particularly curvilinear facies structures, from relative frequencies observed in training images (numerical representation of a sedimentary model that includes facies patterns). According to facies interpretation, the training volume corresponds to a fluvial model with channel-belts showing moderate sinuosity (adapted from Pyrcz et al., 2008); with the following size x=256 y= 256 and z=128 nodes. Five main facies associations are distinguished from outcrop data: fluvial ribbon-shaped channel-belt sandbodies, tuffaceous sheet-floods with scoured bases, ash-fall deposits, lacustrine mudstones and pyroclastic flows (ignimbrites). Maximum thickness is 350 m in the Southern region, and the area is 46000 km2. Alluvial architecture of the succession is characterized by the absence of discontinuity surfaces or truncations, so the model possesses a constant number of layers. The used algorithm is SNESIM (Single Normal Equation) defined by Strebelle (2002), and conditioning data comes from five outcrop profiles. Observed orientation of sandbodies was used in training images to guide simulation. The cell size is x=300 m, y=300 m, z=1.75 m. General facies proportion (72% sheet-floods, 22%  channels, 6% the remaining facies) from original logs is respected in the resulting simulated model, where proportion of channels varies between 9-45%, 5-29% and 17-28% respectively in the lower, middle and upper sections. The model predicts that channel sandbodies would be more frequent in the Central area; and upward in the upper section, which also displays the lower variation. Lacustrine facies would only occur in middle and upper sections, while ignimbrites and ashfalls in the upper section. This model can be improved with more information from outcrops and wells.