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Sedimentology and Reservoir Architecture of a Synrift Lacustrine Delta, Southeastern Mongolia

¹ Department of Geology and Geophysics, University of Utah, 135 South 1460 East, WBB 609, Salt Lake City, Utah 84112-0011, U.S.A.; cjohnson{at}mines.utah.edu
² Department of Geological and Environmental Sciences, Building 320, Stanford University, Stanford, California 94305-2115, U.S.A.

In the East Gobi basin of southeastern Mongolia, non-marine strata fill Jurassic-Cretaceous intracontinental rift basins. Amphitheater-style cliff exposures at Har Hotol on the northern margin of the Unegt subbasin provide more than three kilometers of continuous outcrop, displaying early synrift strata that thicken and coarsen upward from prodelta lacustrine deposits to channelized fluvial sandstone. Outcrop investigations include photopanorama mapping and correlating measured sections, followed by interpretation of lithofacies units and architectural elements. Lithofacies distinctions are based on differences in grain size and bedding style, including massive to planar-bedded mudstone, and lenticular sandstone bodies with erosional bases and common soft-sediment deformation features. Architectural elements are categorized by first-order sedimentary structures (microscale, < 0.1 m), second-order bedforms (mesoscale, 0.1-1 m), and third-order depositional assemblages (macroscale, 1 m to tens of meters). Maps of lithofacies and architectural elements overlaid on photopanoramas reveal complex sedimentologic and geometric relations, including lenticular sand channels and a short lateral transition (< 600 m) from fluvial sandstone into prodelta lacustrine mudstone in the inferred paleocurrent direction. This parasequence is interpreted as a prograding perilacustrine delta, which was eventually overwhelmed by input of volcaniclastic detritus. Several similar progradational cycles are found in stacked succession at Har Hotol, indicating that repeated shifts in the lacustrine shoreline characterized Late Jurassic sedimentation in this area. Cyclic stacking of parasequences reflects autogenic processes (e.g., delta lobe shifting), as well as allogenic controls on the depositional system, including climatic shifts, episodic input of volcanic material, and possibly tectonic activity. The sequence described here is an analog for petroleum reservoirs in the poorly known East Gobi basin, as well as in similar nonmarine rift basins worldwide. Results demonstrate the high degree of vertical and lateral lithologic heterogeneity in this deltaic assemblage, and highlight the utility of detailed, outcrop-based studies for visualization and prediction of subsurface reservoirs.

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