Transition Between Deep-Water Channel-Belt and Overbank Deposits of the Upper Austrian Molasse Basin Using 3-D Seismic-Reflection and Core Data

Masalimova, Larisa ¹; Lowe, Donald ²
(1) GES, Stanford, Stanford, CA. (2) GES, Stanford, Stanford, CA.

The proposed study addresses the depositional mechanism, environment and evolution of the sediments in Atzbach, Schwanenstadt and Zell-am-Pettenfirst gas fields in the Upper Austrian Molasse Basin. The gas-bearing sandstones of these fields were interpreted to be crevasse-splay and overbank deposits to the main axial channel. The narrow configuration of the Molasse Basin controlled the space available for overbank deposits. This research interprets the depositional setting, documents the spatial variation of main bodies and the lithological heterogeneity of out-of-channel deposits.

The 3D seismic reflection data was used to map the main horizons of the Puchkirchen Formation. RMD maps from mapped horizons in order to characterize geometry and distribution of the main sand bodies. The description of core, detailed measurement of sand to mud ratio and degree of bioturbation were made in 32 wells to observe the heterogeneity from proximal to channel to distal setting.

The main lithofacies include thin-bedded sandstone and mudstone (Lf1), thick-bedded sandstone (Lf2), sometimes with mud clasts (Lf2a) or pebbles (Lf2b), thick packages of laminated mudstone (Lf3), and a very few slurry-beds (Lf4). Multiple fining- and thinning-upward cycles are present in almost every single well at scale of 10-20 m. The lithofacies Lf2 (a,b) and Lf4 are present in proximity to channel and represent crevasse-splay deposits. The sand to mud ratios decrease moving from channel to distal setting, the degree of bioturbation doesn’t have a clear trend but in some wells increase in distal setting. The maps generated form seismic-reflection data show fan-shaped bodies of sand, present in overbank area and elongate channel-like features that are perpendicular to channel and might represent the tributary channels.

The narrow configuration of the basin controlled the distribution of the overbank deposits of the Puchkirchen Formation. The resulting deposits accumulated on the slopes of the basin as overbank wedges and as crevasse splays and overbank lobes. The mud-rich sandstones and slurry beds are the signs of the levee crevasse. The trend of sand to mud ratio shows the decrease moving from channel to distal setting. It proves that the channel was the main source of sediments and the flows were waning with time and distance. The degree of bioturbation shows increase in distal setting that can be explained by a more quite and thus preferable environment for organisms.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.