Intrabasinal syntectonic control in Cretaceous fluvial deposits. San Jorge Basin, Argentina

GEORGIEFF, S.M.; SOSA GÓMEZ, J.A.; FERREIRA, L.; SANTANGELO, A.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

The San Jorge Basin is the oldest oil basin in Argentina, the main hydrocarbon reservoirs are Cretaceous rocks belonging to Chubut Group. This Group shows different sedimentary environments: lacustrine, delta and fluvial deposits at the base (Pozo D-129 Formation), followed by volcaniclastic deposits (Castillo Formation) and fluvial deposits at the top (Bajo Barreal Formation). The Chubut Group show many examples of syntectonic control in both outcrops and subsurface deposits. The fluvial channel patterns are very sensitive to the floodplain slope changes, due to extra- and/or intra-basinal syntectonic activity. Systematic mapping of channel sandstone bodies in different sections of Chubut Group show an alternating of quiet and active tectonism, as abrupt changes in the channel sinuosity (2.0 to 1.2) as in the floodplain width (tens of kilometers to less than 1 km), which indicate insights of transversal tectonism regard to paleocurrent directions. The fluvial architecture studies in outcrops also suggest changes in the lateral and vertical stacking of sandstones, which are mainly related to floodplain narrowness defined by tectonic paleovalley and changes in the sediment supply rates. The faults are oriented WNW-ESE and NW-SE, consequently transpressive and transtensive movements were contemporaneously active during Middle Cretaceous. A huge amount of volcaniclastic and piroclastic material were distributed from these shear systems affecting the local slope and sedimentation rates of floodplains. The tectonic control defines three kind of fluvial channel movements, sandstone reservoirs and porosity – permeability quality: 1) free migration, high sinuosity channels (floodplains: 5 – 10 km width), good to very good poro-perm quality (channels are distant from faults supplying of volcaniclastic materal); 2) partial migration, channels were control by a single fault, more or less parallel to paleocurrent directions, good poro-perm quality; and, 3) restricted mi - gration, narrow channels (floodplain = tectonic valleys, less than 1-2 km width), poor poro-perm quality (close to faults supply volcaniclastic material).