Volcano-sedimentary settings: petrology and petrophysic in sandstones hydrocarbon-bearing, Bajo Barreal Formation (Cretaceous), San Jorge Basin, Argentina

FERREIRA, L.; GEORGIEFF, S.M.; ARNOSIO, M.; SANTANGELO, A.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

The outcrops of the Bajo Barreal Formation (Middle Cretaceous) extend along the Deseado River (Santa Cruz, Argentina) for several tens of kilometers and are 100 to 200 m thick. The lower section of the Bajo Barreal Formation shows volcano and sedimentary deposits, this section produces heavy and middle oils. The strata strike N160ºE and dip 10º. The lower Bajo Barreal Formation is a continental unit composed by the alternating of middle and coarse sandstones, siltstones, tuffs, volcaniclastic materials and thin lens of basalt. Sandstones and siltstones represent fluvial deposits, permanent rivers and flooplains respectively; while volcanic, volcaniclastic and piroclastic deposits are associated to intra- and extra-basin tectonic activity. The sandstone bodies are mainly thin lens of 0.3-0.5 m thick and less than 10m width but some few sandstone bodies reach 2.5-3 m thick and more than 100 m width. The trough cross stratification, ripples and parallel lamination were recorded in channels and bars deposits. The mudstones are mainly light brownish gray siltstones in tabular to lenticular massive beds, some of these fine deposits are altered and partially reworked tuff falls. The piroclastic deposits are white grayish tabular beds, fining upward with a glassy matrix. Some fractures oriented W-E and 0.2-0.5 m width show piroclastic and volcaniclastic materials as fill. This study allowed understanding and quantifying the quality of oil reservoirs in tuff sandstones from petrographic and field studies in outcrops, cores and cuttings. The high proportion of volcanic rock fragments produced a decreasing in the porosity and permeability, and hence in the reservoir quality. The laying relation among clastic, piroclastic and volcaniclastic materials indicates contemporaneous accumulation of these deposits and a strong effect in the quality of reservoir in the sandstones hydrocar - bon-bearing.