Fluid migration models in Cretaceous fluvial sandstones bodies

GEORGIEFF, S.M.; IBAÑEZ, L.M.; OVEJERO, R.; FERREIRA, L.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

The San Jorge Basin has been producing gas and oil since 1907 and its production has reached more than 150Mm3 of oil from 1999 to 2009. The hydrocarbon plays belong to Cretaceous rocks related to lacustrine, delta and fluvial deposits. The main hydrocarbon reservoirs are formed by fluvial channels and bars, which consist in fine to coarse sandstones with trough cross stratification as the principal sedimentary structure. These channel belt deposits present 2.5 to 4.0 m thick, 100 to 350m width and, low sinuosity (1.2 to 1.5); though some cases of high sinuosity (2.0 and 2.5) were also recorded. The major sandstone bodies (in thickness and width) overlie the top of a thick mudstone deposit (around 50m); these sandstone bodies also show the highest sinuosity, which decreases upward. In order to propose a fluid migration model, more than 400 samples belonging to seven sandstone bodies were taken from outcrops. The measurements of porosity and permeability were carried out in laboratory from 350 samples (in cubes of 1.000 cm3) and the characteristic of the porosity was analyzed in 50 petrographic thin sections. The sandstone bodies show heterogeneous distribution of porosity and permeability, fluvial bars are more homogeneous in their petrophysical features than channels are due the fact that, in some cases, channel tops are made up by fine sediments related to avulsion and abandonment processes. The classic traps in these oil fields are structural and combined, but stratigraphic traps were observed in some particular anticlines (where the paleocurrent directions are normal to structural dip). Oil migrated along synthetic and antithetic faults and many of these faults show a strike slip component. In general, the porosity measured in laboratory varies from 8 to 24.9% and the permeability was less than 1 mD. The permeability is affected by sandstone composition, e.g., a high proportion of volcanic rock fragments, and the different kind of cements which were observed in petrographic thin sections. It is difficult to propose a single fluid migration model because, even in near oil fields, the tectonic plays a strong control in the dimensions, sinuosity of sandstone channel belts. Besides, petrographic studies show how volcaniclastic materials are affecting the primary porosity and permeability of sandstones closed to transpressive or trastensive faults areas. Hence, two different fluid migration models were proposed for major sandstone bodies according to their relation with synsedimentary tectonic activity: 1) sandstone bodies related to strike slip faults or shear stress areas are poor reservoirs; 2) sandstone bodies distant to strike slip faults or shear stress areas are better reservoirs.