Abstract: Variation in Oilfield Salinity as an Indicator for Fluid Flow Migration in Campos Basin

Chang, H.K. - UNESP; Trevisan, O. V. - UNICAMP; Rodriguez, M.R. - Petrobras/E&P

Fluid migration is most often invoked as a mechanism of solute transport and as source of diagenetic reactions and hydrothermal mineralization. In Campos Basin, the fact that oil generated by lacustrine source rocks is presently trapped in reservoirs located in the open marine sequence is convincing evidence of fluid migration. The migration pathways of fluids expelled from deeply buried continental and transitional sedimentary sequences into post-rift shallow marine carbonate and deep water siliciclastic rocks invariably have to cross a thick evaporitic section (restricted marine sequence) as shown in figure 1. Therefore upward-moving fluids likely uptake a considerable amount of dissolved ions as water passes through evaporites.

The present study offers a revealing evidence of large scale and focused upward fluid flow in sedimentary basin originated in passive margin setting, based on chemical composition of produced oilfield water. Water samples from twelve wells have been collected from an Albian carbonate oilfield, located in Campos Basin, which have not yet undergone water injection. Chemical compositions of produced waters range from 74,000 mg/L to 92,000 mg/L of total dissolved solids. Ion concentrations increase systematically from NW to SE, reaching the highest values near the SE border fault (Fig. 2). Chlorine and sodium ions show a linear variation, away from the southeast bounding fault. Along a distance of 3 km, in a westward direction, Cl^- and Na⁺ values change from 53000 mg/L to 46100 mg/L and 30700 mg/L to 26700 mg/L, respectively.

The linear chemical distribution of these 2 ions have been tested against a simple diffusion model of electrolytes in aqueous solution. The straight line fit of both ions is suggestive of steady-state equilibrium profiles. Calculations using diffusion coefficients varying from 3 x 10^-5 cm²/s to 5 x 10^-5 cm²/s for both ions indicate the minimum time necessary to reach equilibrium to be within 40 to 70 Ma.

Based on these estimates, fluid flow along this NE/SW trending normal fault has been active for at least 40 Ma, i.e., since Oligocene, using a diffusion coefficient of 5 x 10^-5 cm²/s. If one uses a coefficient of 3 x 10^-5 cm²/s, fluid flow would have been active since Maastrichtian (70 Ma). Therefore, the onset of this saline water migration along the normal fault is compatible with the timing of oil migration in the area. This fault may well have been used for hydrocarbon migration that charged the studied oilfield.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil