Hydrothermal Flushing and Calcite Precipitation as Main Modifiers of a Pre-Salt Reservoir in Kwanza Basin (Angola)

Abstract

Microbial carbonates and high energy shoals were originally the best pre-salt reservoir facies in Locosso-1ST oil discovery well from deep waters Block 22 in the proximal tectonic domain from the outer Kwanza Basin, but a complex diagenetic history affected the depositional textures of these pre-salt carbonate reservoirs.

The reservoir interval is interpreted as two prograding shallow benches; both regressive cycles grading upwards from packstones with minor Mg-clays at the base, to intraclast rudstones, boundstones and packstones with polygonal fitted grains and pisolites at top. The top of both cycles is characterized by scarce early calcite cement (Cc1), dolomitization, oxidation and incipient karst processes. After chemical compaction, a minor corrosion event affected the entire bench reservoir rocks. A first oil migration filled the reservoir up to the first paleo-OWC.

After oil-1 accumulation, late calcite cement (Cc2) reduced porosity. Cc2 crystal core presents variable oil + low-density CH4 + CO2-clathratre contents suggesting oil-1 remigration or water flushing. Cc2 C and O isotopic compositions display a burial trend, with ⁸⁷Sr/⁸⁶Sr values (0.7127-0.7125) slightly less radiogenic than the enclosing sedimentary rocks (0.7132), but more radiogenic than both the upper Late Aptian anhydrites (0.7075-0.7077) and basal Barremian basalts (0.7081). Fluid inclusions microthermometry data in Cc2 suggest the mixing of two brines with similar salinity and contrasted temperatures, a colder (T_h≈55°C; 22 %wt eq. NaCl) connate (?) brine, and a hotter (T_h≈140°C; ~23.5 %wt eq. NaCl) brine. Secondary aqueous-bearing fluid inclusions indicate a late temperature decrease towards the onset of oil-2 migration (T_h: 96-82°C) and present-day accumulation.

Profundal lacustrine facies, underlying the bench reservoir facies, present low indigenous vitrinite Ro values (0.62%) indicating that these rocks were not equilibrated in temperature with the hydrothermal fluids. This suggests a low-T hydrothermal event, with a possible hot fluid inflow of a carbonate-buffered residual brine after interaction with other nearby pre-salt bench carbonates, lacking the pervasive silica events seen to affect distal domains in Kwanza. Ar/Ar ages of 114.5 ±2.9 Ma (latest Aptian) and 106.9 ±1.6 Ma (mid Albian) in basalts (127.4 ±2.3 Ma) probably recorded the vanishing stage of this hydrothermal activity, after the anhydrite unit deposition.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018