DEPOSITIONAL ENVIRONMENT, DIAGENETIC HISTORY, AND RESERVOIR GEOLOGY OF THE SANTIAGO MEMBER SANDSTONES OF THE POJUCA FORMATION (LOWER CRETACEOUS) IN THE ARACAS OIL FIELD, RECONCAVO BASIN, BRAZIL

The Santiago Member of the Pojuca Formation consists of sandstone bodies (named S-5 to S-1 in the stratigraphical order) interbedded with shales deposited during the early Aratu Stage (Lower Cretaceous). It belongs to a deltaic system and in the small area of the Araca field, represents deposition in an interdistributary bay area through breaking and breaching of the main distributaries that surrounded the oil field. Core analysis, sand isolith maps, shape of SP curves, and comparison with recent depositional models defined crevasse mouth bar, distal bar, bay fill, and transitional (distal bar/bay) environments. The best potential reservoirs occur in crevasse mouth bar deposits.; Ten microfacies were defined, namely, two wackes, six arenites which are the most frequent in all sandstone bodies (S-1 to S-5), and two carbonates. Arenite microfacies consist predominantly of moderate to very well sorted fine-grained quartzose feldspathic arenite (less then 15% feldspar) and feldspathic arenites (15% of feldspar or more) containing different amounts of matrix (<5, 5-10, 10-15%).; The diagenetic evolution of the various microfacies of the Santiago sandstone is very similar and comprises: destruction of primary porosity by compaction and burial cementation (quartz overgrowth, feldspar overgrowth, and calcite); development of secondary porosity (up to 35%) by leaching of cements (mainly cal- cite); decrease in secondary porosity (to 20%) by late cementation (kaolinite, chlorite, feldspar overgrowth, and pyrite) and compaction; and preservation of secondary porosity due to oil migration which halted further diagenetic evolution.; Framework grains, cements, and porosity in the S-2, S-3, and S-4 reservoirs display different distribution patterns. Framework grains display patterns similar to those of depositional environment whereas cements and porosity patterns reflect predominantly diagenetic processes. They show closed geometry indicating the effect of a dome-like structure of compaction origin which probably controlled the circulation of pore fluids. Highest values of porosity occur in the least cemented areas with exception of the matrix-rich interval (S-4) where the reverse values of porosity and cement are not observed.; The distribution pattern of permeability is complex, and does not follow that of porosity. It does not seem to be directly related to any observable variation in framework grains or cements.