The Lower Cretaceous "A" Sandstone of the Brazilian Reconcavo basin is a complex of bar-shaped lenses formed on ancient intra-basin shelves and shoal ridges. The sedimentary unit is not a blanket sandstone as is commonly thought, although it is a widespread continental deposit. Sandstone pinch-outs are common on the flanks of the major oil-producing structures and in intervening areas that coincide with early Lower Cretaceous subaqueous lows of the basin floor.

The "A" Sandstone is composed predominantly of quartzose sandstone, although interbeds of shale and siltstone in a few localities comprise as much as 10 per cent of the section. The sedimentary unit is divided into upper and lower sandstone bodies and intervening transition beds on the Mata-Catu trend and adjacent areas on the east. The upper sandstone and transition beds are absent in broad areas west of this trend. A maximum recorded thickness of 47 meters of "A" Sandstone was drilled in the SP-1-BA well, Sao Pedro field. The thicknesses of the lower and upper sandstones and transition beds vary independently although the three units generally thin uniformly to a knife-edge thickness on the flanks of structures. Thick-bedded, massive sandstone is predominant, but thin-bedded and cro s-bedded sandstone occurs locally.

The "A" Sandstone was first known to be petroleum-bearing when the I-2-BA well in the Itaparica field was completed as a gas producer in 1942. At the time of writing this report, 100 wells, in nine fields, produce an average of 25,000 BOPD from the "A" Sandstone. The average API gravity of the oil is 39°.

Petroleum migration was controlled by the topography of the original deposition basin. Oil migrated into the porous sandstone on the crests of synchronous highs at an early date, probably shortly after the deposition of the lower and middle Candeias shales. The search for new oil fields in the "A" Sandstone must take into consideration the paleo-structural features expressed by the isopachous contours of the Itaparica Formation because the thickest and most porous sandstone was deposited on these growing structures.