Most of the hydrocarbon reservoirs in the Pendencia Formation, onshore Potiguar basin, northeastern Brazil, are fine- to coarse-grained deltaic and turbiditic lacustrine feldspathic sandstones which contain quartz and feldspar overgrowths, calcite, secondary porosity, dolomite, and chlorite as the principal diagenetic elements. Porosity in these rocks is a complex association of reduced primary porosity (5%), carbonate dissolution porosity (7%) and framework-grain dissolution porosity (6%). Each porosity type displays characteristic textural features which permit recognition and quantification in thin sections. The secondary porosity apparently was produced by corrosive fluids generated from adjacent or deeper shales. The effect of the diagenetic phases on permeability was distinct from their effect on porosity. Initially, quartz and feldspar overgrowths partially obstructed the pore throats, significantly reducing permeability. Later, replacement by calcite of grain or overgrowth boundaries followed by calcite dissolution generated the 8-mu -diameter pore throats that control the present permeability system. Among the late diagenetic phases, chlorite was the most harmful to reservoir permeability, affecting especially fine-grained sandstones. The present pore-fluid distributions in the Pendencia reservoirs show that high irreducible water saturations (S wi around 40%) are common. These high S wi have generally been attributed to chlorite aggregates. However, analysis of the pore structure reveals that microscopic heterogeneity related to the geometrical relationship between preserved primary porosity and diagenetically produced secondary porosity is the main cause of the observed high water saturation. Irreducible water tends to remain held within discontinuous patches of primary porosity as oil fills a continuous network of secondary porosity.