Deep reservoir potential of Pendência Formation onshore Potiguar Basin

Abstract

This evaluation of the deep-reservoir potential of the Pendência Formation, onshore Potiguar Basin, was based on an analysis of the depositional environment texture, mineralogical composition (detrital and authigenic), and spatial distribution of the different reservoirs. Special attention was paid to variations in porosity and permeability values as a function of depth. The clastic sequence of the Pendência Formation (Lower Cretaceous) in the onshore part of the Potiguar Basin consists of a thick packet of conglomerates, feldspathic sandstones, and shales deposited in fan-deltaic, fluviodeltaic, and turbidite systems, associated to a lacustrine environment. These reservoirs tend to display sharp decrease in porosity and permeability as depth increases. This behavior is directly related to basin burial history, aspects of the depositional environment the immature detrital composition of the reservoirs (feldspathic sandstones), and diagenesis. In the fluvial channel deposits, rates of porosity and permeability decrease less than in other genetic reservoir-types (i. e., fan-deltaic, deltaic, and turbidite). The former deposits constitute the best exploratory targets as far as deeper wells in the basin. The fan-deltaic deposits display little deep-reservoir potential, as reflected in their texture (poor sorting) and diagenesis (strong compaction and calcite and kaolinite cementation). The turbidite/deltaic reservoirs are of intermediate quality, with those located at depths of less than 2,000 m displaying the best porosity and permeability. Some of the turbidite deposits tend to present low permeability values, even at shallow depths. In general, they are poorly sorted, micaceous, and shaly, which favored mechanical compaction and destroyed most pore space. This characteristics point to low reservoir potential at any depth. Contradicting the general tendency for porosity to decrease as a function of depth, reservoir with abnormally high porosity are found at the base of the Pendência Formation. Their composition is quartz-feldspathic, and they are located in the middle of a volcano-sedimentary sequence characteristic of most of this basal interval. Various hypotheses have been raised as to the causes of the anomaly but since these reservoirs have not been cored, no conclusive diagnosis is possible.