Abstract

The quality of the reservoirs of the Lower Cretaceous Pendencia Formation of the Potiguar basin, northeastern Brazil, is directly controlled by depositional facies-related carbonate cementation and compaction. The study of the interplay of these processes in the reservoirs offers an opportunity to unravel the diagenetic patterns of clastic sequences in interior rifts and, in particular, the role of carbonate cementation in poorly understood continental systems.

The Pendencia Formation is a thick sequence of fan-deltaic, fluvial-deltaic, turbiditic, and lacustrine sandstones, conglomerates, and shales deposited during the rift stage of the basin. The sandstones are predominantly arkoses (average Q49F40L11), with subordinate plutonic and volcaniclastic feldspathic litharenites. Compaction and cementation had similar importance in the destruction of porosity, with a dominance of cementation in the turbidites and of compaction in the fluvial deposits.

Carbonate cementation in Pendencia reservoirs increases progressively with depth and is facies controlled. Eodiagenetic, nonferroan calcite I (δ18OPDB -10.7 to -4.0%o; δ13CPDB -17.5 to +8.5%o), mesodiagenetic rhombohedral ferroan dolomite/ankerite (δ18OPDB -9.3 to -3.9%o; δ13CPDB, -1.7 to +1.1%o), and ferroan calcite II (δ18OPDB -17.2 to -6.8%o; δ13CPDB -13.6 to +2.3%o) were precipitated at three distinct temperature intervals calculated from the δ18O values: 21 to 58°C, 70 to 79°C, and 85 to 150°C, respectively. According to the δ13C values, dissolved carbonate for calcite I was derived from oxidation and methanogenic fermentation of organic matter and from methane oxidation. Ferroan mesogenetic cements were derived from thermal decarboxylation of organic matter. The shales were a major source of dissolved carbonate, as indicated by the isotopic similarity between their calcite (δ13CPDB -0.2 to +1.8%o; 87Sr/86Sr ~ 0.719) and most cements in the sandstones and by the peripheral cementation along the contacts of interbedded sandstones. As a result of this cementation pattern, thin turbiditic and deltaic sandstone beds are pervasively cemented.

The best reservoir quality potential is encountered in the partially cemented fluvial sandstones at moderate depths. Deltaic and turbiditic sandstones are more pervasively cemented by carbonate derived from the interbedded shales. Alluvial-fan conglomerates and sandstones were flushed by telogenetic meteoric waters close to the borders of the basin and to the proximity to the postrift unconformity. However, porosity enhancement was very limited, due to the precipitation of kaolinite and the intense compaction related to the compositionally immature detrital framework.

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