It has long been thought that the anhydrites present in exploration wells drilled offshore Angola adjacent to fault planes in the Pinda Formation reservoir were a direct result of burial diagenesis. Anhydrites can reduce the porosity of a good-quality reservoir as much as 10%, minimizing the prospect value of some newly explored fields and possibly leading to field abandonment. However, a complete dataset now available from the formation has extended reservoir understanding. Comprising cores, high-resolution microresistivity images, seismic and outcrop data, the dataset is serving as a model for planning future offshore wells. Our analyses, made using the integration of this dataset, indicated the anhydrite traveled to the reservoir rocks through faulting. With an aqueous solution of calcium sulfate in the underlying salt deposits, it was easy for the solution to migrate through the fault planes and invade the pore space. We found the influx of anhydrite to be localized around the fault zones. Our outcrop analysis of the Pinda caprock indicated a fractured and faulted nature with all fractures filled with anhydrite. In addition to our analyses, we have computed a continuous porosity and permeability log using high-resolution microresistivity images; it was an excellent match to the core data. This achievement opened up an efficient way to reduce coring costs in the expensive offshore drilling environment.
This new information is changing the way exploration wells are planned in the offshore Pinda reservoir. In the future, wells will be drilled with sufficient distance between them and the fault planes to reduce the anhydrite content in the near-well formation. Also, the porosity and permeability logs computed from microresistivity images will complement the logging programs, but they are not intended to replace the other log measurements.
During the Early Cretaceous (Aptian) the rifting of the supercontinent Pangea had already started. Africa at this time was separated from South America by a restricted marine depositional environment. This environment eventually evaporated and led to the deposition of massive salt deposits, today known as the Loeme Formation. By Albian times, the separation of the two continental plates was well underway and open-to restricted marine environments were intercalating. This intercalation led to the deposition of what we know today as the complex Pinda Formation. According to Carlos and Inkollu (2002), extensive Pinda exploration began during the 1980s within the Cabinda area. By 2002 Pinda was the primary producing reservoir of Angola (Figure 1). Although today much of the focus for oil exploration is in the deep and ultradeep waters of Angola for the Tertiary turbidite play, companies do continue to explore for hydrocarbons in Pinda in the shallow waters of Angola (Tako, 2008).
Only subsurface data is available for studies of Pinda. Because there are no known outcrops within West Africa, we studied the outcrops of the overlaying Vermelah Formation (caprock and source rock), which were found in Barra Do Dande and other nearby areas. Studies from a number of wells have observed that the anhydrite occurrence is highly variable and unpredictable.
