Fault-Related Hydrothermal Diagenesis, Cretaceous Pinda Group Carbonates and Siliciclastics, Offshore Angola
The Albian Pinda Formation carbonates and siliciclastics of Block 2, offshore Angola, were deposited on the newly rifted, structurally-active African margin during a period of high thermal subsidence. The best fields are composed of shallow marine carbonates that were dolomitized and subsequently leached. In the northern part of the Block, the Pinda overlies thick salt and the fields form in rafted blocks set up by listric faults rooted in the salt. The Pinda there is predominantly composed of limestone and is only dolomitized in areas where basement-rooted wrench faults visible on seismic cut through the salt. In the southern and central parts of the Block, there is no underlying salt and fields occur on fault-bounded basement highs. In this area the shallow marine Pinda carbonates are almost always dolomitized and cut by numerous basement-rooted extensional and strike-slip faults that were active during and just after deposition. The lack of dolomitization in areas where thick salt underlies the Pinda and common dolomitization in heavily faulted areas where there is no underlying salt or where basement-rooted faults cut through the salt strongly suggest that dolomitization is dependent on upward hydrothermal fluid flow through faults from below the formation.
<p>Geochemistry and fluid inclusion analysis show that most of the dolomite and authigenic feldspar found in the siliciclastics formed at temperatures between 60-160C from hypersaline (12-24 wt%), Fe- and Mn-rich fluids. The dolomitization and feldspar precipitation (and most of the subsequent diagenesis) occurred prior to oil migration which occurred when the Pinda was buried to a temperature of ~85C, making the majority of the dolomite hydrothermal in origin. Subsequent dolomite and feldspar dissolution greatly enhanced porosity and permeability. Solution-enlarged fractures and associated patchy dissolution fronts suggest that the corrosive fluid was also fault-derived. This dissolution was followed by precipitation of pyrite and anhydrite which plugged many fractures and locally decreased porosity and permeability. The abundance of sulfide and sulfate suggest that the earlier corrosive fluid may have been sulfuric acid, likely sourced from underlying units. Most of this diagenesis appears to have occurred as a result of evolving brine chemistry and upward hydrothermal fluid flow through faults and possible mixing with in situ fluids.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009