Early vs. Late
Carbonate Reservoir Porosity: a Paradigm Shift
Esteban, Mateu1, C. Taberner2
(1)
Reservoir porosity is traditionally perceived
as predominantly inherited from depositional textures, with important
diagenetic modifications in near-surface environments. Commonly, burial
diagenesis is not seen as significantly altering this pattern, but only causing
a progressive porosity reduction with increasing burial depth.
However, there is consistent evidence of
substantial porosity generation or enhancement in burial environments. A wide
range of case histories display a common diagenetic pattern that is summarized
with the following stages: A) Standard carbonate cementation sequence with
increasing burial. B) Local thermobaric event, commonly associated with
hydrofracturing. C) Major corrosion event affecting late cements, stylolites,
dissolution seams, fractures and hostrocks, producing a wide range of pore
types commonly associated to “exotic” paragenesis of authigenic dickite,
quartz, fluorite, pyrite-marcasite, barite-celestite, K-feldspars, sphalerite,
galena and others. These minerals are present as traces and require a
deliberate search as “fingerprints” of burial diagenetic reactions. D) Terminal
events, with variable amounts of carbonate or sulphate cements and
hydrocarbons.
Burial corrosion may enhance remains of
early porosity, particularly in stratabound reservoirs. However, non-stratabound
reservoirs strongly argue for a predominant burial porosity. In any event, a
paradigm shift is in process. Prediction and quantification of burial corrosion
porosity requires careful study of paragenetic sequences coupled with reactive
transport models, not only at the reservoir but also in source rocks and seals.
Furthermore, correlations with selected seismic attributes appear to validate
and provide promising new perspectives on these diagenetic models in the
context of an integrated petroleum system.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California