Early vs. Late Carbonate Reservoir Porosity: a Paradigm Shift

Esteban, Mateu¹, C. Taberner² (1) REPSOL-YPF, Madrid, Spain (2) Shell International Exploration and Production B.V, 2288 GS Rijswijk (ZH), Netherlands

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.