The Role of Paragenetic Patterns on the Quality of Clastic Reservoirs

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AAPG Annual Convention and Exhibition 2011

Abstract

The quality of clastic reservoirs is impacted by the sequence and intensity of diagenetic processes. Characteristic paragenetic patterns and porosity and permeability values were distilled for 9 compositional petrofacies recognized from 121 cases. The porosity of non-marine feldspathic sandstones depends on a balance among eogenetic clays, compaction, quartz or calcite cementation, and feldspar dissolution. Their permeability is controlled by intensity of compaction, quartz, illite or chlorite cementation. Marine arkoses-subarkoses commonly display less intense eodiagenesis and better reservoir quality, but may show stronger porosity reduction by mesogenetic calcite cementation than their continental equivalents. Non-marine quartzose sandstones (quartzarenites) typically show eogenetic Fe-oxide, smectite and dolomite, and mesogenetic quartz, calcite or dolomite, dissolution, and illite or chlorite. Chlorite rims preserve porosity by inhibiting quartz overgrowths, but reduce permeability. Marine quartzarenites show limited eodiagenesis, and mesodiagenesis similar to non-marine equivalents, with reservoir quality controlled by quartz cementation and pressure dissolution. Acid and basic-intermediate volcanic litharenites commonly display eogenetic smectite, zeolites and silica or calcite, and mesodiagenesis inhibited by intense compaction of altered volcanic fragments, implying in low porosity and permeability. Intense eogenetic carbonate cementation, dissolution and chemical compaction of aragonite and Mg-calcite grains characteristic of carbonate hybrid arenites promotes limited mesodiagenesis, and low porosity and permeability values. This same effect is also observed in carbonate litharenites (calclithites). Berthierine, glauconite, silica and carbonates form during the eodiagenesis of non-carbonate hybrid arenites. Their mesodiagenesis is commonly inhibited by strong compaction of clay ooids, peloids or intraclasts, with consequent very low porosity and permeability. The diagenesis in chertarenites is characterized by microquartz rims, inhibition of quartz overgrowths and pressure dissolution, and consequent porosity preservation. The evolution of litharenites rich in low-grade metamorphic fragments (phyllarenites) is abbreviated due to the intense compaction, promoting very low porosity and permeability. The systematic record and interpretation of paragenetic patterns offer a valuable guideline for the development of reservoir quality predictive models.

L. F. De Ros(1), K. Goldberg(1) and A. R. D. Elias(2)

(1) Geosciences Institute, UFRGS, Brazil;
(2) Petrobras UN/Rio, Rio de Janeiro, Brazil;

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