Ferroan calcite cement dissolution

Pyrite occurs both as early and late cement in Hibernia Field. Early pyrite is present in early calcite cement, fossil fragments and siderite nodules. Late pyrite is found in intergranular pores and fractures. The association of pyrite with a porous zone in Hibernia Sandstone and its absence in adjacent ferroan-calcite cemented sandstone suggests that this pyrite formed after the dissolution of the ferroan calcite. 

Secondary porosity contributes more than half (6.7%) to the total porosity (average.of 11%, excluding tight-sandstone zones) of the Catalina Sandstone. Of the available drill cores from wells 0-35 and K-18, 45.7 and 17.5%, respectively, are completely cemented by early ferroan calcite. The difference between the two wells may be due to the fact that the sandstone beds are thicker in well K-18. Where the early ferroan calcite is absent, mechanical compaction and quartz overgrowths have reduced the primary porosity further. In K-18, the framework grains are largely coated by a micritic-calcite rim, approximately 25 pm thick, which prevented silica cementation. Dissolution of the early ferroan calcite cement contributed most of the... 

Porosity reduction due to recementation af ter the dissolution event is generally small (1-2%) and involved kaolinite, quartz, pyrite and ferroan cal-cite and dolomite cements. Locally, as much as 10% of late cements have been precipitated. Kaolinite usually reaches no more than trace amounts, although locally up to 6% has been observed. Late quartz overgrowths are negligible. Late pyrite generally does not exceed 1% however, locally it may occlude all available pore space. Late ferroan calcite cement has completely filled the available pores at a few levels, but the total porosity loss by this cement and by late ferroan dolomite is not significant. 

Late cements causing post-dissolution porosity reduction include minor kaolinite and late ferroan calcite. Late ferroan dolomite is an important cement in all porous sandstones of the 0-35 well, where it reduced porosity by 4.2% on average (ranging from trace amounts to 20%). In the diage-netic maturity classification of Schmidt MacDonald (1979a) the Catalina Sandstone is semimature. 

Fig. 21. Distribution of porosity and carbonate cements in the Catalina Sandstone (= B Sandstone), 0-35 well. The sandstone beds are largely cemented with early ( ) ferroan calcite, except for a few centimetres above shale interlayers, where cement dissolution has generated significant secondary porosity.

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