Ferroan dolomite cement

Available drill cores from the 0-35 well show very low thin-section porosity (<5%). Advanced mechanical compaction, including shale-clast deformation, together with ferroan dolomite cement and quartz overgrowths, has occluded most of the porosity. 

Internally complex, ferroan dolomite-cemented fractures at Ballycastle are closely associated with regional normal faulting and are genetically linked with cataclastic textures typical of brittle deformation in porous sandstones. Cemented fractures described in this paper are distinguished from the principal slip planes ( faults ) on the basis of minimal displacement (centimetre scale at most), and the definition encompasses the tectonodiagenetic products of initial cataclasis, cementation of the... 

It is noteworthy that there is very little evidence of dedolomitization in the studied sand body at Ballycastle, in marked contrast to that described by Wang (1992) from reddened sandstones in the same succession. Given the interpretation that fracture-related cementation was Late Carboniferous in age, a possible implication is that flushing of unconformity-sourced oxidizing meteoric fluids in the Latest Carboniferous/Early Permian was impeded by the structural-diagenetic compartmentalization of the sand body, compared with those in which ferroan dolomite cement was more homogeneously distributed. 

Internally complex, ferroan dolomite-cemented fractures are a prominent feature within a fluvial sand body at Ballycastle, on the margins of the Rathlin basin in northeast Ireland. The cemented fractures display a tight modal orientation that is coincident with the dominant local normal faulting trend, and are interpreted to have formed in the same tectonic regime. However, there is no clear trend in the spatial distribution or width of the fractures with respect to a fault plane situated at one end of the outcrop. 

Dolomite cementation has had a minuscule impact on the net/gross ratio in the studied sand body, but will have severely partitioned it in terms of potential fluid throughflow. Preservation of the dolomite and lack of reddening, in contrast to other formerly ferroan dolomite-cemented sandstones in... 

Choquette, P.W. (1971) Late ferroan dolomite cement, Mississippian carbonates, Illinois basin, USA. In Carbonate Cements (Ed. Bricker, O.P.). Johns Hopkins University Studies in Geology, 19, 339-346. 

The Kuparuk A formation is characterized as very fine-grained, vertically stratified, imbricated sandstones with relatively low permeability (horizontal air permeabilities on the order of tens to hundreds of millidarcies). They are interpreted as sand and mud deposits from storm-driven waves and currents deposited in a lower shoreface setting. The Kuparuk A sands consist of six separate sandstone bodies (Fig. 2), A1-A6 (base-to-top), that are interpreted to represent the erosional remnants of shorefaces. These remnant shorefaces are oriented generally SW-NE and step basinward, i.e. prograde, to the SE (Masterson Paris 1987). Ankerite (ferroan dolomite) cement locally occludes porosity, particularly in units A3 and A4. 

Ferroan dolomite precipitation is also prominently localized around partially dissolved detrital K-feldspars (Fig. 7B). Localization is not strictly within the volume formerly occupied by the K-feldspar, but rather is crudely centred on the feldspar, extending also into the surrounding pore space. Ferroan dolomite clearly postdates quartz cementation. In a few samples there is petrographic evidence suggesting that ferroan dolomite pre-dates the formation of the late calcite. 

Fig. 3. Photomicrographs of (A) microcrystalline non-ferroan dolomite at the very top of a sandbody with partial replacement of detrital silicate grains, and (B) grain-rimming quartz cement (Q) and pore-filling ferroan dolomite (DOL) enclosing the quartz cement. Remnant porosity (0) is minor and occupies pore centres. Scale bars 200 pm.

The absence of dolomite cement at sandbody bases and its abundance at sandbody tops, the reported organic carbon isotope signal in the rhombic ferroan dolomite and the mixture of pedogenic dolomite textures and burial diagenetic textures in the sandbodies suggests that options 1 and 4 together are probably responsible for, the distribution of dolomite in the Chaunoy sandbodies. 

Volumetrically minor authigenic ferroan dolomite is present locally as overgrowths on distinctive cores of partially dissolved detrital dolomite (Fig. 5). Prominent zoning of Fe and Mg in these overgrowths is readily observed on back-scattered electron images. Dolomite precipitation clearly precedes the formation of calcite cement. 

Fig. 5. Authigenic ferroan dolomite overgrowth (o) on a core of fractured detrital dolomite (d) from the Bismantova Formation. Overgrowth development preceded precipitation of calcite cement. Similar dolomite overgrowths are observed in the Borello Formation. Back-scattered electron image.

Dolomite cements include minor non-ferroan, early ferroan, late ferroan and replacement dolomite. 

Early ferroan dolomite occurs in an 18m thick fine-grained unit of the Hibernia Sandstone in well B-27, where in thin zones (<1 m thick) it occluded all available pore space and replaced detrital quartz. The early diagenetic nature of this cement (crystal size 30-200 pm, average 50 pm) is indicated by a high minus-cement porosity (>30%) and the lack of quartz overgrowths or other cements. 

Replacement dolomite forms euhedral to subhedral rhombs up to 85 pm in diameter in the clay matrix of sandstones and in early ferroan calcite cement. 

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. 

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. 

Stable isotope analysis of intergranular, vein-fill and poikilotopic dolomite cements produced very similar results (Fig. 12). Overall, dolomite 5 0 values fall between -3.8 and -0.9%o pdb and 5 C values between -4.2 and -9.5%o pdb. The bulk of the data display a crude positive covariance, albeit with several outlying points. However, no consistent trends were detected across individual cemented fractures. Three samples of ferroan dolomite pseudomorphing gastropods in the overlying... 

Carbonate cement/replacement in sandstones of the Denver Basin is most commonly ferroan calcite based on staining techniques. Dolomite may occur, but is volumetrically unimportant except in the Fox Hills Formation where dolomite (mean of 6.7vol%) is more common than ferroan calcite (mean of 4.5vol%). Ferroan calcite averages 2.8vol% for sandstones of the Middle Pierre Shale compared with less than 1 vol% for dolomite. Sandstones in the two Codell cores are quite different in terms of the amount of ferroan calcite cement, but average 2.1vol% with no dolomite. The Dakota J Sandstone has only trace amounts of ferroan calcite and dolomite cement. The Lyons Sandstone contains trace amounts of calcite and no dolomite. 

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