SANDSTONE DIAGENESIS & PROVENANCE
Roger Higgs of Geoclastica Ltd is highly experienced in thin-section interpretation of sandstone diagenetic history (porosity evolution & prediction) and provenance (basin analysis). See below for an example of the level of detail attainable.
Download Image (jpg 916
Photos 23, 24. 4686.8', PP & XN. Porous sandstone (point-counted porosity 21%). Patches of
calcite (stained pink) and dolomite cement (and/or replacive dolomite) are
present; both are remnants of decarbonatization (carbonate dissolution, producing secondary porosity), as indicated by ragged borders (B3, GH5). The dolomite is seen to be stained speckled-blue at
higher magnification (Photo 75), indicating ferroan dolomite (or possibly
ankerite; see introduction above). Dolomite and calcite are intimately
interwoven at CD/3-4 with sharp interfaces, but there are no textural
indicators of which mineral preceded which, or whether the dolomite replaced
the calcite or was precipitated separately. Angular holes in the calcite patch
at GH5 suggest dissolution of dolomite crystals. Calcite (or dolomite)
precipitation may have been followed by partial dissolution, making space for
dolomite (or calcite) precipitation, followed by further dissolution of both.
Bitumen-lined pores, including an oversized pore at L-N/4-5, are the only pores
which existed at the time of a '1st oil' charge, which separated two episodes
of decementation. Feldspar grain at D2 (twinning ofuncertain microcline or plagioclase type) is interpreted to have shrunk by dissolution, based on its embayed, 'skeletal' form and lack of visible grain contacts ('floating'); the dissolved portion may first have been replaced by carbonate. Complete dissolution of this grain would have produced an oversized pore.