Abstract

A regular sequence of mineral paragenesis is observed in low-permeability (‘tight’), very fine-grained sandstones of brackish marine to fresh water origin in portions of the Cretaceous of the Rocky Mountains, North America. From pore throat to pore centre in these rocks, the common authigenic sequence is illite (pore throat), kaolinite, silica (pore centre), and siderite (pore centre but after silica). Associated fine- to medium-grained sandstones exhibit a sequence of quartz followed by kaolinite plus traces of calcite from pore throat to centre.

Comparison of Gibbs' free energies of formation calculated by the method of Tardy & Garrels show values decreasing in the same order as paragenesis (illite, kaolinite, silica, siderite) observed in the low permeability very fine-grained sandstones. Diagenetic sequences in the coarser sandstones do not match the order of free energies of formation. This implies that early precipitation in very fine-grained sandstones probably occurs under nearly static conditions, whereas constant fluid throughput in coarser grained rocks leads to a markedly different diagenetic sequence. Thus, two potential sandstone reservoirs, one coarse and one very fine, commonly will have different authigenic mineral suites though their original pore waters were broadly identical.

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