Diagenesis of tight sandstone reservoirs in the Upper Triassic Yanchang Formation, southwestern Ordos Basin, China

In order to better predict the petrophysical properties of the tight sandstone reservoirs in the Upper Triassic Yanchang Formation, Ordos Basin, China, the nature of the diagenetic system involved was investigated using a suite of petrographic and geochemical techniques including thin section and X-Ray Diffraction analysis, scanning electron microscopy, MAPS mineralogy, Cathodeluminescence, electron probe microanalysis and fluid inclusion analysis on a set of selected tight sandstone samples. The sandstones investigated are texturally mature but compositionally immature with an average framework grain composition of Q(32)F(41)L(27). Authigenic quartz, calcite, and chlorite coatings are the major cements, while feldspars are partially dissolved. Two groups of quartz cement are present in the tight sandstone reservoirs. Smectite to illite conversion provided silica source for the first group of quartz overgrowth (Qo-I), while the silica source for the second quartz overgrowth (Qo-II) was mainly originated from feldspar dissolution. Calcite cements consist of two types, namely Ca-I and Ca-II. The Ca-I calcite cement contains no chlorite coatings and have higher concentrations of Fe2+ and Mg2+, whereas the Call calcite cement developed over the chlorite coatings. Plagioclase dissolution appears to be always accompanied by calcite cementation, while K-feldspar albitization forms euhedral albite locally. The burial diagenesis processes were, in most cases, not episodic but occurred as slow adjustments in response to increased burial depths and temperatures. In all the studied diagenetic minerals, elemental distributions appear to be strictly constrained by the interaction of authigenic minerals involved, indicating that the chemical reactions during the diagenesis comply well with the general principle of mass balance. The diagenesis in the Upper Triassic Yanchang Formation tight sandstones was thus mainly developed in a closed geochemical system, where mineral dissolution and precipitation are approximately balanced, and diagenetic fluids were not affected by external sources significantly. Mechanical compaction has played a more important role in destroying the primary porosities of the Upper Triassic Yanchang Formation than the cementation.