Geochemical characteristics of diagenetic fluid and densification model of tight gas sandstone reservoirs in Linxing area, eastern margin of Ordos Basin, China

Quasi-continuous tight gas sandstone reservoirs have been discovered widely in the eastern margin of Ordos Basin (Linxing area) within the marine-continent transitional strata in the upper Paleozoic. Differences in formation water chemical properties, water-rock reactions, and diagenetic evolutions between the Taiyuan Formation and the Lower and Upper Shihezi Formations are attributed to various degrees of densification of sandstones, which significantly restrain the exploration and development of tight gas reservoirs. This study utilizes the petrographic features, electron probe, X-ray diffraction, and chemical properties of formation water to establish the diagenetic sequence of tight sandstone reservoirs and finds that the diagenetic systems of the Taiyuan Fm. were more closed than those of the Shihezi Fms., with enrichments in Ca2+and depletions in Mg2+ and Fe2+. Here, we suggest that the dissolution of feldspars and rock fragments, the siliceous cementation, and the transformation of clay minerals are controlled by the K+/H+ ratios of the formation water. In the Taiyuan Fm., the low K+/H+ ratios resulted in a complete illitization reaction in the closed CaCl2 type formation water, and the migration rate of K+ controlled the proportion of the illite contents in the open NaHCO3 type formation water. In the Lower Shihezi Fm., the rapid consumption of H+ caused the elevation of the K+/H+ ratios, leading to the cease of illitization reaction and preservation of kaolinites and K-feldspars. In the Upper Shihezi Fm., the illitization reaction was inefficient, and smectites were transformed to chlorites. Hence, differentiated sandstone densification models are established to characterize different water-rock reaction systems. Typically, clay minerals and ferrous carbonate cements could form seepage barriers around the thick sandstone layers and promote the formation of independent closed high-pressure micro-diagenetic systems. Consequently, the overpressured reservoirs that underwent relatively weak compaction, dissolution, and cementation (Type Ⅰ) and the chlorite-enriched overpressured reservoirs (Type Ⅱ) tend to preserve partially primary pores and form secondary pores, which should be treated as the “sweet spot” target area for the further exploration of tight gas.