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Origin and enrichment of diagenetic calcite in Upper Triassic Xujiahe sandstones, Northeastern Sichuan Basin


Calcite is one of the dominant cements in Xujiahe sandstones, northeastern Sichuan Basin. It can be concentrated in certain zones as cements and replacing minerals to form tightly calcite- replaced and cemented intervals, and has an obvious damage to the sandstone quality. In this work, we conducted petrology and logging geology to characterize the distribution of tightly calcite- replaced and cemented intervals, and used the geochemical data, including whole rock X-ray diffraction, stable carbon and oxygen isotopes and trace elements, to discuss the formation mechanisms of diagenetic calcite enrichment. The calcite-replaced and cemented intervals are mainly distributed in the west part of the study area usually being observed in the middle of the thick sand body with a thickness of 1~2 m. The occurrence frequency of the calcite-cemented intervals show a gradually decreasing trend from source rocks of the three member of the Upper Triassic Xujiahe Formation (T3x3) to upper and lower formations. The distribution of the calcite-cemented intervals are controlled by both depositional framework and rock component. The depositional framework controls the zone of calcite cements enrichment, while rock component controls the interval of calcite cements enrichment. Diagenetic calcite are mainly formed in eodiagenesis B stage(with temperatures of 60-80°C), corresponding with the main period of organic acid generation. Carbonate minerals are abundant in the T3x3 source rock. During the organic acid generation stage, the fluids with abundant Ca2+ and organic acids, expelled from the T3x3 source rock, flowed into adjacent sandstones, which provide the Ca2+ and HCO3- source for calcite cementation and replacement, and finally control the distribution of the calcite-cemented intervals. It is noted that not all sandstones adjacent to T3x3 formed as tightly calcite-cemented intervals, and calcite cement was selectively concentrated in feldspar or/and carbonate rock fragments abundant layers. It indicates that only sandstones with high alkaline mineral contents can effectively buffer the organic acid, allowing calcite to precipitate as both replacing minerals or cements, and thus finally controlling the location of calcite-cemented intervals.