Occurrences, Distributions, Origins and Forming Mechanisms of Calcite Cements in Upper Triassic Yanchang Formation Sandstones, Ordos Basin

In view of the important influence of calcite cements on the physical properties of sandstone reservoirs, their material sources and formation mechanisms have always been a research hotspot but remain unresolved. Taking the sandstone reservoir of Yanchang Formation of Upper Triassic in Ordos Basin as an example, this paper studies the occurrences, distributions, material sources and formation mechanisms of calcite cements. The Yanchang Formation sandstones, according to the color property of the cores, are divided into the grey sandstone and the grayish brown sandstone. The calcite cements in these sandstones, based on their occurrences by optical microscopy, can be divided into three groups: the disseminated calcite cement in the grey sandstone, the patchy calcite cement in the grayish brown sandstone, and the calcite vein in the fracture. In a sand body, the grayish brown sandstone with the patchy calcite cement is surrounded by the grey sandstone with the disseminated calcite cement. Although the occurrences of these calcite cements are different, all of them originate from the adjacent mudstones during the compaction. The patchy calcite cement forms by the episodic fluid from the adjacent muddy sediment with the fracture acting as the mass transfer path and subsequently the calcite vein fills in the fracture. Then the disseminated calcite cement precipitates with the steady flow in the contact of the nearby muddy-sandy sediments. This investigation of the calcite cements can well explain the varieties of their occurrences and distributions and compositions in Upper Triassic lacustrine Yanchang Formation sandstones, which is beneficial to confirming the information of calcite cements in other lacustrine sandstones.