Abstract
Deep-lacustrine sediments deposited by gravity flows are important, though low-permeability constituents of tight oil reservoirs in the Late Triassic Yanchang Formation (southern Ordos Basin, China). Petrographic, mineralogical, and geochemical data were investigated in order to better constrain the controls on reservoir quality. In particular, sandy debrites and turbidites have been addressed. They were significantly altered by diagenesis, due to compaction, authigenic clay formation, and carbonate and quartz cementation and dissolution. Volcanic ashes provided silica for quartz cementation and easily transformed to authigenic smectite. The negative δ13C and δ18O values of carbonate cement in the sandstones reflect decarboxylation of organic matter as an important carbonate source.
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