Sedimentological and diagenetic control on the reservoir quality of deep-lacustrine sedimentary gravity flow sand reservoirs of the Upper Triassic Yanchang Formation in Southern Ordos Basin, China

The Upper Triassic deep-lacustrine sedimentary gravity flow sandstones (Yanchang Formation, Southern Ordos Basin) form an important oil reservoir, however, it is considered as a low permeability to tight reservoir. This study combines petrographic, mineralogical and geochemical data in order to better constrain the controls on reservoir quality. Two major lithofacies types (lithofacies 1 - sandy debrite; lithofacies 2 - turbidite) were identified and their origin relating to deep-lacustrine gravity flows have been addressed. The gravity flow sandstones are significantly diagenetically altered, including mechanical compaction, authigenic clay formation, carbonate and quartz cementation and dissolution. In addition, volcanic tuffaceous dust alteration is one of the important processes during burial, which provided silica for quartz cementation and that easily transformed to authigenic smectite. It also formed poorly crystallised kaolinite and random mixed-layer I/S clays. The relatively negative delta C-13 (with an average of -2.3 parts per thousand) and delta O-18 values (with an average of -17.0 parts per thousand) of carbonate cements in the gravity flow sandstones reflect decarboxylation of organic matter as one of the important carbonate sources. The factors controlling reservoir quality relate to depositional and diagenetic processes. Depositional factors (e.g. grain size, grain sorting) had a stronger influence on lithofacies 2 sandstones compared to lithofacies 1, which made lithofacies 2 sandstones more easily compact and lose most of its primary porosity. Samples with less intergranular volume (IGV) indicate that mechanical compaction plays an important role in reducing reservoir quality. However, cementation (clay minerals and carbonate cementation) affected sandstones of lithofacies 1 to a stronger degree, heavily destroying reservoir quality while in this lithofacies dissolution (e.g. feldspar, volcanic fragments and minor quartz) significantly improved reservoir characteristics in contrast to lithofacies 2 sandstones. Quartz cements could stabilize the framework and preserve the open porosity, however, they are present with relatively low contents and have limited effect in influencing the reservoir quality. In summary, lithofacies 1 sandstones possess a slightly better reservoir potential in the study area. All these factors have great implications for tight oil exploration and production of the Yanchang Formation in the Southern Ordos Basin which can serve as an example for other tight sandstone gravity flow reservoirs elsewhere in the world.