Abstract
Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China. These differences in their origins led to different sandstone distributions which control the scale and connectivity of oil and gas reservoirs. Numerous cores and outcrops were analysed to understand the origins of these sandstones. The main origin of these sandstones was analysed by statistical methods, and well logging data were used to study their vertical and horizontal distributions. Results show that the sandstones in the study area accumulated via sandy debris flows, turbidity currents and slumping, and sandy debris flows predominate. The sandstone associated with a single event is characteristically small in scale and exhibits poor lateral continuity. However, as a result of multiple events that stacked gravity flow-related sandstones atop one another, sandstones are extensive overall, as illustrated in the cross section and isopach maps. Finally, a depositional model was developed in which sandy debris flows predominated and various other types of small-scale gravity flows occurred frequently, resulting in extensive deposition of sand bodies across a large area.
Highlights
Discussion of the origin of deep-water sandstones began with ‘‘Turbidity currents as a cause of graded bedding’’ (Kuenen and Migliorini 1950)
Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China
Other scholars noted that sandy debris flows tend to be deposited in the form of lumps, which modelled as interrupted sand bodies (Shanmugam and Moiola 1997) and tongue-like sand bodies (Zou et al 2012)
Summary
Discussion of the origin of deep-water sandstones began with ‘‘Turbidity currents as a cause of graded bedding’’ (Kuenen and Migliorini 1950). Bouma (1962) proposed the ‘‘Bouma sequence’’, which summarized the vertical sequence of sedimentary structures in turbidites and became the criterion for recognizing turbidites. Based on these criteria, other scholars contributed valuable insights related to this type of deposit and improved turbidity current theory (Yang et al 2015a). In the same time frame, many depositional models were developed, such as the ancient fan model, the modern submarine fan model and the general fan model (Shanmugam 2000) All these models suggest that turbidity-related sand bodies feature fan-shaped deposits. This consensus has been widely applied in the exploration of deep-water sandstone reservoirs.
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