A Late Triassic gravity flow depositional system in the southern Ordos Basin

Based on the analysis of numerous drill cores and drilling data, lacustrine gravity flow depositional systems were analyzed comprehensively in the Chang6 and Chang7 oil members (Triassic Yanchang Formation) in the southern part of the Ordos Basin. The gravity flow depositional systems in these members are made up of slides, slumps, sandy debris flows, liquefied flows, turbidity current etc, forming well developed units in the study area. Successive beds characterized by, from bottom to top, massive bedding (MB), graded bedding (GB) and horizontal bedding (HB) form well developed sequences; parallel bedding (GB) and ripple bedding (RB) are rare. It turns out that the depositional sequences are quite different from turbidites with a Bouma sequence: (1) MB represents a sandy debris flow, (2) GB deposits in a turbidity current, (3) PB and RB are deposits that were reworked by bottom currents (traction flow), and (4) HB represents a deep-water environment rather than gravity flows. Deposits in the proximal part of the subaqueous lacustrine fan consist mainly of slides, slumps and massive sandy debris flows. Deposits at the middle part of the fan are characterized by an MB-GB-HB sequence of massive sandy debris flows, graded turbidites and horizontally bedded lacustrine mudstones. Deposits at the end of the subaqueous lacustrine fan were mainly graded turbidites and horizontally bedded lacustrine mudstones (GB-HB sequence). Sandy gravity flow deposits mainly developed on the delta front and in the basin plain, extending for dozens of kilometers. They directly cover the source rock in the Chang7 oil members, which has the advantage of near-source oil accumulation. The sandstones at the bottom of each sedimentary cycle are worth further exploration because of their good reservoir properties and high oil content.