264 / 2019-07-01 10:23:19

A Special Sedimentary Pattern on Bar in Shore-shallow Lake: A Case from X Wellblock in Early Pliocene, Neogence in Qaidam Basin, China

Sedimentary pattern on bar in shore-shallow lake, high evaporation, Salinized lake, Qaidam Basin, Early Pliocene

During early Pliocene, Neogene (N21), the west Qaidam Basin was with a topography declining eastward, so lots of clastic from west mountains were transported to Zhahaquan area to build up a series of large-scale deltas and beach-bars in shore-shallow lake. X wellblock just covers a part of a large compound beach-bar.
On principle of the same bottom height, the similar logging curve shape and the law of low water and high oil in one single sand body, single bars are correlated from well to well in isochronous stratigraphic framework, and then are projected onto a plane to draw single bar sedimentary microfacies map of all stages.
Single bar’s characteristics is as followings from sedimentary microfacies maps: (1) Most bars’ strike is SE, WE or NE by rose diagrams of single bar strike; (2) Bar’s strikes become diversity eastward in plane; (3) The strikes also become more diversity from shallow to deep water in vertical direction; (4) Some bars are characterized by not funnel-shaped but typical bell-shaped in GR logging curve.
The beach-bars are deposited just under the following background: (1) The supply of source from the west mountains is very enough to form lots of large-scale deltas and beach-bars; (2) X wellblock’s gradient is so gentle that less than 0.5° based on paleogeomorphology from seismic inversion; (3) It is a salinized lake due to high evaporation and acrid climate in N21.
On flat terrain, high evaporation induces the lake-level to fall and shoreline to migrate in long-range toward lake center, which means that previous deltas will be migrating eastward and that lots of branch channels and mouth bars are built on the way of delta migration. On the contrary, occasional precipitation induces the lake-level to rise and shoreline to migrate in large-range toward the shore, so the branch channels and mouth bars will be flooded by transgression. To different extent, the wave during transgression will transform the branch channel and mouth bar of delta into beach-bar of shore-shallow lake. Shoreline migrating westward and eastward frequently will build lots of bars in shore-shallow lake.
When the water deepens due to environment change in vertical direction (or accessing half-deep lake eastward in the plane), the wave transformation to branch channel and mouth bar will be weakened to some extent, so the diversity of bar strike will be preserved more. On the contrary, in shallow water the transformation of wave will be improved, and the diversity will be suppressed. In addition, some branch channels transformed weakly will keep more original sedimentary sequence in vertical the other bars, and show bell-shaped in logging curves.
This special pattern just like tidal flat deposition is built under the background of enough supply, high evaporation and almost flat terrain, and well explains the single bar’s characteristics above-mentioned.