Application of sequence stratigraphy to Triassic terrestrial strata in Tahe area of Tarim Basin

Based on a synthetic geological study of drilling, well logging and seismic data, core observations and geochemical analysis, it is recognized that Triassic sedimentary facies in the Tahe area of Tarim Basin include braided river deposits filling erosional valleys, and sublacustrine fan, canyon and delta facies. Braided river deposits filling erosional valleys are dominated by coarse-grained lithic quartz sandstone with oblique bedding, and represent the most important sedimentation type of sandstone in the study area. Sublacustrine fan and canyon facies are mainly distributed in the Middle Oil Member. Most delta sediments are deposited in highstand system tract (HST). Because of frequent changes in base level, delta sediments are commonly eroded and rarely preserved. Sedimentary cycles are clearly reflected by lithology, sedimentary structures and well logging data, and are closely related to the changes in lacustrine level. In accordance with the basic principle of sequence subdivision, seven type-I boundaries can be recognized in Triassic strata and six type-I sequences are subdivided correspondingly. In general, lowstand system tract (LST) is well developed within stratigraphic sequences and forms the main body of reservoir sandstone in this area; highstand system tract (HST) and transgressive system tract (TST) are often eroded by upper sequences or missed. Although various factors have different influences on terrestrial sequence stratigraphy, the classical sequence stratigraphy theory proposed by VAIL can be applied to terrestrial strata.