Sedimentary facies and sequence stratigraphy of the Silurian at Tabei uplift, Tarim Basin, China

Abstract

Sequence stratigraphy division and comparison of the Silurian in Tarim Basin were a hot research field in oil industry and academia. However, basic geological problems limited the exploration needed for further research. In this paper, 21 lithofacies and 5 facies associations were identified based on the grain size of sediments, sedimentary characteristics, and bioturbation conditions: (1) fluvial-dominate delta front facies association; (2) tidal flat facies association; (3) tidal channel facies association; (4) offshore-transition facies association; (5) shoreface facies association. The seismic, outcrops, and logging data were involved to divide the Silurian (including upper Ordovician Tierekeawati Fm.) at Tabei uplift into five sedimentary sequences. SQ1 (Tierekeawati Fm.) is mainly characterized by tidal flat facies association, while delta front facies association locally develops; SQ2 (the lower Kepingtage Fm.) generally consists of offshore-transition facies association; SQ3 (the upper Kepingtage Fm.) is mainly characterized by shoreface and delta front facies association. For SQ4 (Tataaiertage Fm.), the transgressive system tract (TST) is dominated by shoreface facies association, while the fluvial-dominate delta facies association widely develops in highstand system tract (HST). SQ5 (Yimugantawu Fm.) is mainly characterized by tidal flat facies association. From SQ1 to SQ2, an overall sea level transgressive process is shown, while an overall sea level regressive process is found from SQ2 to SQ5. The results are consistent with the progradation and regression trends of large regions reflected by sequence framework pattern. As to SQ3 sequence, TST and HST sandstones are the main reservoir intervals in the Silurian. Hercynian movement led to the strong uplift and extensive erosion in the Silurian at Tabei and Tazhong uplift, and is favorable to the formation of strata erosion unconformable traps.