Lacustrine to fluvial depositional systems: The depositional evolution of an intracontinental depression and controlling factors, Lower Cretaceous, northern Tarim Basin, northwest China

Abstract

The Lower Cretaceous (Baxigai and Bashijiqike Formations) of the northern Tarim basin contains over 1000 m of siliciclastic fluvial-deltaic deposits and is of great significance for oil and gas exploration. Based on integrated cores, outcrops, well-logs, and seismic data, the sequence stratigraphy, depositional architecture and sedimentary infilling process from a shallow lake basin to a braided fluvial-dominated basin, as well as their controlling factors, were documented. Four major depositional facies, including lacustrine and braided-river delta in Baxigai Formation, braided river, and sheetflood, sand-bed river in Bashijiqike Formation, were identified based on the analysis of lithofacies and facies associations. Four third-order sequences（SQ1 in Baxigai Formation, SQ2-4 in Bashijiqike Formation）bounded by regional unconformities are recognized in the deposits. SQ1 consisting of deltaic deposits can be classified into Lowstand (LST) + Transgression (TST) and Highstand (HST) systems tracts based on recognition of regressive-transgressive cycle. The third-order sequences in Bashijiqike Formation consisting of fluvial deposits can be divided into Low and High Accommodation Systems Tracts （LAST and HAST） according to the degree of channel amalgamation. Sheetflood, sand-bed river deposits developed in LAST, and is characterized by highly amalgamated channels; the HAST is dominated by braided-river and floodplain deposits, with a higher floodplain ratio and more isolated channels. The tectonic uplifts at the early deposition of Bashijiqike Formation caused the overcompensation of sediments, and thus the depositional system shifted from deltaic to fluvial. The unconformity at the base of the Bashijiqike Formation was attributed to tectonic uplifts occurred in the study area. The cycle of arid to semi-arid paleoclimate led to the periodic changes from sheetflood, sand-bed river in LAST to braided-river in HAST of Bashijiqike Formation. The delta front sandstones comprise the potential favorable reservoirs in the study area.