Paleo-Environment Control on Lake Deltaic System Evolution: A Case Study from the Cretaceous Kapushaliang Group, Northern Tarim Basin

Abstract

The Cretaceous Kapushaliang Group of the northern Tarim Basin contains more than 800 m delta-lacustrine deposits, essential for oil and gas exploration. Substantial research has been carried out previously on the deltaic system in the Kapushaliang Group, but the paleo-environment and its influence on the delta evolution are poorly understood. Herein, we determine deltaic system distribution within sequence stratigraphy and investigate how the paleo-environment controls delta evolution. Sequence boundaries are identified through unconformities in the seismic profiles and abrupt changes in the well-logging curves, identified five 3rd-order sequences (Sq1-Sq4 in the Yageliemu and Shushanhe Formations and Sq5 in the Baxigai Formation). Facies associations and delta distributions are determined using well-logging profiles, cores, outcrops, relative sand content, and seismic attributes. The paleo-environment is documented by mudstone sample testing in the Kuchehe outcrop. The gradual aridification climate and decreased lake levels during the early stage of the Kapushaliang Group led to an aggradation pattern delta in the southern part. In addition, the uplift of the South Tianshan Mountains resulted in increased accommodation and exhibited a retrogradation pattern delta in the northern part. During the early stage of the Kapushaliang Group, increased lake level caused delta retrogradation before maximum transgression. The delta shows a progradation pattern after maximum transgression in the southern part of the study area. Increased accommodation and stable sediments resulted in delta retrogradation in the northern part. In the late stages of the Kapushaliang Group, accommodation gradually decreased in the southern and northern parts. The dry condition accelerated the water evaporation, and increased sediment supply led to delta progradation in the southern part and aggregation in the northern part. The results show that the deltaic system originating from Altyn Mountain has a weak tectonic background and is more sensitive to climate and lake level changes than the delta from the South Tianshan Mountain within a 3rd-order sequence stratigraphy.

Keywords: controlling factors, Deltaic system evolution, Paleo-environment, Kapushaliang Group, Tarim Basin