Sedimentary characteristics and development model of the bedded evaporites in the Paleogene Kumugeliemu formation, Kuqa depression, Northwestern China

Abstract

In recent years, extensive exploration has contributed to significant advancements in the geological formations of the Kuqa Depression. In particular, the Paleogene Kumugeliemu Formation exhibits the development of expansive bedded evaporites. The combination of red sandstone, red mudstone, and gypsum-salt layers holds immense potential as a reservoir and cap association, thereby offering promising prospects for oil and gas exploration. However, our understanding of the sedimentary system and model governing this formation remains limited, impeding progress in the field of oil and gas exploration. To address this gap, this study delved into a detailed examination of the stratigraphy, depositional system, and model of the Kumugeliemu Formation through the application of core samples, logging data, and seismic analyses. The primary objective of this study was to establish a comprehensive theoretical foundation for future oil and gas exploration efforts targeting the Kumugeliemu Formation. The findings revealed a distinct division of the Kumugeliemu Formation into two 3rd order sequences, each characterized by a lowstand systems tract, transgressive systems tract, and highstand systems tract. Moreover, all systems tracts were further subdivided into 14 parasequence sets. During the depositional period of the Kumugeliemu Formation, alluvial fan, delta, and salt lake deposition systems developed from the periphery to the central region of the Kuqa Depression. The alluvial fan mainly exhibited braided channel sedimentary microfacies, whereas the delta region indicated plain distributary channels, submerged distributary channels, and interdistributary sedimentary microfacies. The salt lake area is characterized by shore-shallow lacustrine mud, beach-bar, salt mud flat, and salt flat sedimentary microfacies. Throughout the deposition of the Kumugeliemu Formation, the climate of the Kuqa Depression oscillated between arid and humid. During arid periods, the depression experienced diminished water body extent, heightened salinity levels, and extensive distribution of salt and salt mud flats within the basin, with alluvial fans and delta deposits primarily confined to the basin margins. Conversely, during the humid period, the depression experienced an expansion in the extent of the water bodies, leading to a decrease in salinity levels. The distribution of salt flat and salt mud flat deposits within the basin diminished, whereas alluvial fan and delta deposits advanced extensively across the basin. The establishment and spatial arrangement of the sedimentary system within the Kumugeliemu Formation were governed by a complex interplay of certain factors, such as the semi-closed paleotectonic pattern, paleoclimatic conditions of both dry and wet alternation, and the salt source supply brought by the transgression of the Paleo-Tethys Ocean. The findings of this study offer not only a valuable reference framework for the prospective exploration of hydrocarbon resources within the Kumugeliemu Formation of the Kuqa Depression but also potential insights into the evolution of similar depressions in other basins.