Controls on the facies and architecture evolution of a fan delta in Qinghai Lake, China

Abstract

Deltaic sedimentary systems form the most favorable hydrocarbon reservoirs in continental faulted lacustrine basins, and their types and controlling factors directly affect the distribution of hydrocarbons. The systematic study of typical modern delta deposition provides significant guidance regarding the distribution of oil and gas reservoirs in the subsurface. For this reason, the Heima River delta in Qinghai Lake, which features multiple sediment sources and clear sedimentary evolution stages, was selected for this research. A detailed study of the sedimentology and architectural characteristics of the Heimahe delta in Qinghai Lake was conducted. A total of 4 types of gravel facies, 4 types of sand facies, and 2 types of mud facies were identified. This study also focuses on recognizing the architectural elements within channels and bars. The delta plain features debris-flow, switched, and migrated channels and vertical and bilateral aggradation bars. The delta front features migrated and filled channels and bilateral and lateral aggradation bars. Twenty-two representative outcrop sections were selected. Detailed observation and analysis of these sections revealed three stages: the progradation to aggradation (PA) stage, in which the deposits show evidence of sigmoid-type and coarse-grained sedimentation; the retrogradation (R) stage, which is characterized by imbricated regression; and the aggradation to progradation and degradation (APD) stage, which is characterized by a terraced-stepping, progression stacking pattern. Based on the integrated analysis of the sedimentary environment, outcrop lithofacies associations, architecture stacking patterns, fossils and bioclasts, we identified diverse depositional associations and constructed a sedimentary evolution model of the depositional system in this area. We suggest that the depositional system transitioned from an early single-provenance gravel-rich fan delta to a multi-provenance mud-rich delta and that two factors mainly controlled the transition: the southern boundary fault activity and lake level variations. The contemporaneous activity of the fault increased the accommodation in the low-stand systems tract, which resulted in continuous coarse-sediment deposition.