Depositional characteristics and evolution of the shallow water deltaic channel sand bodies in Fuyu oil layer of central downwarp zone of Songliao Basin, NE China

Abstract

Relatively few subsurface studies have been conducted on shallow water deltaic channel characteristics and evolution in Songliao Basin’s Fuyu oil layer. Using data from 2000-m cores and 60 wells, the shallow water deltaic channels sand bodies in the Fuyu oil layer are reconstructed. The main controlling factors over depositional system evolution are identified. Twelve lithofacies and four main facies associations are summarized. Four facies associations are identified, including (1) a simple distributary channel association, (2) type A point bar distributary channel association, (3) type B point bar distributary channel association, and (4) an underwater distributary channel association. The depositional evolution of the channels is derived from a lacustrine regressive–transgressive cycle that is driven by climate variability and tectonics, and linked to changes in relative lake-level. During periods of rapid drops in relative lake-level or rises in the semi-arid lower shallow water delta plain, channel, sand bodies are type B point bar distributary channel of laterally aggregational deposits. When relative lake level reaches to its lowest value, it results in a large-scale erosive bounding surface juxtaposed with type A point bar distributary channel sand bodies that are infilled with thick bedding in the upper shallow water delta plain. When high relative lake levels generate regional high accommodation/sediment supply ratios, the thin-bedding underwater distributary channel sand bodies are deposited in the humid shallow water deltaic front. Shallow water delta fills are capped by laterally extensive and continuous deep lacustrine dark mudstone and oil shale elements as the relative lake level rises to its highest value.