Recognition of Marginal to Shallow Marine Deposits in Shahejie Formation (Palaeogene) Dongpu Depression and Its Reservoir Significance

Key words: 
• Dongpu depression /
• Shahejie Formation /
• ichnology /
• marine transgression sequence /
• oil reservoir

Abstract: Marine deposits in Shahejie Formation of Dongpu Depression whether it is true or not remains a question. However, Ichnology integrated with sedimentology and palaeontology could be a useful tool in solving this problem. In Well Kai 31, marine regression records indicated by flaser bedding (Well depth 3 553.30 m) followed by double clay drapes(3 552.70 m) and hummocky cross-bedding as well as plant roots(3 551.50 m) showing the environment from foreshore, shoreface shift to backshore. Another marine regression sequence existed in Well Xinhu 4 where marine trace fossils of Diplocraterion, Palaeophycos, Terebellina (2 969.10 m, 2 966.40 m) are also covered by plant roots(2 931.93 m) indicated the sea level fall. In contrast to marine regression records marine transgression sequence recorded by lenticular bedding, plant roots, horringbone cross bedding from Well Kai 34 are interpreted as foreshore environment. Trace fossils of Thalassinoides, Macaronichnus suggesting a shoreface environment occur. Strong bioturbated beds and body fossils of coccoliths are considered as offshore (shallow marine) environment. Studies show that the environment from backshore to offshore can be recognized by means of ichnology integrated with sedimentology and palaeontology in Dongpu Depression. Studies also show that the discoutinuous surface indicating by firm-ground substrate controlled ichnofacies associated with marine transgression sequence are helpful in forming discordant trap.