东濮凹陷古近系沙河街组边缘海—浅海相识别与油气储层

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

东濮凹陷古近系沙河街组有无海相沉积久争不决。痕迹学、沉积学和古生物学综合研究,识别出那里存在海退与海进层序。海退层序在开31井3 553.30 m和3 552.70 m处分别见到压扁层理、双泥披(double clay drapes)。3 551.70 m处,出现丘状交错层理(表现为洼槽状向上弯曲平行纹理和凸起状向上弯曲平行纹理),显示前滨和临滨环境。3 551.50 m处,代表后滨环境的植物根迹出现,完成一个海退层序。另一个海退层序出现在新胡4井,从见到海相痕迹化石Diplocraterion、Palaeophycos、Terebellina开始(2 969.10 m;2 966.40 m)到出现植物根迹(2 931.93 m)结束。以上两个海退层序反映东濮凹陷沙四段沉积期,海水影响所及,仅限于边缘海的前滨和临滨深度范围。与海退层序不同,海进层序显示海水显著加深。文72-104井指示三角洲边缘海湾环境的痕迹化石Teichichnus(3 146 m),过渡带痕迹Rhizocoralliun(3 109.28 m)至滨外生物痕迹Zoophycos(3 050.10 m),记录到一次具有滨外环境的海进过程。开34井3 424 m至3 260 m依次出现透镜层理、植物根迹、透镜层理、羽状交错层理,解释为前滨环境。3 267~3 260 m由压扁层理(3 267.00 m)双向前积层理(3 266.81 m)为标志的前滨环境到具有Thalassinoides(3 266.25 m)、Macaronichnos痕迹化石所指示的临滨环境,再到生物扰动达70%并具颗石藻所代表的滨外浅海环境,结束一次海进过程。海进层序被固底底质控制痕迹相所指示的沉积不连续界面所限定,有利于不整合圈闭的形成。

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.