Abstract:

This paper focuses on the tight marlstone reservoir in lower 3^rd Member of the Shahejie Formation in central and southern Shulu sag,Bohai Bay Basin.The reservoir is of great economic potential,and has the same characteristics as tight oil reservoir,such as reservoir-source coexisting,continuous distribution,oil-bearing but unclear boundary.Owing to its tight physical property and complex sedimentary environment,it is challenging to precisely predict distribution of the marlstone reservoir.In this paper,we presented a comprehensive geophysical workflow for prediction of tight marlstone reservoir.Firstly,the geological and geophysical features of the reservoir are studied.Secondly,multiple wells correlation and well-to-seismic calibration are carried out.Thereafter seismic-based sequential stratigraphy frameworks are established,and depositional units are identified.Then qualitative interpretation for the target reservoir is carried out by using techniques including geophysical modeling,basic seismic attributes analysis and post-stack impedance inversion.Finally,spatial and temporal reservoir distribution is analyzed with the integration of strata slicing technique,seismic attributes analysis and sequential boundary constraints.The results show that the lower 3^rd Member of Shahejie Formation can be considered as a complete third-order sequence,and the distribution range of the target reservoir becomes wider as the depositional stages changes from low system tract,to transgressive system tract and finally to high system tract.The reservoir is thickest in central sag,while it is relatively pure and the most extensive in late stage of transgressive system tract.The inner slope has well-developed faults with NEE strike,and interbedded conglomerate and marlstone, thus is a favorable area for further exploration.

Key words: tight oil, reservoir distribution, marlstone, Shulu sag, Bohai Bay Basin