Study Of Sedimentary Systems And Reservoir Evaluation Of The Yingcheng Formation In Baojia Sub-sag Of Dehui Depression

Cretaceous Yingcheng Formation in the east of the Dehui Depression was studied using drilling,well logging and 3D seismic data and theories of sequence stratigraphy,sedimentology and reservoir geology to well understand the complicated structures,sequence stratigraphy,sedimentary characteristics and depostional system,reservoir characteristics.Some progresses have been achieved as following aspects.Firstly,the sequence stratigraphic framework of the Yingcheng Formation in Baojia Sub-sag was established by combining well with seismic.Based on sequence stratigraphic interface identification,the Yingcheng Formation(K₁yc)is divided into two second-order sequences and six third-order sequences.As a result,the first and second member of Yingcheng Formation were composed as one second-order sequence and three third-order sequences respectively.Considering the low resolution of seismic data and the difficulty in identifying of slope break,only two system tracts are identified,including the LST-TST and the HST,for every third-order sequence.Secondly,on the basis of the mapping principle called"two-step calibration and three-step combination",the identification of sedimentary systems and their spatial and temporal prediction are realized.Using the restoration of residual stratum thickness,the thickness of intrusive rock was removed,and the amount of denudation in the denudation zone was restored,which realized the restoration of the original sedimentary stratum thickness and the reconstruction of palaeo-geomorphology.In addition,the study on fault combination pattern and its sand control law was carried out.By calibrating core facies and logging facies(two-step calibration)and combining faulting pattern,paleo-geomorphology and seismic attribute data(three-step combination),the sedimentary system distribution and sedimentary evolution of system tracts in six three-order sequences were compiled.The eastern and western provenance systems developed in the early and middle periods of first members of the Yingcheng Formation,and the eastern provenance was reduced by the emplacement of western ejecta in the late period.After entering the second member of the Yingcheng Formation,the south sub-sag disappeared and the north sub-sag subsidence center moved further to the north.In the early stage,the provenance of the gentle slope in the southwest and northwest were relatively developed.After the middle stage,the basin expanded and the scale of the sedimentary system in the south and north gentle slopes expanded.Finally,the petrological characteristics and reservoir development rules are discussed in study area.It is determined that the reservoir in this area is mainly composed of pebbly sandstone and tuff pebbly sandstone,which are poorly rounded and transported by both gravity flow and traction flow.Diagenesis and structural fractures played key roles in reservoir quality in the Yingcheng Formation when it was buried in the depth more than 2500m with the porosity of less than 10%and the permeability of less than 0.1md.According to the analysis of the dominant factors,the reservoirs are divided into three categories and the favorable reservoir development areas are predicted.This study put forward the technology of the sedimentary facies in the reformed depression,which provided important support for the exploration of the natural gas in the Yingcheng Formation of Dehui Depression.