Characteristics And Evaluations Of Dense Carbonate Cap Rock Of Ordovician Yingshan Formation In Tahe Oilfield

The quality of cap rock determines oil and gas exploring potential,tectonic movements influenced Tahe oilfield in geological history many times,carbonate rock shows great heterogeneous characteristics,therefore identification and evaluation of dense carbonate cap rock are of importance for petroleum prospection.By utilizing outcrop,borehole,core,well-logging and imaging-logging data,and combined with thin sections and micropore analysis,we are able to do the Yingshan Formation carbonate dense cap rock characteristic conclusion,in both macroscopic and microcosmic scales,as well as the vertical and lateral distributed characteristics study of dense carbonate cap rock in Tahe oilfield.Through describing dense carbonate cap rock characteristics,interpreting cap rock in individual wells and predicting lateral distributing of Tahe area Yingshan Formation,we draw conclusions as follows:(1)Based on the lithologic study of Tahe area,four dominant lithologic types of dense carbonate cap rock been recognized,including eleven sublithologic rock types.Micrite occupies the major proportions among dense carbonate cap rock,followed by arene micrite,dolomite limestone,and fine crystalline dolomite.(2)Studying depositional settings and diagenetic characteristics of dense carbonate cap rock show that dense carbonate cap rock commonly distributes in interbeach sea and intraplatform marsh land,a small amount is found between interbeach sea and intraplatform shoal,rarely not found in intraplatform shoal.Cementation,compaction and micritization increase sealing quality of dense cap rock during diagenesis process.(3)Both the porosity and permeability of dense carbonate cap rock from Yingshan Formation has extremely low value ranging from 0.22%to 1.2%,and from 0.005 to 0.05md respectively.Dense carbonate cap rock can be divided into four categories according to their different micropores structures,and the characteristic of cap rock characteristics have been summarized in the micro-level.(4)The numbers of dense layer numbers in different members vary in a large scale,as well as the individual layer thickness,while the third member within the Yingshan Formation has the thickest and best preserved individual dense layer.For instance,six individual dense layers occurs in the well S88,and the thickest layer reaches 57.2m;Influenced by multiple tectonic movements and karst solutions,dense cap rock is poorly developed in the 4^thmember of the Yingshan Formation.(5)Using seismic mean-square root amplitude attribute data and fracture density in Tahe area,we find that five dense cap rock sets of Tahe area have some similar characteristics,class III cap rock with poor sealing capacity is mainly present in northern part of the area,however classes I and II of carbonate cap rock with excellent sealing capacity commonly distribute in the eastern and southern parts of this area.