Ordos Basin (Chang 7 and Chang 6 member) has a total number of resources for 1.99 billion tons; According to the development plan of Changqing Oil field, there will be 0.8 billion tons production coming from tight oil reservoirs with permeability less than 0.3 mD and it will be a significant resource for long term production. Thus, this research is focus on analysis of the effect of diagenesis on reservoir properties and building a new evaluation method and standard for permeability less than 0.3mD reservoirs, which is different from 0.3~2.0 mD permeability reservoirs established standard.
According to microscopic water displacement test, phase permeability test, NMR, rate-controlled mercury penetration, high resolution FESEM and X-CT scan used in core samples from research area, comprehensively analyze Chang63-Chang72 reservoirs characteristics and diagenesis. The results show that reservoirs consist mostly of lithic arkose and feldspathic lithic sandstone with lower component maturity, and mainly develop primary pore with an average porosity from 8.25% to 9.03% and permeability from 0.12 to 0.14 mD, which present as ultralow porosity and permeability tight reservoirs. Through many thin section observations under microscope analyze diagenesis in the study area, which represent medium-strong compaction, medium cementation and weak dissolution. Calculate the parameters of apparent compaction rate, apparent cementation rate and apparent dissolution rate to quantitatively evaluate diagenesis, divide diagenesis rank according to classification of diagenesis intensity. Combine pore type, cements and physical property characteristics parameters, divide diagenetic facies into seven types, and build up diagenetic facies division standard in the study area. Fine describe distribution characteristics of diagenetic facies in Chang63-Chang72 reservoirs.
Applying porosity and depth relational model and analysis of diagenesis, build up pore evaluation pattern. Then get the results that tectonic environment, sedimentation, sedimentary fabric and diagenesis are four major factors controlling tight reservoir quality of Chang63-Chang72 in research area. Using the results of NMR and centrifuge experiments, establish throat radius and saturation of mobile fluid relational model. It suggests that 0.24μm throat radius is a lower limited value of effective reservoir throat. Based on cluster analysis and Fisher discriminant analysis, the study area reservoirs are divided into three types. At last, combining reservoir parameters such as diagenesis, sedimentary characteristics, and property characteristics, a new comprehensive evaluation standard were developed for Chang63-Chang72 tight oil reservoirs in Ordos Basin to help find favorable zones for petroleum accumulation.
Key words: tight oil reservoir; diagenesis; comprehensive evaluation; cluster analysis; Ordos Basin
