DIAGENETIC EVOLUTION OF MONTMORILLONITE AND PRIMARY MIGRATION OF PETROLEUM

In this paper the author presents the evolution features of montmorillonite to illite during the diagenetic process of the Cretaceous shales of Songliao Basin and their effects on primary migration of petroleum. 1. During the diagenetic process, the conversion of montmorillonite into illite can be characterized by a high speed at the early stage and a relatively stable one at the late stage. 2. In Songliao Basin, the depth at which the "negative anomaly" occurred well matches with the top depth where montmorillonite transformed into illite, resulting from the diagenetic changes of montmorillonite. The occurrence of the "negative a-nomaly" marked the beginning of the conversion of montmorillonite into illite and the primary migration of petroleum in Songliao Basin. 3. The rapid conversion of montmorillonite into illite provided a favourable condition for hydrocarbon migration. The vertical variations in mudstone porosity, the lower maturation of crude oil and the excellent consistency between the rapid conversion stage of montmorillonite into illite in main source beds and the hydrocarbon migration stage in Songliao Basin have indicated that the rapid conversion stage of montmorillonite into illite is also the most important stage of hydrocarbon migration in Songliao Basin. 4. The study on the porous features of shale and the vertical variations in oil- water properties has shown that the conditions for hydrocarbon expulsion from source beds vary at differant conversion stages of montmorillonite into illite.At the early rapid evolution stage,because of the greater variations in volumes of mineral grains, more rapid dehydration, larger porosity and variations of shales, crude oil could migrate in any status. Thus the rap compaction process of shale was also the resqueezed-out process of oil-gas-water from pores and fractures of shale. At the late relatively stable stage, the source beds were further compacted, the shale porosity decreased and the rigidity increased,the microfractures developed.At this time, specific gravity and viscosity of oil have become lower than those of water, the water within shale existed in the form of irreducible water,while crude oil migrated in the form of oil phase along fractures and faults. 5. Sufficient pressure gradient and favourable path are the key to primary migration of hydrocarbon. However,since the source beds generally have abnormal pressure,the hydrocarbon migration from the source beds mainly depends on the suitable le path for migration.The influence of the hydrocarbon migration by the diagenetic olution of moutmorillonite is shown in the first place in the shale porosity. The rapid conversion of montmorillonite into illite, not only produced large quantity of interstitial water but also provided some suitable path for hydrocarbon migration. In this paper, the author puts emphasis on the study of variation in the relationship between porous features of source rocks and oil-water at different evolution stages,while estimating the influence of conversion of montmorillonite on hydrocarbon migration. And, there are now many obstacles in this study and some problems remain uncertain.