Formation Process and Distribution of Laumontite in Yanchang 3 Reservoir of Fuxian Exploration Area in North Shaanxi Province and the Controls of the High Quality Reservoirs

The degree of laumontites development and the status of laumontites dissolution are important indexes for the prediction of potential sedimentary belt in extra-low permeability formations of Yanchang group in Ordos basin. Laumontite is not only distributed in the deep reservoir such as Chang 6 of the northeast Ordos basin, but also extensively distributed in the shallow reservoir such as Chang 3 of Fuxian exploration area in the southeast basin. The main reason is that the Chang 3 reservoir of Fuxian exploration area has a great quantity of plagioclase. In the later period of early diagenetic stage, laumontite and the alkaline pore water which was rich in Na were interactional, and the laumontite cement was formed. Through measuring inclusions in laumontite, we find out that its forming temperature is between 85-100℃. The development of laumontite was controlled by sedimentary micro-facies and the sequences stacked of sandbody, and it was mostly distributed in the delta plain and delta front stacked of the distributary channels compound sandbody. In the early period of later diagenetic stage, laumontite were dissolved by organic acid which was generated in decarboxylation of kerogen, and a great quantity of secondary pores were formed. The total effect of the laumontites sedimentation and dissolution led to the increased porosity in the Fuxian exploration east area and the reduced porosity in the west area. The development of laumontite dissoluted secondary pore was the main control factor of the high quality reservoirs in Yanchang Formation of Fuxian exploration area.