FORMATION OF DIAGENETIC TRAPPIND RESERVOIR OF VAN CHANG FORMATION.IN NORTHERN SHAANXI AND ITS SIGNIFICANCE

The Upper Triassic Yan-Chang Formation.in the region of Zichang,Ansai, and Wuqi counties, the south of Shan-Gan-Ning Basin is a set of deltaic clastic sediments of interior fresh-water lake basin with a thickness of 1, 000-1,500m.The oil-rich Yan-Chang Fm. is one of major source reservoir beds in the southern Basin?although its intensive diageneses, extremely low porosity and permeability have led to its poor production. In recent exploring in Ansai area, the production of several wells has risen to 5-47Ton./day,and the porosties and permeabilities of productive strata Yan-Chang Fm.C6 sandstone, are relatively high. In the present paper author concentrates the attention to sedimentary facies and diageneses of Ce sandstone and considers that in the study region C6 reservoirs formed under diageneses are diagenetic trapping reservoirs, The secondary porosity growth due to dissolution of laumontite cements in sandstone heightened the porosity and permeability of Ce sandstone. The chief conclusions followed have been obtained. 1. The diagenetic processes and porosity evolution of C6 sandtone, demonstrates that,on the base of deposition, the diageneses are the principal geological agent that resulted in the regional low permeability of C6 sandstone; the creating and dissolving of laumontite cements during diageneses brought, in turn, on the occurring of C6 laumontite secondary porosity sand bodies and diagenetic tight zones, The formation of the Ce diagenetic trapping, laumontite secondary porosity sandstone reservoirs discovered in this region underwent a complex diagenetic process as mentioned above. The significance of the conclusive point is that there is some promise of success on searching for relatively high permeable trapped reservoirs in a low permeability reservoir beds region with lacking of structural trap. 2. The formation and distribution of laumontite sand bodies secondary porosity sandbodies,and diagenetic tight zones in Yan-Chang Fm.were mainly cont-trolled by source and facies of sediments associating with an available regular-ty. pressure solution-secondary enlargement type of diagenetjc tight zone occur-ed along the transitional belt between delta front and delta plain distributary Channel; Laumontite-cementing type of diagenetic tight zone appeared common-nly along theside edge zone of delta front sand bodies; and secondary porosity snnd bodies derived from dissolving of laumontite cements grew in the core part of sand bodies. Based on the studing of sand bodies diageneses, combining with analysis of source and facies of sediments, author has made a prediction on distribution of permeable sand body araes which have been conformed by the recent explorings and drillings. 3. The reaction of pore water on abundant plagioclase in C6 sand bodies compounded laumontite cements during a period of temperature lower than 80C.The vjew that laumontite occurs under a condition of high tempperature (190°C-230°C) and the appaerance of laumontite suggests a low metamorphic environment and a deadline for lrquid hydrocarbon existence has prevailed for a long time throu ghout the geological field, but it is an one-sided view. On the contrary, the creation and dissolution of laumontite jn Yan-Chang Fm.of the study region had played an extreme important role in formation of reservoirs. Moreover, a consider able amount of reserve and production have been acquired from this kind of reservoirs.