Origin and Distribution of the Laumontite in the Mesozoic Sandstones of Four Basins, Northern China

It has always been considered that laumontite forms in high temperature (more than 200oC). And laumontite is generally as a typical metamorphic mineral in many metamorphic petrology text books. However there are most of... [ view full abstract ]

It has always been considered that laumontite forms in high temperature (more than 200oC). And laumontite is generally as a typical metamorphic mineral in many metamorphic petrology text books. However there are most of laumontite filling in the intergranular pore as cement or replacing feldspars in Mesozoic sandstones of the four oil-bearing basins in northern China, such as in upper Triassic sandstone of Ordos basin, in lower Jurassic sandstone of Sichuan basin, in lower Cretaceous sandstone of Songliao basin, and in middle Triassic sandstone of Tulufan-Hami basin. The sedimentary environment of the sandstones are fluvial and delta. With measuring homogenization temperatures of the fluid inclusions in the laumontite of the four basins, it turns out that the laumontite in the sandstones of the four basins formed in relatively low temperature (60-1200C). Moreover a number of the laumontite cements are intensely dissolved by organic acid water to generate the secondary pores and most of oil is reserved in the secondary pores. Therefore, in order to explore more oil reserve in the sandstone reservoirs, it is quite important to study origin and distribution of the early diagenetic laumontite.
In attempting to understanding the origin and distribution of the laumontites, we turn our main attention to analysis sedimentary provenance, sedimentary facies, clastic composition, diagenesis of the sandstones containing laumontite, and also to analysis the relationship between laumontite and reservoir property of the sandstones. It is found that provenance and sedimentary facie are two major factors to control the origin and distribution of the laumontite. Specifically the more volcanic rock fragments and plagioclase fragments the sandstones contain, the more laumontite the sandstone contain. For example, there are two big deltaic sedimentary provenance in late Triassic of Ordos basin. It is found that the laumontite cement mainly develops in the deltaic sandstones which contain more volcanic rock and plagioclase fragments. But another daltaic sandstones contain much less laumontite because there are much less volcanic rock and plagioclase fragments. Through studying the relationship between distribution of laumontite and sedimentary facie, it turn out that the laumontite mainly distributes in the thick stacking river channel or distributary channel sandstones. Moreover, most of the laumontite occurrence in the Mesozoic sandstones of northern China appears to be of early diagenetic phreatic origin. The reasons for this interpretation are evidenced by homogenization temperature, vitrinite reflectance (Ro%), sporopollen color (TAI) and transformation of clay mineral.