塔里木库车坳陷中新生界储层砂岩成岩期钠长石化

库车坳陷中新生界以陆相碎屑建造为主，在该区储层砂岩中，钠长石为广泛存在的自生矿物之一。显微结构证据显示，成岩期钠长石交代长石等碎屑，或呈细小自形晶填隙产出，在化学成份上，钠长石与被交代的长石存在过渡关系。在碎屑长石钠长石化过程中，母体相的溶解速率大于钠长石的沉淀速率，并伴有石英，高岭石等矿物的消耗，因此有助于次生孔隙的发育及储层物性的改善。从高岭石的垂向分布，以及钠长石相对含量与埋深的关系，可以看出钠长石化作用主要发生在探成岩(mescdiagenesis)，对应于4 000-5 500m埋探，和90℃ ～110℃ 地温带 在库车和塔北地区，主要工业油气藏亦集中分布在这一深度带。

DIAGENETIC ALBITIZATION IN THE MESOZOIC AND CENOZOIC RESERVOIR SANDSTONES FROM KUCHE DEPRESSION, NORTH TARIM, XINJIANG, CHINA

The Mesozoic and Cenozoic in Kuche Depression is predominantly a set of terrestrial detrital formation. In the reservoir sandstones in the area, albite is one of the widely developed authgenic minerals. Micro-structural evidence shows that diagenetic albite occur as euhedral crystals in the pores, and “places feldspar and other detritus along cleavage and micro-cracks. EPMA analysis indicates that same of the alteration product has a chemical composition transitiona1 between the pure albite and the replaced detrital feldspar. For example, composition of replaced alkali feldspar is between microcline and albite, and An content of altered plagioclase decrease continuously. This has been regarded as a direct evidence for fluid replacement, because the feldspar crystallized from solution or magma usually has a fixed chemical composition. On the basis of mineral assemblage observed by SEM , EPMA and XRD , there exist three kinds of albitization: (1)Ab replaces Kf; (2)Ab replaces the assemblage of Kf, Kaol and Q, forming iliite simultaneously; (3)Ab replaces Pl and Q. In reactions(2)and(3), the volume of resultants much less than that of thereactants. And in reaction (1), dissolution of Kf is more apparent than precipitation of Ah, so that the albitization is in favor of development of secondary porosity and improvement of the physical properties of the reservoir. Aagaard et.al.(1990) found that in eodiagenetic zone(2 km)albite makes up 3 to 5％ of the total volume of the rock, while in mesodiagenetic zone(4-5km)it is about 9％ This means that 30～50％ of detrital Kf is replaced by albite. In Kuehe Depression, albite distribution is also correlated to burial depth. XRD analyzing result shows that in the reservoir sandstone below 4000m , the albite content increases with increasing depth, but in the shallower levels, no such relation is found, Taking into account of the distribution tendency of kaolinite. XRD results show that in the reservoir sandstones above the burial depth of 3000 m. 38.9％ samples were found with XRD detectable kaolinite, but in the sandstones below the depth of 4000 m. only 11 5％ samples contain kaolinite. Both of them suffered no surface weathering, with no significant difference in source materials and sedimentation environment. It seems that more kaolinite was consumed in the deeper reservoirs during diagenetic process. It can be concluded that albitization described by reactions (1)to (3)is dependent on geothermal gradient, and mainly takes place in 9O℃～l10℃ thermal girdle. This is in consistent with the view point of Morad et. a1. (1990) and Saigal et a1. (1988)made on the reservoir sandstones in Norwegian North Sea oil field. In North Tarim, most of the proved oil and gas accumulations are concentrated within the burial depth of 4000 to 5500m , corresponding well to the geothermal zone of albitization. At present we can riot say that this means any causal relationship between them , but there is no doubt that albitization contributes a lot for the development of secondary porosity which in turn improves the physical properties of the reservoir.