酒西盆地碎屑岩胶结物的形成问题

酒西盆地下白垩统陆相碎屑岩胶结物的形成关系到油气储集的重要问题,富铁的洪积扇沉积体系和非富铁的河流一湖泊沉积体系形成两种不同的胶结物组合类型,平面上可分为三带胶结物:混合胶结带、铁镁胶结带和浓聚成岩带。盆地边缘带是以铁、泥、钙为主的混合胶结物,中间为以碳酸铁镁矿物的白云石、铁白云石为主的胶结物,盆中带形成胶结物的浓聚成岩层,事实证明过渡的铁镁胶结带是最富于聚集油气的地带。本文还在纵向上探讨了泥质粘土矿物和碳酸盐胶结物的成岩变化。

FORMATION OF CEMENT OF CLASTIC ROCKS IN THE JIUXI BASIN

At present, cement of clastic rocks in interior basins has been considered an important factor affecting the accumulation of oil and gas. The author systemtically researches this problem by studying the Lower Cretaceous in the Jiuxi Basin. The results show that the cement, depending on its sedimentary environment, has two types of cementintergrowthing-combination namely rich in iron and poor in iron. In the sedimentary system of iron-rich alluvial fan, a lot of cement is formed around iron, such as ferruginous, calcareous and argillaceous mixed cement; ferrodolomite, gypsum, silica and ferruginous layer. But in the sedimentary system of iron-poor river or lake, a lot of cement is formed around carbonate, for example, argillaceous and calcite, argillaceous and ferruginous mixed cement, ankerite, dolomite, calcite, and ankerite layer. The horizontal distribution of cement is divided into three zones: mixed cement zone, ferromagnesian cement zone and diagenetic zone. Argillaceous and ferruginous cement in interior basins is formed during early diagenetic buried stage, and a series of changes of clay minerals in argillaceous cement occur with their buried depth, but the depth at which clay minerals such as kaolinite, montmorillonite, chlorite changeinto illite is different owing to the difference of geothermal gradient. The study that ferruginous, magnesian and calcareous carbonate form calcite, ferrocalcite, dolomite and ankerite cement minerals is a very interesting and worthwhile subject. So far, only a small quantity of the reports regarding this subject of interior basins have been published. However, it has been found that in the Jiuxi Basin there are lots of ankerites and dolomites, which are in the ferromagnesian cement and distribute widely. Most of them are automorphic crystals: a few of them are allotriomorphic crystals. The same result is got by staining in thin section, infra-ree absorption spectrum and X-ray diffraction analysis. In the Jiuxi Basin, there are lots of thin layers of ferruginous, magnesian and calcareous carbonate both in well and outcrop area. Usually, the carbonate thin layers are composed of one, two or three kinds of intergrowth minerals of the four kinds of minerals mentioned above and they have quite close origin relations. During early diagenetive buried stage, calcites formed earliest are replaced by Mg, Fe anions to form dolomite, ankerite and ferrocalcite minerals. During late diagenetic stage, mineral autogeny and recrystallization take place and result in the formation of allatriomorpjic ankerite and dolomite. If there are spaces, autogenetic dolomite rhombohedron can be formed within biological cavity; however, autogenic and bright ankerite crystals can be formed in pore space of sandstone. According to the studies of other people by electron microscope, there are some dissolution pores, intergranular pores and intercrystal pores within solomite. Dolomite rocks and dolomite crystal cement are very useful for the reservoir of oil and gas. It has been confirmed that dolomite rocks and ferromagnesian cementation zones in the Jiuxi Basin are of high yield oil-porducing zones.