Abstract:

All secondary enlargement of quartz and albitization of potash feldspar in water-saturated zone (water layer) are stronger than those in oil-saturated zone. Homogenization temperatures of fluid inclusions in quartz overgrowth (60～140℃) shows that both the temperatures of fluid inclusions in oil-saturated and water-saturated zones are very similar,which proves that the enlargement process of quartz in oil-saturated zone has been continuous after the charging of oil,but the process has been restrained in contrast to that in water-saturated zone. There are mainly five kinds of diagenetic changes for feldspar minerals,including dissolution,carbonatization,kaolinization,illitization and albitization.The pH of pore water and concentration of carbon dioxide (fractional pressure) play an important role in feldspar dissolution.Three different results would occur after feldspar dissolution:(1) all the dissolved substances would migrate along with the solution to other places,which is advantageous to the formation of secondary pores.Migration capacity of dissolved substances depends on the transport capacity of pore water or percolating capability of pore water.Both the poroperm characteristics and oil saturation of the reservoir can affect the percolating capability of pore water.Besides,it is also affected by the property of the pore water.The organic acid in the pore water is favorable to the formation of metal organic complex,thus accelerating the migration of dissolved substances.At the same time,the increase of organic acid concentration would cause the precipitation of SiO₂ and the formation of some clay minerals. (2) K⁺、Na⁺ and Ca²⁺ cations in the pore water lead to the formation and transformation of clay minerals, such as illite; while SiO₂ and Al₂O₃ precipitate nearby as kaolinite and SiO₂, cause the albitization of potash feldspar, or even become authigenic albite or albite overgrowth. (3) The remaining SiO₂ then become the pore filling material, silicon framework or “silicon ball” accumulation. It is the alteration of pore water property caused by oil charging that creates this phenomenon. Since there is a certain amount of organic acid in the oil, and when oil migrate into the reservoir,some of the water soluble substances, such as organic acid, dissolve in formation water, which reduce the pH of formation water and lead to the increase of acidity of pore water, associated with oil with a pH of 3～5, thus boost the dissolution of feldspar. The SiO₂ generated from the dissolution of feldspar, cannot easily migrate in the acidic water, it thus precipitates as the so-called “silicon ball”. But if the oil saturation is continuously increasing so that the reservoir to be rich in oil or immersed in oil, the flowing of pore water would then be restrained, and the diagenesis, including dissolution and precipitation processes, would be impeded. Similarly, the development of calcite and dolomite is better in water-saturated and low oil-saturated zones than in high oil-saturated zone.Theoretically,charging of hydrocarbons alters the property of fluid in the pores, and the diagenetic environment alters accordingly and leads to the ceasing of illite growth. Therefore, the amount of illite is always different in sands containing varying amount of oil. In oil-saturated or oil-rich zone, illite is relatively low, while in sands with small amount of oil, such as the oil patch sands,or with only trace of oil and water layer, illite content is relatively high. The relative content of chlorite in sands varies similarly as that of illite. All mentioned above show that oil charging can change the microenvironment for growing of minerals in reservoir pores, and thus would restrain,to some extent, the diagenesis of reservoir rock.

Key words: oil charging, reservoir mineral, diagenetic evolution, oil-saturated zone, water-saturated zone, fluid inclusion