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 SiO

2

and the formation of some clay minerals. (2) K

+

、Na

+

and Ca

2+

cations in the pore water lead to the formation and transformation of clay minerals

such as illite; while SiO

2

and Al

2

O

3

precipitate nearby as kaolinite and SiO

2

cause the albitization of potash feldspar

or even become authigenic albite or albite overgrowth. (3) The remaining SiO

2

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 SiO

2

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.