Authigenic Minerals Research In Sandstone Reservoir: Xujiahe Formation, Xiaoquan-Xinchang-Hexingchang Area

The analyses of authigenic minerals content,type and distribution regular in Xujiahe formation sandstone reservoir of the middle section of Western Sichuan Sag are based on Thin-section analysis,Scanning electron microscope,X-ray Diffraction,Carbon and oxygen isotope analyses.The analyses lay a foundation for discussing the formation mechanism and the influence to reservoir,forcasting and assessing the quality of the reservoir.Xujiaheâ…¡andâ…£are the main reservoir,authigenic clay minerals,authigenic carbonate cements and authigenic siliceous cements are the main type of authigenic minerals in sandstone reservoir.Authigenic minerals in Xujiahe Formation contains mainly Chlorite and Kaolinite.The average content of chlorite is 0.21%,and it exist as pore-lining cements.Chlorite distribute both in the Xujiaheâ…¡andâ…£vertically,but the samples that contain chlorite more than 2%only found in Xujiaheâ…¡,the provenance may be relevant with dark-colored ferromagnesian mineral.On plain distribution,the well drillings which lean to north-east contain more chlorite,maybe it's caused by higher cuttings in North of Xujiahe Formation.Most of the authigenic chlorite deposit at earlier diagenetic stage,and most clay coatings in Xujiaheâ…£are more thin than Xujiaheâ…¡,which deposit at early stage as Xujiaheâ…¡,earlier than the dissolution of feldspar.Authigenic Kaolinite has low content 0.29%of sandstone reservoir in Xujiahe Formation.Kaolinite's distribution is influenced obviouslly by strata,it distribute only in sandstone of the top of Xujiaheâ…£or near the top.Maybe these Kaolinite is the product of some particular stratic interface or near the interface,and may be the index mineral of these particular interface.The research result of X-ray Diffraction show that almost all the kaolinite in sandstone is authigenetic,which dominanted by dissolution of aluminosilicate,such as feldspar.From the angle of reservoir evolution,kaolinite is the index mineral of feldspar's dissolution and secondary porosity's development.The average content of carbonate cements(the most abundant authigenic minerals this area)access to 6.02%,which occupy 84%of all the cements.Carbonate has four main types:calcite,dolomite,forrocalcite and ankerite.The two sandstone member,Xujiaheâ…¡andâ…£contain the carbonate cements,which don't have much difference on vertically distribution,but the average content of Xujiaheâ…¢andâ…¤(mudstone member)is higher than sandstone member accordantly.The lower strata has higher domomite cements content,the average content of dolomite in Xujiaheâ…¡is higher than Xujiaheâ…£obviously,and the average content of dolomite in Xujiaheâ…¢is higher than Xujiaheâ…¤.But the tendency of calcite cements is converse with dolomite vertically,the average content of calcite Xujiaheâ…£in is higher than Xujiaheâ…¡,and the average content of calcite in Xujiaheâ…¤is higher than Xujiaheâ…¢obviouslly.Dolomite cements in Xujiaheâ…¡deposit at shallow burial depth,cause they have higher deposition temperature(according to oxygen isotope analysis),and the value of minus cement porosity is very low.Based on carbonate isotope analysis,marine provenance may one the reason that dolomite forming.Combine with the oxygen isotope analysis,the calcite cements in Xujiaheâ…£may deposit at early diagenetic stage,for they have lower depostion temperature.By carbonate isotope analysis,maybe the calcite is relavent with dissolution of feldspar.Most the calcite's provenace may be relavent with the conversion of clay minerals.The authigenic seliceous cements in the sandstone reservoir is low,the average content is 0.90%.Most the authigenic seliceous cements present around the periphery of detrital quartz,the widen section is composed with several quartz which have the same optic orientation.Finally,these quartz connect as a big crystal and form the quartz overgrowth to block up some of the pore space,but quantitative seliceous cements present in secondary porosity as microlite after the dissolution.it conclude that feldspar's dissolution is the one of most important provenance of seliceous cements.The distribution of seliceous cements are controlled by strata obviouslly,the content of Xujiaheâ…¡is 1.42%and it decrease to 0.13%of Xujiaheâ…¤.Compare with Xujiaheâ…£,the feldspar's dissolution decrease apparently,both the absolute magnitude and relative magnitude of Xujiaheâ…¡are very low.The thin-section analysis show some samples don't have the dissolution of feldspar completely,but the quartz overgrow generally.So the seliceous cements in Xujiaheâ…¡have destructive infulence,while the seliceous cemented at earlier diagenetic stage,they don't play obviously destructive influence to reservoir.