Effect of Hydrocarbon Emplacement on Diagenesis of Silurian Sandstone of Central Tarim Basin
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摘要: 塔中地区志留系具有较广的沥青分布,为研究烃类侵位对矿物成岩作用的影响提供了良好的实例。通过岩矿观察,并综合油田水化学、氢氧锶同位素和流体包裹体均一化温度等资料,提出该区志留系烃类侵位后因淡水注入而使烃类被氧化,所产生的有机酸促进了钾长石、石英等矿物的溶解,导致了次生孔隙的发育;并为石英次生加大提供所必需的Si的来源。Si的质量转移可能是因浓度梯度而引起的扩散作用进行的。烃类侵位还使成岩介质还原性增强,使得胶结物富含还原的 2价铁;但未能阻滞石英的次生加大.Abstract: Silurian bitumen-hearing sandstoncs in Central Tarim are shown to he a good case to study influence of hvdrocarhon emplacement on diagcncsis.We have carried it out by means of observation of thin sections, SEM of sandstoncs in congwnction with analysis of water chemistry, organic acid anions and δD,δ18O,87Sr/86Sr of oilficld water and homogenization tcmpcraturcs of fluid inclusions of the sandstoncs. It can he found that quartz secondary overgrowth coexists with reservoir bitumen.Bitumen may occur between quartz overgrowth and grains in coarser sandstoncs or occlude in the centre of pore in finer sandstoncs.Homogenization temperaturcs of organic inclusions are divided into 2 groups: 59.5-82℃ and 103-125℃,suggestive of at least 2 phases of oil emplacement.The latter is close to the present temperatures of the bore hole hottom, indicating that the late oil and gas cmplacement took place in the late Yanshan to early Himalava Orogcny and quartz overgrowth is not inhibited by oil cmplaccmenu and ccmcntation continucds to present day. Relationships between Br and Ca,Mg,Sr,Cl,CF(CF=Ca+Mg+Sr-SO4-HC03,meq/L) show that oilficld wato is evolved by suhacrial evaporation of scawatcr and subsequent mixing with meteoric water evidenced by relationship between δD and δ18O.Relative enrichment of Ca,Sr,K and depletion of Mg is explained to he a result of dissolution of K- feldspar,dolomitization of calcite and alhitivation of feldspar.87Sr/86Sr of 14 water samples range from 0.710 44 to 0.712 58,much larger than Silurian seawater, indicating that Rh-enriched dctrital minerals such as I}-feldspar, mica and illitc from sandstone or mudstonc contribute significant amount of 87Sr to oilficld waters. It is proposed that organic acids and anions } hydrocarbons he generated by as a result of meteoric water influx after oil emplacement. Organic acids enhance dissolution of minerals such as K-feldspar, quartz leading to generation of secondary porosity and supply additional Si necessary for quartz overgrowth. Si mass transport is through diffusion due to concentration difference as a result of Si precipitation on dctrital quartz.And this is the reason why quartz overgrowth was not inhibited.And Fe II in authigcnic minerals such as Fe-dolomite Fe-calcite and pyrite is mainly derived from reduction of Fe203 due to low Eh resulting from hydrocarbon emplacement. It can he found that bitumen-hearing sandstone is grey, dark grey, even black in comparison with color brown in most non- bitumen-hearing sandstone, suggesting that the former was bleached.~
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