四川盆地龙马溪组页岩中碳酸盐矿物形成机制

Origin of Carbonate Minerals in Organic Matter-rich Shale of Longmaxi Formation in the Sichuan Basin

碳酸盐胶结物是四川盆地龙马溪组页岩中重要的自身矿物,但对其形成机制目前尚无深入研究。以四川盆地龙马溪组A 井和 B 井为研究对象,综合应用全岩 X 射线衍射测试、扫描电镜分析了页岩中自身碳酸盐矿物的类型,并利用分成分碳氧同位素对其形成机制进行了研究。结果表明:四川盆地龙马溪组页岩中碳酸盐矿物沉积水介质主要为陆相淡水。A 井的方解石δ 13 C平均值为-1. 14‰,δ 18 O 平均值为-12. 63‰,B 井的方解石δ 13 C 平均值为-2. 40‰,δ 18 O 平均值为-12. 84‰,氧同位素表明两口井方解石均主要形成于早成岩 B 期,碳主要来源于有机质脱羧反应;A 井白云石δ 13 C 平均值为- 1. 84‰,δ 18 O 平均值为-7. 56‰,其形成主要与早成岩阶段硫酸盐还原作用密切相关;B 井白云石的δ 13 C 平均值为-4. 48‰,δ18 O 平均值为-11. 12‰,主要与中成岩 A 期有机质脱羧反应密切相关,铁白云石主要形成于晚成岩阶段;黏土矿物转化提供了研究区碳酸盐矿物沉淀所需要的钙离子、镁离子及铁离子。该研究成果对进一步分析四川盆地龙马溪组页岩成岩演化具有重要指导意义。

Carbonate cement is an important authigentic mineral in organic rich shale of Longmaxi Formation Sichuan Basin but no further study has been conducted on its formation mechanisms currently. The shales of A and B wells in Longmaxi Formation are taken as a research object southwestern part of Sichuan Basin. Based on analy sis of X diffraction scanning electron microscope combined with carbon and oxygen isotopes formation mecha nisms of carbonate minerals were revealed. The results show that the authigentic carbonate minerals were mainly af fected by atmospheric fresh water. The average of δ13 C of calcite in well A is - 1 14‰ and the δ18 O is -12 63‰ theaverageofδ13CofcalciteinwellBis -2 40‰andtheδ18Ois -12 84‰ theoxygenisotope showed that calcite in both wells was mainly formed in the early diagenetic B stage The average of δ13C of dolomite inwellAis -1 84‰andtheδ18Ois -7 56‰ itsformationwasmainlyaffectedbysulfatereductioninearlydi agenesisstageandprecipitatedberorecalcite Theaverageofδ13CofdolomiteinwellBis -4 48‰andtheδ18O is -11 12‰ its formation was mainly affected by organic decarboxylation in the middle diagenetic A stage An keriteswereformedinlatestageofdiagenesis. TheCa2+ Fe2+andMg2+requiredofcarbonateprecipitationwere mainly derived from the transformation of clay minerals. This research will play an important significance for further analysis of the diagenetic evolution in study area.