摘要
四川盆地东部长兴组—飞仙关组礁滩相储层为重要的天然气勘探目的层,优质储层的岩性多为白云岩,且主要分布于开江-梁平陆棚的东西两侧。川东兴隆场地区位于开江-梁平陆棚西侧有利相带的东南端,长兴组白云岩层段为该区主力产气层,且白云岩主要分布在台地边缘礁滩相等易于发生暴露的高能沉积相带。明确了白云岩宏观分布的控制因素,以岩石结构及Mg/Ca值和有序度为基础,结合岩石地球化学特征,对川东兴隆场地区上二叠统长兴组白云岩成因进行分析。研究结果表明兴隆场地区长兴组白云岩是在埋藏条件下,同层位的泥晶灰岩及泥灰岩中封存的浓缩海水通过压实作用排挤进入碳酸盐沉积物交代灰岩形成的。有如下证据:①飞仙关组沿缝合线发育的自形白云石化学组成(CaO含量均值30.94%,MgO含量均值20.76%)与长兴组白云石化学组成(CaO含量均值30.51%,MgO含量均值21.28%)相近,表明二者成因及白云化流体具有相似性,推断压实流体可能为长兴组白云岩白云化流体;②长兴组白云岩δ13C介于1‰～2.5‰之间,δ18O介于-6‰～-4‰之间,分布在同期古海水的δ13C(0～5.8‰)、δ18O(-7.5‰～-3.5‰)变化范围内,并与全球典型埋藏白云岩δ13C、δ18O分布区间一致,说明白云化流体具有同期海水性质,且形成于埋藏成岩环境中;③长兴组白云岩的87Sr/86Sr值(0.707108～0.707507)覆盖在了川东北晚二叠世海水Sr同位素(0.706620～0.707742)的变化范围内,证明白云化流体来源于同期海水;④长兴组白云岩稀土元素表现为LREE亏损、HREE富集的海水稀土元素特征,表明白云化流体具有海源流体的性质;⑤长兴组碳酸盐岩Fe含量均较低(<2000×10-6),但长兴组白云岩相对于灰岩具有较高的Mn含量(69×10-6～90×10-6,均值为76×10-6)和较强的阴极发光性,反映了白云岩形成于埋藏的还原环境中。
The reef and shoal facies reservoir in the Changxing and Feixianguan formations are the important targets for natural gas exploration in Eastern Sichuan Basin.The lithology of the favorable reservoirs isdolomitic rocks ,which are located on both sides of Kaijiang-Liangping trough.The Xinglongchang area of East Sichuan Basin is located on the western side of Kaijiang-Liangping trough.The main gas producing formations are dolomite reservoir in Changxing Formation ,which distribute in the high-energy reef and shoal facies zone prone to exposition.Since the control factors of the distribution of dolomite have been clarified , based on the analysis of rock structure , Mg / Ca ratio and the degree of order , the geochemical characteristics , the genesis of dolomite of Upper Permian Changxing Formation in the Xinglongchang area of East Sichuan Basin was studied indetail.Results indicate that the dolomite is the product of replaced dolomitization during burial diagenesis period.The dolomitization fluid is probably the coeval condensed seawater trapped in the limestone and marlstone compelled by compaction.The conclusion comes from the following evidences ：（ 1 ） The chemical composition of the euhedral dolomite developed along the suture line in the Feixianguan Formation is similar to that of thedolomite in the Changxing Formation , indicating that the two kinds ofdolomites have the same origin and the dolomitization fluid may be the compaction fluid ;（2 ） Theδ 13 C of dolomite is 1‰～2.5‰ （ PDB ） and the δ 18 0 is -6‰～-4‰ （ PDB ）,which is accorded with the composition of δ 13 C （ 0～5.8‰ , PDB ） andδ 18 0 （ -7.5‰～-3.5‰ , PDB ） of the coeval paleo-seawater and the distribution ofδ 13 C andδ 18 0 of the typical burial dolomite supposing that the dolomitization fluid is probably the coeval seawater and the dolomite formed during burial ;（3 ） 87 Sr / 86 Sr ratio of thedolomite （ 0.707108～0.707507 ） is under the changing range of paleo-seawater of the Late Permian （ 0.707108～0.707507 ） proving that the dolomitization fluid is coeval seawater ;（4 ） The rare earth elements of dolomite represent the same characteristic with the seawater that the loss of LREE and the enrichment of HREE ,which demonstrates that the dolomitization fluid is seawater derived ;（ 5 ） The carbonate in Changxing Formation has the very low Fe concentration （ 2000×10 -6 ）, but the dolomite has the higher Mn concentration （ 69×10 -6 ～90×10 -6 , averages 76×10 -6 ） and the brighter cathodoluminescence relative to the limestone , indicating that the dolomite formed under the reducing conditions during burial.
出处
《地质学报》
EI
CAS
CSCD
北大核心
2013年第7期1031-1040,共10页
Acta Geologica Sinica
基金
国家科技重大专项(编号2011ZX05005-003-009HZ)
国家自然科学基金项目(编号41103020)
中国石油大学(北京)基金(编号KYJJ2012-01-07)资助的成果
关键词
兴隆场地区
长兴组
白云岩
C、O、Sr同位素
微量元素
白云化流体
Xinglongchang area
Changxing Formation
dolomite
carbon
oxygen and strontium isotope
trace element
dolomitization fluid
