Abstract:

The diagenetic sequence and pore evolution of ancient carbonate rocks in deep or ultra-deep burial depth is one of the hot topical in reservoir evolution. Based on analysis of the petrographic, geochemical, and U-Pb dating of diagenesis minerals of the Dengying Formation at Chuanshen 1(CS1), Jinshi 1(JS1) and other wells, the authors has adopted the comparative methods, in which compare the matrix dolomites with different dolomite cements, as well as the other diagenetic minerals, occurred in cement or fillings in vugs & fractures with regard to the data of carbon, oxygen, and strontium isotopes, homogenization temperatures, U-Pb dating of dolomites and calcites, either in the Dengying Formation of Ediacaran, or Xianrendong Formation of the Early Cambrian, or Maokou Formation of Middle Permian. The diagenetic sequence has been reasonable modified: fibrous or fibrous growth→column fibrous or foliated or blade(length-fast cement) or dogtooth shaped→coarser fibrous or bundle shaped of marine cementation and bundle shaped cement or fasicular or radial dolomite cements. i.s botryoids or comb shell shaped or primary dolomite biomorphs of nanoscale spheroids→micro-stalactitic or vadose silt of meteoric cementation→cryptocrystalline or microcrystalline quartz(1^st)→fluorapatite, barite and fluorite(1^st)→semi-automorphic micro-crystalline dolomite→fine crystalline dolomite→fine and medium crystalline quartz(2^nd) or medium to coarse dolomite→Saddle dolomite(1^st)→medium to coarse crystalline calcite(1^st)→hydrocarbon discharge(episodes)→fluorite(continuous dissolution. 2^nd or 3^rd)+coarse calcite(2^nd)→coarse quartz(3^rd)+Galena+Sphalerite+Pyrite+Fluorite(4^th)+coarse calcite(3^rd)+coarse crystalline dolomite(or Saddle dolomite(2^nd)→pyrobitumens(pyrolysis or TSR, methane)→coarse calcite(4^th)+quartz(4^th); and the medium to coarse dolomite in some of the fractures or cavities is of early hot fluid dolomite origin, with relatively low ${\delta }^{13}{\mathrm{C}}_{\mathrm{P}\mathrm{D}\mathrm{B}}$(-1.78‰~-1.69‰) and ${\delta }^{18}{\mathrm{O}}_{\mathrm{P}\mathrm{D}\mathrm{B}}$(-9.28‰~-9.32‰) and moderate. homogenization temperatures(Th=130.9 ℃, 135.3 ℃, 116.6 ℃) and salinity(12.63wt% NaCl); the U-Pb Concordia Age of dolomite(D_Ⅱ-Ⅲ)is between 474.0±74.0 Ma to 482.0±56.0 Ma, By integrating analyses of burial history, the geological-time of dolomite(D_Ⅱ-Ⅲ) is mostly like to occur in the late Ediacaran to Cambrian(No later than Early Ordovician). The weak extension during the late Ediacaran to Cambrian period, interval by multiple episodes of tectonic compression and exhumation, provided the conditions for the meteoric water percolated downward along the faults or fractures, heated by deep circulation as well as mixing with formation water, lead to precipitate of dolomite. The diagenesis effect on pore evolution display that the early dolomite cementation developed in hot fluid had a smaller proportion and had little effect on pore reduction, in compared with diagenesis such as large-scale marine cementation, strong silicification, compaction, and dedolomitization.

Key words: geochemical characteristics, U-Pb dating age, diagenetic sequence, the early dolomite cement origin in hot fluid, Dengying Formation, Sichuan Basin

摘要：

深层—超深层古老碳酸盐岩的成岩序列与孔隙演化是研究热点与难点。基于川深1井、金石1井等灯影组碳酸盐岩岩相学、地球化学及碳酸盐矿物U-Pb定年数据,对比缝洞胶结物(微区)与围岩碳、氧及锶同位素值,并结合仙女洞组、茅口组碳酸盐岩成岩矿物中的均一温度与地质年代,建立并完善了四川盆地埃迪卡拉系灯影组碳酸盐岩的成岩序列,探讨了其与孔隙演化的关系。灯影组碳酸盐岩成岩序列为: 纤状或纤状生长带→纤柱状或刀刃状→较粗的纤维状、放射状束状海水胶结(葡萄状、皮壳状或栉壳状)、纳米级微球粒原生白云石→重力悬垂、渗流砂及大气淡水方解石胶结→隐晶质、微晶石英→氟磷灰石、重晶石及萤石→半自形粒状粉晶白云石→细晶白云石→中细晶石英或中粗晶白云石(或鞍形白云石)→中粗晶方解石→幕式充注的烃类→萤石+中粗晶方解石→中粗晶石英+方铅矿+闪锌矿+黄铁矿+萤石+中粗晶、巨晶方解石+中粗晶、巨粗晶白云石或鞍形白云石→焦沥青(甲烷,TSR)→中粗晶或巨晶方解石+中粗晶石英。结合构造埋藏史,初步厘定部分中粗晶白云石胶结物为埃迪卡拉纪晚期—寒武纪(或早奥陶世)的热水成因,其特征为: δ¹³C、δ¹⁸O分别为-1.78‰~-1.69‰和-9.28‰~-9.32‰,相对较低; 均一温度为116.6~135.3 ℃,盐度为12.63wt%NaCl_Equ,碳酸盐矿物U-Pb协和年龄为474.0±74.0~482.0±56.0 Ma。在埃迪卡拉纪晚期—寒武纪伸展构造背景下,桐湾运动Ⅰ幕、Ⅱ幕的挤压抬升导致大气淡水沿断裂下渗并参与热循环,促使缝洞中早期白云石沉淀。与大规模海水胶结、强烈硅化、压实、去白云化等成岩作用相比,该期热水胶结作用规模较小,对原始孔隙变化影响有限。

关键词: 地球化学特征, U-Pb年龄, 成岩序列, 早期热水白云石, 灯影组, 四川盆地

CLC Number: 

• P588.2