塔里木盆地寒武-奥陶系白云岩结构构造类型及其形成机理

Types and origin of Cambrian-Ordovician dolomites in Tarim basin.

塔里木盆地寒武—奥陶系白云岩厚度大、分布广,是重要的油气储集层。根据岩石学研究并结合地球化学特征将其分为4类,其特点如下:(1)泥晶一粉晶白云岩,白云石晶体小、晶形差,有序度在4类中最低(0.73);Na、K、Sr 含量在4类中最高;δ^(13)C 值及 Z 值较高,分别为-0.94‰、122.3;Fe、Mn 含量较低;阴极发光为亮桔红色;常呈纹层状,具暴露及成化标志。这些特征表明其结晶较快,形成环境蒸发强烈、盐度较高、氧化性较强,是在萨布哈环境中由浓缩海水发生白云石化所形成。(2)藻纹层白云岩,白云石晶体小,晶形较好,有序度较高(0.85);藻纹层基本保留了其原始微细结构;Na、K、Sr 含量较低;δ^(13)C 值在4类中最高,平均为-0.50‰;含有较多黄铁矿。这些特征说明其可能形成于成岩作用的稍早期阶段且其成因与藻类活动密切相关,成岩环境盐度较低,可能是在藻类作用下,由富藻层浓集的镁与残留海水在浅埋藏环境中共同作用所形成。(3)雾心亮边白云岩,白云石普遍具雾心亮边结构,雾心含少量包裹体及灰质残余,亮边则少见或无;阴极发光下,雾心发亮桔黄色光,亮边发光暗或不发光;局部见此类白云石呈星散状分布于亮晶颗粒灰岩或泥晶灰岩中;白云石晶体较大,晶形较好。此类白云岩的δ^(13)C 及 Z 值在4类中最低,K、Na、Sr、Fe 含量相对较低,Mn 含量最高;有序度较高,均值为0.83。这些特征表明此类白云岩中白云石的雾心可能是在早期浅埋藏环境中,由富镁孔隙水交代周围灰质所形成;亮边则形成于埋藏较深的环境,其所需镁可能来源于粘土矿物的转化。(4)重结晶白云岩:白云石晶体粗大且污浊,接触紧密;局部见白云石晶体具环带结构,局部见颗粒幻影结构;阴极发光较暗;Fe 含量在4类中最高,Mn 含量较高;有序度最高,平均为0.95;包裹体均一温度平均为110.2℃。这些特征表明其可能是在还原性较强的深埋藏环境中,由早期形成的白云岩经较强的重结晶作用所形成。

Cambrian-Ordovician dolomites in Tarim basin, which are characterized by thickly and extensively distributed horizon, are very important hydrocarbon reservoirs. Detailed petrographic investigations have documented 4 dolomite types and their origins are figured out based on petrographic, chemical, isotopic studies, X-ray diffraction and cathodoluminescence analysis. （ 1 ） Mud-silt-sized dolomite consists of anhedral and less ordered micritic crystals and exhibits bright-pink luminescent. It yields the highest Na, K, Sr contents, high δ^13C, Z values （with the average values of -0.94‰ and 122.3, respectively） and low Fe,Mn contents. It always presents as laminas together with evaporated signatures. These characteristics reveal that this kind of dolomite was formed in hypersaline and oxidizing environment and involved fast dolomitization process. It was dolomitized by evaporated seawater in sabkha environment. （2） Algal laminated dolomite （ALD） is composed of euhedral and finely crystallines （〈0.05mm） and its main primary fabric of algal lamination can still be identified. Its δ^13C （-0.50‰） and Z （123.2） values are the highest, however, Na, K, Sr contents are the lowest. The appearance of sedimentary pyrite suggests that sulfate reduction was active in these sediments shortly after deposition. These characteristics indicate that ALD was dolomitized during early diagenesis and algae possibly played an important role. The ALD was likely formed under mediation of organic matter and Mg^2＋ were supplied by magnesitim concentrated algal laminites and sea water. The highest δ^13C and Z values don＇ t represent the hypersaline condition but possibly suggest the mediation by organic matter. （3） The dolomite which consists of crystals with cloudy centers and clear rims （CCCR） presents mainly as patchy occurrence, and most of its crystals are euhedral and microsucrosic. Patchy are represented by the distribution of CCCR crystals in the vicinity of or within stylolites. The cores of the dolomites are always rich in inclusions and contain some limestone remnants, but the rims are relatively inclusion-free. The cores exhibit bright-pink luminescent, however, the rims are dull or nonluminescent. The dolomite yields the lowest δ^13C, Z values and the highest Mn content, and its K, Na, Sr, Fe contents are low, and moreover, it is relatively highly ordered. These characteristics reveal that this kind of dolomite was formed under reducing conditions and involved less hypersaline fluids. It can be inferred that the pore fluids which were rich in Mg^2＋ were responsible for the formation of cloudy cores in shallow early diagenetic environments. Whereas the clear rims were likely formed in subsequent burial into deeper subsurface environments, and the Mg^2＋ needed for further dolomitization possibly was supplied by the transformations of clay minerals. （4） Neomorphic dolomite consists of coarse, turbid crystals and exhibits sucrosic and mosaic textures. Saddle dolostones are present sometimes indicating it involved a deep burial diagenesis. Zonation and grain-ghost were observed occasionally suggesting strong recrystallization. Dull luminescent together with the highest Fe and high Mn contents, low Na, K, Sr contents point to a reducing and less hypersaline environment. Low δ^13 C, the lowest Z value and the highest order degree （ average in 0.95 ） possibly resulted from the recrystallization. The average homogeneous temperature （110.2℃） also indicates deep burial diagenesis. Collectively, these characteristics demonstrate that the neomorphic dolomite was likely formed by recrystallization of pre-existing dolomites during deep burial.