Dolomite Genesis of Miocene Meishan Formation in the Well Xike-1,Xisha Area,South China Sea

银河

时志强

硕士

成都理工大学

白云岩;;地球化学;;储层;;西科1井;;南海

南海具有较为特殊的构造背景与复杂的地质演化特征,因其特殊的地理方位及国家能源战略需要,一直是地质研究的热点。本文以前人研究为基础,通过现场岩心采集和描述,薄片鉴定,扫描电镜,阴极发光等手段对岩石的组构进行分析,通过主、微量元素,碳、氧同位素,稀土元素以及氢同位素等地球化学分析,试图揭示位于南海西沙地区西科1井中新统梅山组白云岩成因,并利用梅山组孔、渗及压汞数据判断梅山组白云岩储集性能,为后期西沙地区的开发提供参考。西科1井新生界碳酸盐岩发育,中新统梅山组白云岩仅局部发育,且普遍白云化不彻底。白云岩的固结程度比灰岩好,且白云岩分布井段多在古暴露面附近。本次研究将白云石平均含量大于75%者划分为白云岩层,梅山组一共识别出4层白云岩层,且在梅山组二段中部(896.7m～910.72m)发现一套白云质灰岩与灰质白云岩并夹有一层很薄的白云岩,其白云石平均含量在50%～75%之间,本文将其视为特殊的白云岩层。整体白云石以粉晶-细晶、平直面半自形-自形晶为主。在显微镜下,梅山组白云岩共识别出两种白云石:一种是晶体表面普遍较脏,呈云雾状的粉-细晶白云石,具雾心亮边结构;另一种为干净明亮的亮晶胶结物白云石,通常沿孔隙分布。在阴极发光中粉-细晶白云石的干净边缘与亮晶白云石显示相同的阴极发光特性,均不发光。扫描电镜下可见部分白云石表面发育溶蚀现象和嵌晶结构。白云岩具有低Sr、低Fe、低Mn、低δCe和趋于正常海水的δEu的特点,表明其所处为氧化的海水环境。白云岩氧同位素倾向于正值,主要由于微蒸发的氧化环境;Sr元素值偏低,可能由于白云化过程是丢锶的过程。白云岩的碳同位素趋正表明白云化过程中对原始灰岩中的碳的继承性;白云岩的碳、氧同位素均为正值,暗示白云石形成流体的性质主要为微蒸发浓缩的海水,但不排除有正常海水参与的可能。结合梅山组白云岩的组构特征,碳、氧同位素数值,古盐度(Z=130.19)以及主、微量元素的分析认为梅山组白云岩以准同生期渗透回流白云化作用为主。白云岩中较高的红藻含量使白云石化较易或更早发生,形成白云化彻底的白云岩。埋藏期对白云石过饱和的孔隙水是梅山组亮晶胶结白云石普遍发育的主要原因。梅山组白云岩储层具有高孔隙度、高渗透率以及大孔喉的特点,成岩作用影响较小,白云岩相较灰岩来说储集性能较好。整体来说白云化作用有利于梅山组碳酸盐岩形成良好的储集层。

The South China Sea has a special tectonic background and complex geological evolution characteristics.Because of its special geographical location and national energy strategy needs,it has always been a research hotspot.Based on the previous research,this study analyzes the rock petrological characteristics by means of collecting rock samples and description of core,rock slice identification,scanning electron microscopy,cathodoluminescence,etc.By the geochemical analysis of major and trace elements,carbon,oxygen isotopes,rare earth elements and hydrogen isotopes,the origin of dolomite in Miocene Meishan formation of Xike-1 well,Xisha Area,South China Sea(SCS)is revealed.The porosity,permeability and mercury data of the Meishan Formation can be used to judge whether the dolomitization of Meishan Formation is conducive to good reservoirs.It provides an example for the later development of Xisha area.The dolostones of Miocene Meishan Formation in the Well Xike-1 is generally not completely dolomitized.From the observation of cores in the Well Xike-1,it is found that the consolidation degree of dolomite is better than that of limestone,and the dolostones are mostly near the exposed boundary.In this paper,the layers with dolomite average value of more than 75% are commonly thought as dolostone.Four dolomite layers are identified in the Meishan Formation.In the middle of the second member of Meishan Formation(896.7m～910.72m),a set of dolomitic limestones and limy dolostone containing a very thin layer of dolomite is found.The average content of dolomite is between 50% and 75%,which is considered as a special dolostone layer in this study.The dolostones are mainly composed of the silty-fine crystalline dolomites,semi-automorphic crystal and flat crystal margins.There are two types of dolomites recognized in the dolomite of Meishan Formation,one is the generally dirty powder fine crystal dolomites with secondary enlarged edge and fog center bright edge structure,the other is the cement dolomites with clean,bright crystal,which is usually distributed along the pores.In cathodoluminescence,the secondary enlarged edge shows the same optical properties as the bright dolomite.Under scanning electron microscope,dissolution phenomenon and crystallographic structure are developed in the surfaces of some dolomites.The dolostones in Well Xike-1 are characterized by lower Sr,Fe,Mn,δCe and δEu values comparing to normal sea water,indicating that it is from an oxidizing seawater environment.The oxygen isotopes of dolomite tend to be positive,which is mainly caused by the environment of slightly evaporation and oxidation.The value of element Sr is low,which may be caused by the dolomitization process.The trend of carbon isotopes in dolomite indicates the inheritance of carbon in the original limestone during dolomitization.The carbon and oxygen isotopes are both positive,suggesting that the dolomite-forming fluid is mainly by the slightly evaporated seawater.But it is not ruled out that the normal sea water is involved.Combined with the structural characteristics,carbon and oxygen isotopic values,paleosalinity(z = 130.19)and the analysis of major and trace elements of dolomite in the Meishan Formation,it is considered that the dolomite in Meishan formation is dominated by the infiltration and circumfluence dolomitization in the quasi syngenetic period.The high content of red algae in interbedded dolomite makes dolomitization easier or earlier to occur and form more pure dolomites.The supersaturated pore water of dolomite during the burial period is the main reason for the cement dolomite in Meishan Formation.The dolostone reservoir of Meishan Formation has the characteristics of high porosity,high permeability and large pore throat.The dolomite layers have better reservoir performance than limestone.As a whole,dolomitization is conducive to the formation of good reservoirs in the carbonate rocks of the Meishan Formation.