中国南方地区灯影组白云岩储层流体溶蚀改造机制

Fluid alteration mechanism of dolomite reservoirs in Dengying Formation,South China

对南方地区震旦系灯影组未遭受溶蚀改造作用的泥粉晶白云岩、遭受溶蚀改造的富含溶蚀孔隙白云岩储层(包括粗晶砂糖状白云岩)以及孔洞中充填的粗晶白云石开展了碳、氧和锶同位素以及稀土元素分析,以判别白云岩储层发育经历的流体改造作用类型、过程和机制。未溶蚀白云岩的δ13 C、δ18 O和87Sr/86Sr比值平均分别为3.0‰、-3.5‰和0.708 779;稀土元素组成上无显著的Ce和Eu异常,属于正常的海水成因白云岩。溶蚀改造后的砂糖状白云岩的δ13 C、δ18 O和87Sr/86Sr比值平均分别为1.7‰、-7.7‰和0.709 579,与未溶蚀改造白云岩相比具有略偏轻的碳、氧同位素组成和较高的87Sr/86Sr比值;稀土元素组成上,具有较高的REE含量和显著的Ce负异常,δCe平均值为0.5,表明多孔的砂糖状白云岩储层是大气降水和含油气流体溶蚀改造的产物。孔洞中充填的粗晶白云石的δ13 C、δ18 O和87Sr/86Sr比值平均分别为0.3‰、-11.3‰和0.710 334,具有显著偏轻的碳、氧同位素组成和显著高的87Sr/86Sr比值;稀土元素组成上,具有显著的Eu正异常,δEu平均值为3.0,最大达到9.8,表明孔隙充填白云石是从热液流体中沉淀出来的,并受地层中有机成因流体的影响。根据所确定流体作用类型并结合构造地质演化背景,探讨了灯影组白云岩储层流体改造过程和机制。南方地区在震旦纪灯影组沉积时期发育海水台地相泥粉晶白云岩沉积,局部见藻纹层结构。震旦纪末期遭受大气降水溶蚀改造,下渗至地下深处,促使白云岩溶蚀和重结晶作用;在此后的埋藏过程中,进一步遭受含油气流体的溶蚀改造作用,最终形成富含溶蚀孔隙和沥青的中粗晶白云岩储层。埋藏过程中,受盆地深部热液的改造作用,在孔隙中形成白云石充填,使白云岩储层发生了一定程度的致密化作用。

The analyses were carried out on carbon,oxygen and strontium isotopes and rare earth elements(REEs)of the unaltered muddy-micritic dolomites,pore-rich dolomite reservoirs altered by corrosion(including coarse crystalline sucrosic dolomite)and pore-filling coarse crystalline dolomites in Sinian Dengying Formation of the southern China,so as to identify the type,process and mechanism of fluid alternation in the development process of dolomite reservoirs.The average values ofδ13 C,δ18 O and 87Sr/86 Sr of unaltered dolomites are 3.0‰,-3.5‰ and 0.708 779 respectively.There is no significant Ce and Eu anomaly in REE composition,indicating normal dolomites formed by the action of seawater.The average values ofδ13 C,δ18 O and 87Sr/86 Sr of altered sucrosic dolomites are 1.7‰,-7.7‰and 0.709579 respectively.The carbon and oxygen isotope compositions are slightly lighter and 87Sr/86 Sr ratio is higher than unaltered dolomites.Moreover,altered dolomites have higher REE content and significant negative Ce anomaly(δCe average= 0.5).It is proven that pore-rich sucrosic dolomite reservoirs are the products of meteoric water and oil-bearing fluid alteration.The average values ofδ13 C,δ18 O and 87Sr/86 Sr of pore-filling coarse crystalline dolomites are 0.3‰,-11.3‰ and0.710 334 respectively.The carbon and oxygen isotope compositions are significantly lighter,while 87Sr/86 Sr ratio is markedly higher.Meanwhile,there is significant positive Eu anomaly(δEu average=3.0,maximumδEu=9.8).It is demonstrated that pore-filling dolomites were precipitated from hydrothermal fluids and affected by organically generated stratigraphic fluids.Based on the identified fluid types and geological evolution background,the fluid alteration process and mechanism of dolomite reservoirs in Dengying Formation were discussed in this study.Muddy-micritic dolomite sediments with seawater platform facies were developed in Sinian Dengying Formation of the southern China,where laminar algal structure was shown in some zones.At the end of Sinian period,the dolomites in Dengying Formation were altered by meteoric water,and then infiltrated to deep subsurface,promoting dolomite dissolution and recrystallization.In the subsequent burial process,these dolomites further underwent the dissolution and alteration of oil-bearing fluids.As a result,coarse crystalline dolomite reservoirs were formed rich in dissolution pores and bitumen.Due to alteration of hydrothermal fluids in the burial process,pores were filled by dolomites,leading to densification of dolomite reservoirs to a certain extent.