四川盆地普光气田飞仙关组白云岩储层成因探讨

下三叠统飞仙关组储层为普光气田重要的天然气储集层,储层厚度大、物性好。优质储层形成机制直接与碳酸盐岩成岩作用有关,其中白云岩化是重要因素之一。通过对岩石结构的分析,结合白云石的有序度、碳氧同位素地球化学特征及沉积环境,对普光气田飞仙关组白云岩成因进行了研究,表明飞仙关组白云岩主要为准同生期渗透回流的成因类型,证据如下:①飞仙关组白云岩明显地保持原岩矿物结构特征,反映了相对快速的白云化过程;②以微泥晶组成鲕粒的亮晶鲕粒白云岩与准同生微泥晶白云岩有相似的低有序度;③白云岩的碳氧同位素组成特征、古盐度值表明白云化流体的高咸度特征,明显有别于混合水成因白云岩;④飞仙关组台地边缘鲕滩毗邻局限台地环境,容易获得大量高Mg/Ca比咸水,有利于白云岩发育,因此,更有利于形成大量早期白云岩。由此认为,飞仙关组白云岩主要为准同生期渗透回流白云岩,并提出毗邻局限台地或蒸发台地等高盐度环境的礁滩相更有利于优质储层的发育。

Discussion on origin of dolomite reservoirs in Feixianguan Formation,Puguang Gas Field,Sichuan Basin

As an important natural gas reservoir in the Puguang Gas Field, the Feixianguan Formation in the Lower Triassic is featured by big thickness and good physical property. The formation mechanism of high-quality reservoirs is directly related to carbonate diagenesis, and the dolomitization of carbonate rocks is one of the key controlling factors. Based on rock structure analysis and combined with the order degree of dolomites, the geochemical characters of carbon and oxygen isotopes and sedimentary environments, it has been analyzed in this paper the dolomite genesis of the Feixianguan reservoirs in the Puguang Gas Field. It has been concluded that the dolomites in the Feixianguan Formation generated from seepage reflux during penecontemporaneous stage with the following evidences: 1) The structural characteristics of the original rock minerals remain obviously in the dolomites in the Feixianguan Formation, indicating for rapid dolomitization. 2) The sparry oolitic dolomites composed by micritic oolites have the same low order degree with the contemporaneous micro-micritic dolomites. 3) The carbon and oxygen isotopes composition and the paleosalinity of dolomites show that the dolomitizing fluids have high salinity, which is obviously different from the mixing water dolomitization. 4) The platform edge oolite beach facies of the Feixianguan Formation is adjacent to restricted platform, so it is easy to obtain quantities of saline water with high Mg/Ca ratios. What's more, this environment is beneficial to the development of dolomites and the formation of quantities of early dolomites. Thereby, the dolomites in the Feixianguan Formation mainly came from seepage reflux during penecontemporaneous stage. The reef-bank facies adjacent to the high-salinity environments such as restricted platform or evaporate platform facies are favorable for the generation of high-quality reservoirs.