四川盆地元坝地区下侏罗统介壳灰岩储层分布预测

Distribution Prediction of Lower Jurassic Shell Limestone Reservoir in Yuanba Area of Sichuan Basin

四川盆地下侏罗统自流井组大安寨段一亚段(大一亚段)灰岩是元坝地区主要产气层。岩心、薄片、物性资料、试油、录井、测井、地震资料综合研究表明,大一亚段灰岩主要为介壳灰岩和含泥介壳灰岩,介壳灰岩平均孔隙度2.36%,平均渗透率为0.02×10^(-3)μm^2,含泥介壳灰岩平均孔隙度为4.59%,平均渗透率为0.21×10^(-3)μm^2。介壳灰岩为浅湖亚相高能滩微相沉积,含泥介壳灰岩为浅湖亚相次高能滩微相沉积。地震波形分类上,以介壳灰岩为主的高能滩微相为高连续低频中强波谷反射,以含泥介壳灰岩为主的次高能滩微相为高连续低频中弱振幅波谷反射。高连续低频中强振幅波谷反射和高连续低频中弱振幅波谷反射以SW-NE向分布于研究区中央地带。以伽马值大于46 API为门槛值剔除泥岩、泥质粉砂岩等,得到介壳灰岩和含泥介壳灰岩的伽马数据体。精细刻画了高能滩微相介壳灰岩和次高能滩微相含泥介壳灰岩的分布范围,新增天然气有利区面积518 km^2,为元坝地区大安寨段天然气储量和产量的增加奠定了坚实的基础。

The shell limestone in the first submember of the Da＇anzhai member（ D1sm） of the Lower Jurassic Ziliujing Formation is an important natural gas reservoir in Yuanba area,Sichuan Basin. In this paper,we conducted an integrated study based on cores,thin sections,porosity and permeability data,oil test data,well logging data,and seismic data in order to predict the distribution of the shell-shoal limestone reservoir in the D1 sm. The D1 sm are composed of mainly shelly limestones and argillaceous shelly limestones. The shelly limestones have an average of porosity of 2. 36% and an average of permeability of 0. 02 × 10^-3μm^2,while the argillaceous shelly limestones have an average of porosity of 4. 59% and an average of permeability of 0. 21 × 10^-3μm^2. The shelly limestones in the D1 sm comprise the high-energy shoal microfacies that developed in the shallow lake subfacies,and the argillaceous shelly limestones are interpreted as moderate-energy or subhigh-energy shoal microfacies. The classification of seismic waveform shows that the reflection characteristics of the high-energy shoal microfacies are well continuity,low frequency,moderate to large amplitudes,and wave troughs in the seismic profiles,and that the reflection characteristics of the moderate-energy shoal microfacies are well continuity,low frequency,moderate to small amplitudes,and wave troughs. These two types of waveforms distributed from southwest to northeast along the central part of the study area. The natural gamma data cube of the shelly limestones and argillaceous shelly limestones was constructed after rejecting the mudstones and muddy siltstones by giving the threshold value of 46 API. The distribution of shelly limestones（ high-energy shoal microfacies）and argillaceous shelly limestones（ moderate-energy shoal microfacies） was precisely mapped; the result shows that the favorable exploration area of natural gas has enlarged by about 518 km^2. The conclusions of this study provide a solid foundation for the increase of the natural gas reserves and production of the Da＇anzhai member in Yuanba aera.