重庆北碚地区下侏罗统自流井组大安寨段沉积特征

Sedimentary characteristics of the Da'anzhai member of the Lower Jurassic Ziliujing Formation in the Beibei district, Chongqing

关键词:  北碚地区  大安寨段  介壳灰岩  滨浅湖

Key words:  Beibei district  Da'anzhai member  shell limestone  littoral-shallow lake

摘要:

大安寨段介壳灰岩作为四川盆地主要的非常规产油层,岩性及沉积特征较为特殊。通过野外露头观察及岩石薄片、铸体薄片、扫描电镜等分析手段,对重庆北碚地区下侏罗统自流井组大安寨段的沉积特征进行了系统分析。结果表明:重庆北碚地区侏罗系大安寨段岩性复杂,以亮晶介壳灰岩、粉晶白云岩、泥质介壳灰岩和泥页岩为主,次为生屑云岩和砂屑灰岩。大安寨段属受陆源碎屑影响明显的湖相碳酸盐岩沉积,可划分出滨浅湖亚相及高能介壳滩、高能砂屑介壳滩、滩间洼地、低能介壳滩等多个微相。其中,高能砂屑介壳滩水动力条件强、原生孔隙较发育。虽受埋藏成岩作用影响而使原生孔隙消耗殆尽,但后期溶蚀作用和构造作用改造仍可形成特低孔、低渗的非常规碳酸盐储层。

Abstract:

The shell limestones from the Da'anzhai member occur as the unconventional source rocks in the Sichuan Basin. A detailed study of sedimentary characteristics of the Da'anzhai member of the Lower Jurassic Ziliujing Formation in the Beibei district, Chongqing is presented on the basis of field outcrops, thin sections, cast sections and scanning electron microscopy (SEM). Lithologically, the Da'anzhai member of the Lower Jurassic Ziliujing Formation dominantly consists of sparry shell limestone, silty dolostone, muddy shell limestone, and mudstone and shale, with subordinately bioclastic dolostone and sandy limestone deposited in the siliclastic-dominated lacustrine carbonate sedimentary environments. The sedimentary subfacies and microfacies include littoral-shallow lake subfacies, and high-energy shell bank, high-energy sandy shell bank, interbank depression and low-energy shell bank microfacies. Of which the high-energy sandy shell bank has highly hydrodynamic conditions, and welldeveloped primary porosities. In spite of the disappearance of the primary porosities by burial diagenesis, the paulopost erosion and tectonism may still facilitate the generation of unconventional carbonate reservoirs with extraordinarily low porosity and low permeability.