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

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.