It is generally accepted that the Da＇ anzhai member of the Lower Jurassic Ziliujing Formation in Sichuan Basin is tight reservoir and has great difficulties in prospecting. Based on the description of field sections and drilling cores, analysis of thin sections and FESEM, the reservoir characteristics of Da＇anzhai member were studied in combination with the data of lithofacies paleogeography, logging, porosity and permeability. It is identified that the Da＇anzhai member is a typical unconventional reservoir with ultra-low porosity and permeability, and the reservoirs have fracture-dissolved pores, caves and fractures. The lake slope facies between shallow lake shell beach and semi- deep and deep lacustrine are favorable for reservoir development. The reservoir space is composed of different sizes of various fractures, dissolved pores, caves and dissolved fractures, grain boundary pores （fracture）, intragranular dissolved pores, intercrystalline pores and so on, as well as nm-p.m level organic pores （fracture） and interlamination pores （fracture） of clay minerals. The development conditions of reservoirs can be summarized as three major elements of ＂layer, facies and position＂. The layer element refers to the second sub-member, which has stable regional distribution, large thickness, high content of organic matters and fragile minerals in mud shale and well-developed nm-μm level organic pores and interlamination pores （fracture） of clay minerals in substrate rock. The facies element refers to the lake slope facies belt composed of interbedded black shale and shell limestones with uneven thickness. in which high-density fi＇actures formed during the diagenesis stage due to differential compaction. The position element refers to where muhi-type and multi-stage fracture and dissolution diagenesis were superposed, it is the location of relatively high quality reservoir in the tight reservoirs, and is also the ＂sweet spot＂ of the oil shale gas （oil） in l）a＇ anzhai member.
Northwest Oil ＆ Gas Exploration
shale gas （oil）
lake slope facies belt