Geological Evaluation Of The Shale Gas Potential Of Upper Permian Longtan Formation And Lower Jurassic Ziliujing Formation,Central West Chongqi

The shale in the lower Jurassic Ziliujing formation and the upper Permian Longtan formation are the new target for the shale gas exploration in the Chongqing region.Previously,these shale were studied mostly for their conventional oil and gas potentials and with a very little emphases on their shale gas ability.The current study was aimed to provide the detail characteristics of shale gas distribution,organic matter type and reservoir quality of these shales.Based on various analysis,the shale gas potential of the lower Jurassic Ziliujing formation and the upper Permian Longtan formation in central-west of Chongqing has been evaluated quantitatively.The sedimentary environment proposed for Longtan formation shale is transitional marine(tidal flats).The organic matter abundance(TOC)is mainly distributed in the range of 0.72% to 11.7% with an average of 3.46 %,and is therefore,considered to be a good source rock.Organic matter type is type ?,and Ro is ranges 1.88%-2.14% showing these rocks are in mature to over mature stage.The shale is exposed in syncline and buried in anticline with the depth range of 0-4000 m.The depocentres of the shale is Yongchuan-Tongliang-Jiangjin areas has the thickness of more than 90 m with 9100 square kilometers distribution.The porosity is mainly distributed in the range of 0.40% to 3.31% with an average of 1.6%.Whereas,the permeability is mainly distributed in the range of 0.02×10-3mD to 30.21×10-3 mD with an average of 8.29 ×10-3mD.Longtan Formation shale is particularly low porosity and permeability reservoirs in which the pore type is mainly the clay minerals crack.The shale gas content of Longtan Formation is 0.05-4.29 m~3/t and the coal gas content is up to 23.34 m~3/t.These gas content can be related to the distance from the coal.As the thickness of single layer is thin,and the shale,sandstone and coal are symbiotic,so the Longtan Formation is the important layer for integrated exploration on Shale gas-Coal bed gas-Sandstone gas.The resource estimations of these three kind of gas reservoirs are 12995 × 108m~3,43781 × 108m~3 and 20868× 108m~3 respectively.The sedimentary environment of Ziliujing Formation shale is continental fluvial – lacustrine.The organic matter abundance(TOC)is mainly distributed in the range of 0.05-2.68%,with an average of 0.98%,proving it as a medium source rocks.Organic matter type is Type?and Ro is between 1.28-1.39%,which shows the source is in the mature stage and lies within the oil window.Burial depth is shallower and is generally less than 2000 m.The thickness generally shallows from the center of anticline to the center of syncline and gradually become shallower near to earth surface.Dark mudstone is only present in Daanzhai Section.The Daanzhai shale have two depocentres,the Fuling area and Kaixian-Yunyang area.The thicknesses are all above 90 meters with a total of 4700 square kilometers exposure area.The porosity is mainly distributed in the range of 0.02% to 5.28%,with an average of 1.3%.The permeability is mainly distributed in the range of 0.057 mD to 1.071 mD,having an average of 0.244 mD.Ziliujing Formation shale is also low porosity-permeability reservoirs with inter-crystalline pore spaces.Daanzhai Section shale gas content is 0.005-1.81 ×108m~3.Even Daanzhai Section shale has some gas-bearing coal,but it is not up to the core area shale gas standards i.e.2m~3/t.Overall,the Ziliujing Formation shale lies within the oil window with a lesser amount of gas.The exposure area is also limited and the field desorption gas is low,mostly is 1m~3 / t or less.The preliminary estimates of the shale gas resources is 3087 × 108m~3.Based on all these observed facts,it can be concluded that the set of these shale gas should not be the main layer in the next shale gas exploration in Chongqing.