Research On Geological Characteristics Of Shale Of Lower Silurian Longmaxi Formation In Yudong Area

Shale gas exploration and research in east Chongqing is still in its early stage except Jiaoshiba area.This paper aims at the marine shale of lower Silurian Longmaxi Formation in east Chongqing.Based on large amounts of relevant research results at home and abroad,guided by stratigraphy,paleontology,sedimentary petrology,geochemistry,unconventional oil and gas geology and petroleum geology,used outcrop data,drilling,seismic,logging materials and organic geochemistry data,combined with regional geological evolution background,the thesis has selected sedimentary facies,mineral composition,TOC content,organic matter maturity,physical property,gas content to be the key parameters,then forecasted favorable zones according to the threshold value of these parameters.The study provides significant theoretical and technical supports to the extraction of mineral shale gas in east Chongqing Longmaxi Formation.The thickness of Longmaxi Formation is 200~600m in research area,and well JY1—C7(Shizhu—Zhongxian—Wanzhou)area is the thickest region,up to 600 meters,and its residual thickness is getting thinner in surrounding area.Silty Mud Shelf microfacies and Mud Shelf microfacies are the dominant sedimentary facies in the study area.The types of kerogen in research area are mainly sapropel types(I),with little humic sapropel types(?1);the organic carbon content is from 0.06%to 9.64%and the average content is 1.96%;the maturity is already reach high-over-mature stage,with an average content of 2.42%.The distribution of effective shale is accord well with sedimentary facies and TOC,Silty Mud Shelf microfacies and Mud Shelf microfacies are in high TOC value and thick effective shale.The mineral composition of Longmaxi Formation in study area is mainly quartz,clay minerals and calcite,with some feldspar,dolomite and pyrite;the brittleness index is over 50%by calculation,the brittleness index which over 40%is good for creating net-shaped fractures in extraction;the reservoir space is dominated by intergranular pore and organic pore;the porosity is from 1%to 5%,and the permeability ranges from 100 nanodarcy to 0.1 millidarcy,so,shale is an extra-low porosity and extra-low permeability reservoir.The gas content obtained from desorption method show that the gas content is high with good exploitation prospects.By observing the normal log curves,gas-bearing shale(mudstone)has obvious logging characteristics—"Four High Three Low":high GR valu?.high AC value?high CAL value?high CNL value?low resistivity?low DEN value?low Pe.Regard triassic gypsum rocks as the cap rocks and compact rocks conformably contacting top and bottom of Longmaxi as roof and floor,good sealing effect of shale,favorable structure condition and over pressured in formations,all above the mentions provide good storage conditions for Longmaxi shale gas.By selecting sedimentary facies,organic carbon content,maturity,burial depth,siliceous and clay minerals content,effective thickness porosity,established a set of evaluation standard of shale gas exploration in east Chongqing and 3 target exploritation areas were selected:east-Fuling area?Baoluan-Fengdu-Zhongxian area?Lichuan area.