Reservoir Characteristics And Formation Mechanism Of The Lower Cambrian Maidiping-Qiongzhusi Formation In Southern Sichuan

With the development of shale gas exploration in Sichuan basin,Maidiping formation-Qiongzhushi formation of Lower Cambrian Series in South Sichuan basin are the most favorable shale gas exploration formations following after Longmaxi formation of Lower Silurian Series.It is necessary to study the characteristics and formation mechanism of shale gas reservoir systematically.Based on the materials of outcrop-well-seismic data,analysis of stratigraphic sequence,lithofacies characteristics,source-reservoir characteristics and controlling factors of shale gas reservoir development is systematically conducted and favorable prospects of high quality shale gas reservoir are predicted.The sedimentary of Maidiping formation-Qiongzhushi formation in research area is mainly controlled by Tongwan orogeny in the end of Sinian and XingKai orogeny in the early Cambrian.Tongwan orogeny has a character of tectonic compression,and Xinggai orogeny resulted in structural inversion,which has a character of extension.In the process of structural inversion,Mianyang-Changning intracratonic sag is formed by the depression which controls the development and distribution of black shale in Lower Cambrian Series.On the basis of the research on lithostratigraphy,biostratigraphy and chronostratigraphy,the materials of outcrop-well-seismic data,global sea level change curve,geochemistry and paleontology are applied to identify five third-order sequence boundaries,divide four third-order sequences and distinguish transgressive systems tract and regressive systems tract in every third-order sequence without differentiating highstand systems tract and lowstand systems tract.The sequence stratigraphic framework is established.SQ1 is equivalent to Maidiping formation;SQ2 is equivalent to lower Qiongzhushi formation;SQ3 is equivalent to lower part of upper Qiongzhushi formation;SQ4 is equivalent to upper part of upper Qiongzhushi formation.Lithofacies characteristics of phosphorite and shale are depicted in detail through the observation of core microsection.According to the structure-origin classification principle of phosphorite lithofacies,phosphorite in research area can be divided into four types:chemical origin?micritic phosphorite?,mechanical origin?grained phosphorite?,biogenic origin and diagenetic origin?cored phosphorite and metasomatic phosphorite?.The name principle based on components?silicic-carbonate mineral-clay mineral?and granularity?sandstone-silty sand-clay?is adopted to divide the shale into four types:claystone,mudstone,siltstone and sandy mudstone,and further subdivide into nine types of lithofacies combined with sedimentary structure and organic content.In the light of lithofacies association characteristics,paleotological features,geochemical signature and logging characteristics,facies and subfacies in research area such as platform and shelf are identified and facies spatial distribution in sequence frame is analyzed.Based on the test data of organic geochemistry,petrology mineralogy,scanning electron microscope,pore structure and gas content,source-reservoir characteristics of targeted formations is studied intensively.It is suggested that the organic type of shale is I type sapmpelic kerogen and condensed section has high abundance of organic matter which has TOC range from 1.06%to 4.89%and the thermal evolution degree is high in which T_maxax is between 520 and 530?and R_o is between 2.35%and 5.5%,and productivity index?PI?is high which is between 0.1 and 0.48.The average content of brittle minerals?quartz,feldspar and carbonate mineral?is more than 50%,the average content of clay minerals is lower than45%which is dominated by illite smectite mixed layer and illite and has high brittle.According to the dynamic Young's modulus and Poisson's ratio analysis,SQ2,SQ3 and SQ4TST/C.S have a character of low dynamic Young's modulus and low Poisson's ratio brittle section,and SQ1-RST is tight section,and SQ4-RST is plasticity section with high dynamic Young's modulus and high Poisson's ratio which formed favorable top and bottom closure.The microscopic reservoir space of shale can be divided into three types:organic pore,inorganic pore?intercrystalline pore of clay mineral,intergranular pores of brittle mineral,intragranular dissolution pore and pyrite intergranular pore?and microfrature?diaclase,diagenetic contraction fracture and dissolution fissure?and further subdivided into eight types.The pore volume of shale condensed section is big?average at 0.00743cm³/g?and BET specific surface is large?average at 10.11m²/g?,and the pore morphology is dominated by thin neck ink bottle pore,and has a small quantity of slit-type pore and cracks between two parallel plates.The micropore mesopore are prominent?average at 65.81%?,and micropore takes the second place?average at 25.04%?,and macropore is few?average at 9.11%?.The pore volume of shale regressive systems tract is small?average at 0.00575cm³/g?and BET specific surface is small?average at 2.575m²/g?.The pore morphology is mainly tablet slit structure,crack and wedge-shaped structure.The mesopore?average at 49.33%?and macropore?average at 41.12%?are dominant and micropore is less?average at 9.5%?.Phosphorus containing sequence SQ1 has lower porosity?average at 0.69%?and lower permeability?0.101md?than shale sequence SQ2-SQ4 which has an average porosity of 1.82%and permeability of 0.104md.The porosity?average at 3.25%?and permeability?average at0.23md?of condensed section is higher than regressive systems tract which has an average porosity of 1.35%and permeability of 0.0623md.The permeability of regressive systems tract is extremely low which indicates that rock texture is relatively hard and microcrack is not common.The gas content is gradually increasing from the bottom up vertically,in which the gas content of condensed section is obviously higher than regressive systems tract and gas content is positively associated with TOC.From the point of tectonic evolution,sedimentary fill and buried hydrocarbon generation coupling,abundance of organic matter,brittleness,the thickness of shale,the development of pore and the controlling factors of shale gas accumulation of favorable shale lithofacies are analyzed.?1?The four fundamental controlling factors of organic accumulation are paleoclimate,primary productivity,the rate of sedimentary and conservation environment of organic matter,and organic accumulation model is established.It is suggested that yangtze plate was located in subtropics in early Cambrian,during which the climate was warm and upwelling was formed in transgressive period and it prompted the formation of high primary productivity in ocean surface and anaerobic environment in ocean floor,and black shale is widely distributed under the background of moderate sedimentary rate.?2?After analysis of brittle mineral evolution in sequence frame,it is considered that the supply of carbonate mineral and biogenic silica in transgressive period,and the inject of terrigenous clast in regressive period are the reasons for the high brittleness of shale reservoir.?3?The type of shale reservoir pore and distribution differences are controlled by abundance of organic matter,thermal maturity,diagenesis and pore fluids pressure.The abundance of organic matter is the prerequisite for development of organic pore.The organic pore formed in hydrocarbon generation process is the most favorable reservoir space in south Sichuan basin.The intercrystal pore of clay mineral formed in diagenesis increase the ability of shale reservoir.In addition the overpressure in stratum can protect pore in some extent.?4?The south Sichuan basin experience multiphase tectonic movements.Qiongzhushi formation in B1 wellblock has big thickness of roof and floor of favorable shale reservoir and stable distribution and tight lithology and high formation pressure and good sealing condition.The floor of Maidiping reservoir is relatively thin because of the paleocrust of weathering in Tongwan period.“Five degree and three conditions”?thickness,abundance of organic matter,maturity,brittleness,paleo-water-depth,tectonicconditions,geopressureconditions and prosity-permeability conditions?of favorable shale reservoir is proposed.Geological condition evaluation index of shale gas is established.Condensed section of SQ2-SQ4 in sequence frame is?-?type favorable shale reservoir.The thickness of condensed section of SQ1 is big,but the hydrocarbon generation potential is weak and the reservoir condition is poor and the floor is thin,which belongs to type?reservoir.Combined geology with geophysical prospecting,it is suggested that the type?-?reservoirs distribution in trough and depression of Maidiping formation-Qiongzhushi formation in lower Cambrian series are the favorable shale gas exploration area.