| In shale gas exploration activities at home and abroad now,the shale gas exploration of highly over-mature shales(the shales in which the organic matter is highly metamorphic and vitrinite reflectance is greater than 3.5%)are not ideal.Whether the highly over-mature shales are suitable for shale gas enrichment and how to make a breakthrough in such shale gas exploration have become the important theoretical and practical problems existed in shale gas geological research and exploration activities.Taking the Qiongzhusi black shales in the southwest Yangtze plate as an example,this paper attempts to clarify the characteristics,formation and evolution laws of the highly over-mature shale gas reservoirs,so as to provide advices for shale gas exploration of this type black shales.Based on the data from outcrop,drilling,geochemical,scanning electron microscope,low temperature nitrogen adsorption,mercury injection and other data,the mechanism of organic matter enrichment,diagenetic evolution and reservoir characteristics of the highly over-mature shale gas reservoirs of Qiongzhusi Formation in the study area have been comprehensively studied by using the principle or methods of sedimentology,geochemistry,unconventional reservoir geology and so on.Some issues about the highly over-mature shale gas reservoirs such as the causes for reservoir quality reduction and factors that have influence on reservoir evolution,the origin of low gas content(most of the gas is nitrogen),and shale gas exploration potential were discussed.The influence factors of pore evolution and the diagenesis-pore evolution model of the highly over-mature shale gas reservoirs were also discussed.The main conclusion as follows:(1)Organic rich shales are well developed in Qiongzhusi Formation in the study area.In the Mianyang-Changning intracratonic sag the thickness of shales with TOC more than 2% is up to more than 70 m,and the mechanism of organic matter enrichment in black shales can be explained by the irregular bottom topography model.In the eastern area,the thickness shales with TOC greater than 2% is generally 30 m ? 50 m,and the mechanism of organic matter enrichment in black shales can be explained by the upwelling model.(2)The black shales of Qiongzhusi Formation have undergone the superimposed transformation of compaction,cementation,metasomatism,dissolution,organic matter maturation and other diagenesis,and the evolution of pores in the shales can be divided into three stages: primary pore failure(Ro < ?1.1%-1.3%),organic pore formation(Ro = 1.1%(1.3%)? 2.0%)and late pore failure(Ro > ?2%).During the primary pore failure stage,there is porosity reduction by mechanical compaction and pore filling by asphalt.During the organic pore formation stage,a large number of organic matter pores were formed and porosity increased.During the late pore failure stage,as the formation of organic matter pores gradually weakened,porosity reduces under compaction.Organic matter pores are the main secondary pores in the black shales.Dissolution and other diagenesis have no or little contribution to the secondary pores.(3)Thermal metamorphism of organic matter in the shales of Qiongzhusi Formation were mainly affected by temperature that increased with formation burial depth.It is the large maximum burial depth not the Emei mantle plume of magma activity that accounts for the high maturity of organic matter in the shales of the Qiongzhusi Formation.The Emei mantle plume of magma activity had an effect on the thermal metamorphism of organic matter but cannot lead to the present maturity.(4)Reservoir quality of the Qiongzhusi black shales is poor.Compared with the high-quality shale gas reservoirs,the Qiongzhusi black shale is characterized by high maturity of organic matter,low porosity,poor development of inorganic and organic pores,low pore connectivity,low specific surface area and pore volume.Low porosity and specific surface area lead to its poor properties for shale gas storage.(5)Compaction is the leading factor for pore evolution of the highly over-mature shale gas reservoirs.Organic matter in shale gas reservoirs is easily affected by compaction because it is under pressure during burial.So the effect of compaction on the pores of shale gas reservoir is still considerable during deep burial.The higher the TOC is,the stronger the impaction is.Reduction of organic matter pores in shale gas reservoirs is mainly related to compaction rather than graphitization of organic matter.Organic matter in shale gas reservoirs is easy to deform and migrate under pressure.Pores in organic matter are gradually destroyed,become irregular in shape,and even disappear with the progress of compaction.As the formation of pores in organic matter gradually weakens after Ro > ?2%,compaction becomes the leading factor for pore evolution,resulting specific surface area and porosity of the shale gas reservoirs monotonically decrease.There is a depth at which black shales become non-reservoirs as the pores are seriously damaged and the porosity drops to less than 2%.This depth can be defined as the lower depth limit of shale gas reservoirs.A burial depth of 8000 m ? 8500 m might be the lower depth limit of shale gas reservoirs of Qiongzhusi Formation in the study area.(6)There is a burial depth window for shale gas exploration.In the case of no tectonic-thermal events,there is a parabola relationship between the porosity of shale gas reservoir and the depth or Ro after the oil generation peak,indicating that there is a burial depth window in which effective shale gas reservoirs or high-quality shale gas reservoirs are developed.This burial depth window is also the largest potential window for shale gas enrichment,which is called the depth window for shale gas exploration in this article.The threshold depth for shale gas exploration is consistent with that of the wet gas window.The threshold depth ensures the gas source for shale gas enrichment and is also the depth at which the high-quality shale gas reservoirs start to develop.The lower depth limit for shale gas exploration is a depth at which black shales becomes to be non-reservoirs for shale gas enrichment.In the case of no tectonic-thermal events,the maximum buried depth corresponds to Ro well,so Ro can be used to calibrate the exploration window(that is,Ro window for shale gas exploration).The concept of exploration window has practical significance as lower depth limit for shale gas exploration is within tectonic subsidence range in some areas.(7)The special burial history of the shales accounts for the low shale gas content(most of the gas is nitrogen)in Qiongzhusi Formation in the study area.The unconformity at the bottom of Qiongzhusi Formation and the reginal shale gas preservation conditions are the secondary influence factors.There are mainly two stages of shale gas loss in the Qiongzhusi Formation,that is hydrocarbon expulsion caused by compaction and formation pressure drop(Himalayan tectonic movement).The low shale gas content(most of the gas is nitrogen)in Qiongzhusi Formation in the study area and adjacent areas is the result of the strong hydrocarbon expulsion.(8)The highly over-mature shales can be potential shale gas reservoirs.There is low exploration potential of such gas shales in the normal formation pressure areas because the pores in the shales have been seriously damaged.However,in the abnormal high formation pressure areas,as the destruction of the pores in shales caused by compaction lags behind that in normal pressure areas,the pores in shales are well preserved when the organic matter is highly over-mature(at least in the early of this stage),so the shales are still good potential shale gas reservoirs. |
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