| The gas in the organic-rich shale formation is named the shale gas. The shale gas is mainlyfilled in the natural fractures and intergranular pore， or adsorb on the surface of kerogen or clayparticle， or even dissolve in kerogen and bitumen. The gas， from thermal maturation or thecontinuous biogenicagency or both， has gradually become an important unconventional gasresources.The exploration and utilization of the shale gas indicates that it will become an attractiveprospect resources. Foe one thing， the shale gas will not only conform to the requirements ofthe national industrial policy and the development of recycling economy， for another thing， itwill also bring considerable economic， environmental and social benefits for the industries. Ifthe shale gas become a productive resource， it will play an important role in solving theproblem of the substitution of oil and gas in China.In sedimentary basins， about80%sediments of various geological periods are shale. Theshale reservoir consists of dark. organic-rich shale interbedded with silty mudstone， muddysiltstone. The shale reservoirs combination varies from basin to basin， because there aredifferent significantly geological and geochemical characteristics for different shale reservoirs indifferent petroliferous basins.The origin of the shale gas is closely connected with theconventional oil and gas accumulation， which indicates that the shale gas has a considerablepotential of exploration. As the coin has two sides，the shale gas also has it’s positive factors andnegative factors. For the positive factors， the continuous enrichment of the shale gasaccumulation charactered by a thick source rock and a wide distribution in the basin， and arelative long production lead time. For the negative factors， the exploitation of the shale gas isdifficult， for the porosity of the shale gas reservoirs is low.The shale gas is mainly enriched in marine shale in southern China.The marine shale occur in various strata of different geological history and they are well developed. Most of the marineshale is abundant in the syncline area or the lower portion of the uplift area.The study area islocated in the deposition depression area of the late paleozoic, which is developed from theepimetamorphic rocks basement of the lower part of the paleozoic. In the study area，theabundance of organic matter of the shale is high, which is matched with the basic geologicalconditions of the generation of shale gas.The exploration shale gas in China has reached an early stage. the shale gas resourcepotential evaluation. It is difficult to estimate the potential shale gas accurately throughconventional oil and gas evaluation system,due to the evaluation of the shale gas resourcepotential is not accurate.This paper is guided by modern sedimentology and sequencestratigraphy theory, and is combined with the background of regional tectonic setting. The aimsof this paper are as the fellows: first of all, we will try to analysis the depositional environmentof the shale formation from the southeastern and northeastern Hunan Province; secondly, we willrecover the litho-facies palaeogeography of the study area; finally, we will locate the organic-richshale intervals of the study area. In this paper, the potential resource evaluation system of marineshale gas will be conducted. The thickness of shale reservoirs, geochemical indicators shalereservoirs, brittle mineral content shale reservoirs, physical properties shale reservoirs are usedin the forming of the evaluation criterion for shale reservoirs. This paper describes thevolumetric method in potential resource evaluation of the unconventional shale gas and thegeneral rule of selecting the parameters which is involved in. In this paper, the potentialresources of the shale gas is evaluated, and the prediction of the enrichment zone of the shale gashas been conducted. This paper provides scientific basis for the resource evaluation of the shalegas and for the selection of the preferred target of the shale gas exploration in the study area.The sedimentary facies of the study area in the devonian-middle triassic was the platformfacies-the peripheral continental facies. There exists four types of sedimentary facies whichbenefits the deposition of the shales,namely, platform basin facies, coastal marsh facies, tidalflat facies and table facies. These sedimentary facies distribute in the lower part of thetransgressive system tract and the upper part of the highstand system tracts，which are connectedwith the development and distribution of groove platform.The development of the platformbasin facies,in the study area, is mainly occurred in the Qiziqiao stage and the Shetianqiao stagein the late devonian. Lithologically consists of argillutite and marlstone. The development of theseamarsh facies is mainly occurred in the coal-forming period of Ceshui time, Datang stage inthe early Carboniferous and the Longtan stage in the late Permian.In the study area, most of the sediments of Devonian have reached the high maturity stage-over mature stage. The Ceshui formation, Datang stage of the lower Carboniferous havebasically reached the high mature-over mature stage. The degree of evolution of Longtan Formation and Dalong Formation in upper Permian is relatively low, but it has reached highmaturity stage. The evolution of the study area shows a character of reaching the high maturitystage-over mature stage.The mineral composition of argillutite in the study area is very complicated, which consistsof clay minerals, quartz, plagioclase, potassic feldspar, calcite, dolomite and other minerals, andalso gypsum is observed in some samples. Most of the argillutite contains pyrite, which shows aneffect of the anoxic strong reducing environment during the deposition of the argillutite. Theaverage content of the brittle mineral quartz is low, which shows a greater effect on thecrack-forming ability of the shale gas reservoirs.The matrix porosity of shale and micro-fracture development, gas-bearing properties andfracturing treatment methods are dominated by brittle mineral content in the shale. The lowcontent of clay mineral and high content of brittle mineral, like quartz, feldspar, calcite, in theshales, will turn the shale into brittle shale. If external force is applied, there will be more naturalfractures and induced cracks, such as the tree-shaped cracks or net-shaped cracks in the brittleshale, which benefits the exploitation of the shale gas. In contrast, the high content of clayminerals in the shales, will turn the shale into plastic shale,which can absorb the majorityexternal force applied on the shale. If external force is applied, there will be more planar cracks,which has negative effect on the post-transformation of the shale.Although in the study area thethe average content of quartz is low, but the content of all fragile mineral reached51.57%,it’s infavor of shale gas reservoirs in the late fracturing treatment, which is conducive to the fracturingtransformation of shale gas reservoirs.The shale reservoir is characterized by the development of many types of micro-pore and ithas extreme low porosity and extreme low permeability. These micro-pore can be divided intothe following types: grid holes, corrosion holes, holes of organic matter, organisms bore andmicro-cracks, etc. The shale gas is mainly found in micro-pore, for high surface to volume ratio,the complex structure of micro-pore and the large internal surface area enable micro-pore to holdlarge amounts of adsorbed gas.On the basis of the test experiment, combined with the gas production characteristics ofdifferent types of kerogen in different stage of organic matter evolution, systematic analysis theTOC, Ro geochemical parameters and specific (pore volume, pore size, pore throat sorting, etc.)impact factors on the gas content, determine the correspondence between the reservoir property,mineral content, geochemical characteristics and the gas containing of the shale reservoir.Determined the gas content of the key mud shale bed, provided information on the basis of shalegas resource potential assessment in the study area.First calculated the resource potential of the mud shale system in central Hunan,southeastern Hunan, Hunan northeastern region by volume method, preferred favorable shale gas exploration and development blocks. In the study area, XiangYe1well is successful with naturaldeliverability of shale gas, on-site desorption with the highest gas0.7334cm3/g, confirmed thebroad shale gas exploration prospects in the study area.The findings and the methodology adopted in the study can provide important guidancereferences for the exploration of the shale gas in the central of Yangtze region or even insouthern China. |
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