Accumulation Model And Gas-bearing Evaluation Of The Shanxi Formation Shale In Fuxian Area Of Ordos Basin

The marine-continent transitional shale is widely distributed and covers a large area in China,with favorable geological conditions for shale gas accumulation and broad resource prospects,which is also well developed in the Ordos Basin.This paper takes the transitional Shanxi Formation shale from the Fuxian block on the southeastern margin of the basin as the study object.Combining with organic geochemical parameters,pore-fracture structure and gas content,this paper discusses the gas-bearing change and enrichment factors,establishes shale gas accumulation model of the Shanxi Formation shale gas,and selects the beneficial shale gas intervals.The main conclusions are as follows:The organic matter enrichment model of the Shanxi Formation shale in Fuxian area is established.The Shanxi Formation shale is formed in the transitional facies environment characterized by delta front facies,and develops clay minerals mainly composed of kaolinite and illite,followed by quartz content.The Shanxi Formation shale was in oxic water environment with stable paleoproductivity during the depositional period.The climatic condition changing from warm-humid to hot-arid and terrestrial input intensity are the main controlling factors of the organic matter enrichment model of the Shanxi Formation.Under this model,type III kerogen is enriched.Total organic content(TOC)varies from 0.3%to 7.5%,and tends to decrease from bottom to top.The thermal maturity(Ro)of organic matter is 2.6%?3.1%,which is in the over-mature stage.The reservoir physical characteristics and main controlling factors of the Shanxi Formation shale are pointed out.The Shanxi Formation shale mainly develops mineral inter-granular pore and intra-aggregate dissolution pore with less organic pores.Pore diameter is generally small,mainly micro and mesopores.Core fractures are developed,including bedding fractures,shear fractures and tectonic stress fractures,while micro-fractures mainly include mineral shrinkage fractures,organic matter shrinkage fractures and tectonic stress fractures.The shale has low porosity,permeability,specific surface area and pore volume which represents poor physical properties.The pore-fracture structure is complex which is dominated by microfractures,and the development of microfractures affects the preservation of pores.The hydrocarbon generation of organic matter and the compaction of the overlying strata are the main controlling factors of the pore-fracture structure in the Shanxi Formation shale.The gas-bearing characteristics and main controlling factors of the Shanxi Formation shale are analyzed.The in-situ desorption experiment results indicate that the Shanxi Formation average shale gas contents of the Yan 2156 well and the Chang 96 well are 1.41 m~3/t and0.81m~3/t,respectively,and the gas content decreases from bottom to top.The proportion of the free gas ranges from 26.62%to 81.1%which is calculated by the isotope distribution model,and the maximum theoretical recoverable coefficients of two wells are 89.43%and 77.26%which are calculated by the gas content distribution model.Stable structure,high TOC value,over-mature stage,stable distribution of shale reservoirs,and good preservation conditions all together lead to the accumulation of shale gas in the Shanxi Formation.The Shanxi Formation shale gas accumulation model is established,and favorable intervals is optimized.The shale gas is mainly composed of methane,containing a small amount of ethane and rare non-alkane gas,which belongs to the late mature thermal gas.According to the oil-type gas found in the Shanxi Formation,this paper suggests two possible sources of the Shanxi Formation shale gas that one kind is from the interior of Shanxi Formation,and the other kind is from carbonate rocks in the Taiyuan Formation.Considering the factors of reservoir physical properties,structural characteristics and preservation conditions,this paper establishes the monoclinic mixed gas source accumulation model of the Shanxi Formation shale gas,and points out that the bottom of Shan 1 section and the middle and upper of Shan 2 section are favorable intervals of the Shanxi Formation shale gas.