Evaluation And Prediction Of “Engineering Desserts” Of Lower Paleozoic Marine Shale Gas In Complex Structure Area

In south China,some adverse factors,such as strong tectonic deformation,large difference of shale brittleness,complex stress state,obscure distribution of natural fractures,seriously affect the effect of hydraulic fracturing in shale reservoirs,which makes it difficult to realize the commercial exploitation of shale gas in “geological desserts”.Hence,this paper takes Guizhou province Cen'gong block as an example.Based on structural geology and Geomechanics,the fracture system,the development characteristics and main controlling factors of natural fractures,the characteristics and distributions of shale brittleness,the characteristics and distributions of the ancient and modern stress fields,the characteristics of formation pressure and the distributions of natural fractures in the lower Cambrian Niutitang formation are deeply studied using drilling,well logging,seismic data,cores,thin sections,experiments and so on to establish a set of evaluation parameters system and prediction method for the "engineering desserts" of Lower Paleozoic marine shale gas in complex structure area of south China.The main research results are as follows:The high dip-angle reverse faults in Cen'gong block are well-developed,which are dominated by NE and NNE.The NNE fault system is mainly formed in the period of Yanshan movement and Himalayan movement,and the NE fault system is a complex fault system formed in many stages.The structure in the centre and south of the study area is stable,with relatively few faults and folds.The folds are mainly trough-like folds.Back thrust faults are often developed on the two wings of the wide and slow anticlines,and hedging thrust faults are often developed on the two wings of the closed synclines;The Niutitang Formation mainly develops structural fractures,including high dip-angle shear fractures,high dip-angle tension fractures,high dip-angle twisting fractures and low dip-angle detachment fractures.Non-structural fractures are mainly bedding fractures.The fracture development is mainly influenced by structure stress,structure position,rock mechanical properties,TOC,mineral component,diagenesis and other factors,of which structure factor and rock mechanical properties are the dominant factors;Compared with various methods,matrix modulus method is considered to be more suitable for extracting the shear wave slowness of shale reservoir to calculate dynamic rock mechanics parameters.Mineral component,rock mechanical parameters,log interpretation and 2D seismic data are used to characterize shale brittleness from different angles,and the Niutitang shale brittleness is comprehensively evaluated both vertically and horizontally;The Niutitang formation pressure in study area is generally not high and presents low pressure or normal pressure.Some gas-bearing layers are characterized by micro-high pressure.The present maximum horizontal principal stress in study area is NE-SW direction based on the borehole breakout method and drilling-induced fracture method.The present principal stress and the difference coefficient of horizontal principal stress are calculated using logging data;The tectonic stress field simulations of the Himalayan period and present period show that the present horizontal principal stress difference in the middle of the study area is smaller than that in east and west,and the development of natural fractures gradually becomes high from the SE to the NW.A set of evaluation method and index system for the "engineering desserts" of the Lower Paleozoic marine shale gas in complex structure area of south China is established based on four key parameters "fracture structure,natural fracture,shale brittleness and in-situ stress field",and the "engineering desserts" of the Niutitang formation in study area is selected.