Study On Shale Gas Accumulation Process And Enrichment Mechanism Of Wufeng-Longmaxi Formation In Changning Area

The study of shale gas accumulation process and enrichment mechanism is the basis for efficient shale gas exploration.This thesis focuses on the Wufeng-Longmaxi Formation marine organic-rich shale in the Changning area.Based on data collection,field investigation,and series of experiments,this thesis clarified the organic geochemical characteristics of the source rock and the mineral composition of the reservoir,and finely characterized the multi-scale pore structure of the shale reservoir and the characteristics of shale gas storage and permeability.By combining low-temperature thermochronology and basin simulation,the generation and transport history of shale gas evolution is restored under the control of tectonic.Based on the shale gas multi-scale migration model,the thesis calculates the shale gas emission in the geological history,and reveals the shale gas accumulation process and enrichment mechanism in the Changning area.The main conclusions are as follows:(1)The thickness and organic carbon content are higher in north and lower in south in the study area.The types of macroscopic fractures of shale mainly include high-angle shear fractures,fractures parallel to the shale bedding,and a few small-scale tensile fractures perpendicular to the bedding.The organic carbon content is positively correlated with the micropore volume and pore specific surface area,which is the core internal factor that affects the gas storage capacity of the reservoir.The permeability of the target layer is 0.008?0.037 m D,and the diffusion coefficient of the isobaric pressure method is 9.0×10^-13-6.3×10^-12 m²/s.Based on the characteristics of pores and fractures and the relationship between porosity and permeability,the thesis proposes that the isobaric pressure diffusion coefficient is suitable for characterizing the capping ability of shale,and diffusion coefficient of pressure decay method is suitable for characterizing the gradual desorption process of shale gas source outward.(2)Two periods of paleofluid activity were identified,corresponds to the last hydrocarbon generation stage and the generation of large-scale fractures in the Paleogene.In the wide and gentle Jianwu syncline,high-conductivity fractures are not developed at the distal end of the denudation line,and the paleo overpressure can be maintained.At outside and the edge of basin,shale gas escapes through the fracture interconnection network in closed anticlines and the proximal areas of large-scale open faults.(3)The oxygen-containing functional groups and aliphatic chains of kerogen molecules are basically consumed during the thermal evolution process,while ductility and orientation of aromatic units increase through aromatization,ring condensation and stacking.33%to 54%of the aromatic layers are concentrated in the main direction.The small molecules depolymerized by hydrocarbon generation form internal pores of organic matter,while the residual organic matter shrinks to produce edge pores.When the hydrocarbon generation is weakened,with the increase of the order and stacking of the aromatic structure,the gap between the fringes decreases and the organic pores gradually decrease.(4)The shale gas accumulation process of the target layer in the study area can be divided into four stages:shallow burial,initial accumulation,main accumulation and accumulation adjustment period.The first two stages are dominated by compaction,with drainage reduction and clay mineral transformation.The main accumulation stage is dominated by hydrocarbon generation,with large-scale gas generation,increase in organic pores,filling of its own reservoir space,and episodic hydrocarbon expulsion.The adjustment stage of hydrocarbon accumulation is dominated by differential structural deformation.After moderate uplift,shale gas is easy to preserve in large-scale continuous reservoirs,which is a key stage determining shale gas enrichment.(5)The simulation results reveal that even with a very small permeability(>10^-21m²)the shale gas will escape within a short period(relative to the geological history)after the formation of a connected fracture network.It is proposed that the effective preservation distance is the sum of the fracture connection distance and the scale diffusion distance.The continuous reservoir and scale diffusion distance are the key to distinguish the preservation conditions of shale gas.From the perspective of reservoir pressure evolution,the influence of structural uplift on shale gas preservation is analyzed.When the burial depth is<1 km and the shale gas does not escape on a large scale,the reservoir pressure coefficient>2.5,which is prone to tension fracture.It is not conducive to the preservation of shale gas.The shale gas enrichment mechanism is revealed for the study area:black shale deposition and moderate burial are the basis for hydrocarbon generation and storage,and the key to shale gas retention and preservation is to maintain large-scale continuous reservoirs through structural transformation.This thesis proposes a shale gas enrichment model based on structural styles,constructs an evaluation parameter system including sedimentary material base,burial depth,and effective preservation distance,and selects 12 shale gas enrichments areas.