Research On The Mechanism Of Quality Differentiation And Effective Reservoir Distribution Of Low-Permeability To Tight Gas Reservoirs

Low-permeability to tight oil and gas reservoirs are one of the hydrocarbon reservoirs attracting significant attention from the world.However,due to the strong heterogeneity and complex controlling factors of pore structures,the distribution of effective reservoirs is unclear,which seriously restricts the efficient development of such reservoirs.In this study,the low-permeability to tight reservoirs of the Huangyan area in the south-central region of the Central Inverted Structural Belt in the Xihu Sag of the East China Sea Basin are selected as the research object.Multiple technologies such as casting thin sections,scanning electron microscopy,porosity and permeability test,pressure-controlled mercury injection,rate-controlled mercury injection,nuclear magnetic resonance,and micro-CT are applied to analyze the macro and micro reservoir quality of the reservoirs in the study area.The study reveals the main controlling factors of reservoir quality and the mechanism for the development of effective reservoirs.Besides,a reservoir quality classification scheme is established,and the distribution characteristics of effective reservoirs is predicted.（1）The macroscopic and microscopic reservoir quality differences between different blocks of the Huagang Formation in the study area are elucidated.The porosity and permeability of Huagang reservoirs in the study area exhibit strong heterogeneity,with the best properties in the south,inferior properties in the northwest,and the poorest properties in the northeast.Among the 20 main gas-producing layers within the Huagang Formation,10%of the layers exhibit comparable permeability in the northern and southern reservoirs.The permeability disparity between the northern and southern blocks is over an order of magnitude for 25%of the layers.Additionally,65%of the layers in the southern blocks have reservoir permeabilities ranging from 2 to 7 times that of the northern blocks.There are also significant differences in pore and throat types between different blocks.The southern reservoirs are characterized by compound pores,residual primary intergranular pores and contracted and wide-sheeted throat.The northern E₃h_sreservoirs mainly develops intergranular dissolution pores,residual primary intergranular pores and narrow-sheeted throats.The northern E₃h_x reservoir mainly develops small intergranular and intragranular dissolution pores,intracrystalline pores and curved lamellar and pipe-like throat.（2）The sedimentary microfacies distribution and the petrological differences of the shallow-water delta reservoirs in the Huagang Formation of the study area are elucidated.The controlling mechanisms of sedimentary conditions on the variations in initial reservoir quality among different blocks within the same single layer is revealed.In the strong hydrodynamic layer,the study area develops wide-belt to continuous channel sandbodies in shallow-water delta plains.The poor sorting,and abundant fine sand in the northern reservoirs control the lower initial porosity in the north compared to the south.Under weak hydrodynamic force,the northern area develops wide-belt channel sandbodies in shallow-water delta plains,while the southern area develops narrow-belt channel sandbodies in shallow-water delta front.The fine to medium sand in the southern reservoirs control the higher initial porosity in the north compared to the south.（3）The controlling mechanisms of diagenetic evolution on the variations in reservoir quality among different blocks within the same single layer,among different blocks at the same depth,and within the same block,is revealed.In the same layer,the deeper burial depth in the north and the shallower burial depth in the south result in stronger compaction in the northern,which is the most important factor for the superior reservoir quality in the south and inferior quality in the north.The higher clay content in the northern E₃h_s reservoir magnifies the difference in porosity between north and south,and the higher illite and illite-smectite mixed layer content in the northeast E₃h_x reservoir increases the permeability difference between north and south by an order of magnitude or more.Within each block or among different blocks at the same depth,medium to fine-grained thick channel bars,point bars,and main channels sandbodies developed under strong hydrodynamics,exhibit better reservoir properties than other microfacies sandbodies.The fundamental reason is that early-stage calcite cementation at the top of thick sandbody and coating chlorite in the middle of thick sand bodies provide strong resistance to compaction.（4）The controlling mechanisms of structural inversion on differential diagenetic intensity among different blocks within the same single layer in the study area is revealed.The higher subsidence amplitude in north and stronger structural inversion intensity in the south result in the current deeper burial depth of the northern Huangyan area.This not only activates early-stage faults in the northeastern and southern blocks,facilitating the communication between source and reservoir rocks and the migration of organic acids along faults to the principal layers during middle diagenetic stage,forms strong dissolution zones in thick sandbodies near faults,but also causes uplift of the middle Huagang reservoirs in the northern block,and lowered the layer temperature below the threshold for the abundant transformation of kaolinite to illite,thereby delaying the transformation process from kaolinite to illite during later diagenetic stage,and indirectly reducing the illite content.（5）The genetic mechanisms of effective reservoirs in the Huagang Formation of the study area are summarized,a classification scheme for effective reservoirs is established,and the spatial distribution patterns and controlling factors of effective reservoirs are identified.Four types of reservoirs,namely Type I,Ⅱ,Ⅲ,andⅣ,are developed in the study area,with the first three being effective reservoirs and Types I andⅡbeing high-quality reservoirs.In the same layer,the higher abundance of high-quality reservoirs in the south compared to the north is mainly controlled by deeper burial depth and higher content of illite and illite-smectite mixed layer in the north.The development of high-quality reservoirs in the H2 Sand group of northern blocks is determined by the strong hydrodynamics and chlorite coatings which effectively resist compaction.Within a block,the distribution of high-quality reservoirs depends on the distribution of channel bars,point bars,and main channels sandbodies with early-stage calcite cementation.The southern E₃h_sreservoirs are presented as banded Type I and continuous TypeⅡreservoirs,the southern E₃h_xreservoirs are presented as narrow-banded TypeⅡand banded TypeⅢreservoirs.The northern E₃h_sreservoirs are presented as banded TypeⅡand continuous TypeⅢreservoirs,the northern E₃h_xreservoirs are presented as banded TypeⅢand bordered TypeⅣreservoirs.This study provides a theoretical reference for the analysis of low-permeability to tight gas reservoirs distribution with the background of structural inversion.It also offers theoretical support for the refined development of similar reservoirs in other regions.