煤系致密砂岩储集层特征及控制因素:基于塔西南坳陷中—下侏罗统不同岩性组合的对比OA

Coal-bearing tight sandstone reservoirs exhibit pronounced heterogeneity in lithologic associations,leading to substantial variability in reservoir quality.However,the diagenetic mechanisms governing this heterogeneity and the key geological controls on pore evolution remain inadequately constrained.This study focuses on the Lower-Middle Jurassic coal-bearing tight sandstone reservoirs in piedmont thrust belt of Southwestern Tarim Depression.Integrating core descriptions,thin-section petrographic analysis,and scanning electron microscopy(SEM)observations,we conducted a systematic comparative evaluation of reservoir characteristics across distinct lithologic associations.Based on coal seam distribution patterns and the architectural stacking of coal-sand-mud successions,three principal lithologic associations were delineated:Association A(braided delta plain),Association B(braided delta plain),and Association C(shore-shallow lacustrine to braided delta front).Diagenetic histories and pore-system attributes differ markedly among these associations.Association A experienced moderate mechanical compaction and pervasive early-stage calcite cementation sourced from primary depositional fluids;dissolution was limited,yielding a low total surface porosity of～1.05％.In contrast,Associations B and C are strongly modulated by grain size,mud content,and critically humic acids leached from adjacent coal seams,resulting in intensified compaction.Late-stage ferroan dolomite and calcite cements in these associations correlate temporally and spatially with organic matter maturation.Moreover,organic acids generated during coal-measure hydrocarbon generation significantly enhanced the dissolution of feldspars and unstable volcanic lithic fragments,giving rise to well-developed intragranular dissolution pores and consistently higher total surface porosities(＞2.0％).The palaeoenvironment of coal accumulation governs both the spatial configuration of coal seams and the lateral extent of lithologic associations.Associations A and B predominantly occur within the braided delta plain,where coal seams are thin and laterally discontinuous;Association C is confined to the shore-shallow lacustrine to braided delta front,characterized by moderately thick,laterally continuous coal seams.While initial depositional lithology and mineralogy exert foundational control on sandstone properties,the influence of coal seams on post-depositional diagenesis particularly through acid-driven inhibition of early carbonate cementation and promotion of feldspar/lithic dissolution during the mesodiagenetic stage is the dominant factor differentiating reservoir quality across associations.