Abstract:

Global petroleum exploration practice has shown that tight gas is the most important and promising unconventional resource currently. Sedimentary environments enriched in tight gas, particularly tight sandstone and conglomerate gas, are primarily fluvial-deltaic and lacustrine shoreline systems, which exhibit an inextricable genetic link to coal-bearing intervals. Within these coal-bearing intervals, two primary source-reservoir assemblages are recognized, commonly resulting in a resource configuration where coalbed methane, tight gas, and conventional natural gas coexist. Tight gas reservoirs within sandy sedimentary facies typically exhibit extensive lateral distribution but strong internal heterogeneity, posing significant challenges for sweet spot prediction. Predicting the size and geometry of tight gas reservoirs characterized by interbedded sand and mud, and mud-encased sand bodies, remains a key challenge in sweet spot prediction. Analysis of typical blocks in the Ordos Basin demonstrates that sedimentary environment and facies belt variations within coal-bearing intervals, differences in sand body genetic types and sedimentary architecture, and the distribution of dominant framework sand bodies are key sedimentary factors controlling the diagenetic differences and reservoir heterogeneity in such tight gas reservoirs. Sweet spot prediction for tight gas reservoirs in coal-bearing intervals should emphasize the following: First, integrate well and seismic data utilizing sedimentary facies-constrained prediction methods. Second, combine sedimentation and diagenesis studies to constrain the delineation of diagenetic facies belts using sedimentary facies belts. Third, accurately assess primary porosity loss in tight reservoirs of coal-bearing intervals and clarify sand body connectivity through genetic sand body correlation. When conducting comprehensive evaluation and predicting gas-bearing potential, it is essential to be aware of potential discrepancies arising from different evaluation schemes. For calculating gas reserves/gas-in-place (GIP), the parameter uncertainty inherent in the volumetric method should be considered, whereas geostatistical methods offer a more robust approach to parameter estimation and reserve calculation. Additionally, in sedimentary facies-controlled geological modeling of tight gas reservoirs, the strong heterogeneity necessitates the selection of reasonable petrophysical cutoff values.

Key words: coal-bearing intervals, sedimentary facies controlled tight gas reservoir, assemblage of source rock and reservoir, reservoir heterogeneity, sweet spot prediction