Geological modeling of subsalt carrier system and hydrocarbon migration simulation in the Majiagou Formation in the Gaojiabu area, eastern Ordos Basin

The Ordovician subsalt carbonates in the central and eastern part of the Ordos Basin are a hot topic for deep marine strata exploration. The subsalt strata of the Majiagou Formation are gas-bearing over a large area but show complex gas/water relationships and unclear enrichment patterns restricting future gas exploration. Taking the Gaojiabao area as an example, this study combines geological and geophysical methods to establish an architectural heterogeneity model of the subsalt reservoirs with multiple-constrained unit reservoir filling modeling. On this basis, this study establishes carrier system models during different key migration and accumulation periods, which precisely depict architecture, transport properties and fluid connectivity at different scales of carrier beds. Hydrocarbon migration and accumulation and late adjustment processes are simulated, and main controlling factors and rules of migration and accumlation are summarized. The results show that the subsalt carbonate reservoirs of the Majiagou Formation in the Gaojiabao area are mainly composed of mound-shoal composite unit reservoir bodies that laterally migrate and are superimposed vertically. Impulsive faulting and fracturing play an important role in improving the connectivity between these reservoir bodies. Oil and gas migration and accumulation mainly occur in the fault-related, interconnected reservoir bodies with good transport performance, and gas accumulations present a series of small "potato-shaped" accumulations. Under influence of late tectonic adjustments, gas invariably migrates upward along faults, which may lead to an increase in gas accumulations in the reservoirs interbed with salt deposit in the lower part of the Ma 5 member. The formation and enrichment of subsalt gas accumulation are mainly determined by reservoir architectural heterogeneity and fault-related adjustment process during late tectonic uplift. Except for the western fault zone, the Ma 4 member and the lower part of the Ma 5 member in the southeastern part of the study area are favorable targets for future exploration.