Under deep- ultra- deep geological conditions, the porosity and permeability physical properties and fluid dynamic connectivity of reservoir rocks determine the flow state of oil and gas in the reservoir, which also determines the dynamic conditions of oil and gas migration, the mechanism and process of reservoir formation. This study has investigated architectural heterogeneity of deep- ultradeep clastic reservoirs. Based on the new understanding, this study has examined hydrocarbon accumulation mechanism and process in deep- ultradeep basins, has proposed multistage composite accumulation mode, and explored deep- ultradeep hydrocarbon distribution laws. Deep clastic reservoirs have commonly strong architectural heterogeneity, that is characterized by regular rock facies stacking determined by the depositional structure. In deep burial process, the different rock facies in the reservoirs experienced contrasting diagenetic evolutions and hydrocarbon charge. In carrier beds with architectural heterogeneity, oil and gas generally migrate upwards. Affected by the distribution of sand bodies, tight interlayer structures, and connectivity modes, the migration pathways become very complicated and hydrocarbon can accumulate anywhere in the reservoir bed during migration. In deep burial process, multistage tectonic activities forced already accumulated hydrocarbons to migrate and adjust in the updip direction, or to migrate up through opening faults to be leaked into faulted- related effective reservoirs to accumulate. Deep- ultradeep exploration has a wide range of targets, as both subsags and slopes, may become favorable exploration areas. Practical target areas in deep- ultradeep exploration include extension belt toward sources from hydrocarbon accumulations in structural highs, secondary accumulations formed by migration of deep- ultradeep hydrocarbons up faults into shallower reservoirs, deep- ultradeep hydrocarbon source- related slope belts, and effective reservoir areas within or near the deep- ultradeep source rocks.