Abstract:

It’s found that the Wushenqi paleo-uplift has a significant impact on controlling the hydrocarbon accumulation of the Ordovician Majiagou Formation in the northern Ordos Basin with a batch of deep wells successfully drilled in Da’niudi gas field there. The study makes full use of the drilling， seismic and laboratory data from the northern part of the basin to discuss the controlling effects of the paleo-uplift on reservoir forming and hydrocarbon accumulation in the Lower Paleozoic. The results are shown as follows. First， the Wushenqi paleo-uplift was formed in the Late Proterozoic and continued to be active until the end of the Ordovician. It underwent five stages of evolution featuring alternated development above and under water. Second， the continuous activities during the Early Paleozoic resulted in the development of fault system to the east of the paleo-uplift， which served as fluid channels for the confined aquifer karstification during the Caledon and Hercynian periods. Deep karst reservoirs were developed thereby. Third， the sedimentary water body on the paleo-uplift was shallow， favorable for the deposition of microbial rocks and enecontemporaneous dolomitization， and prone to generate porous dolomite reservoirs. Fourth， the faults to the east of the paleo-uplift acted as major passages for natural gas migration during accumulation. Multiple reservoirs get connected by these faults， featuring “hydrocarbon charging from three directions， fault-controlled reservoir formation and multi-layered hydrocarbon accumulation”. The study is of a new model to the exploration of the Lower Paleozoic and Ordovician in the Ordos Basin.

Key words: dolomite, reservoir, paleo-uplift, karst gas reservoir, Majiagou Formation, Ordovician, Da’niudi gas field, Ordos Basin