Karst Palaeogeomorphology And Gas Accumulation Mechanism Of The Middle Ordovician Majiagou Formation In The Eastern Ordos Basin,Central China

The Karst paleogeomorphology of the middle Majiagou Formation of ordovician in the eastern Ordos basin as well as the accumulation mechanism of natural gas of the tight dolomite have long been the hotspots and difficult ones in the Study on Regional Natural Gas Geology.Also they are of great significance to understand the palaeogeographic pattern of the North China Craton in the Middle Ordovician and the potential of oil and gas resources in the new field of tight dolomite.On the basis of the field geological profile survey and the core observation,firstly,the karst palaeogeomorphology of the Majiagou period in the researching area was re-examined with the help of various analytical methods such as the microscopic thin section identification,scanning electron microscopy,cathode luminescence,CT tomography,EPMA,icp-ms and computer simulation.Secondly,On this basis,the control of the reservoir and the natural gas accumulation was discussed.The study of the palaeogeomorphology in Karst period showed that in the whole study area,overall,it has got the characteristics of high in the north and low in the south,whichwas very different from the previous thought that the whole area was in the karst depression zone.The paleogeomorphologic features were further refined and the distribution of karst grooves was re-described.Mainly there are three types of diagenesis-reservoir evolution relationships under the control of karst palaeogeomorphology.Respectively,they were the Formation of reservoirs before the densification,marginal densification into reservoir and densification first followed by the reservoir formation.Type one was the most efficient and The second type was the most extensive.Two major factors of the regional gas accumulation were also revealed in the paper.Firstly,the regional tectonic faults happened and traces of hydrocarbon and hydrothermal fluid activities in reservoirs were found along the faults.It showed that the fractures and micro-fractures generated by the structure can be used as a good gas migration channel to communicate the relatively isolated pore space in the dolomite reservoir,which is conducive to the formation of natural gas reservoirs.The hydrothermal activities that came along also caused the metasomatism and recrystallization of the reservoir bedrock to a certain extent,and improved the physical properties of the reservoir.Secondly,the concept of reservoir spatial dispersion was put to use.The reservoir space was grouped into two types:the high discrete space and the low discrete space.By means of CT modeling and the computer simulation,it was proved that the low discrete space had higher gas filling efficiency.And the main factor determining the reservoir spatial dispersion is the mineral filling type in the dolomite karst caves.At last,the problem of the discrepancy between the reservoir physical properties and the gas production in the study area was explained.With these understandings,It is expected that the understanding of the natural gas accumulation regularity of Majiagou Formation of Ordovician in Ordos Basin can be enriched and deepened,In particular,the understanding of new types of Compact Gas Reservoir,for example the tight dolomite.It is also hoped to be of general reference significance for the formation of karst hydrocarbon reservoirs at both home and abroad.