| Multiple exploration wells including well Mitan 1 have obtained high production natural gas flow in the
dolomite of the fourth member of the Ordovician Majiagou Formation(O1m4) in the northeast of Ordos Basin,
demonstrating that the O1m4 of the region has good exploration prospects. However, there is still controversy over whether
large-scale dolomite reservoirs are developed in the O1m4 in the northeastern basin, and the unclear distribution of
dolomite reservoirs restricts the next exploration deployment. Based on comprehensive data such as core, thin sections,
field emission scanning electron microscopy and geochemical characteristics, a systematic study is conducted on the
characteristics and development controlling factors of the O1m4 dolomite reservoir in the northeast of Ordos Basin. On this
basis, the distribution of the O1m4 dolomite reservoir in the northeast of the basin is predicted. The research results indicate
that: (1) Tidal flat type dolomite reservoir, mound-shoal type dolomite reservoir and bioturbation type dolomite reservoir
are developed in the O1m4 in the northeast of Ordos Basin. The storage space of the three types of dolomite reservoir is
mainly intergranular pores with a small number of micro cracks, and the average porosity is 4.77%, 5.12% and 2.52%
respectively. (2) The formation of dolomite reservoirs is related to mound-shoal complex, bioturbation and
dolomitization. The development of tidal flat type dolomite reservoir is related to dolomitization, which is the key to the
formation of intergranular pores in such reservoirs. The development of the mound-shoal type dolomite reservoir is
controlled by the mound-shoal complex and dolomitization. The mound-shoal complex is the material basis for reservoir
development, and the dolomitization further enhances the compressive capacity of the reservoir, which is conducive to the
preservation of intergranular pores. Bioturbation and dolomitization control the development of bioturbation type dolomite
reservoir. The burrows formed by bioturbation lay the foundation for reservoir development. Dolomitization forms
intergranular pores and is beneficial for pore preservation. (3) The Shenmu-Jingbian area in the northeast of the basin is
located on a low uplift belt during the O1m4 period, which is conducive to the development of shoal-mound bodies,
biological disturbance and dolomitization. The developed dolomite reservoirs can provide a large-scale storage space for
natural gas enrichment. |
