Reservoir characteristics and main controlling factors of the fourth member of Ordovician Majiagou Formation in the central and eastern Ordos Basin

Abstract

Recent exploration practice shows that the leopard porphyry dolomite reservoir in the fourth member of Ordovician Majiagou Formation in the central and eastern Ordos Basin has good exploration and development potential. Therefore, based on drilling cores, rock slices and analysis and laboratory data, the reservoir characteristics and main controlling factors of this section were analyzed. The results show that: (1) The reservoir rocks of the fourth member of Majiagou Formation mainly include leopard porphyry dolomitic limestone, leopard porphyry limy dolomite, crystalline dolomite and clotted dolomite, and the reservoir space types mainly consist of intergranular (dissolution) pores, with a small amount of lattice pores and microcracks. (2) The reservoir in the study area is generally characte-rized by low porosity and medium-low permeability pore type reservoir, in which leopard porphyry limestone dolomite and crystalline dolomite are the best reservoir rocks with wide distribution range. However, leopard porphyry dolomitic limestone is developed on a large scale, with poor reservoir performance as a whole, while clotted dolomite has good reservoir performance, but its development frequency is low. The reservoir development in the fourth member of Majiagou Formation in the central and eastern Ordos Basin is mainly controlled by sedimentary microfacies, bioturbation, dolomitization and early diagenetic karstification: as the material basis for reservoir formation, favorable sedimentary microfacies control the horizontal distribution of reservoirs; bioturbation is for dolomitization; dolomitization is the key to reservoir formation, which is conducive to the preservation of reservoir pores; early diagenetic karstification has an important contribution to the improvement of reservoir quality. In the longitudinal, the favorable reservoirs in the fourth member of Majiagou Formation are mainly located in the middle-upper part of the high-frequency upward shallowing sequence; on the plane, the favorable reservoirs are mainly concentrated in the line of Wushen Banner-Jingbian-Zhidan in the west and the two relatively independent areas of Shenmu and Mizhi in the east of the study area.

Keywords

• reservoir characteristics
• main controlling factors
• fourth member of majiagou formation
• ordovician
• central and eastern ordos basin