Sedimentary characteristics and reservoir origin of the mound and shoal microfacies of the Ma51+2 submember of the Majiagou Formation in the Jingbian area

Abstract

Abstract: The carbonate rocks of the Majiagou Formation contain extensive oil and gas resources. To determine the rock type, sedimentary characteristics and sedimentary environment of the Ma51+2 submember, its sedimentary microfacies are distinguished via core observation, thin section analysis, scanning electron microscopy and cathodoluminescence analysis. On the basis of the sedimentary microfacies division, the sedimentary microfacies sequence is analyzed, and a sedimentary model is established to study the reservoir. The results show the following: i) The Ma51+2 submember is believed to be a 4th-order sequence, which can be divided into several 5th-order sequences according to the suborder sea level change, consisting of micritic dolomite with gypsum crystalline casting pores, grain dolomite, micritic dolomite and microbial carbonate. ii) The Majiagou Formation is concluded to have been deposited in a restricted evaporite platform sedimentary environment, which can be divided into four subfacies, namely, grain shoal, microbial mound, platform flat and lagoon subfacies. Specifically, the grain shoal subfacies is primarily composed of intershoal sea, shoal flank and shoal body microfacies, with the microbial mound subfacies largely divided into mound base, mound body and mound cap microfacies. iii) According to the 5th-order sea level changes and lithological characteristics, four typical vertical sedimentary microfacies sequences can be identified, and all are shallowing-upward sedimentary sequences. iv) The pore types in the study area are mainly gypsum crystalline casting pores, intergranular pores, intergranular dissolved pores, and dissolved pores, followed by intercrystalline pores, intercrystalline dissolved pores and fractures. v) The development of grain dolomite and microbial carbonate is influenced by sea level change, sedimentary sequence and paleogeomorphology, with the favorable reservoirs mainly affected by the sedimentary microfacies, dolomitization and dissolution.