Formation mechanism of carbonates in the lacustrine muddy shale and it implication for shale oil and gas: A case study of source rocks in Member 4 and Member 3 of Shahejie Formation, Dongying sag

Abstract

Through analysis of microscopic characteristics, mineral components, elements and isotopes, the genetic mechanism of carbonates in the deep lacustrine source rocks in the upper submember of Member 4 and lower submember of Member 3 of Shahejie Formation in Dongying sag, is well investigated. The results show that four types of carbonates in the deep lake, i.e., lenticular coarse crystalline carbonate, lamellar micro-fine crystalline carbonate, lamellar cryptocrystalline carbonate and massive cryptocrystalline carbonate. Of which, the lenticular coarse crystalline carbonate is formed by diagenetic recrystallization. For the lamellar micro-fine crystalline carbonate and the lamellar cryptocrystalline carbonate, through the alga photosynthesis, the carbon dioxide (CO2) is constantly extracted from water, thus the concentration of CO3−2 ion in water increases, and then the CO3−2 ion reacts with Ca2+ ion in lake water surface to form the carbonates; the saline water environment is favorable for preservation of carbonate particles which mostly occur in lamellar micro-fine crystalline; in the brackish water environment, the water is deep, and the carbonate crystalline beneath the carbonate compensation depth surface is usually is dissolved, and most of lamellar cryptocrystalline are preserved. The massive cryptocrystalline carbonate is formed by the sedimentary carbonate which transport from shallow water to deep water by gravity flow. To some extent, the carbonates control reservoir property and compressibility of muddy shale in the upper submember of Member 4 and lower submember of Member 3 of Shahejie Formation in Dongying sag, and provide important information for reconstruction of sedimentary environment of the ancient lake.