Search

Sandy-conglomerate reservoir has gradually become a major target of oil and gas exploration. Complex diagenetic process and diagenetic fluid play a significant role in affecting reservoir heterogeneity. Carbonate cements form at various stages of the diagenesis process and record various geological fluid information. Recently, one-billion-ton sandy conglomerate oil field was exposed in Triassic Baikouquan Formation, Mahu sag, Junggar Basin. Therefore, an integrated study applying casting thin sections, cathodeluminescence, fluorescence, carbon and oxygen stable isotopes, electronic probe microanalysis and aqueous fluid inclusions measurements was performed in order to identify the types of carbonate mineral and its representative diagenetic environment and discuss the influences of different CO2 injections on reservoir quality. The main findings are as follows: The reservoir is mainly composed of 70.33% conglomerate and 16.06% coarse-grained sandstone. They are characterized by low compositional maturity and abundant lithic debris. Four types carbonate cements are identified according to the petrological and geochemical characteristics, including two types of Mn-rich calcite, ferroan calcite, siderite and dawsonite. They display an unusual broad spectrum of δ13C values (−54.99‰ to +8.8‰), suggesting both organic and inorganic CO2 injections. The δ13C values of siderite are close to 0, and its formation is related to meteoric water. The δ13C values of ferroan calcite and the occurrence of dawsonite indicate the trace of inorganic mantle-derived magmatic fluids. The δ13C values and trace elements of Mn-rich calcite record the information of hydrocarbon-bearing fluids. The fluid inclusions measurement data and reservoir properties and oil-test data show that the oil content of reservoir is not only affected by the formation time of different cements, but also by the relative content of dissolution and cementation. For these reservoirs altered by carbonate cements, it does not cause poor oil-bearing due to blockage of secondary minerals. [ABSTRACT FROM AUTHOR]

The characteristics, heterogeneous distribution and origins of carbonate cements within beach-bar sandstones in the Boxing Sag and implications on the mass transfer from mudstone to sandstone during diagenetic process was investigated systematically by an integrated analysis invoking petrography, carbon and oxygen stable isotopes and porosity-permeability, with specific references to the contrasts among the sharp (S)-change lithology (S-mudstone-S-sandstone interbeds), the gradual (G) -change lithology (G-mudstone-G-sandstone interbeds) and the center of sandstones. There are distinct differences in the occurrence, content, distribution, carbon and oxygen isotopic compositions and precipitation temperatures of the carbonate cements among the S-sandstone, G-sandstone and center of sandstone. Material sources of carbonate cements in the S-sandstones are mainly derived from the diagenetic alterations of the nearby S-mudstones through mass transfer with the participation of organic CO 2 , while in the G-sandstones, the material sources are mainly internal and originated from the calcareous matrix. Dissolution of feldspars and conversion of clay minerals provided the main materials for the precipitation of carbonate cements in the center of sandstones. Mass transfer of carbonate cements mainly occurred between the S-mudstones and the S-sandstones with limited transmission distance, which controlled the distribution of carbonate cements in the S-mudstones. There was almost no mass transfer between the G-mudstones and G-sandstones because of the small concentration gradient, instead, the depositional components and recrystallization controlled the carbonate distribution of the G-sandstones. • Material sources of carbonates in S-sandstones were mainly from nearby S-mudstones. • Material sources of carbonates in G-sandstones were mainly from in situ recrystallization. • Mass transfer of carbonate cements mainly occurred through sharp contact between mudstone and sandstone. [ABSTRACT FROM AUTHOR]