Abstract
Abstract Carbonate cements are an important type of cement in Permian tight gas reservoirs in the Ordos Basin. Moreover, the distribution of carbonate cements in sandstone provides an important basis for studying the fluid mechanism and genesis of cements and a scientific basis for investigating the physical properties of reservoirs and the underground distribution of oil and gas. Based on the observations of thin sections and temperature measurements of fluid inclusions, this study concludes that the carbonate cements in the study area are dominated by calcite and ferrous calcite and divided into three phases, where the latter two phases are dominant. The material source of carbonate cements is obtained via the thermal simulations of gold tubes in source rocks, temperature of fluid inclusions in carbonate cements, composition of fluid inclusions in quartz overgrowth, and analysis of carbon and oxygen isotopes. Results show that phase I calcite may be directly precipitated from the supersaturation of pore water by Ca2+ and CO32−. The CO32− in the phase II calcite and ferrous calcite stems from the CO2 generated in the hydrocarbon generation process of organic matter, while the CO32− in phase III calcite stems from the CO2 generated in the decarboxylation of organic acids. Ca2+ is derived from the dissolution of early carbonate cements in sandstone and conversion of smectite into illite in mudstone. The Permian sandstone in the Yishan Slope at the center of the basin is close to the source rock, and the carbon isotopic value of the carbonate cements, which is dominated by organic carbon, ranges between −7.5‰ and −19.0‰. However, the Permian sandstone in the Tianhuan Depression at the edge of the basin is far away from the source rock, with its carbon isotope value ranging from −5.9‰ to −8.7‰, which is probably due to the insufficient supply of organic carbon and a small amount of early inorganic CO2 remains in the reservoir.
Highlights
Carbonate cements are an important type of cement in sandstone reservoirs and play a vital role in sandstone reservoirs because they affect the quality of the reservoirs [1,2,3,4,5,6,7,8]
The main objectives of this paper are as follows: (1) studying the formation time, material source, and distribution of carbonate cements in the Permian tight sandstone of the Ordos Basin has important theoretical guiding significance for the prediction of reservoirs having high carbonate cements content and good physical properties and (2) determining the similarities and differences between carbon sources of carbonate cements in the sandstones of major petroliferous basins in China and establishing the relationship between the carbon sources of carbonate cements in the sandstones of different types of source rocks
The measured homogenization temperature of the aqueous fluid inclusions in carbonate cements ranged between 67.9°C and 147.2°C (Table 2)
Summary
Carbonate cements are an important type of cement in sandstone reservoirs and play a vital role in sandstone reservoirs because they affect the quality of the reservoirs [1,2,3,4,5,6,7,8]. The carbonate cement content affects the hydraulic fracturing efficiency. These parameters are important indicators for oil and gas exploration and development. It is considered that carbonate cements can be formed during both the early and late diagenetic stages but their composition, crystal shape, size, and distribution characteristics differ [3, 11]. The carbonate cements formed during the early diagenesis are favorable for the reservoirs, while those formed during the late diagenesis fill the pores and reduce the porosity [7] and permeability of sandstone, which is not conducive to the reservoirs [11, 12]. The main controlling factors of sandstone reservoir with strong heterogeneity are grain size, composition, and distribution of diagenetic facies.
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