Abstract
Diagenesis is one of the most predominant factors controlling reservoir quality in the deeply buried siliciclastic sandstones of the third member in the Eocene Shahejie Formation (Es3), in the Raoyang Sag, the Bohai Bay Basin. In this study, thin section, cathodoluminescence (CL), scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectrum, carbon and oxygen isotopes, and fluid inclusion analyses are used to restructure paragenetic sequences and detect origins of carbonate cements recorded in this deeply buried member. Based on petrographic analyses, the Es3 sandstones are identified as lithic arkoses and feldspathic litharenites at present, but derived from original arkoses and lithic arkoses, respectively. Geohistorically, the Es3 sandstones have undergone two diagenetic episodes of eogenesis and mesogenesis. Events observed during eogenesis include chemical compaction, leaching of feldspar, development of chlorite coating and kaolinite, precipitation of the first generation of quartz overgrowth (QogI), dissolution of feldspar, and precipitation of calcite and nonferroan dolomite cement. Mesogenetic alterations include chemical compaction, precipitation of kaolinite aggregate and the second generation of quartz overgrowth (QogII), precipitation of ankerite, development of I/S and illite, and formation of pyrite. Carbon and oxygen isotopic data show that calcite cements are characterized by 13C ( δ 13 C PDB ranging from -0.7‰ to 1.0‰ with an average of 0.1‰) and 18O ( δ 18 O SMOW varying from 12.3‰ to 19.0‰ with an average of 16.2‰); these stable isotopic data combined with Z value (from 114.69 to 122.18) indicate skeletal debris ( δ 13 C PDB ranging from -1.2‰ to -1.1‰ with an average of -1.15‰; δ 18 O SMOW varying from 23.0‰ to 23.2‰ with an average of 23.1‰) and ooids in adjacent carbonate beds involved in meteoric water and seawater from outside jointly served as the carbon sources. For nonferroan dolomite, the δ 13 C PDB value of -4.1‰ is a little bit negative than the calcite, and the δ 18 O SMOW of 14.3‰ is coincident with the calcite, which suggest the nonferroan dolomites come from the diagenetic fluids with a similar oxygen isotopic composition to that of the calcite but modified by the external acidic δ 13C-depleted water. However, the ankerites are actually rich in 12C ( δ 13 C PDB ranging from -10.0‰ to -1.2‰, mean = − 4.3 ‰ ) and 16O ( δ 18 O SMOW varying from 10.1‰ to 19.4‰, mean = 14.9 ‰ ), when combined with the distribution of cutting down along the direction pointing to sand-body center from the margin and microthermometric temperature (Th’s) data mainly varying between 115.2°C and 135.5°C with an average of 96.0°C, indicating the main origination from the Es3 source rocks with effective feldspar buffer action for the acidic fluids in the margins of the Es3 sandstones. In addition, the necessary elements for ankerite such as Fe2+, Ca2+, and Mg2+ ions also come from organic matter and clay minerals during thermal maturation of the Es3 source rocks. The study provides insights into diagenetic processes and origination of carbonate cements in the Es3 sandstones; it will facilitate the cognition of predictive models of deeply buried sandstone reservoirs to some extent, which can reduce the risks involved in oil and gas exploration and development.
Highlights
As hydrocarbon exploration and development expand into technically difficult areas, deeply buried reservoirs have become even more important in recent years [1]
The study of diagenetic processes based on identifying diagenetic events and paragenetic sequences, which is essential for the understanding and prediction of reservoir quality, contributes to enhancing the efficiency of hydrocarbon exploration and promotes the yields of commercial development [5, 8, 10,11,12,13]
Reservoir quality is dependent on both the original depositional environment and postdepositional diagenetic modifications, including physical, chemical, and biological processes, which vary with the pressure, temperature, chemistry of fluids, and framework grains related to the evolution history of sedimentary basin [8, 13,14,15,16,17]
Summary
As hydrocarbon exploration and development expand into technically difficult areas, deeply buried reservoirs have become even more important in recent years [1]. Reservoir quality is dependent on both the original depositional environment and postdepositional diagenetic modifications, including physical, chemical, and biological processes, which vary with the pressure, temperature, chemistry of fluids, and framework grains related to the evolution history of sedimentary basin [8, 13,14,15,16,17] For those deeply buried sandstone reservoirs undergoing complex postdepositional alterations, prediction of reservoir quality faced many more difficulties due to the intricate component transformations that are controlled by fluid-rock interaction [5, 18,19,20,21,22]. The third member of the Eocene Shahejie Formation (Es3) at a deep burial depth (>3000 m) is one of the most important sandstone reservoirs in the Raoyang Sag, the Bohai Bay Basin It is mainly made up of deltaic sandstones interbedded with mudstone and lacustrine carbonate rocks, being subjected to a high diagenetic degree. (1) To better identify petrological characteristics and diagenetic types of the Es3 member in Raoyang Sag, Bohai Bay Basin (2) To reconstruct the paragenetic sequence of multiple diagenetic events in order to learn the diagenetic history (3) To investigate the originations of the carbonate cements abundant in the Es3 sandstones
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