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DOI:10.1306/07232423029
Characteristics and diagenetic model of hyperpycnite reservoirs in the gentle slope of a lacustrine rift basin: A case study from the third member of the Eocene Shahejie Formation, Bonan sag, Bohai Bay Basin, eastern China
Tian Yang12 , Rong Cui1 , Yingchang Cao3 , Yanzhong Wang3 , Lingli Zhou4 , and Laixing Cai12
1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology), Institute of Sedimentary Geology, Chengdu 610059, P. R. China
2 Key Laboratory of Deep-time Geography and Environment Reconstruction and Applications of Ministry of Natural Resources, Chengdu University of Technology, Chengdu 610059, China
3 Laboratory of Deep Oil and Gas (China University of Petroleum (East China)), Qingdao 266580, P. R. China
4 Department of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam
Ahead of Print Abstract
Diagenetic processes of hyperpycnite reservoirs are rarely discussed compared to sediment failure-induced gravity-flow reservoirs. This study takes the hyperpycnite reservoirs in the middle of the third member of the Shahejie Formation (43.7–38.2 Ma), Bonan sag, Bohai Bay Basin, eastern China, as an example to reveal the characteristics and diagenetic processes, and propose a diagenetic model. An integrated study including thin section petrography with point counting analyses and grain size measurements, fluid inclusion microthermometry, carbon and oxygen isotopes, SEM-EDS, XRD, CL, BSE-EPMA, LA-ICP-MS analyses was undertaken. The reservoirs are mainly lithic arkoses (Q47F32L21), with average porosity and permeability values of 17.4 % and 209.5 md, respectively. The porosity and permeability of the channel and lobe axis are significantly higher than those of the levee and lobe fringe. Primary pores and secondary pores formed from feldspar grain dissolution are common with pore-throat radii ranging from 0.01 to 100 μm. In addition to mechanical compaction, quartz overgrowths and carbonate cementation are the main factors that reduce the quality of the reservoirs. Silica sources for quartz overgrowths may include feldspar grain dissolution, clay mineral transformation, and detrital quartz grain pressure dissolution. The homogenization temperatures of aqueous fluid inclusions in the quartz overgrowths range from 95 to 160 ºC, and the content of trace elements (Al, Li, Ge) of some quartz overgrowths is higher than that in detrital quartz grains. The enrichment of silica is controlled by the intensity of feldspar grain dissolution in channel and lobe axis architectural elements. The carbon sources for carbonate cements mainly come from the dissolution of lacustrine carbonates by organic acids; diagenetic alteration of detrital phyllosilicates (micas and smectite) into illite is the source of Mg2+ and Fe2+ for carbonate cements. The δ13C values of ferroan calcite and ankerite range from +0.7 to +2.1 ‰ and +1.6 to +2.7 ‰, respectively. High-temperature ferroan calcite (average of 140.7 ºC) and ankerite (average of 128.0 ºC) are mainly caused by overpressure. The enrichment of carbonate cements is controlled by the accumulation of micas in levee and lobe fringe architectural elements. Reservoir properties and diagenesis of hyperpycnite reservoirs in the research area are closely related to hyperpycnal flow transport and depositional processes. The intensity of feldspar grain dissolution may be promoted by the relatively high primary porosity in bedload-dominated hyperpycnite reservoirs, and the distribution of micas is controlled by lofting processes of suspended load-dominated hyperpycnal flow. The characteristics and diagenetic model of hyperpycnite reservoirs documented in this paper could have significant implications for the study of deep-water gravity-flow sandstone reservoirs in lacustrine rift basins.
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