Deep thermal fluid activity and its influence on the diagenesis and pore evolution of reservoirs: A case study from the Miocene Huangliu Formation reservoir in the LDX area, Yinggehai Basin, northern South China Sea

doi:10.13745/j.esf.sf.2022.2.57

Abstract

Abstract:

In order to understand the main types of deep-seated thermal fluid in the Yinggehai Basin, We studied the thermal fluid characteristics, determined the range of thermal fluid activity, and analyzed the thermal fluid influence on reservoir diagenesis and pore evolution. Combined with previous research results, we investigated the Miocene Huangliu Formation reservoir in the LDX area by blue epoxy resin-impregnated thin section, scanning electron microscope, clay mineral quantification by XRD, physical property, electron probe, fluid inclusion homogenization temperature, and stable isotope analyses. Results showed that the deep-seated thermal fluid activity—which is affected by tectonic thermal events, controlled by an abyssal fault, driven by overpressure, and dominated by CO₂ thermal fluid and to a lesser extent by H₂S thermal fluid—mainly influenced the middle and lower parts of the Huangliu Formation below 3900 m. The reservoir is characterized by the accelerated transformation of authigenic clay minerals, vitrinite reflectance mutation, reduced formation water salinity, inclusion homogenization temperature above the normal formation maximum temperature, and hydrothermal mineral development under the influence of deep therrmal fluids. The Huangliu Formation involves two types of reservoirs: overpressure reservoir (middle-upper parts of the Huangliu Formation) and thermal overpressure reservoir (lower-middle parts of the Huangliu Formation). The diagenetic stage of the overpressure reservoir reached substage A₂ of mesodiagenesis, with relatively weak compaction, weak dissolution and strong cementation, and experienced pore evolution in three stages: porosity decrease by compaction, porosity decrease by compaction and cementation, and porosity decrease by cementation combined with porosity increase by organic acid dissolution. Overall, porosity decreased from 38.8% to 7.6%. The diagenetic stage in the thermal overpressure reservoir reached stage B of mesodiagenesis, with relatively strong compaction, strong dissolution and weak cementation, and experienced pore evolution in four stages: stages 1-3 are the same as in substage A₂, and stage 4 is similar to stage 3 except the porosity increase is caused by both organic acid and inorganic acid dissolutions. Overall, porosity decreased from 38.1% to 9.2%. Favorable reservoirs mainly develope in the thermal overpressure reservoirs of the second member of the lower-middle parts of the Huangliu Formation.

Key words: deep-seated hot fluid, diagentic-pore evolution, favorable reservoir, Miocene Huangliu Formation, LDX area, Yinggehai Basin

CLC Number:

TE122.3

CAO Jiangjun, LUO Jinglan, FAN Caiwei, LI Shanshan, WU Shijiu, FU Yong, SHI Xiaofan, DAI Long, HOU Jingxian. Deep thermal fluid activity and its influence on the diagenesis and pore evolution of reservoirs: A case study from the Miocene Huangliu Formation reservoir in the LDX area, Yinggehai Basin, northern South China Sea[J]. Earth Science Frontiers, 2022, 29(4): 412-429.

Figures/Tables 13

Fig.1 Regional structure division (a) and comprehensive stratigraphic histogram (b) map of Yinggehai Basin. Modified after [10,20,26].

Fig.2 The scatter diagram of the relative content of authigenic clay minerals varying with depth in the LDX area, Yinggehai Basin

Fig.3 The scatter diagram of logarithm of vitrinite reflectance (a) and total mineralization of strata water (b) varying with depth in the LDX area, Yinggehai Basin

Fig.4 Distribution characteristics of the homogenization temperature and salinity of fluid inclusions of the Huangliu Formation in the LDX area, Yinggehai Basin

Fig.5 Microscopic photographs of hydrothermal minerals of the Huangliu and Meishan Formations in the LDX area, Yinggehai Basin

Fig.6 CO2 stable carbon isotope of the Huangliu Formation in the LDX area, Yinggehai Basin, varying with depth and its distribution characteristics

Fig.7 Carbon and oxygen isotopes of carbonate cements of the Huangliu Formation in LDX area, Yinggehai Basin, varying with depth and their genesis. Carbonate genetic plate adapted from [39].

Fig.8 Iron isotope of the Huangliu Formation in LDX area, Yinggehai Basin, varying with iron content of pyrite cements

Fig.9 Seismic section (a) and buried thermal history at well LDX-7 (b) in the LDX area, Yinggehai Basin

Fig.10 Comparative diagram of diagenesis and physical properties of different types of reservoirs of the Huangliu Formation in the LDX area, Yinggehai Basin

Fig.11 Overpressure reservoir and thermal overpressure reservoir characteristics of the Huangliu Formation in the LDX area, Yinggehai Basin

Fig.12 Microscopic characteristics of the main minerals and pore types in the diagenetic stage of the Huangliu Formation reservoir in the LDX area, Yinggehai Basin

Fig.13 Comprehensive model of diagenetic-pore evolutions of different types of reservoirs of the Huangliu Formation in the LDX area, Yinggehai Basin. Evolution of ancient pressure modified after [48].

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