Mechanism of Differential Oil-gas Distribution in the Steep-slope Zone (Northwestern Margin of Bozhong Sag): Evidence from Reconstruction of Hydrocarbon Accumulation

doi:10.19657/j.geoscience.1000-8527.2021.029

Abstract

Abstract:

Hydrocarbon distribution is low in the western part but high in the eastern part of the steep-slope zone in the northwestern margin of Bozhong sag (Bohai Bay Basin), and the reservoir-forming conditions are complex. Based on fluid inclusions petrographic observation, quantitative grain fluorescence technology, Fourier transform infrared spectroscopy (FTIR), and crude oil geochemical properties, and combined with the thermal, and burial history and tectonic evolution, the hydrocarbon accumulation process was unraveled from the paleo- and modern fluid characteristics. Besides, the reason of differential oil-gas distribution was also discussed. Our results define two hydrocarbon charging stages in the study area: stage one started at the early deposition of the Minghuazhen Formation (7 Ma), when the lowly-mature oil was sourced from the Shahejie Formation in the southeast. It has experienced short-range migration and formed reservoirs in the Paleogene sequences in the eastern part of the area. The charging scale is low at this stage. Stage two occurred in the middle deposition stage of the Minghuazhen Formation (5.3 Ma). The highly-mature hydrocarbons may have ascended from east to west, and formed large-scale paleo-oil reservoirs in the Paleogene sequences. During the late deposition stage of the Minghuazhen Formation (3.7 Ma), the activity of boundary faults was strengthened, and the activity rate in the western part of the area was higher than that in the eastern part. The faults may have changed from being hydrocarbon seal to fluid conduits. The Paleogene paleo-oil reservoirs in the western part of the area were largely destroyed, and the oil-water interface of the paleo-oil reservoir in the Dongying Formation (3^rd member) moved upward, with the hydrocarbon ascending to shallow formation. During the Quaternary sedimentation (2.2 Ma), the fault activity weakened and the current reservoirs were gradually formed. The hydrocarbon distribution pattern in the steep-slope zone of the northwestern margin of Bozhong sag are likely controlled by the distance from the source rock, the different reservoir formation stages, and the deformation intensity difference of the boundary faults.

Key words: fluid inclusions, quantitative grain fluorescence, paleo-oil reservoir, fault activity rate, accumulation process, steep-slope zone, Bozhong sag

CLC Number:

TE122.3

HAO Peng, YANG Jilei, ZHANG Xudong, ZANG Chunyan, CHEN Rongtao, WANG Bo, SHUI Leilei, WANG Sihui, CAI Tao. Mechanism of Differential Oil-gas Distribution in the Steep-slope Zone (Northwestern Margin of Bozhong Sag): Evidence from Reconstruction of Hydrocarbon Accumulation[J]. Geoscience, 2021, 35(04): 1124-1135.

Figures/Tables 12

Fig.1 Location of the steep-slope zone, northwestern margin of Bozhong sag

Fig.2 Oil reservoir profile of the steep-slope zone, northwestern margin of Bozhong sag

Fig.3 Thermal evolution history, burial history, and hydrocarbon accumulation timing of the steep-slope zone, northwestern margin of Bozhong sag

Fig.4 Quality chromatogram of saturated hydrocarbon in source rocks and crude oil from the northwestern margin of Bozhong sag

Fig.5 Oil migration direction in the steep-slope zone, northwestern margin of Bozhong sag

Fig.6 Photomicrograph of fluid inclusions from the steep-slope zone, northwestern margin of Bozhong sag

Fig.7 Quantitative fluorescence integrated profile of C2 well in the steep-slope zone, northwestern margin of Bozhong sag

Fig.8 Maturity parameter diagram of C29 sterane isomerization of crude oil from the steep-slope zone, northwestern margin of Bozhong sag

Fig.9 FTIR spectra of hydrocarbon inclusions and crude oil in the Paleogene reservoir of well C1

Table 1 Characteristics of fluid inclusions in the Ed2 of the western region, of the steep-slope zone, northwestern margin of Bozhong sag

井名	深度/m	产状	荧光颜色	包裹体期次	GOI/%	QGF指数	λ_max/nm	古油水层解释	现今油水层测井解释
C4	2 071.5	/	/	第Ⅱ期	/	5.2	420.9	古油层	含油水层
	2 139.0	沿切穿砂颗粒的微裂隙成线状或颗粒内点状分布	黄绿色	第Ⅱ期	8.0	/	/	古油层	含油水层
	2 171.5	/	/	第Ⅱ期	/	4.1	461.3	古油层	含油水层
	2 218.0	/	/	第Ⅱ期	/	1.8	388.9	古水层	水层
	2 287.0	/	/	第Ⅱ期	/	4.8	447.5	古油层	含油水层
	2 361.0	/	/	第Ⅱ期	/	2.7	457.5	古水层	含油水层
	2 380.5	/	/	第Ⅱ期	/	1.5	457.2	古水层	含油水层
C7	2 016.5	/	/	第Ⅱ期	/	9.5	416.5	古油层	含油水层
	2 046.0	/	/	第Ⅱ期	/	8.7	413.9	古油层	含油水层
	2 079.5	沿切穿砂颗粒的微裂隙成线状或颗粒内点状分布	淡黄色、黄绿色	第Ⅱ期	5.0	14.6	405.6	古油层	含油水层
	2 160.5	/	/	第Ⅱ期	/	4.5	422.1	古油层	含油水层
C9	2 036.0	/	/	第Ⅱ期	/	7.0	420.1	古油层	含油水层
C9	2 085.0	沿切穿砂颗粒的微裂隙成线状或颗粒内群体状分布	淡绿色、黄绿色	第Ⅱ期	5.0	/	/	古油层	水层

Fig.10 Activity rate of boundary faults in the steep slope zone, northwestern margin of Bozhong sag

Fig.11 Schematic diagram of the differential hydrocarbon accumulation process in the steep-slope zone, northwestern margin of Bozhong sag

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