Geochemical Characteristics and Genesis of Oil and Gas in the Lishuixi Sag, East China Sea Shelf Basin

doi:10.19657/j.geoscience.1000-8527.2022.052

Abstract

Abstract:

The genesis and source of gas and condensate oil of Lishuixi sag in the western East China Sea Shelf Basin remain disputed for many years. Based on geochemical and drill well analyses and seismic exploration, the oil and gas genesis, source and exploration direction of L36 gas-field in the Lishuixi sag are reviewed with comprehensive research ideas and methods of natural gas geochemistry and oil and gas accumulation. The study shows that the natural gas in L36 gas-field is the product of “condensate-wet gas zone” evolution stage, and that the natural gas maturity R_o is no less than 1.1%. The carbon isotope composition of natural gas and the carbon isotope comparison with typical oil-type gas in China offshore revealed that the natural gas in L36 gas field is oil-type, with the estimated natural gas maturity R_o=1.12%-1.14%, which was originated from lower Paleocene lacustrine Yueguifeng Formation source rocks in the sag, which enriched the genetic types and sources of natural gas in the East China Sea basin. The C₇ light hydrocarbons of condensate oil produced in L36 gas-field are dominated by methylcyclohexane, significantly different from the C₇ light hydrocarbon composition of condensate oil from the lacustrine source rock. The lower Paleocene shallow marine Lingfeng Formation source rocks are characterized by < 1 sterane C₂₇20R/C₂₉20R, no C₃₀4-methylsterane and low Gamma wax content. This indicates that the condensate oil of L36 gas-field is from the Lingfeng Formation source rocks. It is confirmed that in addition to the Yueguifeng Formation source rocks, the Lingfeng Formation source rocks were also developed in the Lishuixi sag. The next natural gas exploration direction in the Lishuixi sag may include: (1) lithologic traps in the structural background of fault anticline and fault nose across the sag center; (2) Xianqiao structural belt. This provides theoretical guidance for the exploration of new hydrocarbon resource in the sag.

Key words: genesis, Yueguifeng Formation, lacustrine source rock, Lingfeng Formation, shallow marine source rock, Lishuixi sag, East China Sea Shelf Basin

CLC Number:

ZHANG Yingzhao, HU Senqing, LIU Jinshui, JIANG Yiming, CHEN Zhongyun, QIN Jun, DIAO Hui, WANG Chao. Geochemical Characteristics and Genesis of Oil and Gas in the Lishuixi Sag, East China Sea Shelf Basin[J]. Geoscience, 2022, 36(05): 1382-1390.

Figures/Tables 9

References 34

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区带	井号	井段/m	测试层号	地层	天然气组分/%				干燥系数	碳同位素δ¹³C/‰
区带	井号	井段/m	测试层号	地层	C₁	C_2-5	N₂	CO₂	C₁/C₁-C₅	C₁	C₂	C₃	C₄	CO₂
丽水西凹陷	L36-1	2 250~ 2 283	DST1- S1.2	上古新统明月峰组	54.75	6.96	3.79	34.50	0.89	-46.13	-29.31	-27.07	-26.93	-4.67
丽水西凹陷	L36-1	2 250~ 2 283	DST1- S1.3	上古新统明月峰组	55.45	6.85	3.55	34.15	0.89	-46.30	-29.55	-26.96	-26.86	-5.03
松东凹陷北坡	SF24-1-1	2 122.5	MDT	下古新统三亚组	74.30	9.80	6.77	7.81	0.88	-47.06	-30.92	-29.96	-28.64	-7.61
宝岛凹陷	BF13-1-1	1 573~ 1 580	DST	中中新统梅山组	86.14	10.24	2.46	0.72	0.89	-48.65	-30.53	-31.15	-27.82	-25.54
宝岛凹陷	BF13-3-1	1 740	MDT	中中新统梅山组	84.70	11.40	2.80	0.61	0.88	-48.91	-27.45	-27.93	-27.84

区带	井号	井段/m	测试层号	地层	天然气组分/%				干燥系数	碳同位素δ¹³C/‰
区带	井号	井段/m	测试层号	地层	C₁	C_2-5	N₂	CO₂	C₁/C₁-C₅	C₁	C₂	C₃	C₄	CO₂
丽水西凹陷	L36-1	2 250~ 2 283	DST1- S1.2	上古新统明月峰组	54.75	6.96	3.79	34.50	0.89	-46.13	-29.31	-27.07	-26.93	-4.67
丽水西凹陷	L36-1	2 250~ 2 283	DST1- S1.3	上古新统明月峰组	55.45	6.85	3.55	34.15	0.89	-46.30	-29.55	-26.96	-26.86	-5.03
松东凹陷北坡	SF24-1-1	2 122.5	MDT	下古新统三亚组	74.30	9.80	6.77	7.81	0.88	-47.06	-30.92	-29.96	-28.64	-7.61
宝岛凹陷	BF13-1-1	1 573~ 1 580	DST	中中新统梅山组	86.14	10.24	2.46	0.72	0.89	-48.65	-30.53	-31.15	-27.82	-25.54
宝岛凹陷	BF13-3-1	1 740	MDT	中中新统梅山组	84.70	11.40	2.80	0.61	0.88	-48.91	-27.45	-27.93	-27.84

井名	井深/m	样品	C₇系列/%
井名	井深/m	样品	直链烷烃	支链烷烃	环烷烃
L36-1	2 580~ 2 283	DST1-S1.7	15.26	23.85	60.53
L36-1	2 580~ 2 283	DST1-S1.8	14.17	24.28	61.55

井名	井深/m	样品	C₇系列/%
井名	井深/m	样品	直链烷烃	支链烷烃	环烷烃
L36-1	2 580~ 2 283	DST1-S1.7	15.26	23.85	60.53
L36-1	2 580~ 2 283	DST1-S1.8	14.17	24.28	61.55

盆地	井名	深度/m	地层	天然气C₇轻烃组分
盆地	井名	深度/m	地层	甲基环己烷 (MCH)/%	正庚烷 (nC₇)/%	二甲基环戊烷 (∑DMCP)/%	甲基环己烷指数/%
东海陆架盆地丽水西凹陷	L36-1	2 250~2 283	上古新统明月峰组	61.60	21.20	17.20	61.60
东海陆架盆地丽水西凹陷	L36-1	2 250~2 283	上古新统明月峰组	63.50	19.40	17.10	63.50
北部湾盆地涠西南凹陷	W-1	2 137.5~2 265.8	流一段	33.30	25.90	40.70	33.30
北部湾盆地涠西南凹陷	W-2	2 459.0~2 473.5	涠三段	35.71	32.14	32.14	35.71
琼东南盆地松东凹陷	SF24-1-1	2 122.5	下中新统三亚组	42.60	24.00	33.30	42.60
琼东南盆地宝岛凹陷	BF13-1-1	1 573~1 580	中中新统梅山组	40.00	20.00	40.00	40.00
	BF13-1-1	1 577.3		40.00	17.10	42.90	40.00
	BF13-1-1	1 740	中中新统梅山组	30.40	26.00	43.60	30.40