Provenance analysis of the Pinghu slope belt in Xihu Depression: Evidence from detrital zircon U-Pb chronology and heavy minerals
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摘要: 西湖凹陷是东海陆架盆地重要的含油气凹陷,其中平湖组是该凹陷内重要的勘探目的层段。目前平湖组油气发现多集中于凹陷西部斜坡带,由于其他地区钻井资料较少,对于西湖凹陷平湖组砂体展布以及物源区一直存在争议,严重制约了西湖凹陷平湖组的油气勘探。主要通过碎屑锆石U-Pb定年,同时结合重矿物组合特征、重矿物ZTR指数分析和碎屑矿物成分成熟度特征,对平湖组沉积物源区进行追踪,分析结果显示研究区锆石年龄分布相对分散且跨度大,包括中生代、古生代和前寒武纪3个年龄段,且以前寒武纪为主。平湖组主要发育3个物源区,其中北部虎皮礁隆起为主要物源区,西部海礁隆起与东部钓鱼岛隆褶带为次要物源区。对平湖组物源体系的研究为西湖凹陷的油气勘探开发提供借鉴。Abstract: Xihu Depression is an important oil-bearing depression in the East China Sea Basin, in which Pinghu Formation is an important exploration target formation. At present, oil and gas discoveries in Pinghu Formation are mostly concentrated in the western slope zone of Xihu Depression. However, disputes over the sand body distribution and source area of Pinghu Formation in Xihu Depression have been existed for a long time due to the lack of drilling data, which seriously restricts the oil and gas exploration in Pinghu Formation. In this study, the sediment source area of Pinghu Formation was tracked mainly by the detrital zircon U-Pb dating combined with composition characteristic and ZTR index of heavy minerals and the maturity of detrital mineral composition, so as to reveal the development regulation of sand bodies. The age distribution of zircon in the study area is relatively scattered, with a large distribution range, including the Precambrian, Mesozoic and Paleozoic, and the Precambrian is dominant. The analysis results show that three provenances are developed during Pinghu Formation, including the main provenance of the northern Hupijiao Uplift, the western Haijiao Uplift and the eastern Diaoyu Island upwarping folded zone, which can provide important reference for the oil and gas exploration and development in Xihu Depression.
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表 1 西湖凹陷平湖斜坡带平湖组主要重矿物质量分数
Table 1. Main heavy mineral content of Pinghu Formation in Pinghu slope belt of Xihu Depression
井号 金红石 锆石 电气石 赤褐铁矿 磁铁矿 白钛矿 石榴石 ZTR指数 wB/% A1 0.3 11.3 3.2 1.0 2.9 13.3 59.0 14.8 A2 0.9 16.4 2.7 1.4 5.0 9.6 43.4 20.0 A3 1.0 13.8 2.6 4.8 2.4 6.9 31.0 17.3 A4 2.7 11.8 1.6 2.9 2.7 11.0 49.5 16.1 A5 0.5 7.0 1.7 2.1 3.3 8.4 38.2 9.2 A6 0.1 10.1 1.6 6.0 1.8 4.6 24.9 11.9 A7 0.7 4.4 3.9 19.1 1.0 14.5 5.7 9.0 平均值 0.9 10.2 2.5 5.4 2.8 10.1 36.7 14.0 表 2 西湖凹陷平湖斜坡带平湖组碎屑组分平均质量分数
Table 2. Average content of clastic components of Pinghu Formation in Pinghu slope belt of Xihu Depression
井号 石英 长石 岩屑 成分成熟度 wB/% A1 78.3 12.5 9.2 3.6 A2 83.1 11.7 5.2 4.9 A3 88.3 7.5 4.2 7.6 A4 72.4 17.6 10.0 2.6 A5 82.3 9.1 8.6 4.7 A6 69.8 12.2 18.0 2.3 A8 51.8 14.9 33.2 1.1 平均值 75.1 12.2 12.6 3.8 表 3 西湖凹陷不同井锆石年龄及发育特征
Table 3. Dating results and characteristic of zircons in different wells, Xihu Depression
井号 有效数据 总点数 碎屑锆石年龄范围 锆石特征 A1 24 70 (87.45±2.08)~(2 900±47.22)Ma
主峰段:160~320 Ma;
次峰段:0~160, 640~800, 1 600~1 920 Ma见有前寒武纪、古生代、中生代3个时代的锆石信息。其中前寒武纪锆石主要为变质增生锆石,外形不规则,震荡环带清晰;古生代锆石环带较窄,破损严重;中生代锆石发光亮,自形,环带清晰 A3 33 80 (124.31±2.69)~(2 161.42±236.57)Ma
主峰段:1 595~1 750 Ma;
次峰段:160~320, 1 280~1 430 Ma前寒武纪锆石多为变质增生锆石,部分可见多套震荡环带及扇形分带结构;古生代锆石发光较暗,环带清晰,边缘可见变质增生边,部分锆石环带被切割,可见多套岩浆环带发育;中生代锆石多为自形,环带清晰,锆石较为完整 A4 26 74 (126.42±2.64)~(2 594.76±30.25)Ma
主峰段:100~360 Ma;
次峰段:2 380~2 460 Ma;缺少960~ 1 700 Ma年龄前寒武纪锆石主要呈自形-半自形,发育环带,形状较规则,边缘可见变质增生边,部分锆石被变质重结晶作用改造;古生代锆石自形良好,亮度较暗,外形规则,具有清晰的震荡环带;中生代锆石为自形,亮度较暗,破坏较为严重,部分可见两套环带 A5 38 72 (130.36±2.56)~(2 505.56±60.80) Ma
主峰段:160~320 Ma;
次峰段:320~480 Ma前寒武纪锆石多呈半自形-自形状,新生部分与原有锆石界限清晰,部分锆石核部较亮,形状规则,保留有原始锆石环带;古生代锆石形状规则,自形状,可见有变质增生边和多套震荡环带发育;中生代锆石自形,外形规则,环带清晰,边界平直,多破损 A7 15 72 (19.76±3.23)~(2 398.15±34.26)Ma
主峰段:160~320 Ma;
次峰段:320~480, 1 840~2 000 Ma前寒武纪锆石外形不规则且多破损,环带不清晰,具变质增生边,核部锆石多被变质重结晶作用改造;古生代锆石图像模糊,外形不规则,弱分带;中生代锆石外形规则,边界平直,为自形,环带清晰,见有扇形环带结构,部分锆石被变质作用严重改造 A9 41 72 (72.58±1.49)~(3 088.58±57.56) Ma
主峰段:160~320 Ma;
次峰段:1 600~1 760 Ma前寒武纪锆石外形不规则,环带较清晰,部分原有锆石环带清晰,边界平直,未被改造,具变质增生边,部分原有锆石环带被切割;古生代锆石外形较规则,分带清晰,核部原有锆石保留较好;中生代锆石外形规则,边界平直,为自形,环带清晰,部分原有锆石环带被切割 -
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