地质力学学报  2021, Vol. 27 Issue (3): 441-452
引用本文
谭元隆, 王宗秀, 冯兴强, 肖伟峰, 季长军, 吴林, 周磊, 张林炎. 复杂构造区油气构造保存条件分析: 来自皖泾地1井的构造解析[J]. 地质力学学报, 2021, 27(3): 441-452.
TAN Yuanlong, WANG Zongxiu, FENG Xingqiang, XIAO Weifeng, JI Changjun, WU Lin, ZHOU Lei, ZHANG Linyan. Structural preservation conditions analysis of oil and gas in complex structural area: A case study of structural analysis in the Well Wanjingdi-1, Anhui, China[J]. Journal of Geomechanics, 2021, 27(3): 441-452.
复杂构造区油气构造保存条件分析: 来自皖泾地1井的构造解析
谭元隆1,2,3, 王宗秀1,2,3, 冯兴强1,2,3, 肖伟峰1,2,3, 季长军1,2,3, 吴林1,2,3, 周磊1,2,3, 张林炎1,2,3    
1. 中国地质科学院地质力学研究所, 北京 100081;
2. 自然资源部古地磁与古构造重建重点实验室, 北京 100081;
3. 中国地质调查局油气地质力学重点实验室, 北京 100081
摘要:下扬子地区中生代发生了多期挤压-伸展构造作用,是制约该区域油气勘探取得突破的重要因素之一。为揭示下扬子区中生代构造作用对油气保存的影响,文章通过地震剖面解释资料、野外露头调查、区域构造演化历史分析,并结合皖泾地1井获取的岩芯资料和油气显示情况综合探讨了下扬子南部地区的有利构造保存条件。地震解释剖面和地表调查结果表明下扬子南部地区晚三叠世以来主要经历了两期重要的构造作用:第一期为印支期挤压作用造成褶皱变形和逆冲断层以及隆升剥蚀;第二期为晚白垩世以来的伸展作用,形成断陷盆地和正断层活动。皖泾地1井在二叠系页岩地层中获得页岩气显示,在下三叠统殷坑组灰岩中钻遇轻质油,表明下扬子南部区域具有良好的油气富集条件和资源潜力。构造保存条件分析表明皖泾地1井位于褶皱冲断带内的向斜部位,印支期隆升剥蚀量小,晚白垩世以来伸展拉张幅度小,具有良好的封闭性和成藏条件,是有利的构造保存部位。
关键词复杂构造区    构造解析    皖泾地1井    保存条件    
DOI10.12090/j.issn.1006-6616.2021.27.03.040     文章编号:1006-6616(2021)03-0441-12
Structural preservation conditions analysis of oil and gas in complex structural area: A case study of structural analysis in the Well Wanjingdi-1, Anhui, China
TAN Yuanlong1,2,3, WANG Zongxiu1,2,3, FENG Xingqiang1,2,3, XIAO Weifeng1,2,3, JI Changjun1,2,3, WU Lin1,2,3, ZHOU Lei1,2,3, ZHANG Linyan1,2,3    
1. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;
2. Key Laboratory of Paleomagnetism and Paleotectonic Reconstruction, Ministry of Natural resources, Beijing 100081, China;
3. Key Laboratory of Petroleum Geomechanics, China Geological Survey, Beijing 100081, China
Abstract: The multi-stage tectonism of compression and extension occurred in the southern part of the Lower Yangtze region in the Mesozoic, which is one of the important factors restricting the breakthrough of oil and gas exploration in this area. This paper aims to reveal the influence of Mesozoic tectonisim on oil and gas preservation in the Lower Yangtze area. Based on the seismic profile interpretation data, field outcrop survey, regional tectonic evolution history analysis and the drilling core data and oil-gas display of the Well Wanjingdi-1, we comprehensively discussed the favorable structural preservation conditions in the southern part of the Lower Yangtze region. The results of seismic interpretation profile and surface survey show that the southern part of the lower Yangtze River has experienced two important tectonic processes since the Late Triassic. The first stage is fold deformation, thrust fault and uplift erosion caused by Indosinian compression, and the second stage is extension since the Late Cretaceous, resulting in the formation of fault basin and normal fault activity. Shale gas display was obtained from the Permian shale in the Well Wanjingdi-1, and light oil was found in the drilling of the Lower Triassic Yinkeng Formation, indicating that the southern Lower Yangtze region has good enrichment conditions and resource potential, which points out a new direction for oil and gas exploration in the region. The analysis of structural preservation conditions shows that the Well Wanjingdi-1 is located in the syncline of the fold-depression belt. The amount of uplift and denudation was small in the Indosinian period. Since the late Cretaceous, the depression has a small amplitude of extension and has good sealing and reservoir forming conditions. It is a favorable structural position for oil and gas preservation in this region. Based on the comprehensive analysis of regional structure and sedimentary basin evolution, it is considered that in the southern Lower Yangtze region, the syncline structural belt in the southern margin of the basin with small erosion amount of Triassic strata or the central zone of fault basin overlying thick Cretaceous-Cenozoic strata should be taken as the key target to search for favorable oil and gas zones.
Key words: complex structural area    structural analysis    Well Wanjingdi-1    preservation conditions    
0 引言

近年来油气公司和中国地质调查局在下扬子南部地区开展了大量油气地质勘探工作,对区域二叠—三叠系油气地质条件也进行了详细的调查分析,并部署实施了数十项油气钻井工程,但是目前尚没有取得大的突破和实现工业油气流的开采(郭念发等,2002潘继平等,2011曹骏,2013黄保家等,2013李建青等,2013蔡周荣等,2015宋腾等,2017石刚等,2019; 张恒,2020)。相关学者对该区域二叠—三叠系沉积环境、物质基础、热演化程度等方面开展了大量的研究和测试分析工作,认为该区域具备良好的油气基础地质条件,如累积有效烃源岩厚度达150 m、热演化程度在1.0%~2.2%之间、有机碳含量为0.8%~14.3%、储层物性较为致密(徐伟民,1991郭念发等,2002刘子满,2004潘继平等,2011段宏亮和王红伟,2012陈平等,2013李伶俐和马伟竣,2013潘磊等,2013吴浩等,2013曹涛涛等,2015Pan et al., 2015Jiang et al., 2016Zou et al., 2019)。但是下扬子地区中生代以来发生了多期挤压-伸展构造作用,形成了复杂且强烈的构造变形(安徽省地质矿产局,1987王浩清和陈家治,1993李海滨等,2011徐曦等,2018李锦轶等,2019张岳桥和董树文,2019)。尤其是印支—早燕山期的挤压作用,造成下扬子地区大规模的褶皱变形和隆升剥蚀(姚柏平等,1999丁道桂等,2008李海滨等,2011曹骏,2013;)。区域地质、地震、钻井和地球物理等多方面资料表明,下扬子区大致以长江为界,北部的褶皱带向南东逆冲,而南部的褶皱带向北西逆冲,形成对冲构造(张永鸿,1991姚柏平等,1999李海滨等,2011)。但不同褶皱带具有不同的变形样式,研究者根据构造变形的差异将下扬子区划分为不同的构造单元(李海滨等,2011吴林等,2015)。不同构造单元的隆升剥蚀幅度亦存在显著的差异,曾萍(2005)等采用镜质体反射率、粘土矿物、裂变径迹、包裹体测温等研究方法,对下扬子区构造-热演化进行了系统的研究,结果表明无为地区印支—早燕山期剥蚀量为2550 m,句容地区为3440 m,黄桥北部为3850 m,黄桥南部为1472 m。而白垩纪以来,下扬子区经历了显著的岩浆活动和伸展拉张作用,前期的挤压构造发生了反转,形成了不同规模的断陷或凹陷盆地(李海滨等,2011李建青等,2012蔡周荣等,2015徐曦等,2018)。白垩系—第四系在苏北盆地厚度达5000 m,而在苏南地区亦可达2500 m,但在下扬子西南地区的安庆、铜陵等地则厚度显著减薄,大多在1500 m以内,指示不同区域伸展拉张幅度的差异性(安徽省地质矿产局,1987)。由于构造样式、隆升剥蚀和伸展拉张幅度等不同构造作用对油气的保存具有显著的影响,但目前下扬子地区关于此问题的研究仍未取得很好的进展,这可能是制约该区域油气勘探取得突破的重要原因(张永鸿,1991姚柏平等,1999李亚辉等,2010李建青等,2012彭金宁等,2015)。因此,文章通过地震剖面解释资料和野外构造地质调查分析皖泾地1井周缘构造演化历史,并结合钻井获取的岩芯资料和油气发现情况综合探讨下扬子南部地区的有利构造保存条件,进一步预测该区域有利构造保存区带,为下一步油气勘探提供基础性资料与参考。

1 区域地质概况

下扬子地块南部以江绍断裂带与华夏地块接壤,北部则以郯庐断裂带与华北地块相接。下扬子地块西南部主要以早古生代及更老的地层出露为特征,在地貌上表现为高耸的山地,被研究者称之为江南隆起带;而其东部和北部则在白垩纪以来发生了广泛的伸展作用,形成了广阔的陆源碎屑充填的断陷盆地(安徽省地质矿产局,1987);二者之间以江南断裂带为界。调查区则位于江南隆起带北缘与断陷盆地的过渡地带,区内褶皱与断裂以北东—北东东向展布为主(图 1)。区域地层出露及地质填图资料显示(安徽省地质矿产局,1987),下扬子南部地区自寒武系至第四系沉积总厚7000 m左右,工作区内地层由老到新如图 2,其中除下—中泥盆统及部分古近系地层缺失外,区内地层发育相对完整。地层岩性特征表明,下扬子南部地区主要赋存有下寒武统、上奥陶统—下志留统和中上二叠统共三套富有机质页岩地层。根据地层岩性组合特征和地层接触关系,下扬子南部地区沉积构造演化大致可分为七个阶段(图 2):被动陆缘阶段(震旦纪—中奥陶世)、前陆盆地阶段(晚奥陶世—晚志留世)、稳定台地阶段(晚泥盆世—中三叠世)、挤压凹陷阶段(晚三叠世—中侏罗世)、断陷火山盆地阶段(晚侏罗世—早白垩世)、伸展断陷阶段(晚白垩世—始新世)及陆内凹陷阶段(渐新世至今)。由此可见,下扬子地块自震旦纪以来发生多期构造运动,中生代的构造变形尤其复杂。印支期华北板块与扬子板块的拼合,导致下扬子中三叠世前的沉积盖层普遍褶皱变形。随后进入挤压凹陷阶段(T3—J2),上三叠统—中侏罗统陆相沉积与下伏地层呈角度不整合接触,并被下白垩统不整合覆盖;晚侏罗世至早白垩世(J3—K1),可能受太平洋板块俯冲影响,区内出现了大规模岩浆侵入和火山喷发活动,形成厚度较大的火山岩和火山沉积岩,并被上白垩统不整合覆盖;晚白垩世—古近纪(K2—E1),下扬子南部地区发生大规模的伸展断陷作用,形成了巨厚的河流湖泊相沉积;新近纪—第四纪断裂活动微弱,调查区以陆内坳陷为主,坳陷盆地沉积不整合在下伏层序之上(安徽省地质矿产局,1987)。

图 1 下扬子及周缘大地构造简图 Fig. 1 Tectonic map of the Lower Yangtze plate and surrounding areas

图 2 下扬子南部地区显生宙地层及构造演化阶段图 Fig. 2 Phanerozoic strata and tectonic evolution stages in southern Lower Yangtze
2 皖泾地1井工区构造

皖泾地1井地理位置为安徽省泾县蔡村镇,位于南陵盆地南缘蔡村向斜的南东翼。南陵盆地周围零星出露北东向展布的志留系—三叠系及上白垩统和少量新近系,盆地内部被大面积的第四系所覆盖。区域内褶皱以北东—北东东向展布,断裂以北东向展布为主,控制了区域内地层的分布,而北西向断裂为次级断裂(图 3)。

图 3 皖泾地1井周缘地质图(其中红色五角星为皖泾地1井位置,过井黑色线段为部署实施的地震剖面测线JX2019-DZ-02,黄色AB线段为南东140°构造剖面) Fig. 3 Regional geological map of the Well Wanjingdi-1 (Red pentagram is the location of the Well Wanjingdi-1, black line passing through the well is the seismic profile line JX2019-DZ-02, and the yellow line AB is the structural section by geological survey)
2.1 深部构造解析

JX2019-DZ-02为过皖泾地1井,长度40 km,北西—南东向的地震测深剖面(图 3),并利用工区周边的宝地1井和皖宣页1井钻遇地层对该地震剖面进行了标定和解释,认为工区地质结构清晰。从图 4a中可以看出,二叠系泥页岩夹砂岩的反射波组连续、清晰、信号强,其上为三叠系灰岩地层,且往北东方向三叠系保存厚度更大。三叠系上部由一套高频、连续反射波组构成,为白垩系—新生代砂岩,底界清晰,表明南陵盆地第四系之下广泛分布着古近系及上白垩统,厚度达3000 m,是发育于中晚侏罗世褶皱冲断带之上的中—新生代断陷盆地。同时,地震剖面显示北西侧白垩系—第四系厚度大,而往南东侧厚度逐渐减薄,指示南陵盆地为北断南超的箕状盆地。

a—地震测线解释剖面;b—地震剖面地质解释;c—测线南东段精细解释剖面图 图 4 过井地震测线(JX2019-DZ-02)及解释(测线位置见图 3) Fig. 4 Seismic profile line (JX2019-DZ-02) through the Well Wanjingdi-1 and the interpretation. (a) Interpretation section of the seismic line. (b) Geological interpretation of the seismic profile. (c) Detailed seismic fine interpretation profile of the SE part of the seismic line.

为了进一步揭示盆地深部构造和演化历史,对地震剖面进行了构造解释。由图 4b地质解释剖面分析认为,整个剖面大致可分为三个构造层:下构造层,前震旦系结晶基底和变质基底组成,未卷入褶皱变形;中构造层,主要由古生界—下中生界的海相碳酸盐岩、碎屑岩组成,变形强烈,表现为一系列的褶皱和逆冲断层,断层倾向南东,多个逆冲断层与褶皱组成叠瓦式逆冲构造;上构造层,由白垩系—第四系地层组成,主要表现为地堑、半地堑的充填结构,褶皱不发育,为典型的负反转构造,表现为一系列倾向南东的铲式断层,为部分早期逆冲断层发生发转的结果。因此,区内中生代可能主要经历了两期构造活动,第一期为二叠—三叠系地层的褶皱冲断变形,主要表现为由南东向北西逆冲,可能为印支期扬子地块与华北板块拼合过程中北西—南东向挤压作用的结果;而第二期则主要为伸展拉张作用,造成前期挤压构造的反转,剖面北西侧断距大,上白垩统—新生代地层沉积厚度大,而向南东侧断距逐渐减小,形成由断层夹持的复杂断块群,为晚白垩世以来北西—南东向伸展作用所致。为了更清晰地反映皖泾地1井周边深部地层展布和构造特征,地震剖面JX2019-DZ-02南东段进行了进一步详细解析(图 4c)。解释剖面清楚地反映志留系、二叠系泥页岩的展布特征,其上依次为下三叠统殷坑组、和龙山组、南陵湖组以及中三叠统周村冲组。深部地层展布特征指示为一向斜构造,其南东翼地层相对平缓,而北西翼相对陡倾,皖泾地1井位于向斜的南东翼。其构造活动特征显示早期为褶皱冲断作用,主要表现为南东向北西的逆冲,而晚期则表现为伸展拉张活动,并沉积了部分白垩系地层,指示了北西—南东向的伸展作用。

2.2 皖泾地1井周缘地表构造分析

为了与深部构造进行对比,研究对皖泾地1井周缘的地表构造进行了详细的调查。皖泾地1井所在的蔡村向斜呈北东向展布,向斜核部地层为中三叠统周冲村组灰岩。从野外露头观察可见蔡村向斜核部向南东翼依次零星出露三叠系周冲村组、南陵湖组、和龙山组、殷坑组,二叠系大隆组、龙潭组、孤峰组、栖霞组,以及部分石炭系和泥盆系(图 3)。其中二叠系及三叠系地层平缓倾向北西,在二叠系孤峰组和三叠系南陵湖组地层中测得产状分别为324°∠28°和321°∠20°(图 5a5b)。蔡村向斜北西翼则地层产状变化大,主要向南东陡倾,测得石炭系金陵组地层产状为154°∠76°,依据地层产状的恢复,认为蔡村向斜为南东翼平缓而北西翼陡倾的斜歪水平褶皱。

a—下三叠统南陵湖组灰岩;b—下二叠统孤峰组黑色硅质页岩;c—上泥盆统石英砂岩中的逆冲断层;d—中二叠统龙潭组砂岩中的逆冲断层;e、f—下石炭统金陵组粉砂岩中逆冲断层被正断层所切 图 5 皖泾地1井周缘地层露头及断裂活动表现 Fig. 5 Outcrop and fault activity in the periphery of the Well Wanjingdi-1. (a) Limestone in the Lower Triassic Nanlinghu Formation. (b) Black siliceous shale in the Lower Permian Gufeng Formation. (c) Thrust fault in the Upper Devonian quartz sandstone. (d) Thrust fault in the Middle Permian Longtan Formation sandstone. (e, f) Thrust faults in siltstone of the Lower Carboniferous Jinling Formation cutting through by normal faults.

蔡村向斜内断裂构造发育,走向北东—北东东向,主要表现为两期断裂活动。第一期表现为逆冲断层,在向斜北西翼泥盆系及龙潭组地层测得逆冲断层产状分别为343°∠60°和340°∠47°(图 5c5d),指示由北北西向南南东逆冲,在向斜南东翼逆冲断层则表现为由南东向北西逆冲,可能为向斜同期或其后的北北西—南南东向挤压作用下形成。第二期表现为正断层活动,主要发育于向斜南东翼,断层面倾向北西,在二叠系孤峰组下部测得正断层产状为322°∠57°,指示北西—南东向伸展作用。同时,在向斜北西翼下石炭统金陵组粉砂岩中观察到晚期正断层切割早期的逆冲断层(图 5e5f)。根据野外露头地质调查和地质图综合分析,恢复出井区AB段构造变形剖面(图 6),该剖面图与地震解释剖面图(图 4c)对比,二者具有很好的一致性,反映相似的构造变形表现和活动期次。

图 6 皖泾地1井区构造变形剖面(剖面位置见图 3) Fig. 6 Structural deformation profile in the Well Wanjingdi-1 area(The profile location is shown in Fig. 3).
3 皖泾地1井钻遇地层及油气显示情况

皖泾地1井目的层段为二叠系上统大隆组、中统龙潭组以及下统孤峰组,完钻层位为下二叠统栖霞组灰岩层。通过实钻地层划分(表 1),皖泾地1井钻遇地层为:第四系(Q);白垩系上统赤山组(K2c);三叠系中统周冲村组(T2z);三叠系下统南陵湖组(T1n)、和龙山组(T1h)、殷坑组(T1y);二叠系上统大隆组(P3d)、中统龙潭组(P2l)、下统孤峰组(P1g);栖霞组(P1q)未穿底。钻遇地层产状平缓,倾角在10°~25°之间。

表 1 皖泾地1井实钻地质分层数据表 Table 1 Actual drilling stratification data of strata in the Well Wanjingdi-1

皖泾地1井在实钻过程中于下三叠统殷坑组、中二叠统龙潭组和下二叠统孤峰组见油气显示。殷坑组以灰岩含油为主,龙潭组、孤峰组为页岩气显示,各显示层段累积厚度大、分布层位多。全井共钻遇气测异常显示16层,累计视厚度92.00 m,其中三叠系下统殷坑组含油异常显示6层,视厚度57.00 m,二叠系龙潭组、孤峰组含气异常显示层10.00 m,视厚度35.00 m。在1643~1761 m井段殷坑组中钻遇油浸显示9.00 m、油斑显示16.10 m、油迹显示4.30 m、荧光显示4.00 m,主要表现为灰岩缝洞型含油特征(图 7),原油金刚烷成熟度在1.37%~1.40%之间,显示为凝析油,表明殷坑组是该工区重要的含油气层系。在含气量解吸取样井段1899.23~2097.20 m的孤峰组及龙潭组中,共采取30个样品。所取样品平均含气量为0.0533 m3/t,最高含气量为0.3501 m3/t。

图 7 皖泾地1井下三叠统殷坑组油气显示情况 Fig. 7 Oil and gas display of the Yinkeng Formation in the Lower Triassic of the Well Wanjingdi-1
4 讨论:皖泾地1井构造保存条件分析

印支期华北与华南的拼合,使下扬子地区前三叠纪地层发生广泛的褶皱变形或隆升剥蚀,部分已出露地表(安徽省地质矿产局,1987)。早白垩世,中国东部大面积的岩浆侵入和火山作用可能提高了前期烃源岩系的热演化成熟度。在晚白垩世后,下扬子地区发生广泛的伸展断陷活动,在三叠系之上又沉积了巨厚的白垩系及新生代地层,特别是在下扬子东部的苏北盆地,白垩系及古近系总厚度可达5000 m。已有研究大多关注印支运动导致下扬子地区强烈而复杂的褶皱变形,而忽视了构造活动相对较弱的燕山、喜山运动(王浩清和陈家治,1993姚柏平等,1999丁道桂等,2008宫龙等,2013彭金宁等,2015吴林等,2015)。但部分研究者通过流体包裹体测温、热演化史和油气成藏期等方面的分析发现,下扬子地区二叠系在沉积后,主要的油气充注期在晚白垩世—古新世期间(刘东鹰和王军,2004黎琼,2012李建青等,2013刘光祥等,2014袁玉松等,2016)。因此,印支期的挤压褶皱作用可能控制油气藏的构造样式,但晚白垩世以来的伸展断陷作用对区域油气成藏的影响更大,控制油气的运聚与成藏。

皖泾地1井周缘地震构造解释剖面和地表地质调查分析均表明,该地区构造活动与断裂发育期次主要分为两个阶段:①印支期形成北东—北东东向展布的褶皱及逆冲断层;②白垩纪以来的伸展作用形成北东—北东东向正断层。印支期的构造挤压造就了下扬子南部褶皱变形样式和逆冲断层的发育,奠定了主要的构造格局,同时也使得前三叠纪地层发生隆升剥蚀。皖泾地1井白垩系之下即钻遇中三叠统周冲村组314 m厚灰岩(表 1),据区域地质资料分析,该区域周冲村组总厚约542 m,其上覆地层扁担山组厚608 m,扁担山组与黄马青组呈角度不整合接触,代表印支运动在该区域的构造响应(安徽省地质矿产局,1987)。因此印支期构造隆升造成该区域剥蚀厚度达836 m,相比于盆地中心地带和沿江地区的剥蚀量明显较小(曾萍,2005)。

区域地质调查表明该区大隆组与上覆殷坑组、下伏龙潭组为整合接触关系(安徽省地质矿产局,1987)。皖泾地1井北侧约10 km处的昌桥乡公路旁出露二叠系较为完整的地层剖面,野外实测资料显示大隆组厚61.35 m。在皖泾地1井北东方向约20 km处水东向斜实施的港地1井钻探资料显示,大隆组黑色富有机质页岩地层倾角10°~25°,连续厚度约70 m(石刚等,2019)。大隆组沉积相与沉积厚度展布特征也表明泾县地区大隆组平均厚度约50 m,而皖泾地1井实际钻遇地层结果显示大隆组只有2 m厚,表明该处大隆组缺失厚度约48 m,指示蔡村向斜形成后仍有显著的正断层活动,但断距并不大。钻井揭示白垩系和第四系总厚度为42 m,也表明该区域晚白垩世以来伸展拉张幅度小,沉积厚度薄。这与地震剖面揭露的南陵盆地深部结构特征具有一致性,盆地北部二叠—三叠系褶皱变形强烈,晚期伸展幅度大,晚白垩世以来沉积厚度大,可达2800 m;而南部变形弱,晚白垩世以来伸展幅度小,沉积厚度薄,仅数十米。此外,皖泾地1井在二叠系龙潭组、孤峰组和三叠系殷坑组中仍然有很好的油气显示,表明该向斜构造具备良好的成藏与保存条件。向斜两翼断层及殷坑组灰岩具有良好的封闭性能,向斜东南翼晚期的正断层活动对油气藏并没有显著的破坏作用,可能是正断层中含有断层泥,对断层带起了封闭作用。向斜北西翼主要为逆冲断层,可能对向斜的北西翼形成侧向封堵。综合上述分析,皖泾地1井所在的蔡村向斜两翼均具备良好的侧向封闭条件,是有利的构造保存区域。

5 结论

(1) 文章通过地震剖面解释和地表调查结果表明,下扬子南部地区晚三叠世以来主要经历了两期构造活动和构造改造作用:第一期为印支期挤压作用造成褶皱变形和逆冲断层以及隆升剥蚀;第二期为晚白垩世以来的伸展作用,表现为正断层活动和断陷盆地的形成。

(2) 构造保存条件分析表明皖泾地1井位于褶皱冲断带内的向斜部位,通过地层对比发现该区域印支期褶皱隆升剥蚀量相对较小,在晚白垩世以来伸展拉张幅度也较小。皖泾地1井在二叠系页岩中获得页岩气显示,在下三叠统殷坑组灰岩中钻遇轻质油,表明该区域拉张作用和正断层活动对油气藏并没有显著的破坏作用,说明上述构造部位具有良好的封闭性和成藏条件,是有利的构造保存区域。

(3) 通过区域构造与钻井资料综合分析认为,在下扬子南部区域,针对二叠系—三叠系烃源岩寻找有利油气区带,应以三叠系剥蚀量小、晚白垩世以来伸展拉张作用弱的盆地南缘向斜构造带为重点目标。

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