林玉祥 赵承锦 朱传真 栾伟娜 吴玉琛 李佳 李秀芹
摘 要:基于层序地层学,将华北地台东部下奥陶统划分为新厂阶和道保湾阶。早奥陶世海水较浅,发育浅海和海岸两种沉积环境,自南往北由古陆边缘沉积区过渡为碳酸盐台地沉积区,局限台地和潮上云坪发育。台地呈“中凹边凸”凹面式隆升,南北升降差异逐渐加大,沉积-沉降中心的延伸方向由东西延展向南北方向发展,显示了多种构造应力综合作用而又存在优势方向的特点。相对海平面继承了寒武纪末期的降低趋势,海退大致自NW向SE,台地衰退,云坪和局限台地向东扩展,开阔台地向北、向东退缩。沉积相南北分区的特点使得岩性分区显著,从台地区到潮坪相区,由灰岩过渡为白云岩。需要注意的是,本区云坪亚相准同生白云岩特别发育,中新生代盆地下奥陶统特别是亮甲山组白云岩发育优质储层。
关键词:岩相古地理;层序地层学;早奥陶世;华北地台;油气勘探;有利区
中图分类号:P531 文献标志码:A
Lithofacies Paleogeography Characteristics of Early Ordovician
in the Eastern Part of North China Platform
LIN Yu-xiang, ZHAO Cheng-jin, ZHU Chuan-zhen, LUAN Wei-na,
WU Yu-chen, LI Jia, LI Xiu-qin
(School of Geological Science and Engineering, Shandong University of Science and
Technology, Qingdao 266590, Shandong, China)
Abstract: Based on sequence stratigraphy, the Lower Ordovician in the eastern part of North China Platform can be divided into two Ⅲ-type sequences, which belong to Xinchangian stage and Daobaowanian stage, respectively. During Early Ordovician, there are two types of sedimentary environments including shallow sea and coastal deposit environments with shallow sea water. The archicontinent marginal sedimentary transits to carbonate platform sedimentary from south to north, and the restricted platform and supratidal dolomitic flat develop. Due to the tectogenetic inconsistency of platform, the differential uplift and depression are magnified between the south and the north as well as a continuous regression, moreover, the subsidence-sedimentary center extends from the latitudinal direction to the longitudinal direction. There is comprehensive action of various tectonic stresses and predominant direction. Relative sea level inherits the decrease in the end of Cambrian, and the regression happens from NW to SE. Dolomitic flat and restricted platform expand eastward, thus open platform retreats northward and eastward. The sedimentary facies is divided into south part and north part, so that the partition of lithology is significant, and the limestone transits to dolomite from platform to tidal flat. Specially, the penecontemporaneous dolomite of dolomitic flat subfacies develops, and the dolomite from Liangjiashan Formation of Lower Ordovician can act as high-quality reservoir in Meso-Cenozoic basin.
Key words: lithofacies paleogeography; sequence stratigraphy; Early Ordovician; North China Platform; petroleum prospecting; favorable area
0 引 言
图1 华北地台东部奥陶系地层划分与对比
Fig.1 Stratigraphy Classification and Comparison of Ordovician in the Eastern Part of North China Platform
层序地层单元是等时地质体,其界面是等时物理面,海平面升降、气候、沉积物供给、构造沉降及其综合作用控制了地层单元的几何形态和岩性。以层序地层旋回进行岩相古地理编图能有效打破传统编图单元穿时的局限[1-8],同时提高古地理成图的成因连续性和实用性[3,9]。对于华北地台东部奥陶纪而言,陆表海浅水碳酸盐岩沉积对海平面变化非常敏感,旋回性发育特征突出。于炳松等认为早奥陶世正处在全球海平面上升期的高位阶段[10],构造稳定的地台内部年代地层具有较大的可比性。因此,笔者运用层序地层学观点,建立该区下奥陶统等时地层格架,揭示沉积相和沉积体系的时空配置关系,预测烃源岩、储集体的类型和分布,阐明生储盖组合,研究岩相古地理控制下的油气地质特征,预测油气勘探有利区,指导本区油气勘探。
1 地层格架建立
华北地台东部奥陶系年代地层界面的形成主要受控于海平面的相对下降[11],具有显著的沉积、成岩标志。由于相对海平面下降,亮甲山组顶部沉积间断面发育;在岩性组合上,冶里组和亮甲山组均表现为底部泥质含量高[12],因而可通过自然伽马曲线辅助识别碳酸盐岩地层单元界面;下奥陶统特别是亮甲山组白云岩化作用、硅化作用、岩溶作用非常发育,成岩标志特征明显。
早奥陶世的相对海平面变化包括2个较大的升降旋回[13],发育海侵体系域(TST)和高水位体系域(HST),缺失低水位体系域(LST)。综合考虑海平面变化、古生物标志、不整合面及沉积间断面、沉积及成岩界面标志[14-15],将下奥陶统划分为2个三级旋回(图1、2),分别与新厂阶和道保湾阶对应;参照陈建强等的划分方案[16-17],将中奥陶统划分为5个三级旋回(图1)。
2 地层单元对比及其展布
从地层厚度(表1)和各露头、钻井地层沉积相柱状图(图2~4)来看,与新厂阶地层相比,道保湾阶地层厚度明显增加。除南部古陆缺失下奥陶统地层外,各地层单元发育比较齐全。下奥陶统地层北厚南薄(图5),平面上由北而南、垂向上自下而上,沉积环境由浅海沉积区过渡为海岸沉积区,大致可反映海退方向为自西向东、自南往北面式发展。岩性分区特征显著,平面上自NE到SW、垂向上自下而上,岩性由灰岩过渡到白云质灰岩、白云岩,为典
图2 早奥陶世地层综合柱状图
Fig.2 Comprehensive Column Schematic of Early Ordovician Stratum
表1 早奥陶世地层厚度
Tab.1 Strata Thickness of Early Ordovician
图3 新厂阶实际材料点
Fig.3 Actual Material Map of Xinchangian Stage
型的向上变浅序列。华北地台东部奥陶纪为“中凹边凸”凹面式抬升,早奥陶世发生广泛海退。
研究区道保湾期与新厂期沉积格局具有一定的继承性和明显的发展性。新厂期从北向南地层厚度有减薄的趋势,沉积-沉降中心在研究区东北部。道保湾期厚度明显加大,沉积-沉降中心仍在东北部,但与新厂期相比,向西有所偏移。从整体来看,西北部及东北部厚度最大,向南厚度逐渐减小。需要注意的是,早奥陶世沉积-沉降中心在不同地区发生不同变化,由南往北依次表现为迁移、合并、分化,且道保湾期沉积-沉降中心更具规律性。
3 古地理特征
通过系列的岩相、沉积构造及地球化学等综合研究分析[18-24],本区早奥陶世主要存在潮坪相、局限台地相、开阔台地相台地及潟湖相4种沉积相类型。在海退背景下受本区水动力条件限制,早奥陶世潮上云坪范围扩大,开阔台地相仅存在于研究区东北部,至道保湾期末,主要发育云坪和局限台地,潟湖、台内浅滩仅呈“孤岛”状分布在云坪、台地相内部或边缘,范围非常小。
3.1 新厂期
新厂期地势南高北低,南北升降差异大,以开阔台地、局限台地和潮上云坪环境为主(图6)。受怀远运动影响,本期华北地台南缘地壳抬升,成为陆地剥蚀区,缺失本期沉积。由南向北地层厚度增大,太原—济南一线以南发育潮上云坪,岩性主要为白云岩、泥质白云岩,夹少量竹叶状白云岩、燧石结核白云岩等。北京—沈阳地区为开阔台地,岩性以泥晶灰岩、泥灰岩、白云质灰岩为主,夹少量竹叶状灰岩、砾屑灰岩。云坪与开阔台地之间为狭长的局限台地沉积,岩性以白云质灰岩、灰质白云岩和泥灰岩为主。此外,郑州北侧局部地区发育有潟湖相沉积,岩性以白云岩为主。济南东侧地区发育台地浅滩相,岩性以白云岩为主,顶部为泥质白云岩。本期南侧地势明显升高,云坪范围明显扩大,北侧开阔台地范围明显萎缩,并有向东退缩的趋势。
图4 道保湾阶实际材料点
Fig.4 Actual Material Map of Daobaowanian Stage
图5 早奥陶世地层对比
Fig.5 Stratigraphic Correlation of Early Ordovician
图6 新厂期岩相古地理
Fig.6 Lithofacies-palaeogeography Map of Xinchangian
3.2 道保湾期
与新厂期相比,道保湾期古地理格局总体保持一致,华北地台南部仍然为剥蚀区,中部及北部仍为陆表海沉积,但南北差异更加明显,古海域范围稍有缩小(图7)。
由于海水进一步由南向北退缩,古海域中水体能量减弱,主要发育潮上云坪、局限台地和开阔台地沉积环境,云坪和局限台地分布范围增大,开阔台地分布范围比前期明显变小。云坪占据了中西部大片地区,主要分布于郑州—济南—太原—呼和浩特周围的广大地区,岩性以白云岩、灰质白云岩、泥质白云岩为主,并含较多燧石结核。天津—沈阳一线东南为局限台地沉积,岩性主要为灰岩、燧石结核灰岩、白云质灰岩、砾屑泥晶灰岩,并夹少量竹叶状灰岩、泥质条带灰岩。沈阳以北为开阔台地,形成砾屑灰岩、泥晶灰岩、豹皮状灰岩和泥灰岩沉积。此外,在沈阳东南侧还发育了一个小型竹叶浅滩,岩性以竹叶状灰岩为主。整体来看,本期受怀远运动的影响,海水由南向北逐渐退出,南北升降差异变大,在道保湾期末发生了广泛的海退,绝大部分地区出现不同程度的剥蚀,从而结束了该期沉积。
4 古地理控制下的油气成藏条件
沉积相对烃源岩、储集层具有一定的控制作用,早奥陶世广泛分布的云坪微相以发育白云岩类烃源岩为特征[25-26]。新厂期至道保湾期属碎屑岩-碳酸盐岩台地建造,在海侵初期富含泥质,泥质的加入有利于有机质的保存。早奥陶世白云岩有机质丰度均不是很高,仅少数样品达到了烃源岩的标准,以亮甲山组生烃潜力最大,平均为0.189×10-3。以黄骅凹陷为例,下奥陶统亮甲山组、冶里组总有机碳(TOC)几乎均小于0.2%。冶里组和亮甲山组生烃潜力在(0.1~0.5)×10-3之间的样品占最大比例,小于0.05×10-3和介于(0.05~0.10)×10-3之间的样品次之,仅生烃潜力大于1.0×10-3的样品减少。济阳凹陷下奥陶统冶里组—亮甲山组主要为浅色白云岩,总有机碳和可溶有机物含量都较低,生烃潜力差。奥陶系烃源岩热解参数也只有0.065×10-3,显示出它们的残余生烃潜力已比较低。总体来看,下奥陶统生烃潜力较小,不可能成为大型油气藏的有效烃源岩。
图7 道保湾期岩相古地理
Fig.7 Lithofacies-palaeogeography Map of Daobaowanian
本区潮上云坪和局限台地非常发育,发育于潮上云坪及局限台地边缘的白云岩分布广且厚度大。云坪环境形成的准同生白云岩孔隙度最高,准同生白云岩经过改造形成次生白云岩,是极好的储集相带。道保湾期末的怀远运动使下奥陶统地层遭受不同程度的剥蚀淋滤,风化壳型储层发育且广泛分布;后期遭受多期次构造运动改造,碳酸盐岩被改造成岩溶储集带和断块储集体[27]。本区下奥陶统主要存在孔隙型、古岩溶型及裂隙型3种类型储层。孔隙型储层的储集空间及渗流通道均为孔隙,部分含有微裂缝,孔隙直径一般为数微米到几百微米;裂隙型储层主要有不规则密集网状和分支状的风化裂隙、垂直或高角度展布的部分充填或未充填构造裂隙以及早期裂缝系统产生溶蚀扩大的溶蚀裂隙;古岩溶型储层是指在地史时期,由于地表流水和地下水对可溶性岩石的改造和破坏作用而形成的溶蚀孔洞、裂缝,直径一般为几百微米到数百毫米,潜水面的多次升降变化可以形成多个岩溶旋回。就储层物性而言,白云岩抗压实及抗压溶能力比灰岩强,易于发育裂隙和断裂,使得白云岩的渗透性极大提高;同时,白云岩的次生交代作用有利于孔隙形成。因此,白云岩储层物性要好于灰岩,其在中新生代盆地为有效储层。
该区奥陶系可形成上生下储上盖型组合,即以马家沟组灰岩为烃源岩层,以新厂阶和道保湾阶白云岩为储层,以马家沟组膏岩和泥质碳酸盐岩为盖层的组合,其主要分布在渤海湾盆地。
5 结 语
(1)通过层序地层划分与对比,建立华北地台东部下奥陶统等时地层格架。以阶为等时地层单元,本区下奥陶统可以划分为与之相对应的2个编图单元,分别对应新厂阶和道保湾阶,并按阶编制了实际材料点图和岩相古地理图。
(2)新厂期和道保湾期沉积格局和古地理格局具有一定的继承性和明显的发展性。“中凹边凸”凹面式抬升特别是怀远运动自南部的挤压隆升,使得南北差异加大,地层厚度由南往北增厚,东部、西部厚度大,中间薄,呈相间排列格局。随着海退持续发展,云坪面积扩大,以发育云坪和局限台地为主,开阔台地退至华北地台东北部。
(3)综合考虑岩相、构造组合特征,新厂阶和道保湾阶白云岩储集条件优越,发育古风化壳型储层和孔隙-裂缝型储层。华北地台东部下奥陶统具备有利储集系统,主要分布于渤海湾盆地。
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