摘要
川东北地区作为四川盆地油气勘探中较有优势的后备区块,其以勘探层系之广和资源储备量之大,成为四川盆地油气勘探中的焦点。其勘探成功与否直接关系到能否缓解成都气紧的局面,并为“川气出川”宏伟规划的顺利实施提供充足的资源保障,因此,开展本项目的研究具有明显的经济效益和显著的社会效益。
本文通过借鉴川西致密碎屑岩的成功勘探经验,以大地构造学理论、沉积盆地分析理论、层序地层学理论、沉积学理论和石油地质学理论为指导,以盆地演化-层序演化-沉积演化为主线,以自流井组及其同时异相地层为重点研究层位,通过野外露头剖面、钻井和地震及其它物探资料相结合,野外宏观观测与室内微观分析相结合,盆地整体性研究与重点区块研究相结合,建立相对等时的层序地层格架,完成层序地层格架内层序-沉积相对比。并以三级层序体系域为编图单元,编制具有精确性、等时性、成因连续性和勘探实用性等优点的层序岩相古地理图。在此基础上,结合各体系域期的沉积等厚图,综合认识不同时期层序地层格架内岩相古地理及其演化特征,并尝试探讨层序地层格架内沉积充填的控制因素。最终为认识盆地演化过程中储集体的时空展布规律和预测有利的储层发育区、安排部署下一步勘探计划提供理论支持。
我们将川东北地区下侏罗统地层划为一个构造层序(二级层序),两个三级层序(即SQ1和SQ2),又将各三级层序进一步划分出了湖进体系域(PST)和湖退体系域(RST)。通过综合研究认为:受印支晚期构造运动抬升影响,SQ1-PST期大量沉积物被带入盆地,在盆地短轴方向发育辫状河三角洲相沉积;在盆地长轴方向发育三角洲相沉积。SQ1-RST期研究区西北部物源供给逐渐减少,而研究区东北部物源供给也呈逐渐减少态势,在盆地边缘主要发育冲积平原-三角洲平原,向盆地内部依次发育滨湖-三角洲前缘、浅湖-前三角洲、浅湖、浅湖-半深湖亚相沉积。SQ2-PST期研究区西北部物源供给很少,仅有研究区东北部提供少量物源,研究区主要发育滨浅湖-浅湖-半深湖亚相沉积。SQ2-RST期受盆地边缘构造缓慢抬升影响,来自研究区西北部和东北部的物源供给逐渐加大,加之湖盆沉降,滨湖-浅湖-半深湖的分带更加明显,研究区在有物源注入处发育三角洲前缘亚相沉积,在无物源注入处依次向盆地内部发育滨湖-浅湖-半深湖亚相沉积。
川东北地区下侏罗统层序地层格架内沉积充填的控制是多方面的,但最主要原因是构造活动,其次为气候变化。盆地边缘的抬升与沉降直接关系到沉积物的供给丰富与否,而湖盆基底的构造升降直接影响到沉积相带的迁移。盆地边缘构造升降活动以及湖盆的构造升降共同决定了沉积中心的迁移。而气候变化对层序地层格架内的沉积充填起着一定的辅助作用,即温暖潮湿气候驱使湖平面上升和水域扩大、淡化,大量流水带来了丰富的沉积物供应;而干旱气候使湖平面趋于下降,水域收缩、咸化,河流、三角洲的作用变得不那么明显。
The northeast area of Sichuan province is one of most advantageous reserve block of exploration for oil and gas, has a lot of exploration targets and large oil and gas reserves, and become the force of exploration for oil and gas in Sichuan basin. The success in exploration for oil and gas has the great significance in relieving pressure of LNG supply in the city of Chengdu and ensure success of ambitious plan of“The gas from Sichuan province is supplied to other province”. Therefore, this research has obviously economic benefits and social benefits.
This article based on successful exploration experience in detrital rocks of west Sichuan, guided by the theory of geotectology, study of sedimentary basin, sequence stratigraphy, sedimentology and petroleum geology, followed the main line from evolution of basin, evolution of sequence to evolution of sedimentation. We put emphasis on Ziliujing formation and its correlated strata, combined with the research of outcrops in field, datum of well drilling and seismic exploration, combined micro measurement in field with indoor macro analysis, combined the research of integrity of basin with the research of important areas, established relatively isochronic framework of sequences, finished the correlation between sequence and sedimentary facies in framework of sequences. We utilized the system tract of third-order sequence as the unit for mapping, mapped sequence-based lithofacies and paleogeography map which had many advantages as follow accuracy, isochronism, origin-continuity, exploration practicability and so on. Combined the isopach map of each system tract period, We can comprehensively realize the characteristics of lithofacies paleography and evolution in framework of sequences, then we can try to discuss the factors of sedimentary filling. Ultimate objective is to provide theoretical support for searching and ascertaining potential reservoirs and regularity of distribution, in order to arrange the next program of exploration. We divided lower Jurassic into a tectonic sequence(second-order sequence), two
third-order sequences (SQ1 and SQ2) which can be divided into PST and RST. Through comprehensive research, we believe that : In period of SQ1-PST, affected by tectonic movement, great loads of sediment was carried into basin, shorter axis direction of basin developed braid river deltas, longer axis direction of basin developed (Meandering)deltas; In period of SQ1-RST, the provenance from both northwest and northeast of research area was on the decline, mainly developed alluvial plain and delta plain in boundary of the basin. From boundary to centre of the basin consecutively developed shore lacustrine - delta front, shallow lacustrine - prodelta, shallow lacustrine, shallow lacustrine -semi-deep lacustrine; In period of SQ2-PST, the provenance from both northwest and northeast of research area was a small amount, mainly developed shore lacustrine, shallow lacustrine, semi-deep lacustrine. In period of SQ2-RST, affected by slowly tectonic uplift in boundary of the basin, the provenance from both northwest and northeast of research area was increasingly rising. With depression in basin, it is obvious to zone between shore lacustrine, shallow lacustrine and semi-deep lacustrine. In supplying place of provenance developed deltas and in no-supplying place of provenance consecutively developed shore lacustrine, shallow lacustrine and semi-deep lacustrine towards basin.
The control factors in framework of sequences of lower Jurassic in the northeast area of Sichuan province were various, but the factors were mainly tectonic activity and secondly climate change. Lifting or subsidence in boundary of the basin was related to sediment supply, while lifting or subsidence in basin affected migration of sedimentary facies zones. Both tectonic activity in boundary of the basin and in basin ensure migration of depocenter. The climate change decided the change of relative lake-level. In warm, wet climates, relative lake-level rised, water area expanded, water desalted, with mass stream sediment was carried to basin. While in dry climates, relative lake-level falled, water area shrinked, water salted, streamaction and deltafication were not relatively obvious.
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