松辽盆地古龙凹陷白垩系青山口组页岩层序等时格架下的有机质分布规律

Distribution of organic matter in the Qingshankou Formation Shale,Gulong Sag,Songliao Basin observed within an isochronous sequence stratigraphic framework

松辽盆地古龙凹陷白垩系青山口组陆相纯页岩型页岩油获得高产突破,但古龙页岩有机质分布的非均质性强,主控因素不清,制约有利区和甜点段预测。通过精细分析岩心、露头、薄片、地震、测井、地球化学和古生物等数据,针对湖相深水页岩特点,基于“层序等级”与“湖侵-湖退(T-R)旋回”理论,结合天文旋回研究进展,建立了深湖区高频层序等时格架。通过类比现代湖泊,建立了古湖泊沉积环境判识指标,分析了古生产力、氧化还原环境及沉积速率的耦合关系,探讨了高频层序格架下有机质富集非均质性的成因。研究认为:①古龙凹陷青山口组可划分为4个三级层序,其中,层序1和层序2识别出2个T-R旋回,由13个准层序组(52个准层序)构成;准层序和准层序组的沉积时长分别为约40 kyr和170 kyr。②层序格架下页岩发育3种岩相、4种纹层、5种组合类型、11种纹层组合模式和3种沉积微相;T-R旋回控制了沉积微相、岩相和纹层组合的纵向分布,其中,深湖相静水沉积和泥流沉积的黏土质页岩为有利岩相。③T-R旋回控制了有机质分布,湖泛面附近是有利部位;有利区/段为古龙凹陷的准层序组2以及三肇凹陷的准层序组1—4。该研究成果能够为页岩油有利区和甜点段预测提供沉积学依据。

The Qingshankou Formation shale,Gulong Sag,Songliao Basin,has been gushing out oil in a rate stunning the Chinese oil industry.However,further prediction of sweet spots in the formation remains a challenge due to the highly uneven distribution of organic matter and confusion in the understanding of controlling factors.This study adopted the concepts of“hierarchy in sequence stratigraphy”and“transgressive-regressive(T-R)sequences”,with updated astronomical cycle research results to determine the accurate duration of sequences and re-establish an isochronous sequence stratigraphic framework specifically for lacustrine deep-water shale based on core,outcrop,and thin section observation,as well as seismic profile,well-logging,geochemical,and paleontological data analyses from a microcosmic to macroscopic scale.Subsequently,using modern lakes sedimentation as an analogy made it possible to propose qualitative and quantitative indexes for identifying paleo-water environment,and probe into the origin of the heterogeneity of organic matter enrichment under the sequence stratigraphic framework on the basis of the coupling relation among paleoproductivity,redox conditions,and sedimentation rate.The main conclusions are as follows:1)There are four third-order sequences in Qingshankou Formation Shale,Gulong Sag,Songliao Basin.Among them,SQ1 and SQ2 consist of two T-R sequences of 13 parasequence sets(made up of 52 parasequences).The durations of parasequence and parasequence set are approximately 40 kyr and 170 kyr,respectively.2)Three types of lithofacies,four types of laminae,five laminae combinations,eleven laminae combination patterns,and three sedimentary microfacies have been recognized under the sequence stratigraphic framework.T-R cycles control the vertical distribution of laminae combination,lithofacies and sedimentary microfacies.The argillaceous shales deposited in deep-lake stagnant water and mud flow are prospective lithofacies.3)T-R cycles control the enrichment of organic matter and different orders of flooding surfaces are the favorable locations for organic matter enrichment.Parasequence set 2 in Gulong Sag and parasequence sets 1 to 4 in Sanzhao Sag have been evaluated to be the most promising exploration targets.This study aims to provide sedimentological evidence for shale oil target area and“sweet spots”prediction.