强制海退体系域中烃源岩的类型与测井评价——以尼日尔Termit盆地为例

Source Rock Types and Logging Evaluation in Forced Regressive Systems Tract: Taking the Termit Basin( Niger) as an Example

强制海退体系域由于可发育优质的储集层而备受石油地质学者关注,但关于强制海退体系域中烃源岩的研究仍然十分缺乏。根据岩性录井、测井曲线和地球化学分析数据,以尼日尔Termit盆地为例探讨强制海退体系域中烃了源岩的类型及其识别特征和地球化学特征,并通过改进的ΔlogR方法建立烃源岩总有机碳(TOC)含量测井预测模型,初步分析了不同类型烃源岩的发育特征和质量差异。结果表明:强制海退体系域中可以发育煤/炭质泥岩、滨岸泥岩、三角洲泥岩和滨外陆棚泥岩4种烃源岩,其中滨外陆棚泥岩发育于强制海退体系域的底部,而煤/炭质泥岩以薄层状伴生于盆地边缘附近滨岸沼泽相和三角洲相。基础地球化学数据表明滨岸泥岩主要为II2型干酪根的好—极好烃源岩,三角洲泥岩为非均质性强烈的差—好烃源岩,而滨外陆棚泥岩整体为Ⅲ型干酪根的差—中等烃源岩。应用lgR-Δt交会图求取ΔlogR值,建立TOC测井预测计算模型,计算了其他井的TOC预测曲线,结果表明Termit盆地强制海退体系域中的泥岩基本为好—极好烃源岩,不同构造区块中差别不大,其中滨岸泥岩为主要的烃源岩类型,累计厚度大且生烃潜力高。从强制海退体系域的演化过程及Termit盆地实例研究来看,强制海退体系域中在一定条件下可以发育优质烃源岩,滨岸泥岩可能是其主要的烃源岩类型。

Nowadays,petroleum geologists have paid more attention to the Forced Regressive Systems Tract(FRST)for the development of effective reservoir rocks therein,however,studies focused on potential source rocks of FRST are rare.In this paper,a combination of lithology,well logging and geochemical analysis data of Termit Basin(Niger)was used to investigate the types,well logging identification and geochemical characteristics of source rocks in FRST.Furthermore,the development of different source rocks have been preliminarily analyzed on the basis of the total organic carbon(TOC)prediction model established by an improvedΔlog R method.Results indicate that 4 types of source rocks occur in FRST:coal/coaly mudstone,shoreface mudstone,deltaic mudstone and offshore mudstone.Offshore source rocks vertically distribute in the bottom sections of FRST,and the shoreface source rocks widely occurred in FRST while the thin coal/coaly mudstone existed associating with coastal marsh and deltaic environment.Shoreface mudstones are classified as good to excellent source rocks with type II 2 kerogen and high TOC content;the deltaic mudstones consist of poor to good source rocks with higher heterogeneity than others,while the offshore source rocks are poor due to the dominance of Type III kerogen.In this study,a TOC prediction model was established with the value ofΔlog R acquired from a plot of lg R versusΔt.TOC prediction curves from the model of different wells in Termit Basin indicate that most of FRST source rocks in different tectonic blocks have a good to excellent hydrocarbon potential,mainly consist of shoreface source rocks with larger cumulative thickness and higher hydrocarbon potential than others.Based on the evolution progress and the examples of Termit Basin,we can draw the conclusion that excellent source rocks could be occurred in the FRST,and the shoreface mudstones might be their main types of source rocks.