稳定构造背景下气候驱动的陆相河流层序地层模式研究

It is not appropriate to research a terrestrial fluvial sequence by applying an offshore fluvial sequence stratigraphic model for the different controlling factors, but it is still vacant to build a terrestial fluvial model based on its controlling factors. The fluvial deposits accumulated in the 1st Member of Shanxi Formation and the 8th Member of Xiashhezi Formation in the area north of Sulige Temple, Ordos basin, China, are driven by climatic cycles superimposed on a steady tectonic regime, as will be researched to build a terrestrial fluvial sequence stratigraphic model in this project. (1) Identification marks of paleosol are found by core celibrating log, then parasequences recognization and correlation are carried out in a connecting-well section on the basis of identified paleosol layers as well as flooding surfaces. At the same time, base level cycle correlation is also carried out to examine the above parasequence correlation to confirm the parasequence correlation program. The next is to count the sand/strata ratio, the average grain size of sand rock, the average thickness of single sand body and the matrix content in sand body in every parasequence, then the vertical evolution of sedimentary characteristics is summaried. (2) According to the frequencies of the long and short eccentricity cycle of the key well from the cathment area, the curves relevant to the long and short eccentricity cycle are disassembled from the original gamma-ray (GR) and sprectral gamma-ray (SGR). Then the relevance between the disassembled curves and the paleoclimate indicators is found. On the basis of the relevance, the paleoclimate curves corresponding to the two eccentricity cycles respectively are inversed. (3) The responding relationship between the paleoclimate curves and sequence sedimentary characteristics is analysed, then the terrestrial fluvial sequence stratigraphic model driven by climatic cycles superimposed on the steady tectonic regime is built based on that responding relationship. This model will enrich fluvial sedimentology theory, and will be very important to terrigenous fluvial oil and gas exploration.

近海河流层序模式的应用范围有限，而基于控制因素的陆相河流层序模式尚未建立。鄂尔多斯盆地苏里格庙以北的山1段和盒8段是稳定构造背景下气候控制的陆相河流沉积，本项目拟以此为对象，建立陆相河流层序模式。首先，通过岩心刻度测井建立古土壤层的测井识别标志，结合洪泛面，在连井剖面上划分对比准层序，同时进行基准面旋回对比，相互校验对比结果，确定准层序划分方案。统计各准层序的砂地比、砂体平均粒度、单砂层平均厚度、砂体杂基含量等，分析层序内部沉积特征的垂向变化。然后，根据汇水区关键井的长、短偏心率周期的频率，从其GR和SGR曲线中分解出与长、短偏西率周期对应的曲线，分析其与古气候指标的相关性，据此反演长、短偏心率周期的古气候变化曲线。最后，研究古气候变化曲线与层序内部沉积特征的响应关系，建立稳定构造背景下气候驱动的陆相河流层序地层模式。这一研究将丰富河流沉积学理论，对于陆相河流储层的勘探有重要的参考价值。

陆相河流层序中准层序及体系域的等时划分与对比、内部沉积特征演化及其控制因素是层序地层学研究和油气勘探中的重要课题之一。本项目引入古土壤层建立等时河流层序格架，并探讨了古气候对层序内部建造的影响。对鄂尔多斯盆地中二叠统下石盒子组盒8段辫状河体系中的岩心和测井资料进行详细分析，筛选出高CNL/DEN和高CNL等为在地下识别古土壤的有效测井指标，据此并结合大同辫状河露头特征对盒8段进行等时的准层序划分与对比，然后根据各准层序的砂体厚度、砂地比在垂向上的变化定量划分体系域，从而建立了等时层序地层格架。研究结果表明，与近海河流层序不同，陆相河流层序由于可容空间有限，层序之间切割严重，高位体系域不发育，甚至水进体系域往往也是不完整的。根据GR曲线提取的米氏周期与反应古气候的地化指标之间没有明确的相关性，即古气候变化对陆相河流层序中的准层序级别的沉积建造的影响不明确，但对层序级别的影响是存在的，干旱半干旱气候容易形成以辫状河为主的河流层序。项目还研究了辫状河与曲流河露头特征、辫状河储层流动单元特征以及受气候影响的洪水对河道弯曲度的控制作用。相关研究对于陆相河流层序特征、河流相油气储层开发中水平井的设计与地质导向均有重要的借鉴意义。

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