致密砂岩裂缝充填机制及其对储层物性的影响——以鄂尔多斯盆地三叠系延长组为例

The filling characteristics of fractures in tight sandstone reservoirs directly constrain the connectivity and open degree of fractures and cracks, and affect the characteristics of reservoir and accumulation of oil and gases. However, the filling progresses of fractures are controlled by water-rock interaction in reservoir. So, it is helpful to understand the forming mechanism of fractured-reservoir in tight sandstone by study of filling progresses of fractures in reservoir through fluids and diagenesis analysis. In this project, we take the fractures in tight sandstone reservoir of Triassic Yanchang Formation in Longdong area, Ordos Basin as a case study. Based on the analyses of outcrop, core samples, logging data, and seismic data, the mechanical characteristics of fractures will be described in detail. Meanwhile, combining with series of experimental analyses (including fine petrographic observation, cathodoluminescence, fluid inclusions, trace elements, etc.), the generations of fractures, filling characteristics of different generation fractures will be determined, and the characteristics of temperature field, pressure field, and fluid field in different evolution stages of fractures will be reconstructed. On this basis, we will conduct an in-depth analysis on filling mechanism of different types of fractures from the perspective of water-rock interaction, combining with the reservoir physical parameters, the effect of filling on variations of porosity and permeability in fractures will be analyzed deeply, and the physical properties of fractured reservoir will be classified according to filling characteristics. The goal of this project is to establish a systematic research method about water-rock interaction of fractures in tight sandstone reservoir. We think the results of our study can provide the important theoretical basis and guidance for the fine exploration and production in similar petroliferous basins.

储层裂缝的充填特征直接影响裂缝的连通性和开启程度，进而影响了致密砂岩储层物性和油气成藏，而裂缝的充填过程与水岩作用密切相关，因此，通过水岩作用机理分析储层裂缝充填机制研究有助于深入了解致密砂岩储层成因机制。本项目拟选择鄂尔多斯盆地陇东地区三叠系延长组为研究对象，基于野外露头、岩心、地震与测井资料研究裂缝几何-力学特性，通过精细岩相学、阴极发光、流体包裹体、同位素和微量元素等实验分析，确定裂缝发育期次与形成时间，明确不同期次裂缝的充填特征，恢复裂缝演化有关的温度场、压力场和流体场演化特征；通过水-岩作用机理分析剖析不同类型裂缝的充填机制；结合储层物性参数分析裂缝充填作用对裂缝孔渗变化的影响，评价裂缝对储层物性的影响。通过本项目的研究，以期建立一套系统的适用于致密砂岩储层中裂缝充填机制定量分析的研究方法，并为同类型含油气盆地的致密砂岩储层裂缝研究提供理论依据和实践指导。

本项目以鄂尔多斯盆地陇东地区三叠系延长组（长6、长7和长8）为研究对象，基于露头、岩心和古地磁分析，研究了储层裂缝的几何-力学特性。然后，通过精细岩相学、流体包裹体、元素和同位素分析，重点从水-岩作用角度系统研究了裂缝充填特征和充填机理，探讨了裂缝对储层物性、储层成岩作用和油气成藏的影响和相互关系。取得的认识包括：（1）研究区发育2期共轭剪切裂缝系统，其中第一期北西-南东向和第二期北东-南西向裂缝发育程度高且呈正交型式产出，第一期近东西向和第二期近南北向裂缝发育受限。（2）明确了2期裂缝的宏观发育特征和形成时间。（3）建立了砂岩定向流体包裹体薄片和与之配套薄片的制作方法。（4）明确了储层岩石学及成岩作用特征。其中，储层发育2期碳酸盐胶结物。（5）被充填的裂缝主要是第一期，第二期裂缝基本未充填。长8裂缝充填物为方解石，长6和长7的为含铁方解石。（6）系统总结了裂缝胶结物显微结构特征。其中，开启-愈合结构在长8裂缝胶结物中普遍发育，而在长6和长7中极少见。（7）明确了裂缝形成与充填相关流体特征。长6和长7裂缝充填过程伴随大量烃类流体活动。（8）长8裂缝充填与裂缝开启同步，为侏罗纪末期；长6和长7裂缝充填滞后于裂缝开启期，为早白垩世中-晚期。（9）恢复了裂缝充填阶段地层的温、压特征，明确了不同层段裂缝充填期地层均处于超压。（10）长8裂缝胶结物富Mn、贫Fe，其形成与有机质厌氧发酵产生的碳酸有关。长6和长7裂缝胶结物富Fe、贫Mn，其形成与烃类流体活动有关。（11）综合成脉温压条件、元素组成、物质来源、胶结结构和速率及围岩岩石学特征，剖析了裂缝充填机制。（12）探讨了裂缝空间分布及其充填演化对储层物性的影响。（13）储层仅存在一期原油充注（早白垩世中-晚期），第一期裂缝的发育与充填对油气成藏影响较大。（14）明确了储层成岩作用对裂缝发育的控制以及储层致密化与油气成藏的时序关系。